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Title: Geology, Vol. 1 [of 3]
Geologic processes and their results

Author: Thomas C. Chamberlin
Rollin D. Salisbury

Release date: August 27, 2024 [eBook #74316]

Language: English

Original publication: New York: Henry Holt and Company, 1906

Other information and formats: www.gutenberg.org/ebooks/74316

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*** START OF THE PROJECT GUTENBERG EBOOK GEOLOGY,
VOL. 1 [OF 3] ***

AMERICAN SCIENCE SERIES—ADVANCED COURSE

GEOLOGY
BY

THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY
Heads of the Departments of Geology and Geography, University of Chicago
Members of the United States Geological Survey
Editors of the Journal of Geology

IN THREE VOLUMES

VOL. I.—GEOLOGIC PROCESSES AND THEIR RESULTS

SECOND EDITION, REVISED

NEW YORK

HENRY HOLT AND COMPANY
1909

Copyright, 1904,
BY
HENRY HOLT AND COMPANY

PRINTED IN THE U. S. A.

PREFACE.
In the preparation of this work it has been the purpose of the authors to
present an outline of the salient features of geology, as now developed,
encumbered as little as possible by technicalities and details whose bearings
on the general theme are unimportant. In common with most writers of text-
books on geology, the authors believe that the subject is best approached by
a study of the forces and processes now in operation, and of the results
which these forces and processes are now bringing about. Such study
necessarily involves a consideration of the principles which govern the
activities of geologic agencies. These topics are presented in Volume I, and
prepare the way for the study of the history of past ages, which is outlined
in Volume II.
The general plan of the work has been determined by the experience of
the authors as instructors. Little emphasis is laid on the commonly
recognized subdivisions of the science, such as dynamic geology,
stratigraphic geology, physiographic geology, etc. The treatment proceeds
rather from the point of view that the science is a unit, that its one theme is
the history of the earth, and that the discussions of dynamic geology,
physiographic geology, etc., apart from their historical bearing, lose much
of their significance and interest. The effort has, therefore, been to
emphasize the historical element, even in the discussion of special themes,
such as the work of rivers, the work of snow and ice, and the origin and
descent of rocks. This does not mean that phases of geology other than
historical have been neglected, but it means that an effort has been made to
give a historical cast to all phases of the subject, so far as the topics permit.
Throughout the work the central purpose has been not merely to set forth
the present status of knowledge, but to present it in such a way that the
student will be introduced to the methods and spirit of the science, led to a
sympathetic interest in its progress, and prepared to receive intelligently,
and to welcome cordially, its future advances. Where practicable, the text
has been so shaped that the student may follow the steps which have led to

present conclusions. To this end the working methods of the practical
geologist have been implied as frequently as practicable. To this end also
there has been frankness of statement relative to the limitations of
knowledge and the uncertainty of many tentative conclusions. In these and
in other respects, the purpose has been to take the student into the fraternity
of geologists, and to reveal to him the true state of the development of the
science, giving an accurate and proportionate view of the positive
knowledge attained, of the problems yet unsolved, or but partially solved,
and of solutions still to be attained.
The theoretical and interpretative elements which enter into the general
conceptions of geology have been freely used, because they are regarded as
an essential part of the evolution of the science, because they often help to
clear and complete conceptions, and because they stimulate thought. The
aim has been, however, to characterize hypothetical elements as such, and
to avoid confusing the interpretations based on hypothesis, with the
statements of fact and established doctrines. Especial care has been taken to
recognize the uncertain nature of prevalent interpretations when they are
dependent on unverified hypotheses, especially if this dependence is likely
to be overlooked. If this shall seem to give prominence to the hypothetical
element, it should also be regarded as giving so much the more emphasis to
that which is really trustworthy, in that it sets forth more frankly that which
is doubtful. Hypothetical and unsolved problems have been treated, so far
as practicable, on the multiple basis; that is, alternative hypotheses and
alternative interpretations are frequently presented where knowledge does
not warrant positive conclusions.
In many cases the topics discussed will be found to be presented in ways
differing widely from those which have become familiar. In some cases,
fundamentally new conceptions of familiar subjects are involved; in others,
topics not usually discussed in text-books are stated with some fullness; and
in still others, the emphasis is laid on points which have not commonly
been brought into prominence. Whether the authors have been wise in
departing to this extent from beaten paths, the users of the volumes must
decide.
The work is intended primarily for mature students, and is designed to
furnish the basis for a year’s work in the later part of the college course. By
judicious selection of material to be presented and omitted, the volumes

will be found useful for briefer courses, and by the use of the numerous
references to the fuller discussions of special treatises, they may be made
the basis for more extended courses than are commonly given in
undergraduate work. The attempt has also been made to make the volumes
readable, in the belief that many persons not in colleges or universities will
be interested in following a connected account of the earth’s history, and of
the means by which that history is recorded and read. Antecedent
elementary courses in geology will not be necessary to the use of these
volumes, though such courses may be helpful.
The arrangement of themes adopted is such as to bring to the fore
processes with which all students are immediately in contact, and which are
available for study at all seats of learning. The commoner geologic agents,
such as the atmosphere and running water, have been elaborated somewhat
more fully than is customary, and the common rather than the exceptional
phases of the work of these agents have been emphasized, both because of
their greater importance and their universal availability. The text has been
so shaped as to suggest field work in connection with these topics
especially, since work of this sort is everywhere possible.
After the preliminary outline, which is intended to give some idea of the
scope of the science, and of its salient features, and to show the relations of
the special subjects which follow, the order of treatment is such as to pass
from the commoner and more readily apprehended portions of the subject to
those which are less readily accessible and more obscure. Following the
same general conception, the treatment of the topics is somewhat graded,
the earlier chapters being developed with greater simplicity and fullness,
while the later are somewhat more condensed.
Many acknowledgments are made in the text and foot-notes, but it is
impossible to adequately acknowledge all the sources which have been
drawn upon, since the whole body of literature has been laid under
contribution. The authors especially acknowledge the generous assistance
of Professor J. P. Iddings in connection with the chapter on The Origin and
Descent of Rocks; of Dr. F. R. Moulton, Professor C. S. Slichter, Professor
L. M. Hoskins, Mr. A. C. Lunn, and Mr. W. H. Emmons in connection with
mathematical problems; of Professor C. R. Barnes in connection with the
geologic functions of life; and of Professor Julius Stieglitz in connection
with chemical subjects.

The illustrations have been selected from numerous sources, which are
usually acknowledged in the text. Especial acknowledgment is due to the
U. S. Geological Survey for the use of numerous photographs and maps,
and to Mr. G. A. Johnson, who has made many of the drawings reproduced
in Volume I. The authors are under even larger obligations for assistance in
the preparation of Volume II, for which acknowledgment will be made in
the proper place.
University of Chicago, January, 1904.

CONTENTS.
VOLUME I.
CHAPTER I.
PRELIMINARY OUTLINE.

Subdivisions, 1. Dominant processes, 2.
PAGE

Astronomic Geology 2
The earth as a planet, 2. Its satellite, 3. Dependence on
the sun, 4. Meteorites, 4.
Geognosy 5
I. The Atmosphere 5
Mass and extent, 6. Geologic activity, 6. A thermal
blanket, 7.
II. The Hydrosphere 7
Oceanic dimensions, 8. Geologic activity, 8. Chief
horizons of activity, 9.
III. The Lithosphere 9
Irregularities, 10. Epicontinental seas, 11. Diversities of
surface, 12. The surface mantle of the lithosphere, 12. The
crust of the lithosphere, 13. The interior, 14. Varieties of
rock in crust, 14. Stratified rocks, 14. Conformability, 15.
Relative ages, 15. The crystalline rocks, 16. Four great
sedimentary eras, 17. The Archean complex, 18.
General Table of Geologic Divisions 19

CHAPTER II.

THE ATMOSPHERE AS A GEOLOGICAL AGENT.
The Atmosphere as a Direct Agency 21
I. Mechanical Work 21
Transportation and deposition of dust, 22. Transportation
and deposition of sand, 25. Formation of dunes, 26. Shapes
of dunes, 26. The topographic map, 30. Topography of dune
areas, 32. Migration of dunes, 33. Distribution of dunes,
35. Wind ripples, 37. Abrasion by the wind, 38. Effects of
wind on plants, 40. Indirect effects of the wind, 41.
II. Chemical Work 41
Precipitation from solution, 41. Oxidation, 42.
Carbonation, 43. Other chemical changes, 43. Conditions
favorable for chemical changes, 43.
The Atmosphere as a Conditioning Agency 43
I. Temperature Effects 44
II. Evaporation and Precipitation 50
III. Effects of Electricity. 52
Summary 54

CHAPTER III.
THE WORK OF RUNNING WATER.
Rain and River Erosion 57
Subaërial Erosion without Valleys 58
The Development of Valleys 63
By the growth of gullies, 63. Limits of growth, 67. The
permanent stream, 70. Other modes of valley development,
73. Structural valleys, 77. The courses of valleys, 77. The
development of tributaries, 78.
A Cycle of Erosion. Its Stages 80
General Characteristics of Topography Developed by River
92
Erosion

Special Features Resulting from Special Conditions of Erosion 92
Bad-land topography, 93. Special forms of valleys;
canyons, 94.
The Struggle for Existence Among Valleys and Streams 100
Piracy, 103.
Rate of Degradation 105
Material in solution in river water. 107.
Economic Considerations 108
Analysis of Erosion 110
Weathering 110
Transportation 115
Transporting power and velocity, 115. How sediment is
carried, 116.
Corrasion 119
Abrasion, 119. Solution, 122.
Conditions Affecting the Rate of Erosion 123
The Influence of Declivity 123
The Influence of Rock 124
Physical constitution, 124. Chemical composition, 124.
Structure, 125.
The Influence of Climate 110
Effects of Unequal Hardness 132
Rapids and falls, 132. Rock terraces, 140. Narrows,
141. Other effects on topography, 142. Adjustment of
streams to rock structures, 146.
Influence of Joints and Folds 150
Joints, 150. Folds, 154.
Effect of Changes of Level 132
Rise, 161. Sinking, 170. Differential movement,
warping, 171.
The Aggradational Work of Running Water 177
Principles involved, 177.

The Deposits 181
Types, 181. Alluvial fans and cones, 181. Ill-defined
alluvium, 183. Alluvial plains, 184. Flood-plains due to
alluviation, 186. Flood-plains due to obstructions, 188.
Levees, 188. Flood-plain meanders, cut-and-fill, 190.
Scour-and-fill, 194. Materials of the flood-plain, 196.
Topography of the flood-plain, 196. Topographic adjustment
of tributaries, 197. River-lakes, 198. Deltas, 198. Delta
lakes, 204.
Stream Terraces 204
Due to inequalities of hardness, 204. Normal flood-plain
terraces, 205. Flood-plain terraces due to other causes, 208.
Discontinuity of terraces, 209. Termini of terraces, 210.

CHAPTER IV.
THE WORK OF GROUND (UNDERGROUND) WATER.
Conditions influencing descent of rain-water, 213.
Supply of ground-water not altogether dependent on local
rainfall, 215. Ground-water surface—water table, 215.
Depth to which ground-water sinks, 216. Movement of
ground-water, 220. Amount of ground-water, 221. Fate of
ground-water, 221.
The Work of Ground-water 222
Chemical Work 222
Quantitative importance of solution, 223. Deposition of
mineral matter from solution, 225.
Mechanical Work 226
Results of the Work of Ground-water 226
Weathering, 226. Caverns, 227. Creep, slumps, and
landslides, 231.
Summary 232
Springs and Flowing Wells 234

Mineral matter in solution, 235. Geysers, 236. Artesian
wells, 242.

CHAPTER V.
THE WORK OF SNOW AND ICE.
Snow- and Ice-fields 244
The passage of snow into névé and ice, 246. Structure of
the ice, 247. Texture, 247. Inauguration of movement, 248.
Types of Glaciers 251
The General Phenomena of Glaciers 256
Dimensions, 256. Limits, 258. Movement, 259.
Conditions affecting rate of movement, 261. Likenesses and
unlikenesses of glaciers and rivers, 262.
Surface Features 266
Topography, 266. Surface moraines, 266. Relief due to
surface débris, 268. Dust-wells, 269. Débris below the
surface, 272.
Temperature, Waste, and Drainage 273
The winter wave, 274. The summer wave, 276. The
temperature of the bottom, 276. Temperature of the interior
of the ice, 277. Compression and friction as causes of heat,
278. Summary, 279. Movement under low temperature,
279. Evaporation, 279. Drainage, 280.
The Work of Glaciers 281
Erosion and Transportation 281
Getting load, 282. Conditions influencing rate of
erosion, 283. Summary, 286. Varied nature of glacial débris,
286. The topographic effects of glacial erosion, 287. Fiords,
290. The positions in which débris is carried, 290. Transfers
of load, 292. Wear of drift in transit, 298.
Deposition of the Drift 298
Beneath the body of the ice, 298. At ends and edges of
glaciers, 299.

Types of Moraines 301
The terminal moraine, 301. The ground moraine, 301.
The lateral moraines, 302. Distinctive nature of glacial
deposits, 304. Glaciated rock surfaces, 304.
Glacio-Fluvial Work 305
Icebergs 307
The Intimate Structure and the Movement of Glaciers 308
The growth and constitution of a glacier, 308. The
arrangement of the crystal axes, 312.
The Probable Fundamental Element in Glacial Motion 313
Melting and freezing, 313. Accumulated motion in the
terminal part of a glacier, 316.
Auxiliary Elements 317
Shearing, 317. High temperature and water, 318.
Applications, 319.
Corroborative Phenomena 320
Other Views of Glacier Motion 321

CHAPTER VI.
THE WORK OF THE OCEAN.
Volume and composition, 324. Topography of bed, 326.
Distribution of marine life, 328.
Processes in Operation in the Sea 329
Diastrophism, 329. Vulcanism, 332. Gradation, 333.
Movements of the Sea-water 334
Differences in density and their results, 335. Differences
in level and their results. 335. Movements generated by
winds, 336. Movements generated by attraction, 322.
Aperiodic movements, 338. Summary, 339.
Waves 339
Wave-motion, 339.
Work of the Waves 342

Erosion 342
By waves and undertow, 342.
Topographic Features Developed by Wave Erosion 349
The sea-cliff, 349. Chimney rocks, etc., 350. Sea caves,
350. The wave-cut terrace, 351. Wave erosion and
horizontal configuration, 353.
Transportation by Waves 354
Deposition by Waves, Undertow, and Shore Currents 355
The beach, 355. The barrier, 356. The spit, the bar, and
the loop, 357. Wave-built terraces, 363.
Effect of Shore Deposition on Coastal Configuration 363
Summary of Coastal Irregularities 364
The Work of Ocean-currents 366
Deposits of the Ocean-bed 368
Shallow-water Deposits 369
Littoral deposits, 369. Non-littoral, mechanical deposits
in shallow water, 369. Characteristics of shallow-water
deposits, 373. Topography of shallow-water deposits, 374.
Chemical and organic deposits, 375. Limestone, 378.
Deep-sea Deposits 378
Contrasted with shallow-water deposits, 378. Sources,
380. Mechanical inorganic deposits, 380. Organic
constituents of pelagic deposits, 382. Chemical deposits,
383.
Lakes 386
Changes taking place in lakes, 387. Lacustrine deposits,
388. Extinct lakes, 388. Lake ice, 389. Saline lakes, 391.
Indirect effects of lakes, 392. Composition of lake waters,
392.

CHAPTER VII.
THE ORIGIN AND DESCENT OF ROCKS.

Composition of Igneous Rocks 395
Leading elements, 396. Union of elements, 397.
Formation of minerals, 397. Sources of complexity, 398.
The leading minerals of igneous rocks, 399. The feldspathic
minerals, 400. The ferromagnesian minerals, 400. Summary
of salient facts, 401.
The Nature of Molten Magmas 401
Time required in crystallization, 402. Successive stages
of crystallization, 403.
The Fragmental Products of Sudden Cooling 404
Pyroclastic rocks, 404.
The Glassy Rocks 406
The solid glasses. 406. The first stages of crystallization,
407. The obsidians, 407.
Special Structures 410
Flow structure, 410. Amygdaloids, 411.
The Porphyritic Rocks 411
The Phanerocrystalline Rocks 412
The phanerites, 412. The granites, 413. The syenites,
415. The diorites, 416. The gabbros, 416. The peridotites,
416. The basalts, 417. The dolerites, 417. General names,
418.
Derivation of Secondary Rocks 420
Regolith, 422. Disrupted products: arkose and wacke,
422. Disintegrated products, 422.
Classes of Sedimentary Rocks 422
Shales, sandstones, and conglomerates, 422. Limestones
and dolomites, 424. Precipitates, 424. Iron-ore beds, 425.
Silicious deposits, 425. Organic rocks, 426.
Internal Alterations of Rocks 426
Oxidation and deoxidation, 427. Solution and
deposition, 427. Hydration and dehydration, 428.
Carbonation and decarbonation, 429. Molecular
rearrangements, 431.

The Salient Features of Rock Descent 431
The Reascensional Process 432
Induration under ordinary pressures and temperatures,
432. Cavity filling, 436. Fissure filling; veins, 437.
Solution as well as deposition, 437. Concretions, 438.
Replacements and pseudomorphs, 439. Incipient
crystallization, 439.
Reconstruction under Exceptional Conditions 440
Slaty structure, 441. Foliation, schistosity, 443.
Metamorphism by heat, 446. Metamorphism by heat and
lateral pressure, 448. Deep-seated metamorphism, 449.
Completion of the rock cycle, 449.
Various Classifications and Nomenclatures 449
New System of Classification and Nomenclature 451
The Proposed Field System 451
The phanerites, 451. The aphanites, 452.
The Proposed Quantitative System 454
Reference List of the More Common Minerals 460
Reference List of the More Common Rocks 467
Ore Deposits 474
Concentration, 474. Exceptional and doubtful cases,
474. Original distribution, 475. Magmatic segregation, 475.
Marine segregation and dispersion, 476. Origin of ore
regions, 477. Surface residual concentration, 478.
Purification and concentration, 478. Concentration by
solution and reprecipitation, 479. Location of greatest
solvent action, 480. Short-course action, 481. Long-course
action, 481. Summary, 483. The influence of contacts, 484.
The effect of igneous intrusions, 484. The influence of rock
walls, 484.

CHAPTER VIII.
STRUCTURAL (GEOTECTONIC) GEOLOGY.

The structural phases which rocks assume, 486.
Structural Features of Sedimentary Rocks 486
Stratification, 486. Lateral graduation, 488. Special
markings, 489. Concretionary structure, 490. Secretions,
497.
Structural Features of Igneous Rocks 498
Structural Features Arising From Disturbance 500
Inclination and folding of strata, 500. Joints, 510.
Sandstone dikes, 514. Faults, 514. The significance of
faults, 521. Effect of faulting on outcrops, 522.

CHAPTER IX.
THE MOVEMENTS AND DEFORMATIONS OF THE
EARTH’S BODY (DIASTROPHISM).
Minute and Rapid Movements 526
Earthquakes 527
Points of origin, foci, 527. The amplitude of the
vibrations, 529. Destructive effects, 530. Direction of throw,
531. Rate of propagation, 532. Sequences of vibrations,
533. Gaseous emanations, 533. Distribution of earthquakes,
533.
The Geologic Effects of Earthquakes 534
Fracturing of rock, 534. Changes of surface, 534.
Effects on drainage, 535. Effects on standing water, 535.
Changes of level, 536.
Slow Massive Movements 537
Present movements, 538. Fundamental conceptions, 539.
Nearly Constant Small Movements 540
Reciprocal features, 541.
The Great Periodic Movements 542
Mountain-forming movements, 542. Distribution of
folded ranges, 543. Plateau-forming movements, 543.

Continent-forming movements, 544. Relations of these
movements in time, 545. Relations of vertical to horizontal
movements, 545. The squeezed segments, 546. The
depressed or master segments, 546. The differential extent
of crustal movements, 548.
The Causes of Movement 551
General Considerations 551
1. The centripetal agencies 552
Gravity, 552. Molecular and sub-molecular
attractions, 554. Cohesion and crystallization, 554.
Diffusion, 555. Chemical combination, 556. Sub-atomic
forces 556.
2. The resisting agencies 557
Heat, 557. All resistance perhaps due to motion, 558.
Alternative Views of Original Heat Distribution 559
Thermal distribution on the convection hypothesis, 559.
Level of no stress, 561. Thermal distribution on the
hypothesis of central solidification, 562. Thermal
distribution under the accretion hypothesis, 564.
Computed Pressures, Densities, and Temperatures within the
564
Earth Based on Laplace’s Law
Recombination of material, 568. Comparison of the
hypotheses, 568.
Observed Temperatures in Excavations 569
Explanations of varying increment, 570. The permeation
and circulation of water, 570. Chemical action, 570.
Differences in the conductivity of rock, 571. Compression,
571. Gradients projected, 571. The amount of loss of heat,
572. The amount of shrinkage from loss of heat, 572.
Other Sources of Deformation 574
Transfer of internal heat, 574. Denser aggregation of
matter, 574. Extravasation of lavas, 574. Change in the rate
of rotation, 575. Distribution of rigidity, 578.
Sphericity as a Factor in Deformation 580

The influence of the domed form of the surface, 581.
Theoretical strength of domes of earth-dimensions, 581.
Stress-accumulation independent of sphericity, 583. The
actual configuration of the surface, 584. Concave tracts,
584. General conclusion, 588.

CHAPTER X.
THE EXTRUSIVE PROCESSES.
Outward movements, 590.
Vulcanism 590
Phases of vulcanism, 591.
1. Intrusions 591
The heating action, 592.
2. Extrusions 592
Fissure eruptions, 593. Volcanic eruptions, 594.
Intermediate phenomena, 596. Lunar craters, 598.
Volcanoes 599
Number of, 599.
Distribution of Volcanoes 599
In time, 599. Relative to land and sea, 599. Relative to
crustal deformations, 601. In latitude, 603. In curved lines,
603.
Relations of Volcanoes 604
Relations to rising and sinking surfaces, 604. Relations
to one another, 605. Unimportant coincidences, 606.
Periodicity, 607.
Formation of Cones 608
Lava-cones, 608. Cinder-cones, 608. Subordinate cones,
610. Composite cones, 610. Extra-cone distribution, 610.
Lavas 612
Their nature, 612. Consanguinity and succession of
lavas, 614. Temperature of lavas, 615. Depth of source, 616.

Volcanic Gases 617
Differences in gas action, 617. Spasmodic action, 618.
Kinds of gases, 618. Residual gases in volcanic rock, 619.
The source of the gases, 621.
The Cause of Vulcanism 623
I. On the Assumption that the Lavas are Original 623
Lava outflows from a molten interior, 624. Lavas
assigned to molten reservoirs, 624.
II. On the Assumption that the Lavas are Secondary 625
Lavas assigned to the reaction of water and air
penetrating to hot rocks, 625. Lavas assigned to relief of
pressure, 627. Lavas assigned to melting by crushing, 628.
Lavas assigned to melting by depression, 629. Vulcanism
assigned to the outflow of deep-seated heat, 629.
Modes of Reaching the Surface 631
Additional Considerations Relative to the Gases 633
Thermal Considerations 635

CHAPTER XI.
THE GEOLOGIC FUNCTIONS OF LIFE.
I. The Distinctive Features of Organic Processes 638
The Chemical Work of Life 638
Life material chiefly atmospheric, 638. The non-
atmospheric factors, 639.
(1) Changes in the composition of the atmosphere 639
The consumption and restoration of carbon dioxide,
640. The freezing and consumption of oxygen, 640. The
organic residue, 640. The meaning of the organic residue,
641. The more inert factor, 642. Probable fluctuations of
atmospheric composition, 642. The climatic effects of
organic action, 643.
(2) Aid and hindrance to inorganic action 644

The promotion of disintegration, 644. Protection
against erosion, 644. The influence of land vegetation on
the character of the sediments, 645.
(3) Distinctive deposits 646
Organic rocks, 646. Inorganic rocks due to life, 646.
Fossils 646
The general order of life succession determined by
stratigraphy, 647. Fossils as means of correlation, 647.
Special Modes of Aggregation and of Movement 648
The Mental Element 649
(1) The material effects of the mental element, 649.
Human modification of the animal and vegetal kingdoms,
650. (2) The psychological factors as such, 651.
II. Special Contributions of the Organic Kingdoms 652
Contributions of the Plant Kingdom 652
Reference table of the principal groups of plants, 653.
The contribution of the Thallophytes, 653. The contribution
of the Bryophytes, 656. The contribution of the
Pteridophytes, 657. The contribution of the Spermatophytes,
657. Plant life terrestrial rather than marine, 658.
Contributions of the Animal Kingdom 658
Reference table of the principal groups of animals, 659.
The contribution of the Protozoa, 660. The contribution of
the Cœlenterata, 661. The contribution of the
Echinodermata, 661. The contribution of the Vermes, 662.
The contribution of the Molluscoidea, 662. The contribution
of the Mollusca, 662. The contribution of the Arthropoda,
662. The contribution of the Vertebrata, 663.
III. The Associations and Ecological Relations of Life 663
The Basis of Floras and Faunas 663
Assemblages Influenced by the Mutual Relations of Organisms 664
Food relations, 664. Adaptive relations, 665.
Competitive relations, 665. Offensive and defensive
relations, 665. Implied forms of life, 666.

Assemblages Influenced by Environment 666
Plant societies, 667.
The Influence of Geographic Conditions on the Evolution of
668
Floras and Faunas
The development of provincial and cosmopolitan faunas,
668. Restrictive and expansional evolution, 672.

PLATES.
PLATE FACE PAGE

I. Bathymetrical Chart of the Oceans 10
II. Fig. 1. New Jersey. Fig. 2. Kansas. Fig. 3. Indiana. Fig. 4.
Nebraska 39
III. Fig. 1. Kansas. Fig. 2. Nevada 72
IV. Fig. 1. Illinois. Fig. 2. North Dakota 73
V. Fig. 1. Kentucky. Fig. 2. Virginia 85
VI. Parts of Los Angeles and San Bernardino Counties,
California 87
VII. Kansas 90
VIII. About 16 Miles Southwest of St. Louis, Mo. 91
IX. Niagara Falls 100
X. Fig. 1. Yellowstone Park. Fig. 2. Arizona 101
XI. Part of the Catskills, N. Y. 106
XII. Fig. 1. New Mexico. Fig. 2. Virginia, West Virginia, and
Maryland 107
XIII. Fig. 1. Colorado. Fig. 2. Kansas 162
XIV. Fig. 1. Pennsylvania. Fig. 2. California 163
XV. Near Hahnville, Louisiana 188
XVI. Missouri 189
XVII. Fig. 1. Hunterdon County, N. J. Fig. 2. Near Pikeville,
Tenn. 232
XVII
Fig. 1. Washington. Fig. 2. California
I. 233
XIX. Part of the Big Horn Range, Wyoming 286
XX. Section of the California Coast near San Mateo, Cal. 287
XXI. New Jersey 356

PLATE FACE PAGE

XXII. Fig. 1. Portion of South Coast of Martha’s Vineyard,
Mass. Fig. 2. Portion of the California Coast near
Tamalpais 357
XXII
Fig. 1. Massachusetts. Fig. 2. Maine
I. 364
XXI
Portion of the Coast of Maine
V. 365

TABLES.
Analyses of American River-waters 106
Analyses of American Spring-waters 236
Analyses of the Waters of Inclosed Lakes 392

GEOLOGY

CHAPTER I.
PRELIMINARY OUTLINE.

Geology treats of the structure of the earth, of the various stages through
which it has passed, and of the living beings that have dwelt upon it,
together with the agencies and processes involved in the changes it has
undergone. Geology is essentially a history of the earth and its inhabitants.
It is one of the broadest of the sciences, and brings under consideration
certain phases of nearly all the other sciences, particularly those of
astronomy, physics, chemistry, zoology, and botany. It also embraces the
earlier expressions of mental development and of life-relationships, chiefly
as found in the lower animals.
Subdivisions.—Naturally so broad a science has many special aspects
which constitute subdivisions, in a sense, though they are rather dominant
phases than independent sections. That phase which treats of the outer
relations of the earth is Cosmic or Astronomic geology; that which treats of
the constituent parts of the earth and its material is Geognosy, of which the
most important branch is Petrology, the science of rocks. That branch which
investigates the structural arrangement of the material, or “the architecture
of the earth,” is Geotectonic, or Structural geology; while that which deals
with the surface changes and topographic forms, that is, with the face of the
earth, is Physiographic geology. The study of the fossils that have been
preserved in the rocks, and of the faunas and floras that these imply,
constitutes Paleontologic geology, or Paleontology. The treatment of the
succession of events forms Historical geology. This is chiefly worked out
by the succession of beds laid down in the progress of the ages, which
constitutes Stratigraphic geology. The treatment of causes, agencies, and
processes is the function of Dynamic or Philosophic geology.
Besides these there are special applications which give occasion for other
terms, as Economic geology, which is concerned with the industrial
applications of geologic knowledge; Mining geology, which is a sub-section
of economic geology, relating to the application of geologic facts and
principles to mining operations; Atmospheric geology, Glacial geology, and

others that define themselves, and are for the greater part but limited aspects
of the broad science.
Dominant processes.—Three sets of processes, now in operation on the
surface of the lithosphere, have given rise to most of the details of its
configuration, and even many of its larger features. These processes have
been designated diastrophism, vulcanism, and gradation. Diastrophism
includes all crustal movements, whether slow or rapid, gentle or violent,
slight or extensive. Many parts of the land, especially along coasts, are
known to be slowly sinking relative to the sea-level, while other parts are
known to be rising. The fact that rocks originally formed beneath the sea
now exist at great elevations, and the further fact that areas which were
once land are now beneath the sea, are sufficient evidence that similar
changes have taken place in the past. Vulcanism includes all processes
connected with the extrusion of lava and other volcanic products, and with
the rise of lava from lower to higher levels, even if not extruded. Vulcanism
and diastrophism may be closely associated, for local movements at least
are often associated with volcanic eruptions, and more considerable
movements may be connected with the movements of subsurface lavas,
even when the connection is not demonstrable. Gradation includes all those
processes which tend to bring the surface of the lithosphere to a common
level. Gradational processes belong to two categories—those which level
down, degradation, and those which level up, aggradation. The
transportation of material from the land, whether by rain, rivers, glaciers,
waves, or winds, is degradation and the deposition of material, whether on
the land or in the sea, is aggradation. Degradation affects primarily the
protuberances of the lithosphere, while aggradation affects primarily its
depressions.

Astronomic Geology.

The earth as a planet.—Though supremely important to us, the earth is
but one of the minor planets attendant upon the sun, and is in no very
special way distinguished as a planetary body. Of the eight planets, four,
Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while
three, Mars, Venus, and Mercury, are smaller. There are a host of asteroids,
but all together they do not equal the mass of the smallest planet. The

average mass of the eight planets is more than fifty times that of the earth,
while the largest, Jupiter, is more than three hundred times as massive as the
earth. The earth’s position in the group is in no sense distinguished. It is
neither the outer nor the inner, nor even the middle planet. Even in the
minor group to which it belongs, it is neither the outermost nor the
innermost member, though in this group it is the largest. Its average
distance from the sun is about 92.9 million miles, and this fixes its
revolution at 365¼ days, for its period of revolution is directly dependent
on its distance from the sun, and is necessarily longer than the revolutions
of the inner planets and shorter than those of the outer planets. Its rotation
in twenty-four hours is not far different from that of its neighbor Mars, but
is much slower than the more distant and larger planets, Jupiter and Saturn,
which rotate in about ten hours. Comparison cannot be made with the
innermost and outermost planets, because their rotations are not yet
satisfactorily determined. The plane of the earth’s revolution lies near the
common plane of the whole system, but this is not peculiar, as all of the
planets revolve in nearly the same plane. Only a few of the small asteroids
depart notably from this common plane. This has an important bearing on
theories of the origin of the system, since this close coincidence of the
planes of the orbits is not consistent with any haphazard aggregation of the
material. Of similar importance is the fact that all of the planets revolve in
the same direction and in ellipses that do not depart widely from circles.
The eccentricity of the earth’s orbit is only about ¹⁄₆₀. This eccentricity
varies somewhat, due to the disturbing influences of the other planets, and
this variation has been regarded by some geologists as an influential cause
of climatic changes, but its adequacy to produce great effects has been
doubted by others. The inclination of the earth’s axis, now about 23½°,
holds an intermediate position, some of the planets having axes more
inclined, as Saturn, 26 ⅚ °, and others less inclined, as Jupiter, 3°. The
inclination of the axis is subject to trivial variations at present, and in the
long periods of the past has possibly changed more notably. This possible
change has also been thought to be a cause of climatic variation, but its
efficiency has not been demonstrated.
Its satellite.—The earth is peculiar in having one unusually large
satellite, which has a mass ¹⁄₈₁ of its own. The great planets have several
satellites whose combined mass exceeds that of the moon, and perhaps in

some few cases the individual satellites may be larger than the moon, but
they do not sustain so large a ratio to their planets, for Titan, probably the
largest, is only ¹⁄₄₆₀₀ of the mass of Saturn. There is little doubt that the
moon has played an important part in the history of the earth. It is the chief
agency in developing oceanic tides, and it possibly also develops a body
tide in the earth itself. These tides act as a brake on the rotation of the earth
and tend to reduce its rate, and thereby to lengthen the day. While this may
have been counteracted in some measure by the shrinkage of the earth,
which tends to increase its rate of rotation, it has been held by eminent
physicists and geologists that the rotation of the earth has been greatly
lessened during its history, and that a long train of important consequences
has resulted. If the contraction of the earth has been sufficient to offset this
lessening, the tidal brake must be credited with the prevention of the
excessive speed of rotation which would otherwise have been developed.
The tides are efficient agencies in the shore wear of the oceans, and in the
distribution of marine sediments, and these, it will be seen later, are
important elements in the formation of strata.
Dependence on the sun.—By far the most important external relation of
the earth, however, is its dependence on the sun. The earth is a mere
satellite of the sun, less than ¹⁄₃₀₀₀₀₀ of its mass, and hence under its full
gravitative control. The earth is dependent on the sun for nearly all its heat
and light, and, through these, for nearly all of the activities that have given
character to its history. It is too much to say that all activities on the surface
of the earth are solely dependent on those of the sun, for a certain measure
of heat and light and other energy is derived from other bodies, and a
certain not inconsiderable source of energy is found in the interior of the
earth itself; yet all of these are so far subordinate to that great flood of
energy which comes from the sun that they are quite inconsequential. The
history of the earth in the past has been intimately dependent upon that of
the sun, and its future is locked up with the destiny of that great luminary.
Geology in its broadest phases can therefore scarcely be separated from the
study of the sun, but this falls within the function of the astronomer rather
than the geologist.
Meteorites.—There are a multitude of small bodies passing through
space in varying directions and with varying velocities and occasionally
encountering the earth, to which they add their substance. Some of these

meteorites revolve about the sun much as if they were minute planets, but
some of them come from such directions and with such velocities as to
show that they do not belong to the solar family. Some consist almost
wholly of metal, chiefly iron alloyed with a small percent. of nickel
(holosiderites); some consist of metal and rock intimately mixed
(syssiderites and sporadosiderites); and some consist wholly of rock
(asiderites). The rock is usually composed of the heavier basic minerals,
though some meteorites consist largely of carbonaceous material. Besides
meteorites, there is little doubt that wandering gaseous particles strike the
earth, but this is beyond the reach of present demonstration. The amount of
substance added to the earth by these meteorites and gases in recent times is
relatively slight compared with the whole body of the earth. What
contribution may have come to the earth in earlier times from such sources
is a matter of hypothesis which will be discussed later.

Geognosy.

The constitution of the earth.—Turning from its external relations to
the earth itself, a natural threefold division is presented: (1) the atmosphere,
(2) the hydrosphere, and (3) the lithosphere.

I. The Atmosphere.
The atmosphere is an intimate mixture of (1) all those substances that
cannot take a liquid or solid state at the temperatures and pressures which
prevail at the earth’s surface, together with (2) such transient vapors as the
various substances of the earth throw off. The first class form the permanent
gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen
about 21 parts, carbon dioxide about .03 part, together with small quantities
of argon, neon, xenon, krypton, helium, and other rare constituents. The
second class are the transient and fluctuating constituents of the
atmosphere, chief among which is aqueous vapor, which varies greatly in
amount according to temperature, pressure, and other conditions. To this are
to be added volcanic emanations and a great variety of volatile organic
substances. Theoretically, every substance, however solid, discharges
particles which may transiently become constituents of the atmosphere.
Practically, only a few of these exist in such quantity as to be appreciable.

Dust and other suspended matter are usually regarded as impurities rather
than constituents of the atmosphere, but they play a not unimportant part by
affecting its temperature and luminosity, and by facilitating the
condensation of moisture.
Mass and extent.—The total mass of the atmosphere is estimated at five
quadrillion tons, or ¹⁄₁₂₀₀₀₀₀ of the mass of the earth. It is relatively dense at
the surface of the earth and decreases in density outwards in a manner
difficult of absolute determination, so that the actual height of the
appreciable atmosphere is not positively known. The true conception of the
atmosphere is perhaps that of a tenuous envelope exerting a pressure of
about fifteen pounds per square inch at the sea-level, and thinning gradually
upwards until it reaches a tenuity which is inappreciable, but perhaps not
ceasing absolutely until the sphere of gravitative control of the earth is
passed, about 620,000 miles from the lithosphere. In the lower portion,
according to the kinetic theory of gases, the molecules fly to and fro,
colliding with each other with almost inconceivable frequency, and with
very short paths between successive collisions, but in the upper rare portion
some of the molecules bound outwards, and do not strike other molecules,
and hence pursue long elliptical paths until the gravity of the earth
overcomes their momentum, when they return, perhaps to bound off again
or to force other molecules to do so. This fountain-like nature of the outer
part of the atmosphere makes any sharp definition of its limit impracticable.
Some molecules are believed to be shot away at such speed that they do not
return. Beyond about 620,000 miles from the surface of the lithosphere, the
differential attraction of the sun is greater than that of the earth, and if the
attraction of the earth does not turn the molecules back before reaching this
distance, they are almost certain to be lost to the earth.
The measurement of heights by the aneroid barometer, which is much
used in practical geology, is dependent on the lessening of pressure as the
instrument is carried upward.
Geologic activity.—The atmosphere is the most mobile and active of the
three great subdivisions of the earth, and when its indirect effects through
the agency of water, as well as its direct effects, are considered, it is to be
regarded as one of the most effective agencies of change. It acts chemically
upon the rock substance of the earth, causing induration in some instances,
but more often inducing disintegration and change of composition by means

of which rock is reduced to soil, or soil-like material, and rendered
susceptible of easy removal by winds and waters. When in motion the
atmosphere acts mechanically on the surface of the earth, transporting dust
and sand, and by the friction of these it abrades the surface. It is chiefly
effective, however, in furnishing the conditions for water action. Partly by
its mechanical aid, but chiefly by securing the right temperature, it is a
necessary factor in the action of rains, streams, glaciers, and the various
forms of moving water upon land. So also, on the ocean, wave action is
essentially dependent on the winds. In the absence of atmospheric
propulsion, wave action would be chiefly confined to the tides and to
occasional earthquake impulses, and would lose nearly all its efficiency.
Stream action and wave action, which are the most declared of the
geological agencies, are therefore to be credited as much to the atmosphere
as to the hydrosphere, since the action is a joint one to which both
envelopes are essential.
A thermal blanket.—A function of the atmosphere of supreme
importance is the thermal blanketing of the earth. In its absence the heat of
the sun would reach the surface with full intensity, and would be radiated
back from the surface almost as rapidly as received, and only a transient
heating would result. During the night an intensity of cold would intervene
scarcely less severe than the temperature of space. In penetrating the
atmosphere certain portions of the radiant energy of the sun are absorbed.
Of the remainder which reaches the surface of the earth, a part is
transformed into vibrations of lower intensity, which are then more
effectively retained by the atmosphere. The air thus distributes and
equalizes the temperature. The two constituents of the atmosphere which
are most efficient in this work are aqueous vapor and carbon dioxide, and
the climate of the earth is believed to have been very greatly affected by the
varying amounts of these constituents in the atmosphere, as well as by the
total mass of the atmosphere.
The function of the atmosphere in sustaining life and promoting all that
depends on life is too obvious to need comment.
The special geological action of the atmosphere will be discussed in the
next chapter.

II. The Hydrosphere.

About 1300 quadrillion tons of water lie upon the surface of the solid
earth. This equals about ¹⁄₄₅₄₀ part of the earth’s mass. Were the surface of
the solid earth perfectly spheroidal, this would constitute a universal ocean
somewhat less than two miles deep. Owing to the inequalities of the rock
surface, the water is chiefly gathered into a series of great basins or troughs
occupying about three-fourths (72%) of the earth’s surface. These basins are
all connected with each other and act as a unit, so that anything which
changes the level of the water in one changes the level of all. This helps to
make a common record of all great movements of the earth’s body, for the
level of the ocean determines where the detritus from the land shall lodge,
and hence where the edge of the marine beds shall be formed. This will
appear more clearly when the formation of marine strata is discussed.
Oceanic dimensions.—The surface area of the ocean is estimated by
Murray at 143,259,300 square miles. Of this, somewhat more than
10,000,000 square miles lie on the continental shelf, i.e., lap up on the
borders of the continental platforms. This shows that the great basins are
somewhat more than full. If about 600 feet of the upper part of the ocean
were removed, the true ocean basins would be just full, and the surfaces of
the true continental platforms would be dry land. The area of the true
oceanic basins is about 133,000,000 square miles, and that of the true
continental platforms about 64,000,000 square miles. Under about 20% of
the ocean area, the bottom sinks to depths between 6000 and 12,000 feet;
under about 53% it sinks to depths between 12,000 and 18,000 feet; and
under the remaining 4% it ranges from 18,000 feet down to about 30,000.
The last includes those singular sunken areas known as “deeps,” and
sometimes called anti-plateaus, as they extend downward from the general
ocean bottom much as the plateaus protrude upwards from the general land
surface.
Besides the ocean, the hydrosphere includes all the water which
constitutes the surface streams and lakes, together with that which
permeates the pores and fissures of the outer part of the solid earth; but
altogether these are small in amount compared with the great ocean mass.
Geologic activity.—Of all geological agencies water is the most obvious
and apparently the greatest, though its efficiency is conditioned upon the
presence of the atmosphere, upon the relief of the land, and upon the radiant
energy of the sun. Through the agency of rainfall, of surface streams, of

underground waters, and of wave action, the hydrosphere is constantly
modifying the surface of the lithosphere, while at the same time it is bearing
into the various basins the wash of the land and depositing it in stratified
beds. It thereby becomes the great agency for the degradation of the land
and the building up of the basin bottoms. It works upon the land partly by
dissolving soluble portions of the rock substance, and partly by mechanical
action. The solution of the soluble part usually loosens the insoluble, and
renders it an easy prey of the surface waters. These transport the loosened
material to the valleys and at length to the great basins, meanwhile rolling
and grinding it and thus reducing it to rounder forms and a finer state, until
at length it reaches the still waters or the low gradients of the basins and
comes to rest. The hydrosphere is therefore both destructive and
constructive in its action. As the beds of sediment which it lays down
follow one another in orderly succession, each later one lying above each
earlier one, they form a time record. And as relics of the life of each age
become more or less imbedded in these sediments, they furnish the means
of following the history of life from age to age. The historical record of
geology is therefore very largely dependent upon the fact that the waters
have thus buried in systematic order the successive life of the ages. Aside
from this, the means of determining the order of events of the earth’s
history are limited and more or less uncertain.
The special processes of the hydrosphere in its various phases will be the
subject of discussion hereafter (Chaps. III, IV, VI). Suffice it here to
recognize its great function in the constant degradation of the land, and in
the deposition of the derived material in orderly succession in the basins.
Chief horizons of activity.—The great horizons of geological activity
are (1) the contact zone between the atmosphere and the hydrosphere,
chiefly the surface of the ocean, (2) the contact zone between the
hydrosphere and the lithosphere, chiefly the shore belts, and (3) the contact
zone of the atmosphere and surface waters, with the face of the continents.
It is in these three zones that the greatest external work is being done and
has been done in all the known ages.

III. The Lithosphere.
The atmosphere and hydrosphere are rather envelopes or shells than true
spheres, though in some degree both penetrate the lithosphere. The

lithosphere, on the other hand, is a nearly perfect oblate spheroid with a
polar diameter of 7899.7 miles, and an equatorial diameter of about 26.8
miles more. Its equatorial circumference is 24,902 miles, its meridional
circumference 24,860 miles, its surface area 196,940,700 square miles, its
volume 260,000,000,000 cubic miles, and its average specific gravity about
5.57. The oblateness of the spheroid is an accommodation to the rotation of
the earth, the centrifugal force at the equator being sufficient to cause the
specified amount of bulging there. Computations seem to indicate that the
accommodation is very nearly what would take place if the earth were in a
liquid condition, from which the inference has been drawn that it must have
been in that condition when it assumed this form, and must have continued
essentially liquid until it attained its present rate of rotation, since, if the
earth once rotated at a much higher speed, the flattening at the poles and the
bulging at the equator must have been correspondingly greater. It is thought
by others, however, that the plasticity of the earth is such that it would at all
times assume a close degree of approximation to the demands of rotation,
even if the interior were in a solid condition. By still others it is thought that
the contraction of the earth has tended to accelerate the rotation about as
much as the tides have tended to retard it, and that it has undergone little
change of form.
Irregularities.—It is only in a general view, however, that there is a
close approximation to a perfect spheroidal surface. In detail there are very
notable variations from it. Geodetic surveys seem to have shown that the
equatorial diameters are not all equal, even when the measurements are
reduced to sea-level, but research along this line has not reached a sufficient
stage of completeness to permit satisfactory discussion. It is, however,
highly probable that the ocean surface as well as the average land surface is
warped out of the perfect spheroidal form to some notable degree. This is
very likely due to inequalities in the density of the earth’s interior. The fact
that the larger portion of the water is gathered on one side of the globe,
while the land chiefly protrudes on the opposite side, is very possibly due to
unequal specific gravity in the interior of the earth.
The most obvious departure from a spheroidal form is found in the
protrusion of the continents and in the sinking away of the earth surface
under the oceans. As these inequalities present themselves to-day, they are
known as continental platforms and ocean basins. These do not correspond

accurately with the present land and water surfaces. About the continental
lands there is a submerged border extending some distance out from the
shore, and constituting a sea-shelf beyond which the surface descends
rapidly to the great depths of the ocean. This slightly submerged portion,
known as the continental shelf, belongs as properly to the continent as the
adjacent low lands which are not submerged. The submergence of the edge
of this shelf at present is usually about 100 fathoms, so that if the upper 600
feet of the ocean were removed, the outlines of the land would correspond
quite closely with the border of the true continental platform.

BATHYMETRICAL CHART OF THE OCEANS
SHOWING THE “DEEPS” ACCORDING TO SIR JOHN MURRAY

It is customary to look upon the protrusions of the continents as the great
features of the earth’s surface, but in reality the oceanic depressions are the
master phenomena. In breadth, depth, and capacity they much exceed the
continental protrusions, and if the earth be regarded as a shrunken body, the
settling of the ocean bottoms has doubtless constituted its greatest surface
movement. From the estimates of Murray, Gilbert has derived the following
tables, showing the relative areas of the lithosphere above, below, and
between certain levels.[1]

From these estimates it appears that if the surface were graded to a
common level by cutting away the continental platforms and dumping the
matter in the abysmal basins, the average plane would lie somewhere near
9000 feet below the sea-level. The continental platform may be conceived
as rising from this common plane rather than from the sea-level.
Percent. of Surface Percent. of Surface
Contours.
above. below.
Contour 24,000 feet above sea-level 0.004 99.996
“ 18,000 “ “ “ 0.09 99.91
“ 12,000 “ “ “ 0.7 99.3
“ 6,000 “ “ “ 2.3 97.7
Sea level 27.7 72.3
Contour 6,000 feet below sea-level 42.5 57.5
“ 12,000 “ “ “ 57.3 42.7
“ 18,000 “ “ “ 96.8 3.2
“ 24,000 “ “ “ 99.93 0.07
Percent.
More than 6000 feet above sea-level 2.3
Between sea-level and 6000 feet above 25.5
Between sea-level and 6000 feet below 14.8
Between 6000 and 12,000 feet below sea-level 14.8
Between 12,000 and 18,000 feet below sea-level 39.4
Between 18,000 feet and 24,000 feet 3.1

Epicontinental seas.—Those shallow portions of the sea which lie upon
the continental shelf, and those portions which extend into the interior of
the continent with like shallow depths, such as the Baltic Sea and the
Hudson Bay, may be called epicontinental seas, for they really lie upon the
continent, or at least upon the continental platform; while those other
detached bodies of water which occupy deep depressions in the surface are
to be regarded as true abysmal seas, as, for example, the Mediterranean and
Caribbean seas and the Gulf of Mexico, whose bottoms are as profound as
many parts of the true ocean basin itself.
Diversities of surface.—The bottoms of the oceanic basins are
diversified by broad undulations which range through many thousands of
feet, but they are not carved into the diversified forms that give variety to
land surfaces. The ocean bottoms are also diversified by volcanic peaks,
many of which rise to the surface and constitute isolated islands. Some of
them have notable platforms at or near the surface, cut by the waves or built

up by the accumulation of sediment and of coralline and other growths
about them. Aside from these encircling platforms, the solid surface usually
shelves rapidly down to abysmal depths, so that the islands constitute peaks
whose heights and slopes would seem extraordinary if the ocean were
removed.
The surface of the land is diversified in a similar way by broad
undulations and volcanic peaks, and also by narrower wrinklings and
foldings of the crust; but all of these irregularities have been carved into
diversified and picturesque forms by subaërial erosion. In this respect the
surface of the land differs radically from the bed of the sea. The agencies
which have produced the continental platforms and abysmal basins, and the
great undulations and foldings, as well as the volcanic extrusions that mark
them, are yet subjects of debate. Here lie some of the most difficult
problems of geology, but these cannot be stated with sufficient brevity to
find a place here.
The surface mantle of the lithosphere.—The surface of the lithosphere
is very generally mantled by a layer of loose material composed of soil,
clay, sand, gravel, and broken rock. This loose material is sometimes known
as mantle rock, and sometimes as rock waste. On the land, mantle rock is
often composed of the disintegrated products of underlying rock
formations. It represents the results of the recent action of the atmosphere,
of water, of changes of temperature, and of other physical agencies acting
on the outer part of the rock sphere. The surface of this mantle is being
constantly removed by wind and water, but as constantly renewed by
continued decomposition of the rock below. In some areas, especially in the
northern part of North America and the northwestern part of Europe, the
soil graduates down into an irregular sheet of mixed clay, sand, gravel, and
bowlders, known as drift. From this and other evidence it is inferred that at
a time not greatly antedating our own, ice, chiefly in the form of glaciers,
spread extensively over the high latitudes of the northern hemisphere. In
some parts of the earth the surface is still covered by fields of snow and ice,
comparable to those which formed the drift. In still other places, especially
along the flood plains of streams, the mantle rock consists of deposits made
by streams which were unable to carry their loads of sediment to the sea.
The crust of the lithosphere.—Much of the detritus washed down from
the land finds its way to bodies of standing water, and beneath lakes and

seas the mantle of loose material is made up largely of the gravel, sand, and
mud derived from the land. Before deposition these materials are more or
less assorted and arranged in layers by waves and currents. When
consolidated they constitute rock. The weathering of the rocks of the land,
the wearing away of the resulting detritus, and its deposition beneath
standing water, are among the most important processes of geologic change.
On the land, the mantle of loose material is sometimes absent, and in
such places the surface of solid rock of the crust appears. Bare surfaces of
rock are most commonly seen where the topography is rough, especially on
the slopes of steep-sided valleys and mountains, and on the slopes of cliffs
which face seas or lakes. Solid rock, without covering of soil or loose
material of any sort, is also frequently seen in the channels of streams,
especially where there are falls or rapids.
We have but to note the effects of a vigorous shower on a steep slope, or
of a swift stream on its channel, or of waves on the cliffs which face lakes
and seas, to understand at least one of the reasons why loose materials are
frequently absent from steep slopes. The very general exposure of solid
rock where conditions favor surface erosion suggests that rock is
everywhere present beneath the soil or subsoil. Fortunately there is an easy
way of testing the universality of the crust beneath the mantle. In all lands
inhabited by civilized peoples there are numerous wells and other
excavations ranging from a few feet to several hundred feet in depth, and
occasional wells and mine-shafts reach depths of several thousand feet.
Even in shallow excavations rock is often encountered, and in most regions
excavations as much as two or three hundred feet deep usually reach rock,
and no really deep boring has ever failed to find it. It may, therefore, be
accepted as a fact that the upper surface of the solid rock is nowhere far
below the surface.
Concerning the thickness of the crust, if there be any true crust at all,
little is known by direct observation. The deepest valleys, such as the
canyon of the Colorado, and the shafts and borings of the deepest mines and
wells, give knowledge of nothing but rock. The deepest excavations extend
rarely more than a mile below the surface. It is certain that rock of known
kinds extends to far greater depths.

The interior.—Concerning the great interior of the earth, little is known
except by inference. From the weight of the earth,[2] it is inferred that its
interior is much more dense than its surface. From its behavior under the
attraction of other bodies, it is believed to be at least as rigid as steel, and its
interior cannot, therefore, be liquid, in the usual sense of that term. From
the phenomena of volcanoes, and from observations on temperature in deep
borings, it is inferred that its interior is very hot. Further inferences
concerning its character are less simply stated, and will be referred to later.
The solid part of the earth is therefore composed of (1) a thin layer of
unconsolidated or earthy material, a few feet to a few hundred feet in
thickness, covering (2) a layer or zone many thousands of feet, and
probably many miles, thick, composed of solid rock comparable to that
exposed at numerous points on the surface, and (3) a central mass, to which
the preceding layers are but a shell, composed of hot, dense, and rigid rock,
the real nature of which is not known by observation.
Varieties of rock in crust.—If the mantle of soil, subsoil, and glacial
rubbish were stripped from the land, the surface beneath would be found to
be made up of a great variety of rocks, all of which may be grouped into
two great classes. About four-fifths of the land surface would be of rock
arranged in layers, and the other fifth would be of crystalline rock,
generally without distinct stratification, and often bearing evidence of the
effects of high temperature.
Stratified rocks.—The composition of most stratified rocks corresponds
somewhat closely with the composition of sediments now being carried
from the land and being deposited in the sea. Their arrangement in layers is
the same, and the markings on the surfaces of the layers, such as ripple-
marks, rill-marks, wave-marks, etc., are identical. Furthermore, the
stratified rocks of the land, like the recent sediments of the sea, frequently
contain the shells and skeletons of animals, and sometimes the impressions
of plants. Most of the relics of life found in the stratified rocks belonged to
animals or plants which lived in salt water. Because of their structure, their
composition, their distinctive markings, and the remains of life which they
contain, it is confidently inferred that most of the stratified rocks which lie
beneath the mantle rock of the land were originally laid down in beds
beneath the sea, and that the familiar processes of the present time furnish
the key to their history.

Fig. 1.—Beds of (Cambrian) sandstone, a, are conformable with one another, but
unconformable on beds of (Huronian) quartzite, b, Near Ableman, Wis.

Conformability.—When the stratified rocks exposed by the removal of
the mantle rock are examined, the successive beds are sometimes found to
lie on one another in regular succession, showing that they were laid down
one after another, without change in the attitude of the surface on which
they were deposited. Such rocks are conformable (the beds of series a, Fig.
1). In other cases it would be seen that certain beds overlie the worn
surfaces of lower beds, the layers of which may have a different angle of
inclination (series a, Fig. 1, is unconformable on series b). Such relations
show that the lower series of beds was disturbed and eroded before the
overlying beds were deposited on them. Such series of rocks are
unconformable.
Relative ages.—The structure and relations of rocks lead to inferences as
to their relative ages. In the case of stratified rocks it is obvious that
overlying beds were deposited later than those below, and where there is
unconformity it is evident that an interval of time elapsed between the
deposition of the unconformable series. Another and in some respects more
important means of telling their order of formation is found in the remains
of life entrapped in the water-laid sediments. Whatever life existed in the
waters in which the sediments were deposited was liable to burial, and if it
was possessed of hard parts, such as bones, teeth, shells, hard integuments,
etc., these parts, or at least their impressions, were likely to be preserved in
the sediments. Even tracks and imprints of perishable parts are sometimes
preserved. All these relics, which we call fossils, give indications of the

kinds of life which existed when the beds were formed. The fossils of the
youngest beds show that the life which existed when they were deposited
was quite like that of the present time. The fossils of the next older and
lower beds show greater departure from present types. This series of
changes continues downward as lower and lower beds are studied, until
beds at considerable depths contain no relics of existing species but, in lieu
thereof, forms of more primitive types. Some of these earlier types are
clearly the ancestors of more modern forms, while others seem to have no
living descendants. Going still deeper, the fossils indicate life of more and
more primitive types, until they depart very widely from the living forms,
and seem to be but remotely ancestral. So the beds may be followed
downward until the lowest, which contain distinct evidences of life, are
reached.
It should be understood that it is not possible to proceed directly
downward through the whole succession of bedded rocks, but that the edges
of the various beds may be found here and there where they have been
brought to the surface by warpings or tiltings, or exposed by the wearing
away of the beds which once overlay them. The full series of strata is made
out only by putting together the data gathered throughout all lands, and
even when this is done an absolutely complete series cannot yet be made
out or, at least, has not been.
The crystalline rocks.—The crystalline rocks which would appear if the
mantle rock were removed are of two types, igneous and metamorphic.
Igneous rocks may be loosely defined as hardened lavas. Metamorphic
rocks are those which are greatly changed from their original condition.
Either stratified or igneous rocks may become metamorphic.
Igneous rocks sustain various relations to the stratified rocks, as
illustrated by Fig. 2. From these relations it is possible to tell something of
the order of their formation. Where the stratified rocks are broken through
by lavas, it is obvious that the stratified rocks were formed first, and the
lavas intruded later. Lava sheets intruded between beds of stratified rock
can be told from those which flowed out on the surface and were
subsequently buried, for in the former case the sedimentary rocks, both
above and below the igneous rock, were affected by the heat, while in the
latter case only those below were so affected.

Fig. 2.—Diagrammatic representation of the relations of igneous rock to stratified rock.
The igneous rocks, represented in black, have been forced up from beneath.

More commonly than otherwise the metamorphic rocks (Fig. 3) lie
beneath the sedimentary beds and are often broken through by the igneous
rocks. From their position in many places their great age may be inferred,
but locally, especially where dynamic action has been severe, relatively
young rocks are metamorphic.

Fig. 3. The figure represents a section of the earth about 1000 miles Long. The unequally
thick black line at the top represents on something like its proper scale the depth of the
stratified rock. The area below represents crystalline rock, largely metamorphic.

Four great sedimentary eras.—The water-laid series represents four
great eras in the history of the earth, as shown by the relics of life imbedded
in them. Beginning with the latest, these are the Cenozoic (recent life),
during which the life took on its modern aspect; the Mesozoic (middle life),
during which the life bore a mediæval aspect; the Paleozoic (ancient life),
during which the life belonged to older types; and the Proterozoic (earlier
life), during which it is inferred that much life prevailed, though its record
is very imperfect. It may safely be assumed to have been more primitive
than that of the Paleozoic, as it was earlier. Each of these great divisions
embraced several lesser periods or epochs, and these again are subdivided
more and more closely according to the degrees of refinement to which
studies are carried. The chief of these subdivisions are given in the table on
page 19, and others will come under consideration in the historical chapters.
In these four great series of sedimentary rocks there are, here and there,
intrusions of igneous rocks, and in some places the sedimentary beds have
been metamorphosed into crystalline rocks by heat and pressure. This is
particularly true in the lowest of these series, the Proterozoic, where a large

part of the sediment is metamorphosed, and where there is much igneous
rock, but it is still clear that the main portion of this series was originally
water-laid sediment, and so it belongs to the sedimentary series rather than
the Archean, in which the sediments are the minor rather than the main
factor. It has, however, usually been classed with the Archean, and it is
certainly not always easy to draw the dividing line. In a sense it may be
regarded as a transition series.
The Archean complex.—Beneath the dominantly sedimentary but partly
metamorphic and igneous series there is a very complex group of rocks
largely of metamorphosed igneous origin, though containing some
metamorphosed sediments. These extend downwards to unknown depths.
While all the great formations are occasionally bent and broken, these
lowest ones are almost everywhere warped, folded, and contorted, often in
the most intricate way. They have been very generally mashed and sheared
by enormous pressure, so that they have become foliated, and their original
character is much masked. They therefore form a series of great obscurity
and complexity. As they are at the bottom of the known series, they have
been called the “Fundamental gneiss” and the “Basement complex,” but as
the part which we see is not the true base nor the true foundation, it is safer
to call them simply the Archean (very ancient) complex. As life appears to
have been present during a part at least of the period of its formation is
referred to the Archeozoic era.

Fig. 4.—Diagram to illustrate the relations of the five great groups of formations.
AR = Archean, Pr = Proterozoic, P = Paleozoic, M = Mesozoic, C = Cenozoic.

Beyond and below this series, the structure of the earth is a matter of
inference. Vast as are the preceding series, they together form relatively but
a thin shell on the outer surface of the globe.
The foregoing series are diagrammatically expressed in Fig. 4, and
systematically presented to the eye in the following table.
GENERAL TABLE OF GEOLOGIC DIVISIONS.
Cenozoic Present.

Pleistocene.
Pliocene.
Miocene.
Oligocene.
Eocene.
Transition (Arapahoe and Denver).
Upper Cretaceous.
Lower Cretaceous (Comanche or Shastan).
Mesozoic
Jurassic.
Triassic.
Permian.
Coal Measures or Pennsylvanian.
Subcarboniferous, or Mississippian.
Paleozoic Silurian.
Devonian.
Ordovician.
Cambrian.
Great interval.
Keweenawan.
Interval.
Animikean or Penokean.
Proterozoic
(Upper Huronian of some authors).
Interval.
Huronian.
Great interval.
Archeozoic Archean Great Granitoid Series.
Complex. (Intrusive in the main,
Laurentian.)
Great Schist Series.
(Mona, Kitchi, Lower Keewatin,

Coutchiching, Lower Huronian of
some authors.)
The purpose of this general survey is to bring the salient features of the
earth’s structure into view preparatory to entering in more detail into the
study of particular processes and special formations and to lay a foundation
for the fuller apprehension of the successive stages of the history of the
earth, which constitutes the chief purpose of geological study. It is now
advisable to turn to the detailed consideration of individual processes and
specific structures. The complexity of the actions involved in the history of
the earth is so great that such separate consideration at the outset is helpful.

CHAPTER II.
THE ATMOSPHERE AS A GEOLOGICAL AGENT.

While it is convenient to regard the lithosphere as the earth proper, and
the atmosphere as its envelope, the latter is as truly a part of the planet as
the former, and its activities and its history are as truly subjects of
geological study as the formation of the rocks. This view is in no way
vitiated by the fact that the special study of the atmosphere is set apart
under the name Meteorology, for in the same way the special study of rocks
is set apart under the name Petrology, that of ancient life under the name
Paleontology, and that of other phases of the subject under other names.
The atmosphere is one of the three great formations of the earth, and as a
geological factor takes its place beside the hydrosphere and the lithosphere.
It has played a part in the history of the earth comparable to that of the
water, though its mass is less and its record more elusive. Unsubstantial as
the atmosphere seems when contrasted with the liquid and solid portions of
the earth, its extreme mobility and its chemical activity compensate for its
lightness and tenuity, and give it a function of the first order of importance.
The atmosphere plays a direct part as (1) a mechanical and (2) a chemical
agent, and at the same time serves an indirect function in furnishing
favorable conditions under which (3) solar radiation produces temperature
effects, and (4) evaporation gives origin to precipitation and stream effects,
and furnishes the necessary conditions for land plants and animals, and the
important influences that spring from them.
This chapter is devoted to the work of the atmosphere in these and some
less notable phases. The consideration of the origin and history of the
atmosphere will receive attention later.

A. THE ATMOSPHERE AS A DIRECT AGENCY.

I. Mechanical Work.

The mechanical work of the atmosphere is accomplished chiefly through
its movement. A feeble breeze is competent to move particles of dust, and
winds of moderate velocity to shift sand. Exceptionally strong winds
sometimes move small pebbles, but winds of sufficient force to move larger
pieces of rock are rare. It follows, therefore, that the impact of the wind has
little direct effect except on surfaces covered with dust and dry sand.
The transportation of material by the wind is limited by the size of the
particles to which it has access. Dust particles expose more surface to the
wind relative to their mass than sand grains. Winds which are unable to
carry sand may still carry dust, and winds which are able to shift sand no
more than a trivial distance may blow dust great distances.
The common conception of wind as a horizontal movement of some part
of the atmosphere is not altogether accurate. Every obstacle against which
wind blows causes deflections of its currents, and some of these deflections
are upward. Furthermore, there are exceptional winds, in which the vertical
element predominates. Particles of dust are often involved in these upward
currents, and by them carried to great heights, and in the upper air are
transported great distances.
Transportation and deposition of dust.[3]—The universality of the
transportation of dust by the wind is well known. No house, no room, and
scarcely a drawer can be so tightly closed but that dust enters it, and the
movements of dust in the open must be much more considerable. The
visible dustiness of the atmosphere in dry regions during wind-storms is
adequate and familiar proof of the efficiency of the wind as a transporter of
dust.
Under special circumstances, opportunity is afforded for rough
determinations of the distance and height to which wind-blown dust is
transported. Snow taken from snow-fields in high mountain regions is
found to contain a small amount of earthy matter. Its particles are often
found to be in part volcanic, even when the place whence the snow was
taken is scores or even hundreds of miles from the nearest volcano. There is
probably no snow-field so high, or so far from volcanoes, but that volcanic
dust reaches it. If this be true of all snow-fields, it is probably true of all
land surfaces. In the great Krakatoa[4] eruptions of 1883 large quantities of
volcanic ash (pulverized lava) were projected to great heights into the

atmosphere. The coarser particles soon settled; but, caught by the currents
of the upper atmosphere, many of the finer particles were transported
incredible distances. Through all their long journey, the particles of dust
were gradually settling from the atmosphere, but not until the dust had
traveled repeatedly round the earth did its amount become so small as to
cease to make its influence felt in the historic red sunsets which it
occasioned.[5] Some of this dust completed the circuit of the earth in 15
days.
In various parts of Kansas and Nebraska[6] there are very considerable
beds of volcanic dust, locally as much as 30 feet thick, which must have
been transported from volcanic vents by the wind, though there are no
known centers of volcanic action, past or present, within some hundreds of
miles of some of the localities where the dust occurs. These beds of
volcanic dust, so far from its source, may serve as an illustration of the
importance of atmospheric movements as a geological force.
Volcanic dust is shot into the atmosphere rather than picked up by it. Dust
picked up by the wind is perhaps transported not less widely than volcanic
dust, but, after settling, its point of origin is less readily determined. It
would perhaps be an exaggeration to say that every square mile of land
surface contains particles of dust brought to it by the wind from every other
square mile, but such a statement would probably involve much less
exaggeration than might at first be supposed.
Examples of extensive deposits of dust other than volcanic are also
known. In China there is an extensive earthy formation, the loess,
sometimes reaching 1,000 feet in thickness, which von Richtofen believes
to have been deposited by the wind.[7] This conclusion has, however, not
passed unchallenged.[8] The loess of some other regions has been referred to
the same origin, and some of it is quite certainly eolian.[9]
The transportation of dust is important wherever strong winds blow over
dry surfaces, free or nearly free of vegetation, and composed of earthy
matter. Its effects may be seen in such regions as the sage-brush plains of
western North America. The roots of the sage-brush hold the soil
immediately about them, but between the clumps of brush, where there is
little other vegetation, the wind has often blown away the soil to such an
extent that each clump of brush stands up several inches, or even a foot or

two, above its surroundings (Fig. 5). Such mounds are often partly due to
the lodgment of dust about the bushes.

Fig. 5.—This figure shows the effect of sage-brush or other similar vegetation in holding
sand or earth, or in causing its lodgment, in dry regions.

Where the earthy matter is moist, the cohesion of the particles is great,
and the wind cannot pick them up. Furthermore, if the surface is generally
moist, it is likely to be covered with vegetation which protects it against the
wind. But even where vegetation is prevalent the wind finds many a
vulnerable point. Thus on the edges of plains or plateaus facing abrupt
valleys, the wind attacks the soil from the side, and in such situations all
earthy matter may be stripped from the underlying rock for considerable
distances from the edge of the cliff (Fig. 6). This may be seen at numerous
points on the lava plateaus of Washington.

Fig. 6.—Diagram to illustrate the way in which the wind sometimes strips the soil from the
edge of a bluff. This phenomenon is not rare in the basin of the Columbia River in
Washington.

The presence of dust in the upper atmosphere during a rain-storm is
sometimes the occasion of phenomena which are often misinterpreted. If
there be abundant dust in the atmosphere through which rain-drops or
snowflakes fall, much of it is gathered up by them, and the water is thereby
rendered turbid and the snow discolored. Here is to be found the
explanation of “mud-rain,” “blood-rain” (red dust), etc.
Since dust is carried to a considerable extent in the upper atmosphere, its
movements and its deposition are little affected by obstacles on the surface
of the land. A building or a hedge can only affect the lodgment of that part
of the atmospheric dust which comes in contact with it or is swept into its
lee. Since most obstacles on the surface of the solid part of the earth reach
up but slight distances into the atmosphere, the dust of the greater part of
the air settles without especial reference to them, and is spread more or less
uniformly over the surface on which it falls.

Fig. 7.—Diagram to illustrate the effect of an obstacle on the transportation and deposition
of sand. The direction of the wind is indicated by the upper arrow. The lower arrows
represent the direction of eddies in the air occasioned by the obstacle. If the surface in
which the obstacle was set was originally flat (dotted line), the sand would tend to be
piled up on either side at a little distance from the obstacle, but more to leeward. At the
same time, depressions would be hollowed out near the obstacle itself (see full line).
(After Cornish.)

Much of the dust transported by the wind is carried out over seas or lakes
and falls into them. By this means, sedimentation is doubtless going on at
the bottom of the whole ocean, and at the bottoms of all lakes. While means
of determining the amount of dust blown into the sea are not at hand, it is
safe to say that, were such determinations possible, the result, if stated in
terms of weight, would be surprising.
Transportation and deposition of sand.—In its transportation by the
wind, sand is not commonly lifted far above the surface of the land, and its
movement is therefore more generally interfered with by surface obstacles
than is the movement of dust. A shrub, a tree, a fence, a building, or even a

stone may occasion the lodgment of sand in considerable quantity, though it
has little effect on the lodgment of dust. The effect of obstacles is illustrated
by Fig. 7 (see also Fig. 5). If the obstacle which occasions the lodgment of
sand presents a surface which the wind cannot penetrate, such as a wall,
sand is dropped abundantly on its windward side as well as on the leeward;
but if it be penetrable, like an open fence, the lodgment takes place chiefly
on its leeward side. In cultivated regions cases are known where, in a few
weeks of dry weather, sand has been drifted into lanes in the lee of hedges
to the depth of two or three feet, making them nearly impassable to
vehicles.
Formation of dunes.—In contrast with dust deposited from the
atmosphere, wind-blown sand is commonly aggregated into mounds and
ridges in the process of lodgment. These mounds and ridges are dunes.
Once a dune is started, it occasions the further lodgment of sand, and is a
cause of its own growth. Dunes sometimes reach heights of 200 or 300 feet,
but they are much more commonly no more than 10 or 20 feet in height. On
plane surfaces, there is a limit in height above which they do not rise,
though the limit is different under different conditions. The velocity of wind
at the bottom of the air is not so great as that higher up, and as a dune is
built up, a level is presently reached where the stronger upper winds sweep
away as much sand as is brought to the top. The very even crests of many
dune ridges are probably to be accounted for in this way. Wind-blown or
eolian sand, not piled up in heaps or ridges, is somewhat widespread, but
does not constitute dunes.
Shapes of dunes.[10]—Dunes may assume the form of ridges or of
hillocks. The ridges may be transverse to the direction of the prevailing
wind or parallel with it. Where dunes assume the form of hillocks rather
than ridges, a group of them may be elongate in a direction parallel to the
dominant wind, or at right angles thereto. The shape assumed by a dune or a
group of dunes depends on the abundance of the sand, the strength and
direction of the wind, and the shape of the obstacle which occasions the
lodgment.

Fig. 8.—Dune ridges parallel to the direction of the wind. Southwest part of India. Scale
about 3 miles to the inch. (Cornish.)

Fig. 9.—Dune ridges transverse to the direction of the wind. Scale about 3 miles to the inch.
(Cornish.)

The incipient stages of dune formation are readily seen in many dry,
sandy regions. The dune is likely to start in the lee of some obstacle, and to
be elongate in the direction of the wind, especially if the wind be strong

relative to the supply of sand. This shape is permanently preserved if the
proper relations between the supply of sand and strength and direction of
wind are preserved. In the dune region of the Indian desert[11] the prevailing
winds are alternately the southwest and northeast monsoons, the former
being the stronger. The supply of sand comes from the southwest. Near the
southwest coast the dune ridges are parallel to the direction of the wind
(Fig. 8); in the interior, where the winds are less strong, the dunes are
transverse to it (Fig. 9); while between the districts where these two types
prevail intermediate forms occur. The transverse dune ridges (Fig. 9) are
said to be the result of the lateral growth and erosion of longitudinal dunes.
[12]
In regions of changeable winds the shape of the dunes is subject to great
variation. Dunes are sometimes crescentic, the convexity facing the wind
(Fig. 10).

Fig. 10.—Crescentic dunes in ground-plan, the convexities facing the wind. (Bokhara.)
(Walther.)

Along coasts, dune ridges are often transverse to the wind, and groups of
dune hillocks are frequently elongate in the same direction. Here the source
of supply of the sand is itself an elongate belt, often transverse to the
dominant wind, and the resulting dunes often have great length transverse
to the wind. Where the wind has strong mastery over the sand, the
longitudinal tendency is seen, even along coasts.[13]

Fig. 11.—Section of a dune showing, by the dotted line, the steep leeward (bc) and gentler
windward (ab) slope. By reversal of the wind the cross-section may be altered to the form
shown by the line adc. (Cornish.)

The shapes of dunes in section, like
the shapes in ground-plan, depend on
Fig. 12.—Cross-section of a dune the relative strength and constancy of
showing the profiles developed by the winds and the supply of sand. With
scour of the wind on both flanks. constant winds and abundant drifting
(Cornish.)
sand, dunes are steep on the lee side
(bc, Fig. 11), where the angle of slope is the angle of rest for the sand. It
rarely exceeds 23° or 24°.[14] Under the same conditions the windward slope
is relatively gentle (ab, Fig. 11). If the winds be variable so that the
windward slope of one period becomes the leeward slope of another, and
vice versa, this form is not preserved. Thus, by reversal of the wind, the
section abc, Fig. 11, may be changed to adc. If the winds and the supply of
sand be equal, on the average, from opposite directions, the slopes should,
on the average, be equal, though perhaps unequal after any particular storm.
The steep slopes of new-made dunes are lost after the sand has ceased to be
blown. At some points where the winds erode (scour) more than they
deposit, new profiles are developed (Figs. 12 and 13). The erosion profiles
may be very irregular if the dunes are partially covered with vegetation. The
effect of vegetation in restraining wind erosion is shown in Fig. 14, where
plants have preserved a remnant of a dune.

The topographic map.—Since dunes as well as other
topographic features are conveniently represented on contour
maps, and since such maps will be used frequently in the

following pages, a general explanation of them is here
introduced.
“The features represented on the topographic map are of three
distinct kinds: (1) inequalities of surface, called relief, as plains,
plateaus, valleys, hills, and mountains; (2) distribution of water,
called drainage, as streams, lakes, and swamps; (3) the works of
man, called culture, as roads, railroads, boundaries, villages, and
cities.

Fig. 13.—Diagram showing the outline of dunes in process of destruction.
Seven Mile Beach, N. J. (N. J. Geol. Surv.)

Fig. 14.—Illustrates the protective effect of vegetation against wind erosion.
Dune Park, Ind. (Cowles)

Relief.—All elevations are measured from mean sea-level.
The heights of many points are accurately determined, and those
which are most important are given on the map in figures. It is
desirable, however, to give the elevation of all parts of the area

mapped, to delineate the horizontal outline, or contour, of all
slopes, and to indicate their grade or degree of steepness. This is
done by lines connecting points of equal elevation above mean
sea-level, the lines being drawn at regular vertical intervals.
These lines are called contours, and the uniform vertical space
between each two contours is called the contour interval. On the
maps of the United States Geological Survey the contours and
elevations are printed in brown (see Plate II).

Fig. 15.—Sketch and map of the same area to illustrate the representation of
topography by means of contour lines (U. S. Geol. Surv.)

The manner in which contours express elevation, form, and
grade is shown in the following sketch and corresponding
contour map, Fig. 15.
The sketch represents a river valley between two hills. In the
foreground is the sea, with a bay which is partly closed by a
hooked sand bar. On each side of the valley is a terrace. From
the terrace on the right a hill rises gradually, while from that on

the left the ground ascends steeply in a precipice. Contrasted
with this precipice is the gentle descent of the slope at the left.
In the map each of these features is indicated, directly beneath
its position in the sketch, by contours. The following
explanation may make clearer the manner in which contours
delineate elevation, form, and grade:
1. A contour indicates approximately a certain height above
sea-level. In this illustration the contour interval is 50 feet;
therefore the contours are drawn at 50, 100, 150, 200 feet, and
so on, above sea-level. Along the contour at 250 feet lie all
points of the surface 250 feet above sea; and similarly with any
other contour. In the space between any two contours are found
all elevations above the lower and below the higher contour.
Thus the contour at 150 feet falls just below the edge of the
terrace, while that at 200 feet lies above the terrace; therefore all
points on the terrace are shown to be more than 150 but less
than 200 feet above sea. The summit of the higher hill is stated
to be 670 feet above sea; accordingly the contour at 650 feet
surrounds it. In this illustration nearly all the contours are
numbered. Where this is not possible, certain contours—say
every fifth one—are accentuated and numbered; the heights of
others may then be ascertained by counting up or down from a
numbered contour.
2. Contours define the forms of slopes. Since contours are
continuous horizontal lines conforming to the surface of the
ground, they wind smoothly about smooth surfaces, recede into
all reëntrant angles of ravines, and project in passing about
prominences. The relations of contour curves and angles to
forms of the landscape can be traced in the map and sketch.
3. Contours show the approximate grade of any slope. The
vertical space between two contours is the same, whether they
lie along a cliff or on a gentle slope; but to rise a given height on
a gentle slope one must go farther than on a steep slope, and
therefore contours are far apart on gentle slopes and near
together on steep ones.

For a flat or gently undulating country a small contour
interval is used; for a steep or mountainous country a large
interval is necessary. The smallest interval used on the atlas
sheets of the Geological Survey is 5 feet. This is used for
regions like the Mississippi delta and the Dismal Swamp. In
mapping great mountain masses, like those in Colorado, the
interval may be 250 feet. For intermediate relief contour
intervals of 10, 20, 25, 50, and 100 feet are used.
Drainage.—Watercourses are indicated by blue lines. If the
streams flow the year round the line is drawn unbroken, but if
the channel is dry a part of the year the line is broken or dotted.
Where a stream sinks and reappears at the surface, the supposed
underground course is shown by a broken blue line. Lakes,
marshes, and other bodies of water are also shown in blue, by
appropriate conventional signs.
Culture.—The works of man, such as road, railroads, and
towns, together with boundaries of townships, counties and
states, and artificial details, are printed in black.”[15]

Topography of dune areas.—From what has been said, it is clear that
the topography of dune regions may vary widely, but it is always
distinctive. Where the dunes take the form of ridges (Fig. 1, Pl. II), the
ridges are often of essentially uniform height and width for considerable
distances. If there are parallel ridges, they are often separated by trough-like
depressions. Where dunes assume the form of hillocks (Figs. 2 and 3, Pl.
II), rather than ridges, the topography is even more distinctive. In some
regions, depressions (basins) are associated with the dune hillocks.
Occasionally they are hardly less notable than the dunes themselves. A
somewhat similar association of hillocks and basins is locally developed by
other means, but dunes are made up of sand and usually of sand only, while
the composition of similarly shaped hillocks and depressions shaped by
other agencies is notably different.

In Fig. 1, Plate II (Five Mile Beach, 8 miles northeast of Cape
May, N. J.), the contour interval is 10 feet. There is here but one
contour line (the 10-foot contour), though this appears in several

places. Since this line connects places 10 feet above sea-level,
all places between it and the sea (or marsh) are less than 10 feet
above the water, while all places within the lines have an
elevation of more than 10 feet. None of them reaches an
elevation of 20 feet, since a 20-foot contour does not appear. It
will be seen that some of the elevations in Fig. 1 are elongate,
while others have the form of mounds.
Fig. 2 (Pl. II) shows dune topography along the Arkansas
River in Kansas (near Larned); Fig. 4, dune topography in
Nebraska (Lat. 42°, Long. 103°), not in immediate association
with a valley or shore; and Fig. 3 shows irregular ridge-like
dunes at the head of Lake Michigan. In Fig. 2 the contour
interval is 20 feet. All the small hillocks southeast of the river
are dunes. Some of them are represented by one contour and
some by two. The altitude of the region is considerable, the
heavy contour representing an elevation of 2100 feet; but the
dunes themselves are rarely more than 20 feet above their
surroundings. In Fig. 4, where the contour interval is also 20
feet, there are, besides the numerous hillocks, several
depressions (basins). These are represented by hachures inside
the contour lines. In some cases there are intermittent lakes
(blue) in the depressions. The heavy contour at Spring Lakes in
this figure is the contour of 4300 feet. There are two depression
contours (4280 and 4260) below it. The bottom of the
depression is therefore lower than 4260, but not so low as 4240.
In Fig. 3 the contour interval is 10 feet, and the dune ridges
north of Miller are more than 50 feet high. The dune ridges here
have helped to determine the position of this branch of the
Calumet River, and have blocked its former outlet. The present
drainage is to the westward.

Migration of dunes.—By the continual transfer of sand from its
windward to its leeward side, a dune may be moved from one place to
another, though continuing to be made up, in large part, of the same sand. In
their migration dunes sometimes invade fertile lands, causing so great loss
that means are devised for stopping them. The simplest method (employed
in France and Holland) is to help vegetation to get a foothold in the sand.

The effect of the vegetation is to pin the sand down. As a dune ridge along a
coast travels inland, another may be formed behind it. Successions of dune
ridges are thus sometimes formed.

Fig. 16.—Diagram illustrating the migration of dunes on the Kurische Nehrung. (Credner.)

A remarkable instance of the migration of a sand dune is recorded on the
Kurische Nehrung on the north coast of Germany. The Nehrung consists of
a long narrow neck of land composed of sand, lying off the main coast. At
the beginning of this century there was a notable dune ridge on one side.
Since that time it has migrated a considerable distance, and in its migration
it has been brought into the relationships illustrated in the accompanying
diagrams (Fig. 16). In 1800 the dune ridge was on one side of a church,
which was then in use. In 1839 the ridge had been so far shifted to the
leeward as to completely bury the church, and in 1869, its migration had
progressed so far as to again discover the building.[16]

Fig. 17.—Migration of dunes into a timbered region. Dune Park, Ind. Head of Lake
Michigan. (Meyers.)

When dunes migrate into a timbered region they bury and kill the trees
(Fig. 17). In one instance on the coast of Prussia a tall pine forest, covering
hundreds of acres, was destroyed during the brief period between 1804 and
1827.[17] At some points in New Jersey orchards have been so far buried
within the lifetime of their owners that only the tops of the highest trees are
exposed. Trees and other objects once buried may be again discovered by
farther migration of the sand (Figs. 18 and 19).[18]

Fig. 18.—A resurrected forest. The dune sand after burying and killing the timber has
been shifted beyond it. Dune Park. Ind. (Meyers.)

Eolian sand, not aggregated into distinct dunes, is often destructive. Even
valleys and cities are sometimes buried by it. Drifting sands had so
completely buried Nineveh two centuries after its destruction that its site
was unknown.
Distribution of dunes.—Dunes are likely to be developed wherever dry
sand is exposed to the wind. Their favorite situations are the dry and sandy
shores of lakes and seas, sandy valleys, and arid sandy plains.
Along coasts, dunes are likely to be extensively developed only where
the prevailing winds are on shore. Thus about Lake Michigan, where the
prevailing winds are from the west, dunes are abundant and large on the
east shore, and but few and small on the west. In shallow water, shore
currents and storm waves often build up a reef of sand a little above the
normal level of the water. When the waves subside, the sand dries and the
wind heaps it up into dunes. This sequence of events is in progress at many
points on the Atlantic Coast. Sandy Hook, New Jersey, and the “beaches”
farther south started as barrier ridges. When the waves had built them above
normal water-level, the wind re-worked the sand, piling it up into mounds
and hillocks (Fig. 1, Pl. II). Such dune belts a little off shore are sometimes
turned to good account. They are usually separated from the mainland by a
shallow lagoon. Where land is valuable, the lagoon is sometimes filled in,
making new land, thus anticipating the result which nature would achieve

more slowly. This has been done at some points on the western coast of
Europe.

Fig. 19.—Migration of dune sand exposing bones in a cemetery. Hatteras Island, N. C.
(Collier Cobb.)

Dunes are likely to occur along stream valleys (Fig. 2, Pl. II), if their
bottoms or slopes are of sand, and not covered by vegetation. Dunes along
valleys are usually on the side toward which the prevailing winds blow.
Thus they are more common on the east side of the Mississippi than on the
west. Dunes may be formed in the valley bottoms, but the sand is often
blown up out of the valley and lodged on the bluffs above.
Apart from these special classes of situations, any sandy region the
surface of which is dry is likely to have its surface material shifted by the
wind and piled up into dune ridges or hillocks (Fig. 4, Pl. II). Dunes
probably reach their greatest development in the Sahara, where some of
them take the form of hillocks, and some the form of ridges. Travelers in
that region report that dune ridges are sometimes encountered the faces of
which are so high and steep as to be difficult of ascent, and that parties have
been obliged to travel miles along their bases before finding a break where
crossing was practicable.

Fig. 20.—Wind-ripples. (Cross, U. S. Geol. Surv.)

Wind-ripples.—The surface of the dry sand over which the wind has
blown for a few hours is likely to be marked with ripples (Fig. 20) similar
to those made on a sandy bottom beneath shallow water, under the influence
of waves. Like ripple marks made by the water, wind-ripples have one side
(the lee) steeper than the other. While the ripples are, as a rule, but a
fraction of an inch high, they throw much light on the origin of the great
dune ridges. If the ripples be watched closely during the progress of a wind-
storm, they are found gradually to shift their position. Sand is blown up the
gentler windward slope to the crest of the ridge and falls down on the other
side. The moment it falls below the crest of the ridge to leeward, it is
protected against the wind, and is likely to lodge. Wear on the windward
side is about equal to deposition on the leeward, and the result is the orderly
progression of the ripples in the direction in which the wind is blowing, just
as in the case of dune ridges.
Abrasion by the wind.—While the effect of the wind on sandy and
dusty surfaces may be considerable, its effect on solid rock is relatively
slight and accomplished, not by its own impact, but by that of the material it

carries. The effect of blown sand on rock surfaces over and against which it
is driven is perhaps best understood by recalling the effects of artificial
sand-blasts, by means of which glass is etched. In a region where sand is
blowing, exposed surfaces of rock suffer from a multitude of blows struck
by the sand grains in transit. The result is that such rock surfaces are worn,
and worn in a way peculiar to the agency accomplishing the work. If the
rock be made up of laminæ which are of unequal hardness, the blown sand
digs out the softer ones, leaving the harder projecting as ridges between
them. Adjacent masses of harder and softer rock of whatever thickness are
similarly affected. The sculpturing thus effected on projecting masses of
rock is often picturesque and striking (Figs. 21 and 22), and is most
common in arid regions. Details of wind-carving are shown in Fig. 23.

Fig. 21.—Wind-carved rock. (Green.)
PLATE II.

U. S. Geol. Surv. Fig. 1. NEW Scale, 1+ mile per inch.
JERSEY.

U. S. Geol. Surv. Fig. 3.
INDI Scale, 1+ mile per inch.
U. S. Geol. Surv. Fig. 2. KANSAS. Scale, 2+ mile per inch. ANA.

U. S. Geol. Surv. Fig. 4. NEBRASKA. Scale, 2+ mile per inch.

Sand drifted over loose stones lying on the surface often develops flat or
flattish faces or facets on them. These facets are likely to be three in
number, and the exposed portion of the stone is likely to develop a sort of
pyramidal shape, the three flattish surfaces being mutually limited by
tolerably well-defined lines (Fig. 24). Thus arise the three-faceted stones
(Dreikanter of the Germans) commonly seen where sands have been long in
movement.

Fig. 22.—Wind-carved hillock of cross-bedded sandstone. Missouri River, Montana.
(Calhoun, U. S. G. S.)

Not only does the drifting sand wear the surface over which it passes and
against which it strikes, but the grains themselves are worn in the process.
They are liable to be broken as they strike rock surfaces, and they are likely
to strike one another in the atmosphere. In both cases they are subject to
wear, and so to reduction to a finer and finer state.
The erosion accomplished by the wind is therefore of various sorts. The
impact of the wind itself picks up the fine materials which are already
loosened, thus wearing down the surface from which they are removed; the
materials picked up wear the rock surfaces against which they are blown,
and the transported materials themselves suffer reduction in transit.
Effects of wind on plants.—Another effect of wind work is seen in the
uprooting of trees (Fig. 25). The uprooting disturbs the surface in such a
way as to make loose earth more readily accessible to wind and water. The
uprooting of trees on steep slopes often causes the descent of considerable
quantities of loose rock and soil. Again, organisms of various sorts (certain
types of seeds, germs, etc.), as well as dust and sand, are extensively
transported by the wind. While this is important biologically its geological
effects are remote.

Fig. 23.—Figure showing details of wind-carving on rock surface (rhyolite). Mono Valley,
California.

Fig. 24.—Wind-worn stones (Dreikanter).

Fig. 25.—Shows the disturbance of surface earth and rocks by upturning of trees. (Darton,
U. S. Geol. Surv.)

Indirect effects of the wind.—Other dynamic processes are called into
being or stimulated by the atmosphere. Winds generate both waves and

currents, and both are effective agents in geological work. The results of
their activities are discussed elsewhere.

II. The Chemical Work of the Atmosphere.
The chemical work of the atmosphere (including solution and
precipitation from solution) is principally accomplished in connection with
water, a dry atmosphere having relatively little direct chemical effect on
rock or soils.
Precipitation from solution.—The water in the soil is constantly
evaporating. Such substances as it contains in solution are deposited where
the water evaporates, and where evaporation is long continued without re-
solution of the substances deposited, the surface becomes coated with an
efflorescence of mineral matter. Conspicuous examples are found in the
alkali plains of certain areas in the western part of the United States. Since
the alkaline efflorescence is the result of evaporation it is connected with
the atmosphere, but the material of the efflorescence was brought to its
present position by water. The principle involved is illustrated by the white
efflorescence which frequently appears on brick walls during the dry days
which follow a drenching rain. The water penetrates the brick and mortar
and dissolves something of their substance, and when it is evaporated from
the surface the material in solution is left behind.
In arid regions the deposition of substances other than alkali is common.
The percolating waters dissolve whatever is soluble, and when they
evaporate their mineral content is left. The pebbles and stones of the arid
plains have in many places become heavily coated with mineral matter
deposited in this way, and not infrequently cemented into conglomerate.
One of the commonest mineral substances found in such situations is lime
carbonate. In some cases it was doubtless derived by solution from
limestone beds beneath the surface, but this is not always the case. It often
encrusts the bits of lava on lava plains where it can hardly have been
derived from limestone. The faces of cliffs of granite or gneiss, hundreds
and even thousands of feet above all other sorts of rock,[19] are sometimes
spotted with patches of lime carbonate. In the first case the lime carbonate
was derived by chemical change from the lava, and in the second, from the
granite or gneiss (see Carbonation below), but its present position is the
result of evaporation.

Oxidation.—In the presence of moisture the oxygen of the air enters into
combination with various elements of the soil and rocks. This is oxidation.
No other common mineral substance shows the results of oxidation so
quickly and so distinctly as iron. The oxidized portion is loose and friable,
and a mass of iron exposed to a moist atmosphere will ultimately crumble
away. This change is comparable to other less obvious changes taking place
in many minerals at and below the surface. Oxidation generally involves the
disintegration of the rock concerned. Its effects in this direction will be
referred to in other connections.
Carbonation.—The production of lime carbonate from rock containing
calcium compounds, but not in the form of carbonates, is known as
carbonation, and is one of the important chemical changes effected by the
carbon dioxide of the atmosphere in coöperation with water. In the process
of carbonation the original minerals of complex composition are
decomposed and simpler ones usually formed. Volumetric changes are
involved, which often lead to the disruption of the rock (see Ground water).
Furthermore, carbonates are among the more soluble minerals, and their
production therefore brings some of the rock materials into a soluble
condition, and their extraction through solution tends still further to
disintegrate the rock. The carbonation of crystalline rocks is therefore a
disintegrating process, and will be considered further in its many concrete
applications.
Other chemical changes.—A third chemical process which often
accompanies oxidation and carbonation is hydration. This is effected by
water rather than by air, and will be considered in that connection. In
general it leads to the disintegration of the minerals and rocks affected. The
chemical effects of nitric acid, etc., developed through the agency of
atmospheric electricity, and the corresponding effects of the gases and
vapors which issue from volcanoes, many of them chemically active, are to
be mentioned in this connection.
Conditions favorable for chemical changes.—Conditions are not
everywhere equally favorable for the chemical work of the atmosphere. In
general, high temperatures facilitate chemical action, and, other things
being equal, rocks are more readily decomposed by atmospheric action in
warm than in cold regions. Chemical activity is probably greater where the
climate is continuously warm than where there are great changes of

temperature. Changes of temperature, on the other hand, tend to disrupt
rock, and thus increase the amount of surface exposed to chemical change.
Since nearly all the chemical changes worked by the atmosphere on the
rocks are increased by the presence of moisture, the chemical activity of the
atmosphere is greater in moist than in dry regions.

B. THE ATMOSPHERE AS A CONDITIONING AGENCY.

The most obvious mechanical work of the atmosphere is effected by the
wind, but mechanical results of great importance, conditioned by the
atmosphere, are also effected when the air is still.

I. Temperature Effects.
When the sun shines on bare rock its surface is heated and expanded, and
the expanded particles crowd one another with great force. Since rock is a
poor conductor of heat its surface is heated and expanded notably more than
parts beneath the surface. It follows that strains are set up between the
expanded outer portion and the cooler and less expanded parts within. In the
cooling of the same rock mass it is the outermost portion which cools first
and fastest, and, contracting as it cools, strains are again set up between the
outer part, which is cooled more, and the inner part, which is cooled less.
The result may be illustrated by the effect of cold water on hot glass, or of
hot water on cold glass. In either case the fracture is the result of the sudden
and considerable differential expansion or contraction. Since the heating
and cooling of rock are much slower than the heating and cooling of glass
under the conditions mentioned, the rupturing effects are less conspicuous,
but none the less real. The actual effects of temperature changes are
illustrated by familiar phenomena. The surface portions of bowlders
exposed to the sun are frequently seen to be shelling off (Fig. 26). The
loosened concentric shells may be a fraction of an inch, or sometimes even
several inches in thickness. This process of exfoliation affects not only
bowlders, but bare rock surfaces wherever exposed to the sun (Figs. 27, 28).
It is often conspicuous on the faces of cliffs.

Fig. 26.—Exfoliation. A bowlder of weathering, the rock being granite. Wichita Mountains,
Oklahoma.

Fig. 27.—A weathered summit of granite in the Wichita Mountains. Oklahoma. (Willis,
U. S. Geol. Surv.)

Several conditions, some of which are connected with the atmosphere
and some with the rock, determine the efficiency of this process. Since the
breaking of the rock results from the expansion and contraction due to its
changes of temperature, it follows that, other things being equal, the greater
the change, the greater the breaking; but the suddenness of the temperature
change is even more important than its amount. It follows that great daily,
rather than great annual, changes of temperature[20] favor rock-breaking,

though with changes of a given frequency their effectiveness is greater the
greater their range. A partial exception to this generalization should be
noted. If abundant moisture is present in the pores and cracks of the rock a
change of temperature from 45° to 35° (Fahr.) might be far less effective in
breaking the rock than a change from 35° to 25° in the same time, for in the
latter case the sudden and very considerable expansion (about one-tenth)
which water undergoes on freezing is brought into play. This may be called
the wedge-work of ice. The daily range of temperature is influenced
especially by latitude, altitude, and humidity. Other things being equal, the
greatest daily ranges of temperature occur in high-temperate latitudes,
though to this general statement there are local exceptions, depending on
other conditions. High altitudes favor great daily ranges of temperature, so
far as the rock surface is concerned (see Figs. 29, 30), for though the rock
becomes heated during the sunny day, the thinness and dryness of the
atmosphere allow its heat to radiate rapidly at night. Here, too, the daily
range of temperature is likely to bring the wedge-work of ice into play.
Since the south side of a mountain (in the northern hemisphere) is heated
more than the north, it is subject to the greater daily range of temperature,
and the rock on this side suffers the greater disruption. Similarly, rock
surfaces on which the sun shines daily are subject to greater disruption than
those much shielded by clouds. Isolated peaks, because of their greater
exposure, are subject to rather greater daily ranges of temperature than
plateaus of the same elevation.

Fig. 28.—Exfoliation on a mountain slope. Mt. Starr-King (Cal.) from the north.

The daily range of temperature is also influenced by humidity. Because
of the effect of water vapor in the atmosphere on insolation and radiation, a
rock surface becomes hotter in the day and cooler at night beneath a dry
atmosphere than beneath a moist one. Aridity therefore favors the disruption
of rock by changing temperatures.
Turning from the conditions of the atmosphere which affect the
disruption of rock to the conditions of the rock which influence the same
process, several points are to be noted. In the first place, the disrupting
effects of changes of temperature are slight or nil where the solid rock is
protected by soil, clay, sand, gravel, snow, or other incoherent material. If
the constituent parts of the loose material are coarse, like bowlders, their
surfaces are affected like those of larger bodies of rock. The color of rock,
its texture and its composition, also influence its range of daily temperature
by influencing absorption and conduction. Dark-colored rocks absorb more
heat than light-colored ones, and compact rocks are better conductors than
porous ones. Great absorption and slow conduction favor disruption. A
given range of temperature is unequally effective on rocks of different
mineral composition. In general crystalline rocks (igneous and

metamorphic) are more subject to disruption by this means than
sedimentary rocks, partly because they are more compact, but especially
because they are made up of aggregates of crystals of different minerals
which, under changes of temperature, expand and contract at different rates,
while the common sedimentary rocks are made up largely of numerous
particles of one mineral.

Fig. 29.—Top of Notch Peak, Bighorn Mountains, Wyo. Shows the thoroughly broken
character of the rock on the summit, the absence of soil, vegetation, etc. (Kümmel.)

Fig. 30.—A detail from Fig. 29 showing the size of the rock blocks. (Kümmel.)

Fig. 31.—Peak north of Kearsarge Pass, the Sierras. Shows the way in which serrate peaks
break up into angular blocks.

The freezing of water in the pores of rock is effective in disrupting them
only when the pores are essentially full at the time of freezing. Otherwise
there is room for the expansion attending the freezing. If the pores of the
rock are large, the expansion on freezing may force out sufficient water to
balance the increase of volume, even though the rock was completely
saturated. If the pores be very small the water passes out less readily, and if
the rock is saturated, freezing is more likely to be attended with disruption.
[21]

In view of these considerations the breaking of rock by changes of
temperature should be greatest on the bare slopes of isolated elevations of
crystalline rock, where the temperature conditions of temperate latitudes
prevail, and where the atmosphere is relatively free from moisture. All these
conditions are not often found in one place, but the disrupting effects of
changing temperatures are best seen where several of them are associated
(Figs. 29, 30, and 31).
The importance of this method of rock-breaking has rarely been
appreciated except by those who have worked in high and dry regions.
Climbers of high mountains know that almost every high peak is covered

with broken rock to such an extent as to make its ascent dangerous to the
uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule,
literally crumbling to pieces (Fig. 31). The piles of talus which lie at the
bases of steep mountain slopes are often hundreds of feet in height, and
their materials are often in large part the result of the process here under
discussion. In mountain regions where atmospheric conditions favor sudden
changes of temperature, the sharp reports of the disruption of rock masses
are often heard. Masses of rock, scores and even hundreds of pounds in
weight, are frequently thus detached and started on their downward course.
[22]
Small pieces of rock are of course much more commonly broken off than
large ones. The disruption of rock by changes of temperature is not usually
the result of a single change of temperature, but rather of many successive
expansions and contractions.
The sharp needle-like peaks which mark the summits of most high
mountain ranges (Fig. 32) are largely developed by the process here
outlined. The altitude at which the serrate topography appears varies with
the latitude, being, as a rule, higher in low latitudes and lower in high. But
even in the same latitude it varies notably with the isolation of the
mountains and with the aridity of the climate. Thus within the United States
the sharply serrate summits appear in some places, as in Washington and
Oregon, at altitudes of 6000 to 10,000 feet, while in the isolated Wichita
range of Oklahoma, much farther south, but in a much drier climate, the
same sort of topography is developed at altitudes of 2500 to 3000 feet.
Even in low latitudes and moist climates the effects of temperature
changes are often seen. Thin beds of limestone at the bottom of quarries are
sometimes so expanded by the heat of the sun as to arch up and break.[23] In
desert and arid regions,[24] whatever the altitude, the effects of temperature
changes are often striking.

Fig. 32.—Serrate peaks of granitic rock in Black Hills. (Darton, U. S. Geol. Surv.)

The disruption of rock by changes of temperature is one phase of
weathering. It tends to the formation of a mantle of rock waste, which, were
it not removed, would soon completely cover the solid rock beneath and
protect it from further disruption by heating and cooling; but the loose
material thus produced becomes an easy prey to running water, so that the
work of the atmosphere prepares the way for that of other eroding agencies.

II. Evaporation and Precipitation.
Perhaps the most important work of the atmosphere as a dynamic agent
lies in its function as the medium for the circulation and distribution of
water. Atmospheric temperature is the primary factor governing
evaporation, an important factor in the circulation of the vapor after it is
formed, and controls its condensation and precipitation.
The average amount of annual precipitation on the land is variously
estimated at from forty to sixty inches, the lesser figure being probably
more nearly correct. Since much of this water falls at high altitudes, the
work which it accomplishes in getting back to the sea is great. The water
which falls on the land, if withdrawn wholly from the ocean, would exhaust

that body of water in 10,000 to 15,000 years if none of it returned. The
work of evaporation is of course not done by the atmosphere, though the
atmosphere determines the effect of the solar energy which does the work.
[25]

The precipitation is distributed with great inequality, and this inequality
affects both the rain and the snow. Some regions have heavy precipitation
and some light; some regions have much rain and little snow; others have
much snow and little rain; others have rain and no snow, and still others
have snow and little or no rain. The amount and distribution of rain and
snow determine the size and distribution of streams and glaciers, and
streams and glaciers are the most important agencies modifying the surface
of the land.
It is impossible to separate sharply the geologic work of the water of the
atmosphere from that of other waters; but so long as moisture is in the
atmosphere (including the time of its precipitation) its effects are best
considered in connection with the atmosphere.
The mechanical work of the rain.—In falling the rain washes the
atmosphere, taking from it much of the dust, spores, etc., which the winds
have lifted from the surface of the dry land. Not only this, but in passing
through the atmosphere the water dissolves some of its gases, and perhaps
particles of soluble solid matter. When therefore the falling water reaches
the surface of the land it is no longer pure, and some of the gases it has
taken up in its descent enable it to dissolve various mineral matters on
which pure water has little effect.
As it falls on the surface of the land the rain produces various effects of a
mechanical nature. In the first place, it leaves on the surface the solid matter
taken from the air. The amount of material, thus added to any given region
in any particular shower is trivial, but in the course of long periods of time
the total amount of material washed out of the air must be very great.
Every rain-drop strikes a blow. If the drops fall on vegetation, they have
little effect, but if they fall on sand or unprotected earthy matter they cause
movements of the particles on one another, and this movement involves
friction and wear. While the results thus effected are inconsiderable in any
brief period of time, they are not so insignificant when the long periods of
the earth’s history are considered.

Clayey soils contract and often crack on drying. Falling on such a soil
when it is dry the rain causes it to expand, and the cracks are healed by
lateral swelling. The same soils are baked under the influence of the sun,
and when in this condition are softened and made more mobile by the
falling of rain. Under the influence of the expansion and contraction
occasioned by wetting and drying, the soils and earths on slopes creep
slowly downward. When rain falls on dry sand or dust the cohesion is at
once increased, and shifting by the wind is temporarily stopped.
After the water has fallen on the land its further work cannot be looked
upon as a part of the work of the atmosphere; but any conception of the
geological work of the atmosphere which did not recognize the fact that the
waters of the land have come through the atmosphere would be inadequate.
The work of the water after it has been precipitated from the atmosphere
must be considered in another chapter.

III. Effects of Electricity.
Another dynamic effect conditioned by the atmosphere is that produced
by lightning. In the aggregate this result is inconsequential; yet instances
are known where large bodies of rock have been fractured by a stroke of
lightning, and masses many tons in weight have sometimes been moved
appreciable distances. Incipient fusion in very limited spots is also known
to have been induced by lightning. Where it strikes sand it often fuses the
sand for a short distance, and, on cooling, the partially fused material is
consolidated, forming a little tube or irregular rod (a fulgurite) of partially
glassy matter. Fulgurites are usually only a few inches in length, and more
commonly than otherwise a fraction of an inch in diameter. Strictly
speaking these results are the effect of the electricity of the atmosphere
rather than of the atmosphere itself, but they are best mentioned in this
connection.
Allusion has already been made to the chemical changes in the
atmosphere occasioned by electric discharges.

Fig. 33.—Stratified jointed rock in process of weathering. (Cross, U. S. Geol. Surv.)

Fig. 34.—Represents a later stage of the processes illustrated by Fig. 33. (Darton, U. S.
Geol. Surv.)

SUMMARY.

Weathering.—The result of all atmospheric processes, whether physical
or chemical, by which surface rock is disrupted, decomposed or in any way
loosened, is weathering. This convenient term also includes similar results
effected by ground water, plants, etc. The tendency of weathering is to
produce a mantle of residuary earth over solid rock. Weathering by
mechanical means tends to produce material which, though in a finer state
of division, is still like the original rock in chemical composition.
Weathering by chemical means tends to produce a mantle made up chiefly
of the less soluble parts of the rock from which it was derived. All processes
of weathering prepare material for transportation by wind and water.

Fig. 35.—Details of a weathered rock surface, due partly to wind work and partly to
solution. The particular phase of weathering illustrated by this figure is known as
“honeycomb” weathering. (Gilbert, U. S. Geol. Surv.)

Many considerations determine the thickness which the mantle of
weathered rock (mantle rock) attains. Some of these considerations have to
do with the atmosphere, and some with drainage. Since the latter are, on the
whole, more important, this matter will be discussed in connection with the
work of water (Chapters III and IV).

CHAPTER III.
THE WORK OF RUNNING WATER.

Familiar phenomena, both of land and sea, reveal the constant activity and
importance of water as a geologic agent. Even when there is no precipitation the
moisture in the air influences its activity in certain ways. Just as iron “rusts” more
readily in moist air than in dry, so changes in other mineral substances are
influenced by atmospheric humidity. Where precipitation takes place the results are
more obvious. The passing shower works changes in the surface of the land,
striking in proportion to the rate and amount of precipitation. The rains feed the
streams, and every stream is modifying its bed, and with increasing rapidity as its
current is swollen. Even the moisture which is precipitated as snow works its
appropriate results. Before it melts it protects the surface against other agents of
change; but if it accumulates in sufficient quantity in appropriate situations, it may
give rise to avalanches and glaciers, which, like running water, degrade the surface
over which they pass.
A part of the water which falls as rain, and a part of that which results from the
melting of snow and ice, sinks into the soil and into the rock below, becoming
ground water. It is this ground water which especially justifies the name
hydrosphere, often applied to the waters of the earth, for it literally forms a
spherical layer in the outer portion of the solid part of the earth. During the stay of
the water beneath the surface it effects changes in the rocks through which it
passes, dissolving mineral matter here and depositing it there, substituting one
substance for another in this place, and effecting new chemical combinations in
that. Slow as these processes are, they have worked wondrous changes in the
course of the earth’s history.
When the waters are gathered together in ponds, lakes, and oceans, they are still
active, and the results of their activity are seen along the shores, where winds and
waves produce their chiefest effects. Even the ocean currents, far from land, and the
processes of the deep sea, are not without their effect on the course of geological
history.
The work of the surface waters, ground (underground) waters, standing waters,
and ice will be considered in order.

RAIN AND RIVER EROSION.

Rain and river erosion began when the first rains fell on land surfaces. Neither
the location nor the nature of the first land surface is known. There is little reason
to believe that the ocean was ever universal, but there is reason to believe that most
land areas have at some time or other been covered by the sea. The prevalent
conception that land areas which were once submerged came into existence by
being elevated above sea-level, should be supplemented by the alternative
conception that submerged areas may have become land by the depression of the
ocean basins, thus drawing off the water from the areas where it was shallow. Thus
in Fig. 36 the sinking of the sea-bottom from a to b would lower the surface of the
water from cc′, to dd′, and draw off the water from the surfaces cd and c′d′.

Fig. 36.—Diagram to illustrate the origin of lands by the lowering of the sea-level due to depression
of the sea bottom. If the bottom is depressed from a to b the surface will be drawn down from cc′
to dd′, and the surfaces cd and c′d′ will become land.

Without attempting to picture the character of the original land our study of
subaërial erosion may begin with an area which has just been changed from sea
bottom to land. What is the nature of such a land surface? Of what material is it
composed, and what is the character of its topography? Concerning its constitution
something may be inferred from the nature of the deposits now found at the bottom
of the sea. Near the shore and in shallow water they often consist of gravel and
sand, though other materials are not wanting. Far from shore and in deep water they
consist for the most part of fine sediments, some of which were washed or blown
from the land, some of which came from the shells and other secretions of marine
animals, some from volcanoes, and some from various other sources. The
topography of the newly emerged land may have had some likeness to the
topography of the sea bottom. The numerous soundings which have been made
over large areas of the sea have shown that its bottom is, as a rule, free from the
numerous small irregularities which affect the surface of the land. They seem to
show that a large part of the ocean bottom is so nearly flat that, if the water were
removed, the eye would hardly detect irregularities in the surface. This statement
does not lose sight of the fact that the ocean bottom is, in certain places, markedly
irregular. Volcanic peaks and striking irregularities of other sorts abound in some
places. Nevertheless if the bottom of the sea could be seen as the land is, its most
striking feature, taken as a whole, would be its apparent flatness.

With the topography of the sea bottom the topography of the land is, in its
details, in sharp contrast. In order to get at the history of the latter, we may study
the sequence of events which would follow the emergence of a portion of the
former.

Subaërial Erosion without Valleys.
For the sake of emphasizing the fundamental principles involved in the work of
running water, a hypothetical case will first be studied in some detail, even at the
risk of elaborating processes already understood. The principles themselves will
find application later in relations which are much less simple.
Let it be assumed that the area of newly emerged land is a circular dome-shaped
island. The simplest possible condition is represented by assuming its slope to be
the same in all directions from the center, and its materials to be absolutely
homogeneous. Such an island would be subject to all the forces ordinarily operating
on land surfaces. The chief agency tending to modify land surfaces is atmospheric
precipitation. It will be assumed that the rain falls on the surface of the island with
absolute equality at all points, and that all other forces which affect it operate
equally everywhere.
The rain falling on a land area disappears in various ways; part of it evaporates,
part of it sinks, and part of it runs off over the surface. If the island be composed of
fine and unconsolidated materials, such as clay, the water which runs off over the
surface will carry sediment down to the sea. If the island be composed of solid rock
instead, exposure to the air will cause it to decay, and the products of decay, such as
sand and mud, will suffer a like fate.
For the sake of a clear understanding of the processes involved, two cases may
be postulated; one in which the waters of the sea remove the sediment washed
down from the hypothetical island as fast as it reaches the shore, and one in which
they allow it to accumulate without let or hindrance. In both cases the wear of the
waves will be neglected.
1. In the first case the water flowing off over the surface (the run-off) will
descend equally in all directions. It will constitute a continuous sheet of surface-
water, and both its volume and its velocity will be the same at all points equally
distant from the summit. Erosion accomplished by sheets of running water, as
distinct from streams, is sheet (or sheet-flood) erosion.[26] Since the material of the
surface is homogeneous, the wear effected by the water will be equal at all points
where its velocity and volume are equal. For obvious reasons the depth of the run-
off will increase from summit to base. The gradient (slope) also increases in the
same direction, and the increase of volume and of gradient conspire to augment the

velocity of the water, and therefore of the wear effected by it. If the thin sheet of
water starting from the top of the island with relatively low velocity be able to wash
off even a little fine material from the surface, the thicker sheet farther down the
slope, moving with greater velocity, will be able to carry away more of the same
sort of material, and the increase will be progressive from summit to base. It
follows, therefore, that the surface will be worn equally at points equally distant
from the summit, but unequally at points unequally distant from it. The first shower
which falls on the island may be conceived to wash off from its surface a very thin
sheet of material, but a sheet which increases in thickness from top to bottom. The
run-off will not be stopped immediately on reaching the sea, but will displace the
sea-water to some slight depth, and wear the surface some trivial distance below
the normal level of the sea. The result of successive showers working in the same
way through a long period of time will be to diminish the area of the island and to
steepen its slopes. The results of a considerable period of erosion under these
conditions are shown diagrammatically in Fig. 37, which illustrates both the
diminution in area which the island has suffered, and the increase in the angle of its
slopes. Immediately about it, at the stage represented by aa, Fig. 37, there is a
narrow marginal platform, or submerged terrace, in place of the land area which
has been worn away at or just below the level of the sea. Long successions of rains
working in the same way will give the island steeper slopes, a smaller area, and a
wider marginal terrace. Successive stages are shown by the lines bb and cc, Fig. 37.

Fig. 37.—Diagram to illustrate the effect of rain erosion on an island where there is no deposition or
wave erosion about its borders. The uppermost curve represents the original surface, while aa, bb,
and cc represent successive surfaces developed by sheet erosion, on the supposition that no
material is deposited along the shores.

If rain falls on such an island until it completes the work which it is possible for
running water to do, the island will be reduced essentially to the level of the sea,
and in its place there will be a plain, the area of which will be equal to that of the
original island. Its central point will be at the level of the sea, and its borders a
trifling distance below it (Fig. 38). The island is gone, and in its place there is a
plain as low as running water can wear it. Other agencies might come in to defeat
the result just outlined, but if the island did not rise or sink after its formation, rain
falling upon it would, under the conditions specified, finally bring about the result
which has been sketched. The plain (Fig. 38) which succeeds the island is a base-
level of erosion, though this term is also used in other ways. Under these conditions
the slope of the land would remain convex at all stages, but the convex erosion
profile of the land would meet a nearly straight line just below sea-level. The
relative lengths of these two elements of the profile, the curve above and the

straight line below, vary as erosion progresses, the convex portion becoming
shorter and the other longer, The two parts of the profile taken together are concave
upward at the lower end all the time, and for a greater distance from its lower end
in all the advanced stages of erosion (Fig. 37).

Fig. 38.—Diagram to illustrate the final effect of rain erosion under the conditions specified in the
text. The diagram expresses the final result of the processes suggested by Fig. 37.

In the destruction of the land under these conditions neither valleys nor hills
would be developed, nor would the topography of the land be fashioned to
correspond with the surfaces with which we are familiar.
It is to be distinctly borne in mind that the foregoing is a hypothetical case; it is
not probable that such an island ever existed, or ever will; but that does not
diminish the value of the illustration, since the principles involved are operating on
every land mass, though in less simple relations.
2. The second case differs from the first in that the sediment washed down from
the land is deposited about its borders. This results in the building up of a marginal
platform, as shown in Figs. 39–41. As erosion goes on more sediment is washed
down and deposited, partly on the narrow marginal shelf which has already been
developed, and partly on its outer slope, as shown in the figures. The marginal flat
is thus extended beyond the original shores of the island on the one hand, and
toward its center on the other. As it develops, its inner portion, and indeed all
except its outer edge (ab, Figs. 40 and 41), will be gradually built up above the
level of the water. This marginal lowland is developed at a level as low as running
water, under the conditions then and there present, can reduce the land. Such a
surface may be said to be at grade, since running water neither wears it down nor
builds it up. Its angle of slope is a function of (1) the volume of the water running
over it, and (2) of the load which the water carries.

Fig. 39–41.—Diagrams to illustrate the effect of rain erosion on an island when all the eroded
material is deposited about the shore. The black portions represent deposition. The dotted lines
represent the original surface. The several diagrams represent successive stages in the process.

Since the marginal plain of the above illustration extends beyond the original
shore of the island, the area of land is increased, though both its average elevation
and its mass (above water) are reduced. In case destructive processes did not
operate on the marginal graded plain the spreading and lowering suggested by Figs.
39 and 40 would go on until the central mass of the island was brought down to a
gradient in harmony with that of the gently sloping border, as shown in Fig. 41.
When this had been accomplished there would be a relatively large land area with
low slopes (Fig. 41) in place of the smaller area with steeper ones (compare Figs.
39 and 40). The basal part of the larger island from the center to the original margin
would be made up of the original material in its original position (unshaded part of
Fig. 41). Its surface would be covered, least deeply near its center and most deeply
near the original margin, with débris gradually shifted from higher levels, as shown
in Fig. 41.
Were such an island as that shown in Fig. 41 once formed, the rain falling on it,
and flowing off over its surface, would carry off its surface soil and spread it about
the shores. Though the surface of the marginal flat of Fig. 40 was as low as running
water could bring it at the time it was developed, the conditions of erosion have
changed by the time the land reaches the conditions shown in Fig. 41, and the same
amount of rainfall may now be effective in erosion. In the first case (Fig. 40) the
water descending from the higher part of the land brought down sediment and
started across the flat with a load. Its energy was consumed in transporting what it
had, not in getting new material. In the second case (Fig. 41) the water flowing
over the gently sloping surface has no initial load, and its energy is therefore
available for erosion. Under continued rainfall, the area of the land shown in Fig.
41 would be increased as before by successive marginal deposits (see Fig. 42), and
at the same time its average height would be reduced. The lowering and enlarging
of the island would continue until the whole surface was brought so nearly to the
level of the sea that water would cease to run over it with sufficient velocity to

carry away even the fine material of its surface. Such a surface, brought down as
low as running water can degrade it, is also (see p. 57) a base-level. It will be seen
from the foregoing illustrations that a graded surface may pass into a base-level,
with no sharper line of demarkation than that which separates a mature man from
an old one. In this case, as in the preceding, the island has been base-leveled, but
still without the formation of valleys or hills.

Fig. 42.—Diagram to illustrate the result of the continuation of the processes shown in Figs. 39–41.

Both the preceding hypothetical cases make it clear that, from the point of view
of erosion, every drop of water which runs off over the surface of the land has for
its mission the getting of the land into the sea. Under ordinary conditions surface
drainage must fail to bring a land area altogether to sea-level, the absolute base-
level of subaërial forces; but it is not simply the water which runs off over the
surface which degrades the land. That which sinks beneath the surface contributes
to the same end by slowly dissolving mineral matter below the surface, and finally
carrying it to the sea. In this way the reduction of land areas to sea-level may be
completed.
The rain-water which evaporates from the surface without sinking beneath it
does not effect much wear; but the water thus evaporated is subject to
reprecipitation, so that, in the long run, it may assist in the work which has been
sketched. Thus it is not simply the waters which run off over the surface of the
land, but all which fall upon it, which unite to compass its destruction.

The Development of Valleys.
By the growth of gullies.—Had the slopes of the hypothetical island not been
absolutely uniform the processes of erosion would have been different. Let the
departure from uniformity be supposed to consist of a single slight meridional
depression near the base of the island (Fig. 43). As the rain falls it will no longer
run off equally in all directions. A greater volume will flow through the depression
than over other parts of the surface having the same altitude, and the greater
volume of water along this line will give greater velocity, greater velocity will
occasion greater erosion, and greater erosion will deepen the depression. The
immediate result is a gully or wash (Fig. 44). So soon as the gully is started it tends
still further to concentrate drainage in itself, and is thereby enlarged. The water
which enters it from the sides widens it; that which enters at its head lengthens it by
causing its upper end to recede; and all which flows through it, so long as its
bottom is above base-level, deepens it. The enlarged gully will gather more water

to itself, and, as before, increased volume means increased velocity, and increased
velocity increased erosion. As the gully grows, therefore, its increased size
becomes the occasion of still further enlargement.

Fig. 43.—Diagram showing a slight meridional depression in the surface of an otherwise even-sloped
island.

Continued growth transforms the gully into a ravine, though between a gully and
a ravine there is no distinct line of demarkation. But growth does not stop with
ravine-hood. Water from every shower gathers in the ravine, and, flowing through
it, increases its length, width, and depth, until it reaches such proportions that the
term ravine is laid aside, as childhood names are, and the depression becomes a
valley.

Fig. 44.—Diagram illustrating the development of a gully, starting from the condition shown in Fig.
43.

It was assumed in the preceding paragraphs that the single depression in the
slope was meridional and low on the slope, but almost any sort of depression in
almost any position would bring about a similar result, since it would lead to
concentration of the run-off. Had the original surface been interrupted by ridges
instead of depressions, the effect on valley development would have been much the
same, for a ridge, like a depression, would, in almost any position, occasion the
concentration of the run-off, and so the development of valleys. Under the
conditions represented in Fig. 44 the lengthening of the drainage depression is
effected chiefly at its upper end, the head of the valley working its way farther and
farther back into the land. This method of elongation is known as head erosion. But
the lengthening of the valley is not always wholly by head erosion. The gully
normally begins where concentration of run-off begins, and if this were not at sea-
level, the gully might be lengthening at both ends at the same time. This would
have been the case, for example, had the original depression of Fig. 43 been half-
way up the slope of the island.
If while the slopes of the island were absolutely uniform its surface material
failed of homogeneity, the result would be much the same as if the slopes were
unequal. If the material lying along a certain meridian of the island be slightly
softer than that over the rest of the surface, the run-off, which would at the outset

be equal on all sides, would effect more erosion along the line of the less resistant
material than elsewhere. The result would be a depression along this line, and, once
started, the depression would be a cause of its own growth. If the soft material were
disposed in any way other than that indicated, the final result would be much the
same, for it would quickly give origin to a depression which would lead to the
concentration of the surface-waters, and this is the condition for the development of
a gully, a ravine, and finally a valley.
In the presence of sufficient rainfall,
either heterogeneity of slope or of material
will therefore occasion the development of
valleys. If the lack of uniformity appears at
but a single point there will be but a single
valley. If it appears at many points the
number of valleys will be large. Since it is
incredible that a land mass of perfectly
homogeneous material and of absolutely
uniform slopes ever existed, it is believed
that every land mass, affected for any
considerable length of time by rain, has had
valleys developed in it. The degree of
heterogeneity of material and slope is
usually so great as to lead to the Fig. 45.—Diagram to illustrate the effect of
development of many valleys, even on sheet and stream erosion on the outline of
an island when no deposition takes place
areas which are not large; but for the sake about its borders. The dotted line
of emphasizing the simpler elements of the represents the original outline of the
island, the full line its border at a later
complex processes of stream work, the time. The stream develops a reëntrant
hypothetical case of an island with but a (bay) in the outline.
single valley, and that without tributaries,
may first be studied. Under these conditions two cases may be considered, the one
where there is no deposition about the island, and the other where deposition takes
place.
1. If all the material eroded from the surface of such an island, both in and out of
the valley, were carried well beyond the borders of the land before being deposited,
the edge of the island would recede from its original position toward the center, as
illustrated by Figs. 37 and 45; but the recession would be most rapid where the
valley joins the sea (Fig. 45). At this point therefore a reëntrant would be developed
(a, Fig. 45), and the island would lose its circular outline. Continued erosion would
cause the shore-line to retreat on all sides, but fastest at the lower end of the valley,
and the final result would be a base-level differing from that developed under the

conditions specified on p. 60, in that the last part to be brought low would not be
the center of the original island.
Under the foregoing conditions the
profile of that part of the valley which is
above sea-level (cb) would be convex,
following the analogy of sheet erosion on a
hypothetical island of uniform slopes and
homogeneous material with no marginal
deposition. Its side slopes, likewise
developed under the influence of running
water augmented in volume from top to
bottom, would also be convex.
2. If the sediment washed down from the
land is deposited about its borders, both the
outline of the island and the profile of the
valley will be altered. Deposition at the
debouchure of the valley follows the same
Fig. 46.—Diagram showing the outline of an principles as deposition elsewhere; but if all
island as modified by sheet and stream the sediment brought to the sea be
erosion where eroded material is deposited
at the shore. The dotted line represents the
deposited at the shore, the seaward
original outline; the full line, a later one. extension of the land by deposition would
The excess of deposition at the end of the be more rapid opposite the valley than
valley causes a projection of land into the
sea. elsewhere, and the island would lose its
circular outline, and develop some such
form as is shown in Fig. 46. In this case the profile of the upper end of the valley,
and the upper parts of its side slopes, as well as the upper parts of the extra-valley
slopes of the island, are convex (compare Figs. 39 and 40); but the convexity above
is exchanged for concavity below, the change beginning at the point where
downward erosion of the descending waters is checked. As a valley lengthens, the
larger part of its profile becomes concave (compare the profiles of Figs. 39 to 41),
but the extreme upper end still remains convex. Since the side slopes of a valley are
much shorter than its lengthwise slope, a larger proportion remains convex. Under
the conditions here discussed the change from convexity above to concavity below
would begin at about the point where deposition begins.[27]

Fig. 47.—Diagram representing several meridional valleys developing in a circular island. The
valleys are all young and narrow. All are making deposits at their debouchures.

The deposition at the debouchure of the valley, and later above the debouchure,
will follow the same course as about the island under the conditions already
discussed (pp. 61, 62).
Limits of growth.—In all cases there are limits in depth, length, and width,
beyond which a valley may not grow. In depth it may reach base-level. At the coast,
base-level is sea-level,[28] but inland it rises by a gentle gradient. In length, the
valley will grow as long as its head continues to work inland. In the case
represented by Figs. 45 and 46 the head of the valley would not cease to advance
when the center of the island was reached, though beyond that point head erosion
would not be more rapid than lateral erosion on either side. If but a single valley
affected a land area the limit toward which it would tend, and beyond which it
could never pass, would be the length of the land area in the direction of the
valley’s axis. In width, a valley is increased by the side cutting of the stream, by the
wash of the rain which falls on its slopes, and by the action of gravity which tends
to carry down to the bottom of the slope the material which is loosened above by
any process whatsoever. If there be but one valley in a land area its limiting width
is scarcely less than the width of the land itself.

Fig. 48.—Same as Fig. 47, with the valleys more developed. The dotted line represents the original
outline of the island. Its area is being extended by deposition everywhere, but most at the
debouchures of the streams.

Fig. 49.—A later stage of the island shown in Fig. 48.

Had there been several initial meridional
depressions instead of one in the island, or
had there been several meridional belts
where the material of the surface was less
Fig. 50.—Diagram to illustrate the lowering
resistant than elsewhere, several valleys of a divide without shifting it. The crest of
would have been developed, converging the divide is at a, b, and c successively. If
toward the center (Fig. 47). If the erosion were unequal on the two sides, the
divide would be shifted.
conditions were such as to allow of the
equal development of valleys on all sides of the island, each would be lengthened
by head erosion until it reached the center of the island, where the permanent divide
between their heads would be established. Each would be widened by all the
processes which widen valleys, and their widening would narrow the intervening
areas (Figs. 48 and 49). Under conditions of equal erosion the limits of width for
each valley would be the centers of the ridges on either side, and here the divides
between them would be permanently established. Though erosion would continue
even after the crest of the ridge had been narrowed to a line, the permanence of the
divide would follow from the fact that erosion would be equal on both sides of this
line, and its effect would be to lower the divide, but not to shift it horizontally (Fig.
50). The limits in length and width are therefore not the same where there are
several valleys as where there is but one. The limit in depth, however, remains the
same, and the final result of erosion, proceeding along these lines, would be the
base-leveling of the land, leaving a plain but slightly above sea-level. The plain
would not be absolutely flat, though its relief would be very slight, and the higher
parts would be along the lines of the divides between the streams (Fig. 51.
Compare also Fig. 42). Many valleys would occasion more rapid degradation than
few, and the period of base-leveling would be correspondingly shortened.
Had the initial depressions which gave origin to the valleys had positions other
than meridional, the valleys would have had other and less regularly radial courses,
but the final result of their development would have been the same.
It is not to be inferred that the method of valley development which has been
sketched is the only one. The processes of valley development are complex, and the
history of some valleys has run a different course; yet the processes outlined above
are in operation in all cases, and they were probably the most important ones in the
development of the first drainage system on any land surface. As will be seen in the
sequel the history of valleys is subject to serious accidents, and they are often of
such a nature as to mask the simplicity of the more normal processes.

Fig. 51.—Diagram illustrating the further development of Fig. 49. The land here has been reduced
greatly, though not yet to base-level.

The permanent stream.—From the foregoing discussion, it is seen that a valley
may be developed by the run-off of successive showers. If supplied only from this
source surface streams would cease to flow soon after the rain ceased to fall, and a
valley might attain considerable size without possessing a permanent stream. How
does the valley developed by the run-off of successive showers come to have a
permanent stream? The answer to this question involves a brief consideration of
that part of the rainfall which sinks beneath the surface.
If wells be sunk in a flat region of uniform structure and composition the water
in them is generally found to stand at a nearly common level. The meaning of this
fact is not far to seek. If a hole 60 feet deep fills with water up to a point 20 feet
from the surface, it is because the material in which the well is sunk is full of water
up to that level. When the well is dug the water leaks into it, filling the hole up to
the level to which the rock (or subsoil) is itself full. This level, below which the
rock and subsoil (down to unknown depths) are full of water, is known as the
ground-water level, ground-water surface, or water-table.
The ground-water level fluctuates. In a wet season it rises, because more water
has fallen and sunk beneath the soil; but several processes at once conspire to bring

it down again. Where there is growing vegetation its roots draw up water from
beneath, and evaporation also goes on independently of vegetation. The water is
drawn out through wells and runs out through openings. It may also flow
underground from one region to another where the ground-water surface is lower.
All these processes depress the ground-water surface.
A well sunk to such a level as to be
supplied with abundant water in a wet
season may go dry during a period of
drought because the ground-water level is
depressed below its bottom. Thus either
well shown in Fig. 52 will have water
during a wet season when the water-level is Fig. 52.—Diagram illustrating the
at a; but well No. 1 will go dry when the fluctuation of a ground-water surface. a =
wet-weather ground-water level; b =
water surface is depressed to b. ground-water level during drought. Well
No. 1 will contain water during the wet
The principles applicable to wells are season, but will go dry in times of drought.
applicable to valleys. When a valley has Well No. 2 will be permanent.
been deepened until its bottom reaches below the ground-water level, water seeps
or flows into it from the sides. The valley is then no longer dependent on the run-
off of showers for a stream. It will be readily seen that at some stage in its
development, the bottom of a valley may be below the ground-water level of a wet
season without being below that of a dry one. Thus the valley represented in cross-
section by the line 2–2, in Fig. 53, will have a stream when the ground-water level
is at aa, but none when this level is depressed to bb. If the rainfall of the year were
concentrated in a single wet season, the intermittent stream would flow not only
during that season, but for so long a time afterward as the ground-water level
remained well above the valley bottom. In regions subject to frequent and short
periods of heavy precipitation, alternating with droughts, the periods of intermittent
flow may be many and short. Since the precipitation of many regions varies greatly
from year to year, it follows that a stream may flow continuously one year and be
intermittent the next. Many valleys in various parts of the earth are now in the stage
of development where their streams are intermittent.
As a valley containing an intermittent stream becomes deeper, the periods when
it is dry become shorter, and when it has been sunk below the lowest ground-water
level, it will have a permanent stream (3, Fig. 53). Since a valley normally develops
headward, its lower and older portion is likely to acquire a permanent stream, while
its upper and younger part has only an intermittent one (Fig. 47 and Fig. 1, Pl. III,
near Anthony, Kan. The intermittent part of the stream is indicated by the dotted
blue line). For the same reason the head of a stream is likely to be farther up the
valley in wet weather than in dry. So soon as a valley gets a permanent stream, the

process of enlargement goes on without the interruption to which it was subject
when the supply of water was intermittent.

Fig. 53.—Diagram to illustrate the intermittency of streams due to fluctuations of the ground-water
level. The water level aa would be depressed next the valley 2–2 by the flow of the water into the
valley. The profile of the ground-water surface would therefore be aca rather than aa.

In general a permanent stream at one point in a valley means a continuous stream
from that point to the sea or lake which the valley joins; but to this rule there are
many exceptions. They are likely to arise where a stream heads in a region of
abundant precipitation, and flows thence through an arid tract where the ground-
water level is low, and evaporation great. In such cases, evaporation and absorption
may dissipate the water gathered above, and the stream disappears (Fig. 2, Pl. III,
near Paradise, Nev.).
PLATE III.

U. S. Geol. Surv. Fig. 1.
KANSA Scale, 2+ mile per inch.
S. U. S. Geol. Surv. Fig. 2. NEVADA. Scale, 4+ mile per inch.

PLATE IV.

U. S. Geol. Surv. Fig. 1. ILLINOIS. Scale, 1+ mile per inch.

U. S. Geol. Surv. Fig. 2. NORTH DAKOTA. Scale, 2+ mile per inch.

Other modes of valley development.—If as a new area of land emerges from
the sea its surface has a depression without an outlet, and such an assumption is by
no means improbable, the depression would be filled with sea-water. The inflowing
water from the surrounding land might fill the basin to overflowing, and the
outflow, finding exit at the lowest point in the rim of the basin, would flow thence
toward the sea. Such a stream would develop a valley, the history of which would
be somewhat different from that which has been sketched. Instead of developing
headward from the sea, the valley would be in process of excavation all the way
from the initial basin to the sea at the same time (Fig. 54). The upper end of the
valley might ultimately be cut to the level of the bottom of the basin, when the lake
would disappear. The head of the valley might then work back across the former
site of the lake into the territory beyond. Valleys might have developed above the
lake before it was drained, and after this event, such valleys would make
connections with the valley below (Fig. 55). A valley developed in this manner is
not simply a gully grown big by head erosion, and the valley would not precede the
stream.

Fig. 54.—Diagram to show how a valley may be developing all the way from a water-filled basin
(lake) to the sea at the same time. Small valleys leading to the lake are also developing. The black
area = the sea.

Fig. 55.—The stream leading out from the lake (Fig. 54) has drained the lake, and the valleys above
and below the site of the former lake have united.

If a surface of land were notably irregular before valleys were developed in it,
there might be many lakes, and the flow from a higher lake might pass to a lower. If
the lakes were ultimately drained, the several sections of the valley would be joined
to one another without intervening basins. In certain regions, especially those
which have been affected by continental ice-sheets, this has been a common
method of valley development in post-glacial time. In this case also the stream
precedes its valley, and not the valley its stream. Many post-glacial valleys, on the
other hand, antedated their permanent streams, as in the cases first described.

Fig. 56.—Diagram showing the phases of valley development described in the text.

If the gradient of a slope on which valleys are to develop is notably unequal,
though without basins, the development of valleys may follow somewhat different
lines. If on emergence the seaward part of a new land area assumes the form of a
plain, bordered landward by a steeper slope (Fig. 56), the most notable early
growth of the valleys would be on the latter. The run-off would develop gullies on
the steep slope, but on reaching the plain below the velocity of the water would be
checked, and it would drop much of the detritus washed down from above. This
deposition would build up (aggrade) the surface, and much or even all the water
might sink into and seep through the débris thus deposited, and disappear altogether
from the surface, as at b, Fig. 56. This would be most likely to occur where the
débris is abundant and coarse, and the precipitation slight. If the water disappears at
the base of the mountain (see Fig. 2, Pl. III), the early growth of the valley may be
confined to the steep slope remote from the sea (ab, Fig. 56); but on the slope
where the valley is growing there will be headward lengthening, as in the general
case already considered. If the surface drainage does not disappear at the base of
the steep slope, the run-off will find its way over the plain along the lowest
accessible route to the sea (de, Fig. 56). In this case the valley may be growing
throughout its length at the same time.

The conditions represented by ab, Fig. 56, may
be no more than temporary. Sooner or later a valley
developing headward across the plain (hi, Fig. 56)
may provide a channel for the water descending
from the higher land beyond. In this case the valley
develops in sections, the one on the slope above,
the other on the plain below, and their union
(compare fghi, Fig. 56, with Fig. 57) results from
their growth.
The principles here sketched have been in
operation wherever land areas were so elevated as
to give rise to unequal slopes, and this has perhaps
been the rule rather than the exception. The results
effected by the operation of these principles would
of course be dependent on the varieties of slope, on
the abruptness with which a slope of one gradient
gave place to another, on the texture of the rock,
the amount and distribution of precipitation, etc.,
etc.
Fig. 57.—Diagram representing the In the preceding paragraphs the lengthening of a
further development of the valley at its upper end by head erosion has been
valleys fg and hi in Fig. 56. The
repeatedly referred to. If all valleys began their
head of the latter (Fig. 56) has
worked back until it has reacheddevelopment at the sea and lengthened headward, it
the lower end of the former. might seem that their seaward ends should be their
oldest parts; but since the development of valleys is begun somewhat promptly
after the land appears above the sea, and since the emergence is generally gradual,
that part of a valley which is at the seashore at one time may be far inland a little
later, because the land has been extended seaward. On an emerging land area
therefore the normal growth of a valley involves its lengthening at its lower end as
well as at its upper. The lengthening of a valley, or at least the lengthening of a
stream, also takes place at its lower end if the land in which it lies is being extended
seaward by deposition.
Structural valleys.—In mountain regions valleys are sometimes formed by the
uplift of parallel mountain folds, leaving a depression between (Fig. 58). Drainage
will appropriate such a valley so that it becomes in some sense a river valley. But it
is not a river valley in the sense in which the term has been used in the preceding
pages. It is rather a structural valley. In its bottom a river valley may be developed
(a, Fig. 58).

Fig. 58.—Structural valley with a river valley developing its bottom.

The foregoing illustrations by no means exhaust the list of conditions under
which valleys develop, but they suffice for the present.
The courses of valleys.—River valleys
are rarely straight. To understand why they
are crooked it is only necessary to
understand the methods by which they
grow. In so far as a river valley is a gully
grown big, that is, in so far as its length is
the result of head erosion, its course was
determined by the course of the antecedent Fig. 59. Fig. 60.
gully. If in the case shown in Fig. 59 the Figures to show why the head of a gully (and
therefore a valley) departs from a direct
slope of the surface above the head of the course.
gully is uniform, its material homogeneous,
and the rainfall everywhere equal, more water will come into the gully from the
direction a than from any other. In this case there would be more wear in the direct
line of its extension than elsewhere, and the head would advance in a straight line.
But if there be inequalities of slope about the head of a gully at any stage of its
development more water may come in from some direction other than that in the
direct line of its extension. In Fig. 59, for example, more water may enter from the
direction of b than from that of a. Since most wear is likely to be affected along the
line of greatest inflow, the head of the gully will be turned in that direction (Fig.
60). Started in this course it will continue in the new direction so long as erosion in
this line is greater than that elsewhere; but whenever the configuration of the
surface causes more water to enter the head of the gully from some direction other
than that in which it is headed, the line of axial growth is again changed, as toward
c, Fig. 60. Since new land surfaces are probably more or less undulatory,
crookedness should be the rule among valleys developed from gullies by head
erosion. Streams and valleys the courses of which are determined by the original
slope of the land are said to be consequent.
Inequalities of material, leading to unequal rates of erosion, effect the same
result, in the absence of inequalities of slope. If at any stage of a valley’s

development erosion were equal in two
directions at its head, and at the same time
greater than at points between, two gullies
would result (Fig. 61) diverging from the
point in question.
In the case of a valley developed by
overflow from a lake its course is
determined by the lowest line of flow to
Fig. 61.—Diagram illustrating the
development of two equal gullies from the which the water has access. If this line be
head of one. straight the valley will be straight; if it is
crooked, as it generally is, the valley is crooked also.
The development of tributaries.—Thus far valleys leading immediately to the
sea have been considered, and no account taken of tributaries. As a matter of fact
most considerable valleys have numerous tributaries. It is now in order to inquire
into their mode of development.
So soon as a gully is started, the water flowing into it from either side wears back
the slopes. The least inequality of slope, or the least variation in the character of the
material, is sufficient to make the lateral erosion unequal at different points, and
unequal erosion in the slopes results in the development of tributary gullies. The
oldest tributaries may be nearly as old as the main which they join, and from which
they developed, for the possibilities of unequal side erosion exist as soon as a gully
is opened. While the main gully is developing into a ravine, and the ravine into a
valley, the tributary gullies are likewise developing into maturer stages. Tributary to
a young valley, therefore, there may be gullies near its head, ravines in its middle
course, and small valleys along its oldest portion. It is not to be understood,
however, that the oldest tributaries are necessarily the largest, for because of more
favorable conditions for growth the younger tributaries often outstrip the older.
The position of tributaries with reference to their mains is worthy of note. The
water flowing down a slope follows the line of steepest descent. A gully is usually
wider at its lower end, and narrower at its upper. Wherever this is true the line of
steepest descent down its side is not a line perpendicular to its axis, but a line
slightly oblique to it (ef, Fig. 62), and oblique in such a direction that it meets the
axis with an obtuse angle below and an acute angle above. It is in the direction
corresponding to this line that tributary gullies tend to develop. Thus at the
inception of its history a tributary gully is likely to join its main with an angle
slightly acute on the up-stream side. If the tributary did not begin until after its
main was farther advanced this tendency would be less and less pronounced.
Inequalities of material or slope would often counteract this tendency, which, at

best, would cause the courses of tributaries to depart but
little from perpendicularity to their mains.
After the head of a tributary has worked back from the
immediate slope of its main every condition which
determines the course of a gully is likely to affect it, and
it is by no means certain that it will continue to lengthen
in the direction in which it started. Since the general
slope of the surface into which the tributary works is
likely to be seaward, more water is likely to enter from
the landward than from the seaward side of its head, so
that, except where there are notable irregularities of
slope, its tendency will be to turn more and more toward Fig. 62.—Diagram to
the direction of its main (efg, Fig. 62). illustrate the oblique
position of a tributary
In depth the tributary is always limited by its main. gully at its inception, and
its later normal change of
The principles which determine the length and width of a direction.
main valley determine also the length and width of a
tributary (see p. 67 et seq.).

A CYCLE OF EROSION. ITS STAGES.

From what has preceded it is clear that the topography of a region undergoing
erosion will change greatly from time to time. The first effect of erosion is to
roughen the surface by cutting out valleys, leaving ridges and hills. The final effect
is to make it smooth again by cutting the ridges and hills down to the level of the
valleys.

Fig. 63.—Diagram showing three parallel valleys in a land surface.

Fig. 64.—Diagram to illustrate the lowering of the surface by valley erosion. The successive cross
profiles of the valleys are represented by the lines 1–1, 1–1′, 2–2, 2–2′, etc.

The base-level of erosion has already been defined; but the mode of its
development may now be illustrated in the light of the preceding discussion.
Suppose a land surface affected by a series of parallel young valleys without
tributaries (Fig. 63). Between them there is a series of upland plateaus. The profile
of the surface between two adjacent valleys is represented in section by the
uppermost line in Fig. 64. As the valleys are widened from 1–1 and 1,′–1′, to 2–2
and 2′–2′, the intervening plateau is correspondingly narrowed. When the valleys
have attained the form represented by 3–3 and 3′–3′, the intervening upland has
been narrowed to a ridge, a, and the valley flats have become wide. With continued
erosion the ridge will be lowered (to b and below), and in time the surface will
approach a plain. In this condition it is known as a peneplain (an “almost-plain”).
Finally, when running water has done its utmost, the ridges will be essentially
obliterated and a base-leveled plain (e, e′, e″) results. The figure expresses the fact
that the base-level develops laterally from the axis of the valley. It also develops
headward from the seaward end of the valley. Similarly, taking into account all the
valleys which affect it, the seaward margin of a base-leveled plain is developed
first, and thence it extends itself inland.

Fig. 65.—Diagram showing the dissection of the upland shown in Fig. 64 by tributary valleys.

Tributaries are tolerably sure to develop along each main valley. The heads of the
tributaries work back across the uplands between the main valleys, dissecting them
into secondary ridges (Fig. 65). Tributaries will develop on the tributaries, and
these tertiary valleys dissect the secondary ridges into those of a lower order. This

process of tributary development goes on until drainage lines of the fourth, fifth,
sixth, and higher orders are formed (Fig. 66). Since the process of valley
development under such circumstances is also the process of ridge dissection, a
stage is presently reached where the ridges are cut into such short sections that they
cease to be ridges, and become hills instead. Even then the processes of erosion do
not stop, for the rain-water falling on the hills washes the loose material from their
surfaces, and starts it on its seaward journey. Thus the “everlasting hills”
themselves are lowered, and, given time enough, will be carried to the sea. Under
these conditions, as under those already discussed, the final result of stream erosion
is the reduction of the land to base-level. The base-leveled surface, as before, would
not be absolutely flat. The area reduced by each stream will have a slight gradient
down-stream, and from each lateral divide toward the axis of the valley. The crests
of the scarcely perceptible elevations which remain will be in the position of the
former divides, and these will be highest where most distant from the sea by the
course which this part of the drainage took. Even the insensible divides between
streams flowing in a common direction may disappear, for when valleys have
reached their limits in depth, their streams do not cease to cut laterally. Meandering
in their flat-bottomed valleys, they often reach and undercut the divides (Pl. VII),
whether they be high or low. By lateral planation, therefore, the divides between
streams may be entirely eaten away.

Fig. 66.—Diagram showing tributaries of several orders developed from the conditions sketched in
the text.

It has now been seen that by whatever method erosion by running water
proceeds, whether there be many valleys, or few or none, the final result of
subaërial erosion must be the production of a base-level. It has also been seen that
the base-level is first developed at the lower ends of the main streams, and that it
extends itself systematically up the main valleys and up all tributaries. The time
involved in the reduction of a land area to base-level is a cycle of erosion.

It will have been evident from the preceding pages that the terms “grade,”
“graded plain,” and “base-level” and “base-leveled plain,” are somewhat variously,
and therefore somewhat confusingly, used. “Grade is a condition of essential
balance between corrasion and deposition.”[29] A graded valley is one in which
deposition and corrasion are, in the vertical sense, balanced. Its angle of slope is
most variable, and is dependent on the capacity of the stream for work, and on the
work it has to do. A weak river must have a higher gradient than a strong one; a
stream with much sediment must have a higher gradient than one with little, and a
stream with a load of coarse material must have a higher gradient than one with a
load of fine. Thus the graded valley of the lower Mississippi has an inappreciable
angle of slope, but the graded valleys of many of its tributaries have slopes of
hundreds of feet per mile. Since both the size of the stream and the amount and
coarseness of its load at a given place vary from time to time, it is clear that the
inclination of a graded valley must vary also, and further, that it must be in process
of continual readjustment. With the changing conditions of advancing years the
slope of a graded valley normally decreases. The same principles apply to graded
surfaces outside of valleys.
In the continual readjustment of grades incident to a river’s normal history the
land is brought nearer and nearer to sea-level without ceasing to be at grade. When
the inclination of a graded surface becomes so low that it is sensibly flat, the
surface may be said to be at base-level, although this does not mean that the surface
can never be degraded further. If the term be used in this way, it is clear that there is
no sharp line of distinction between a graded surface and a base-leveled surface,
and as the terms are now commonly applied no such distinction exists.
If the term base-level were made synonymous with sea-level, as has been
proposed,[30] the term might as well be discarded, for sea-level could always be
used in its stead. Furthermore, streams often erode below sea-level. The bottom of
the channel of the Mississippi is below sea-level for some 400 miles above its
debouchure, and locally (Fort Jackson) it is nearly 250 feet below. This deep
channel is the result of the erosive activity of the stream, not of subsidence. Again,
the sea-level is itself inconstant. The extent of its changes cannot now be measured,
but they have probably been more considerable in the course of geological history
than has been commonly recognized. It is true that they take place slowly, as far as
known, but it is also true that the duration of an erosion cycle is sufficiently long
for even very slow changes to reach great magnitude. The sea-level, therefore, can
hardly be accepted as the absolute base-level, unless (1) the absolute base-level is a
variable, and unless (2) the absolute base-level be a surface below which rivers
may cut to the extent of at least 250 feet.

The ocean may be looked upon as a barrier which in a general way limits the
down-cutting of running water; for only very large streams cut much below its
level. Other barriers, such as lakes, and the outcrops of hard rock in a stream’s bed,
have a comparable, though more temporary, effect on the development of valley
plains above. Plains thus developed have been called temporary base-levels. They
differ from other graded plains in being controlled primarily by a barrier below,
rather than by conditions which exist above.
Since river valleys have a beginning and pass through various stages of
development before the country they drain is base-leveled, it is important to
recognize their various stages of advancement. Nor is this difficult. An old valley
and a young one have different characteristics, and the one would no more be
mistaken for the other by those who have learned to interpret them, than the face of
an aged man would be mistaken for that of a child.

Fig. 67.—A gully developed by a single shower. (Blackwelder.)

The cycle begins with the beginning of valley development, and at that stage
drainage is in its infancy. The type of the infant valley is the gully or ravine (Figs.
67 and 68). It has steep slopes and a narrow bottom. Fig. 1 of Plate IV represents
similar, or rather older, ravines in contour (shore of Lake Michigan, just north of
Chicago). With age, the valley widens, lengthens, and deepens, and passes from
infancy to youth. In this stage also the valleys are relatively narrow, and the divides
between them broad. They may be deep or shallow, according to the height of the
land in which they are cut, and the fall of the water flowing through them; but in
any case the streams flowing through them have done but a small part of the work

they are to do before the country they drain is base-leveled. Figs. 69 and 70,
respectively, represent youthful valleys in regions of moderate and great relief. Fig.
2, Plate IV, shows a youthful valley in a region of slight relief (near Casselton, N.
D., Lat. 46° 40′, Long. 97° 25′). The uppermost line in Fig. 64 likewise represents
topographic youth, as shown in cross-profile.

Fig. 68.—A gully somewhat older than that shown in Fig. 67. (Alden.)

Fig. 69.—A young valley in a region of slight relief.

Not only are narrow valleys said to be young, but the territory affected by them
is said to be in its topographic youth, since but a small part of the time necessary to
reduce it to base-level has elapsed. An area is in its topographic youth when
considerable portions of it are still unaffected by valleys. Thus the areas (as a
whole), as well as the valleys, represented on Plate IV, are in their topographic
youth. It is often convenient to recognize various sub-stages, such as early, middle,
and late, within the youthful stage of valleys or topographies. The different parts of
the areas shown on Plate IV, for example, represent different stages of youth.
Youthful streams, as well as youthful topographies, have their distinctive
characteristics. They are usually swift; their cutting is mainly at the bottom rather
than at the sides, and their courses are often marked by rapids and falls.
As valleys approach base-level they develop flats. As the valleys and their flats
widen, and as their tributaries increase in numbers and size, a stage of erosion is
presently reached where but little of the original upland surface remains. The
country is largely reduced to slopes. In this condition the drainage and the
topography which it has determined are said to be mature. Mature topography is
shown in contours in the figures of Plate V, and in the northern part of Plate VI,
where slopes, rather than upland or valley flats, predominate. Fig. 1 of Plate V
represents an area in southeastern Kentucky (Lat. 37° 12′, Long. 83° 10′); Fig. 2, an
area in western Virginia. Plate VI represents an area in southern California,
somewhat west of San Bernardino. The three areas are alike in representing mature
drainage, though not of equal stages of advancement. The striking differences of

topography of the three areas are the result of differences in rock structure and
altitude, and will be considered later. Mature topography is also shown in Fig. 71,
where the relief is moderate, and in Figs. 72 and 73, where it is great. Figs. 72 and
73 illustrate clearly the universal tendency of rivers in regions of notable relief to
develop new flats well below the old surface of the region. At the same time that
these low-lying flats are developing, tributary drainage is dissecting and
roughening the upper surfaces. This process is well shown in Fig. 73. In both Figs.
72 and 73 the summits of the mountains on either side of the valleys appear to have
had about the same elevation. The new flat is therefore developed at the expense of
the old flat. As will be seen in the sequel, the first flat which a stream develops
along its course is usually somewhat above base-level. It is a graded flat.
PLATE V.

U. S. Geol. Surv. Fig. 1. KENTUCKY. Scale, 2+ mile per inch.

U. S. Geol. Surv. Fig. 2. VIRGINIA. Scale, 2+ mile per inch.

PLATE VI.

U. S. Geol. Surv. Scale, 1+ mile per inch.
PARTS OF LOS ANGELES AND SAN BERNARDINO
COUNTIES, CALIFORNIA.

Fig. 70.—The valley (canyon) of the Yellowstone. A young valley in an elevated region.

Fig. 71.—Mature erosion topography in a region of slight relief, Iowa. (Calvin.)

Fig. 72.—Mature erosion in a mountain region. From mouth of Gray Copper Gulch, Silverton,
Colo., quadrangle. (Cross, U. S. Geol. Surv.)

The same processes which have made young valleys mature will in time work
further changes. When the gradients of the valleys have become low and their
bottoms wide, and when the intervening ridges and hills have become narrow and
small, the drainage and the drainage topography have reached old age, and the
streams are in a condition of senility. This is illustrated by Fig. 1, Plate VII (central
Kansas), and in section by the third and lower lines in Fig. 64. Topographic old age
sometimes has a different expression; this is shown in Fig. 74, where most of the
surface has been brought low. The elevations which rise above the general plain are
small in area, but have abrupt slopes. This phase of old-age topography is usually
the result of the unequal resistance of the rock degraded. The effects of unequal
rock-resistance will be considered later.

Fig. 73.—Mature erosion in a mountain region. Silverton, Colo. (Cross, U. S. Geol. Surv.)

The marks of old streams are as characteristic as those of young ones. They have
low gradients and are sluggish. Instead of lowering their channels steadily they cut
them down in flood, and fill them up when their currents are not swollen. They
meander widely in their flat-bottomed valleys (Fig. 1, Pl. VII, Central Kansas) and
their erosion, except in time of flood, is largely lateral.
If the processes of degradation were to continue until the land surface was
brought to sea-level, and this might be done by solution though not by mechanical
erosion of running water, the rivers would no longer flow, and the drainage system
would have reached the end of its history—death.
Not only do valleys normally pass from birth to youth, from youth to maturity,
and from maturity to old age, but a single river system may show these various
stages of development in its various parts. Thus in the area shown in Fig. 2, Plate
VII (north central Kansas), there is a tract (extreme southwest) where the erosion
history is scarcely begun. The zone of land a little farther northeast, and just
reached by the heads of the valleys (same figure), is in its youth. The well-drained
and uneven tract southwest of the flat of the Solomon River is in maturity, while
the flat of the main valley has the general characteristics of old age.

Fig. 74.—A peneplained surface where the elevations are small but steep-sided. Near Camp Douglas,
Wis. (Atwood.)

The age of valleys in terms of erosion is also expressed more or less perfectly by
their cross-sections. The line 1–1 (and 1′–1′) of Fig. 64 represents in cross-section a
narrow V-shaped valley. Such a section is always indicative of youth. The stream
which developed it cut chiefly at its bottom, not at its sides. It was therefore rapid,
and rapid streams are young. The line 2–2, (2′–2′) (Fig. 64) shows the same valley
at a later and maturer stage when downward cutting has nearly ceased. The
widening of the valley by slope wash has become relatively more important than
before, and the stream has so far lost velocity as the result of diminished gradient as
to be unable to carry away all the detritus washed down from the sides. As a result
of deposition at the bases of the side slopes, a concave curve has been developed.
Up the valley from the point where such a section as is represented by 2–2 occurs,
the valley may still have a section similar to that represented by 1–1.

PLATE VII.

U. S. Geol. Surv. Fig. 1. KANSAS. Scale, 2+ mile per inch.

U. S. Geol. Surv. Fig. 2. KANSAS. Scale, 2+ mile per inch.

PLATE VIII.

U. S. Geol. Surv. ABOUT 15 MILES SOUTHWEST OF ST. Scale, 1+ mile per inch.
LOUIS, MISSOURI.

Still later stages of development are represented by the cross-sections 3–3 and 4–
4. Not only has the valley become larger, but the stream has deposited detritus (not
shown in the figure) in the bottom of its valley, developing an alluvial flat. On this
flat the stream meanders, and the valley may be widened by the undercutting of the
bluffs wherever the stream in its wanderings reaches them (Pl. VIII, near St. Louis).
A valley might possess the characteristics shown by the cross-sections 3–3, 2–2,
and 1–1, Fig. 64, in its lower, middle, and upper courses, respectively.

The preceding discussion, and the illustrations which accompany it, give some
idea of the topography which characterizes an area in various stages of its erosion
history. Whether the valleys are deep or shallow, and the intervening ridges high or
low, depends on the original height of the land and its distance from the sea. The
higher the land, and the nearer it is to the sea, the greater the relief developed by
erosion. A plateau near the sea may become mountainous in the mature stage of its
erosion history, while a plain in the same situation would only become hilly. A
plateau in the heart of a continent would have less relief in its maturity than one of
equal elevation near the sea, since the grade-plain in the former position is higher
than in the latter. Plates IV and IX show youthful topography where the relief is
relatively slight, and Plate X shows youthful topography where the relief is great.
Similarly, Plates V and VI show mature topography where the relief is great, and
Fig. 1, Plate III, shows mature topography where the relief is relatively slight.
Topographic youth, topographic maturity, and topographic old age are also
indicated in other ways, and especially by the presence of features which rivers
tend to destroy. If, for example, the surface of the land, well above the valley
bottoms, is marked by numerous ponds and marshes, it is clear that drainage has
not yet progressed beyond its early stages, for, unless the lakes be very deep,
valleys working back into the land will find and drain them before topographic
maturity has been reached. Their presence is evidence that the region where they
occur has not yet been thoroughly dissected by erosion lines, and therefore has not
reached maturity. Still other marks of topographic youth, such as rapids, falls, etc.,
as well as marks of topographic maturity and old age, will be mentioned in the
following pages.

GENERAL CHARACTERISTICS OF TOPOGRAPHIES
DEVELOPED BY RIVER EROSION.

With the characteristics of river valleys and the methods by which they grow
clearly in mind it is easy to say whether rivers have been the chief agents in the
development of a given topography. River valleys are distinguished from other
depressions on land surfaces by their linear form and, leaving out of consideration
the relatively insignificant inequalities in a stream’s channel, by the fact that any
point in the bottom of a river valley is lower than any other point farther up the
stream in the same valley, and higher than any point farther down the stream. The
second point might be otherwise stated by saying that every valley excavated by
erosion leads to a lower valley, or to the sea, or an inland basin. Streams which dry
up, or otherwise disappear as they flow, constitute partial exceptions. If, therefore,
the depressions on a land surface are linear, lead to other and deeper valleys, and

finally to an inland basin, or the sea, and if the elevations between these valleys are
such as might have been left by the excavation of the valleys, it is generally clear
that rain and rivers have been the chief factors in the development of the
topography. If, on the other hand, a surface is characterized by topographic features
which streams cannot develop, such as enclosed depressions, or hills and ridges
whose arrangement is independent of drainage lines, other agents besides rain and
surface streams have been concerned in its development.

SPECIAL FEATURES RESULTING FROM SPECIAL
CONDITIONS OF EROSION.

Many striking topographic and scenic features result from rain and river erosion.
Some of them depend primarily on the conditions of erosion, such as climate,
altitude, etc., while others depend largely on the structure and resistance of the
rock. Between these two classes there is no sharp line of demarkation. Illustrations
of two types, dependent largely but by no means wholly on conditions independent
of the rock, are cited at this point. Others will be mentioned in other connections.

Fig. 75.—Bad-land topography. North of Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.)

Bad-land topography.—To a type of topography developed in early maturity in
certain high regions where the rock is but slightly, though unequally, resistant, a
special name is sometimes given. Such regions are termed bad lands. Some idea of
bad-land topography is gained from Figs. 75 to 78. Bad-land topography is found in
various localities in the West, but especially in western Nebraska and Wyoming,
and the western parts of the Dakotas. The formations here are often beds of
sandstone or shale, alternating with unindurated beds of clay. Climatic factors are
also concerned in the development of bad-land topography. A semi-arid climate,
where the precipitation is much concentrated, seems to be most favorable for its
development. The bad-land topography is most striking in early maturity.

Fig. 76.—Toadstool Park, Sioux Co., Neb. The peculiar topography is the result of erosion working
on jointed rocks of unequal hardness in an arid region of considerable elevation where rainfall is
unequally distributed. (Darton, U. S. Geol. Surv.)

Special forms of valleys; canyons.—Various conditions influence the size and
shape of valleys, especially in the early stage of their development. If the altitude of
the land be great, the gradient of the streams at this stage will be high. A high
gradient means a swift stream, and a swift stream erodes chiefly at its bottom. High
altitudes therefore favor the development of deep valleys. Such valleys will be
narrow if the conditions which determine widening are absent or unfavorable.
Since slope wash is one of the main factors in the widening of valleys, an arid
climate favors the development of narrow valleys, if there be sufficient water to
maintain a vigorous stream. Narrowness and steepness of slopes will also be
favored if the valley is cut in rock which is capable of standing with steep faces.
Thus a stream may develop a narrow valley in indurated rock where it would not do
so in loose gravel, and, other things being equal, it will develop a narrower valley
in rock which is horizontally bedded than in rock the beds of which are inclined.
Aridity, high altitude, and the proper sort of rock structure therefore favor the
development of canyons, and many of the young valleys in the western part of the
United States where these conditions prevail, belong to this class.

Fig. 77.—Detail of bad-land topography. Head of Indian Draw, Washington Co., S. D. Protoceras
sandstone on Oreodon clay. (Darton, U. S. Geol. Surv.)

Fig. 78.—Detail of bad-land topography. Southwest foot of Mesa Verde, Colo. (Matthes, U. S. Geol.
Surv.)

Fig. 79.—Grand Canyon of the Colorado. (Peabody.)

Fig. 80.—Grand Canyon of the Colorado. (Peabody.)

While all canyons are valleys, most valleys are not
canyons. The distinction between a canyon and a valley
which is not a canyon is not sharp. The canyon depends Fig. 81.—Diagram showing
the relations of depth and
for its distinctive character on the relation of depth, width of a valley, the
width, and angle of slope to one another; but any width of which is eight
definition of the depth, width, and angle of slope times the depth.
[31]
necessary to constitute a valley a canyon is arbitrary. In popular usage the rule
seems to be that if a valley is sufficiently deep, narrow, and steep-sided to be
distinctly striking, it is called a canyon in regions where that term is in use.
Whether a valley is deep, narrow, and steep-sided enough to be striking clearly
depends on the observer. The Colorado Canyon (Figs. 79 and 80) is the greatest
canyon known, but it is rarely more than a mile deep, and where its depth
approaches this figure it is often eight, ten, or even twelve miles wide from rim to
rim. Its width at bottom is little more than the width of the stream; that is, a few
hundred feet. Its cross-profile throughout much of its course is therefore not in
keeping with the conventional idea of a canyon. With a depth of one mile and a
width of eight, the slope, if uniform, would have an angle of less than 15°. Such a
valley is represented in Fig. 81. As a matter of fact the slopes of a canyon are not
commonly uniform. The slopes represented in Fig. 82 correspond more nearly than
those of Fig. 81, to the actual slopes of the Colorado Canyon. The inequalities of
slope are occasioned by the inequalities of hardness. It is perhaps needless to say

that to an observer on the rim of the canyon the slopes seem several times as steep
as those shown in the diagrams.
Like all valleys which are narrow relative to their depth, the Colorado Canyon,
great as it is, is a young valley; for it represents but a small part of the work which
the stream must do to bring its drainage basin to base-level.
While aridity and altitude are conditions which favor the development of
canyons, as shown by the fact that most canyons are high and dry regions, they are
not indispensable. Niagara River has a canyon below its falls (Pl. IX), and the
surrounding region is neither high nor arid. The narrow part of the valley has been
developed by the recession of the falls, and is so young that side erosion has not yet
widened the valley or lowered its angle of slope to such an extent as to destroy its
canyon character. This canyon is often called a gorge, a term frequently applied to
small valleys of the canyon type.

Fig. 82.—Cross-section of the Colorado Canyon. (After Gilbert and Brigham.)

Fig. 83.—Detail of erosion in the Grand Canyon. The inequalities of slope are the result of unequal
hardness. The vertical planes which give the architectural effect are the result of joints. (Holmes.)

Plate X shows portions of the canyons of the Yellowstone and the Colorado
rivers respectively. In the first the contour interval is 100 feet, and in the second,
250 feet. The horizontal scale is ¹⁄₁₂₅₀₀₀ (about 2 miles to the inch) in the first, and
¹⁄₂₅₀₀₀₀ in the second. These scales should be borne in mind in interpreting the map.
Falls, rapids, narrows, and other peculiar features, due primarily to inequalities in
the hardness of the rock affected by erosion, will be considered later.

Fig. 84.—A surface illustrating the struggle for existence among gullies. Most of the smaller gullies
shown on the slope can have but little growth before being absorbed by their larger neighbors. A
type of erosion surface common in the Bad Lands. Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.)

THE STRUGGLE FOR EXISTENCE AMONG VALLEYS AND
STREAMS.

It is not to be inferred that every gully becomes a valley, nor that every small
valley becomes a large one. Among valleys, as among living things, there is a
struggle for existence, and fitness determines growth and survival. At an early stage
of its erosion history the number of small valleys in a given area is often great,
while at a later stage the number is less and the size of the survivors greater.

PLATE IX.

U. S. Geol. Surv. NIAGARA FALLS. Scale, 1+ mile per inch.

PLATE X.

U. S. Geol. Surv. Fig. 1. YELLOWSTONE PARK. Scale, 2+ mile per inch.

U. S. Geol. Surv. Fig. 2. ARIZONA. Scale, 2+ mile per inch.

One phase of the struggle for existence is often well
illustrated on a freshly exposed slope of clay. The
number of miniature gullies which develop on such a
Fig. 85.—Diagram slope, even in a single shower, may be very large (Fig.
illustrating the absorption
of one gully by another by
84); but the history of many of them is ephemeral. If two
lateral erosion. The adjacent ones are of unequal depth the widening of the
successive lines represent deeper narrows and finally eliminates the divide between
successive cross-sections.
them, and the two become one (Fig. 85).
Another phase of the struggle for existence is shown in other situations.
Examination of a good map of the north shore of Lake Superior or the west shore
of Lake Michigan shows a large number of small streams and gullies (Fig. 1, Pl.
IV). The valleys are short and narrow, and between and beyond them are

considerable areas untouched by erosion. The drainage near the lake is therefore
young, and each of the small valleys is growing. This condition of things is perhaps
typical of that which has been, is, or will be along the average coast at a certain
stage in its erosion history. No equal stretch of coast-line where erosion is far
advanced can boast of a number of large rivers comparable to that of the many
small ones along the coasts mentioned. It therefore seems evident that of these
many small streams a few only will attain considerable size.
Some of the methods by which the growth of the many is arrested are easily
understood. Some of the young valleys on a given coast will work their heads back
into the land faster than others because of inequalities of slope and material. This
will be true of the tributaries no less than of their mains. If valleys develop in ways
other than by head erosion (see p. 73) the chances are also against their equality of
growth. If two streams, such as a and c, Fig. 86, develop faster than the
intermediate stream b, it is clear that their tributaries may work back into the
territory which at the outset drained into b, so as to cut off the supply of water from
the latter stream (compare a′b′c′, Fig. 87). As a result, the growth of b will be
checked, and ultimately stopped. Similarly other valleys, such as f, will get the
better of their neighbors, and many of the competitors, as b, d, e, and g will soon
drop out of the race. Between the stronger streams competition still goes on. If a′
and f′ develop faster than c′ its prospective drainage territory will be preëmpted by
its rivals (compare Figs. 87 and 88). Thus as the result of the unequal rate at which
valleys are lengthened, the larger number of those which come into existence are
arrested in their development. As a result of growth in the manner indicated, the
basins of even the large streams remain narrow at their lower ends while they
expand above. This is the usual form of a drainage basin the development of which
has been normal.

Fig. 86–88.—Diagrams to illustrate successive stages in the struggle for existence and dominion
among streams.

Did valleys grow in length only, competition would not destroy the small ones; it
would simply limit them. But valleys widen as well as lengthen, and by widening,
adjacent valleys may eliminate the divide between them and become one. The
elimination of the intervening ridge may be by lateral planation (p. 82), or, if the
valleys be of unequal depth, by slope wash (see Fig. 85). By these and other
processes many young valleys are dwarfed, and many others are destroyed.
Piracy.—Streams do not always hold the courses which they establish for
themselves at the outset. If the valley occupied by the stream a, Fig. 89, is
deepened more rapidly than the valley occupied by b, a tributary from the former,

c, may work back across the inter-stream area to e and steal the head waters of that
stream (Fig. 90). The tributary which does the stealing is known as a pirate. Stream
f (Fig. 90) is said to be beheaded, and its upper portion, de, diverted. The beheaded
stream is diminished in volume; or if its total supply of water came in above the
point of tapping it would disappear altogether.

Fig. 89 and 90.—Diagrams to illustrate piracy.

The process may not end even here. If after the diversion of de the point in the
channel to the left is lowered faster than the channel of the beheaded stream f, the
divide between dg and the head of f (Fig. 90) will be shifted down the valley of the
latter, as shown in Fig. 91. The shifting will go on until the divide reaches a
position of stability, that is, until erosion on its opposite sides is equal.

Fig. 91.—Diagram showing the shifting of a divide after piracy.

The foregoing case may be called foreign piracy because the valleys of different
systems are concerned. Domestic piracy may also take place, as illustrated in the
accompanying diagrams (Figs. 92 and 93). Here a tributary to a crooked river may
develop, working back until it taps the main at a higher point, thus straightening the
course of the stream. The change takes place only when the highest point in the
tributary valley is brought below the surface of the water in the main stream at the
point where the tapping takes place. This would be likely to occur only after the
main stream had attained a low gradient, for so long as it is deepening its channel
notably, the small amount of water flowing through the tributary valley would not
be likely to bring it down to the level of the main. In any case the flow of water
from the main stream through the new valley would be likely to be started during
flood, and at such time the erosion in the new channel would be great. The
complete and final diversion of the stream through the new channel might be a
slow process.
Piracy may occur where the material in which the valleys are cut is
homogeneous; but, as will be seen later, heterogeneity of material, by determining
unequal rates of erosion, stimulates the piratical proclivities of streams.
An actual case of piracy is shown on Plate XI. North and South Lakes formerly
drained westward to the Schoharie Creek, the present head of which is in the
extreme northwest corner of the map. The head of Kaaterskill Creek, which had a
much higher gradient, worked back and captured the head of the westward-flowing
stream, diverting the drainage from North and South Lakes to itself. Schoharie
Creek was thus beheaded.
Plaatekill Creek, near the south limit of the map, appears to have beheaded the
creek flowing west and northwest, similarly diverting its head waters. The Dells,

Wis., quadrangle (U. S. Geol. Surv.)
affords an illustration of domestic
piracy.

Fig. 92 and 93.—Domestic piracy. The tributary, a of
Fig. 92, develops headward until it taps the main
stream at b, giving the result shown in Fig. 93.
RATE OF DEGRADATION.

The amount of mechanical sediment which the Mississippi River carries to the
Gulf of Mexico is estimated to represent a rate of degradation for the Mississippi
basin of about one foot in 5000 years. But the mechanical sediment carried to the
Gulf does not really represent the total degradation of the basin, for the water which
sinks beneath the surface is dissolving more or less rock substance, especially lime
carbonate. This material is carried to the sea in solution, and does not appear in the
sediment on which the above estimate is based. Taking into account the matter
dissolved by the water and carried to the sea in solution, the average rate of
degradation for the Mississippi basin is estimated at one foot in 3000 to 4000 years.
It is not to be inferred that this rate is uniform, or even that erosion at any rate
whatsoever is taking place in all parts of the basin. Such is not the fact. On the
whole the rate of erosion is doubtless greatest toward the margins of the basins
where the land is in its topographic youth or early maturity. It is notably less in the
middle courses of the valleys, and erosion is locally exceeded by deposition along
the lower courses of the Mississippi and some of its main tributaries.
The average elevation of North America is not accurately known, but it is
probably not far from 2000 feet. If the present rate of degradation, say one foot in
3500 years, were to continue, it would take something like 7,000,000 years to bring
the continent to sea-level. But this rate of degradation could not continue to the

end, for as the continent became lower streams would become sluggish and erosion
less rapid. Long before the continent reached base-level the rate of degradation, so
far as dependent on mechanical erosion, would become so slow that the time
necessary to bring the continent to sea-level would be almost inconceivably
prolonged. Furthermore, it is quite possible that the land is suffering, or is liable to
suffer, uplift, relative or absolute. If the rate of rise were equal to the rate of
degradation the average height of the continent would of course not be affected.

The amount of sediment carried by streams in suspension varies
notably according to the stage of the water. During a year when the
stream was under careful study the Mississippi at Carrollton (Miss.)
was found to carry ¹⁄₆₈₁ of its weight of sediment during the high-water
stage of June, and ¹⁄₆₃₈₃ during the low-water of October, the average
for the year being ¹⁄₁₈₀₈. The average of a greater number of records
gives about ¹⁄₁₅₀₀ as the average ratio between the weight of the
sediment and the weight of the water. This corresponds to about ¹⁄₂₉₀₀
by volume, the average specific gravity being about 1.9. The amount
of material carried in the upper part of the water was notably less than
that carried at greater depths, but that carried midway between top and
bottom was about the same as that carried at the bottom.[32]
The discharge of the Mississippi River is about 19,500,000,000,000
cubic feet of water per year, and the sediment it carries in suspension is
estimated to weigh about 812,500,000,000 pounds. This is equivalent
to about 6,714,694,400 cubic feet. It is estimated that about
750,000,000 cubic feet of sediment is rolled along the bottom, giving a
total of 7,468,694,400 cubic feet as the aggregate annual load carried
to the Gulf by the river. This would be adequate to cover an area one
square mile in extent to the depth of 268 feet per year.

PLATE XI.

U. S. Geol. Surv. PART OF THE Scale, 2+ mile per inch.
CATSKILLS, NEW
YORK.

PLATE XII.

U. S. Geol. Surv. Fig. 1. NEW MEXICO. Scale, 2+ mile per inch.

U. S. Geol. Surv. Fig. 2. VIRGINIA, WEST VIRGINIA AND MARYLAND. Scale, 1+ mile per inch.

ANALYSES OF AMERICAN RIVER-WATERS.[33]
[Reduced to Parts per 1000 by Dr. H. J. Van Hoesen.]
Hudson,
Name of river Bear Croton Cumberland Delaware James
N. Y.
Collected at Evanston, Reservoir, New Reservoir at Reservoir Richmond
Wy. York City Nashville, at Water
............
Tenn. Trenton, Works, Va.
N. J.
Date Dec., 1873 1881 Oct. 24. 1876,
............ ............ ............ after light
rain
Analyst F. W. Clarke E. Waller N. T. Lupton H. Wurtz C. F. W. H. Taylor
Chandler
Reference Bulletin No. Water supply Am. Chemist, Geol. of Public Health Ann. Rept.
9, U. S. of New York July 16, N. J., Papers, Board of
Geol. Surv., City, 1881 1876, p. 16 1868, p. Vol. I, Am. Health,
p. 30 702 Pub. Health Richmond,
Ass. Va., 1876

Hudson,
Name of river Bear Croton Cumberland Delaware James
N. Y.
Sodium, Na .0082 [34] .01032 .00072 .00244 .00234
.00298
Potassium, K ...... .00154 .00050 .00178 .00058 .00251
Calcium, Ca .0432 .00905 .02987 .01104 .02220 .01284
Magnesium, Mg .0125 .00336 .00280 .00435 .00465 .00377
Chlorine, Cl .0049 .00213 .00299 .00121 .00581 .00105
Carbonic acid, [35] [34]
CO2 .0982 .02248 .05727 .02552 .07278 .02954
Sulphuric acid,
SO3 .0105 .00441 .00563 .00175 .01257 .00363
Phosphoric acid,
H3PO ...... .00172 Trace
Nitric acid,
HNO3 ...... ...... .00511 ...... ...... .00231

Silica, SiO2 .0070 .03360 Trace .00852 .00698 .01024
Alumina, Al2O3 .00041
Sesquioxide of .00047
iron, Fe2O3 ...... ...... ......
Sesquioxides of
iron and
alumina, Fe2O3 ...... .00078 .00671 ...... .00120 ......
and Al2O3
Sesquioxides of
iron and
manganese, .00072
Fe2O3 and
Mn2O3
Carbonates of
iron and
manganese,
FeCO3 and
MnCO3
Oxide of iron,
......
FeO
Oxide of
manganese, Trace ......
MnO
Hydrogen in
.00121
bicarbonates, H
Chloride and
sulphate of
sodium, NaCl, ......
and Na2SO4
Ammonia, NH4 ...... ...... ...... .00001
.01087
Organic matter .00400 .01666 .01197 .00299
Carbonates and ...... ...... ...... ...... ...... ......
sulphates of

Hudson,
Name of river Bear Croton Cumberland Delaware James
N. Y.
Na, K, and Mg
.1845 .08433 .13786 .06795 .14238 .07246

ANALYSES OF AMERICAN RIVER-WATERS (continued).
[Reduced to Parts per 1000 by Dr. H. J. Van Hoesen.]
Rio
Los Maumee,
Name of river Mississippi Ottawa Passaic
Grande del Sacramento
Angeles O.
Norte
Collected at Hydrant, St. Ann’s 4 miles Fort Hydrant, Hydrant at
City ater Lock, above Craig, Sacramento, Los
Works, Montreal, Newark, New Cal. Angeles,
............
New Can. N. J. Mexico Cal.
Orleans,
La.
Date Sept. 8, Mar. 9, 1851 1873 Sept., 1878
............ ............
1878 1854
Analyst W. J. Jones C. F. W. J. Jones T. S. Hunt E. N. O. Loew W. J. Jones
Chandler Horsford
Reference Rept. Cal. Report of Rept. La. Geol. of Geol. of N. J., U. S. Geog. Rept. Cal.
State Toledo State Canada, 1868, p. 708 Surv. State
Board of Water Board of 1863, p. west of Board of
Health, Works, Health, 567 100th M., Health,
1878 1881 1882, p. Vol. III. 1878
370 p. 576
Sodium, Na .02968 .00162 .0310 .00239 .02357 .03220 .00200
Potassium, K ...... .00309 ...... .00139 .00163 .00063 ......
Calcium, Ca .01750 .02645 .0372 .00992 .01459 .01633 .01279
Magnesium,
.02097 .00443 ...... .00161 .00404 .00123 .00121
Mg
Chlorine, Cl .01044 .00250 .0480 .00076 .03192 .03604 ......
Carbonic acid,
CO2 .05635 .04438 .0383 .02255 .02634 .01025 .00887
Sulphuric acid,
SO3 .05724 .01401 .00194 .01716 .04700 .00397
Phosphoric
acid, H3PO .02638 Trace Faint trace .0179
Nitric acid,
HNO3 ...... ...... ...... Trace ......
Silica, SiO2 .02005 .00724 .02060 Trace .03167
Alumina, Al2O3 .00171 ...... Trace Trace .00120 Trace
.01342
Sesquioxide of
iron, Fe2O3 .00100
Sesquioxides of ...... ......
iron and
alumina,

Rio
Los Maumee,
Name of river Mississippi Ottawa Passaic Grande del Sacramento
Angeles O.
Norte
Fe2O3 and
Al2O3
Sesquioxides of
iron and
manganese, ...... ......
Fe2O3 and
Mn2O3
Carbonates of
iron and
manganese, .00443 ...... ...... .01088
FeCO3 and
MnCO3
Oxide of iron,
Trace Trace
FeO
Oxide of
manganese, Trace
MnO
Hydrogen in
bicarbonates, ......
H
Chloride and
sulphate of
sodium, ...... .02431
NaCl, and
Na2SO4
Ammonia, NH4 ...... Trace
Organic matter .00499 ...... .01392
Carbonates and
sulphates of
...... ...... .0154 ...... ...... ...... ......
Na, K, and
Mg
.24475 .10971 .1699 .06116 .13287 .15760 .11484

ANALYSES OF AMERICAN RIVER-WATERS (continued).
[Reduced to Parts per 1000 by Dr. H. J. Van Hoesen.]
St.
Name of river Humboldt Truckee Walker Jordan Mohawk Genesee
Lawrence
South side Collected Battle Mt., Lake Tahoe, Mason Utah Utica, N. Y. Rochester,
Point des at Nev. Nev. Valley, Lake N. Y.
Cascades Nev.
Date Mar. 30, Dec., 1872 Oct., 1872 Oct., 1872 Nov.,
1863 1873
Analyst T. S. T. M. F. W. Clarke F. W. Clarke F. W. C. F. C. F.
Hunt Chatard Clarke Chandler Chandler
Reference Geol. of U. S. Geol. U. S. Geol. U. S. Geol. Bulletin Johnson’s Johnson’s
Canada, Surv., Surv., Surv., No. 9, Cyclopedia, Cyclopedia,

St.
Name of river Humboldt Truckee Walker Jordan Mohawk Genesee
Lawrence
1863, p. Monograph Monograph Monograph U. S. Vol. IV Vol. IV
567 XI, p. 41 XI, p. 42 XI, p. 40 Geol.
Surv.,
p. 29
Sodium, Na .00513 .0467 .0073 .0318 .0178 .0036 .0044
Potassium, K .00115 .0100 .0033 Trace ...... .0009 .0023
Calcium, Ca .03233 .0489 .0093 .0228 .0558 .0318 .0417
Magnesium,
.00585 .0124 .0030 .0038 .0186 .0069 .00896
Mg
Chlorine, Cl .00242 .0075 .0023 .0131 .0124 .0023 .0024
Carbonic acid, [35] [35] [35]
CO2 .06836 .1544 .0287 .0576 .0608 .0569 .0646
Sulphuric acid,
SO3 .00831 .0477 .0054 .0284 .1306 .0187 .0431
Phosphoric
acid, H3PO Trace ......
Nitric acid,
HNO3 ...... ...... ...... ...... ...... ......
Silica, SiO2 .03700 .0326 .0137 0.225 .0100 .0067 .0014
Alumina,
Al2O3 .0013
Sesquioxide of
iron, Fe2O3 ...... ......
Sesquioxides
of iron and
alumina, .0013 .0014
Fe2O3 and
Al2O3
Sesquioxides
of iron and
manganese,
Fe2O3 and
Mn2O3
Carbonates of
iron and
manganese, ......
FeCO3 and
MnCO3
Oxide of iron,
Trace ......
FeO
Oxide of
manganese, Trace ...... ......
MnO
Hydrogen in
bicarbonates, ...... ...... ......
H

St.
Name of river Humboldt Truckee Walker Jordan Mohawk Genesee
Lawrence
Chloride and
sulphate of
sodium, ...... ...... ......
NaCl, and
Na2SO4
Ammonia,
NH4 ...... ...... ......
Organic matter ...... ...... .0234 .0250
Carbonates
and
sulphates of ...... ...... ...... ...... ...... ...... ......
Na, K, and
Mg
.16055 .3615 .0730 .1800 .3060 .1525 019526

The following table[36] gives the percentage of material carried in suspension by
various rivers:
Height in Feet of Thickness of
Drainage Mean Annual Ratio of
Column of Sediment in
Areas in Discharge (in Total Tons Sediment to
River. Sediment with a Inches if Spread
Square Cubic Feet.) Annually. Water by
Base of One over Drainage
Miles. per Second. Weight.
Square Mile. Area.
Potomac 11,043 20,160 5,557,250 1 : 3,575 4.0 .00433
Mississippi 1,244,000 610,000 406,250,000 1 : 1,500 241.4 .00223
Rio
30,000 1,700 3,830,000 1: 291 2.8 .00116
Grande
Uruguay 150,000 150,000 14,782,500 1 : 10,000 10.6 .00085
Rhone 34,800 65,850 36,000,000 1 : 1,775 31.1 .01075
Po 27,100 62,200 67,000,000 1: 900 59.0 .01139
Danube 320,300 315,200 108,000,000 1 : 2,880 93.2 .00354
Nile 1,100,000 113,000 54,000,000 1 : 2,050 38.8 .00042
Irrawaddy 125,000 475,000 291,430,000 1 : 1,610 209.0 .02005
Mean 334,693 201,468 109,649,972 1 : 2,731 76.65 .00614

The composition of rain-water falling near London, as determined by analysis,
was as follows:[37]
Organic carbon .99 part in 1,000,000 of water.
Organic nitrogen .22 “ “ “ “ “
Ammonia .50 “ “ “ “ “
Nitrogen as nitrates and nitrites .07 “ “ “ “ “
Chlorine 6.30 parts in “ “ “
Total solids 39.50 “ “ “ “ “

A comparison of the composition of rain-water with that of springs and rivers
gives some idea of the solvent work of water. From a study of the water of nineteen
of the principal rivers of the world Murray has compiled the following table[38]
showing the amount of mineral matter in average river water:

MATERIAL IN SOLUTION IN ONE CUBIC
MILE OF AVERAGE RIVER WATER.[39]
Constituents. Tons in a Cubic Mile.
Calcium carbonate (CaCO3) 326,710
Magnesium carbonate (MgCO3) 112,870
Calcium phosphate (Ca3P2O8) 2,913
Calcium sulphate (CaSO4) 34,361
Sodium sulphate (Na2SO4) 31,805
Potassium sulphate (K2SO4) 20,358
Sodium nitrate (NaNO3) 26,800
Sodium chloride (NaCl) 16,657
Lithium chloride (LiCl) 2,462
Ammonium chloride (NH4Cl) 1,030
Silica (SiO2) 74,577
Ferric oxide (Fe2O3) 13,006
Alumina (Al2O3) 14,315
Manganese oxide (Mn2O3) 5,703
Organic matter 79,020
Total dissolved matter 762,587
Murray also estimates that the aggregate amount of water flowing into the sea
annually is about 6528 cubic miles, which, on the above basis, would carry about
4,975,000,000 tons of mineral matter in solution.
A large number of analyses of waters of rivers from the United States and
Canada give an average of about .15,044 part in a thousand of mineral matter in
solution, more than one-third being CaCO3. The average amount of mineral matter
in solution in 48 European streams cited by Bischoff[40] is .2127 part in a thousand,
of which CaCO3 is rather more than half. The average mineral matter in solution in
36 rivers cited by Roth[41] (including some of those tabulated by Bischoff) is .2033
part in a thousand, of which CaCO3 is slightly less than one-half.

An average for American and European rivers, so far as determinable from data
at hand, is about .1888 part in a thousand in solution, of which CaCO3 is slightly
less than one-half. These last figures are probably not very far from an average for
river water in general.
The following table shows the total amount of solids carried in solution by the
rivers indicated:[42]
Rhine 5,816,805 tons per year.
Rhone 8,290,464 “ “ “
Danube 22,521,434 “ “ “
Thames 613,930 “ “ “
Nile 16,950,000 “ “ “
Croton 66,795 “ “ “
Hudson 438,000 “ “ “
Mississippi 112,832,171 “ “ “

ECONOMIC CONSIDERATIONS.

Certain considerations of human interest in connection with river erosion are
worthy of note. When a drainage system has reached its mature stage its basin has
the roughest topography which it will have at any time during that cycle of erosion.
At that stage, therefore, road construction is relatively difficult. If the relief be
great, roads must follow the valleys, or the crests of the ridges between them, if
they would avoid heavy grades. In such regions roads are usually few and crooked.
The stage of development of valleys has an influence on the navigability of their
streams. Streams well advanced in life are much more readily navigable than young
ones, because their grades are lower and their volumes of water greater. Old
streams, on the other hand, are sometimes depositing sand or silt along their lower
courses to such an extent as to interfere with navigation.
At certain stages of their development the power of streams is more easily
utilized than at others. Young streams, depending as they do for their supply on the
rainfall of a limited area, are likely to be fitful in their flow, and therefore unreliable
as a source of power. This is especially true where the precipitation is unequally
distributed, and where the slopes are steep and free from forests. Because of their
great volume, old and large streams, though sluggish, have great power, but it is
less easily controlled. Where streams are large enough to be navigable industrial
considerations often prevent the utilization of their power, the streams being more
serviceable as highways than as sources of power. Other things being equal, it

follows that streams are most available for water-power when they are large enough
to have a moderately steady flow, and not so large as to be beyond ready control, or
to be valuable for purposes of navigation.
Streams are subject to more disastrous floods in some stages of their
development than in others. Floods resulting from heavy rains are likely to be
greatest where the slopes above the drainage lines are on the whole greatest, for this
is the condition under which the water is most quickly gathered into the drainage
channels. The most disastrous floods, humanly speaking, are those which affect
wide-bottomed valleys, where the flats are settled. In such cases a relatively slight
rise may flood very extensive areas. In such valleys the most disastrous floods are
generally in the spring, when the waters from the melting snows of the preceding
winter are being discharged.[43] Many other considerations enter into the problem of
floods. The presence of forests and other forms of vegetation on the slopes retards
the flow of water into the valleys, and so tends to prevent floods, or at any rate to
make them less severe. Porous soil and subsoil, or in their absence porous rock,
absorb the rainfall, and prevent its prompt descent into the valleys and so tends to
prevent or diminish floods.
The acreage of arable land within a given area stands in some relation to its
drainage development. At an early stage in its erosion history, before an upland has
been dissected by valleys, nearly all of it may be arable. Later, when drainage is at
its maturity, and when hillsides and ridge slopes constitute a large part of the area,
there is probably the least acreage of arable land. This is especially true if the
slopes are so steep as to allow the soil to be readily washed away. At a still later
stage, when the valley bottoms have become wide and the slopes of the ridges and
hills so reduced as to be available, the area of cultivable land is again increased.
Marshes, ponds, and lakes have some bearing on the resources and industries of
a region, and they stand in a more or less definite relation to the stage of erosion in
which a region finds itself. In its youth ponds and lakes may occupy much of the
surface; in its maturity they will have been largely drained.
These suggestions are sufficient to show that the topography of a region, even in
so far as shaped by erosion, touches human interests at many points.

ANALYSIS OF EROSION.[44]

Erosion is the term applied to all the processes by which earthy matter or rock is
loosened and removed from one place to another. It consists of three sub-processes,
namely, weathering, transportation, and corrasion.

Weathering.
The term weathering is applied to nearly all those natural processes which tend
to loosen or change the exposed surfaces of rock. The lettering of inscriptions on
exposed marble becomes fainter and fainter as time goes by, and finally disappears,
because the rock in which the letters were cut has weathered away. Some of it has
crumbled off as the result of the expansion and contraction induced by changes of
temperature, and some of it has been dissolved by the rain which has fallen upon it.
In this case the weathering is effected partly by the atmosphere and partly by water.
These are the chief, but not the only agents concerned in the general processes of
weathering. Those phases of weathering which are the result of the activities of the
atmosphere, whether physical or chemical, have been discussed in connection with
the atmosphere (pp. 42 and 54).
The rain which falls upon the surface of exposed rock, and that which sinks
through the soil to the solid rock below, dissolves, even if slowly, some of the rock
constituents. Each constituent of a rock composed of several minerals may be
looked upon as a binding material for the others. When one is dissolved the rock
crumbles, much as mortar does when the lime which cements the sand is dissolved.
The solution of mineral matter by ground water, as well as the other chemical
changes it effects, is greatly augmented by the impurities, especially carbonic and
other organic gases, dissolved by the water from the atmosphere and the soil. The
commonest chemical changes effected by the joint action of water and air,
oxidation and carbonation, have been referred to in Chapter II. Hydration is more
exclusively the work of water, and is one of the commonest processes of rock
change, and often of rock disintegration. Numerous other less simple chemical
changes resulting from the activities of ground water are constantly in progress, and
in so far as they lead to the disintegration of rock are processes of weathering.
Many chemical changes involve notable changes in volume of the mineral matter
concerned. Merrill has calculated that in the conversion of the granitic rock of the
vicinity of Washington, D. C., into soil, its volume has been increased 88 percent.,
largely as the result of hydration.[45] Even when the chemical changes do not
themselves directly involve the disintegration of the rock, the accompanying
increase of volume is sometimes sufficient to cause its physical disruption. This
also may be regarded as a phase of weathering.
The weathering accomplished by water, or under its influence, proceeds at rates
which vary with the composition of the rock, the amount and composition of the
water, the temperature, and certain other factors less susceptible of brief statement.
The weathering effected by ground water has a wider range both in area and depth
than that due to changes of temperature, for while the latter is effective only where

temperature changes are considerable, and where coherent material lies at the
surface (p. 45), the former is operative to all depths to which water sinks.

Fig. 94.—Talus accumulation at the base of a steep bluff. Weber Canyon, Uinta Mountains, Utah.
The talus has accumulated since the last glaciation of the valley and is therefore of very recent
origin. (Church.)

There are other processes of weathering not due directly either to the atmosphere
or to water. The roots of trees and smaller plants frequently grow into cracks of
rocks, and, increasing in size, act much like freezing water (p. 45) in similar
situations. This wedge-work of roots is a phase of weathering.
From the faces of steep cliffs masses of rock frequently fall. However dislodged,
their descent is effected by gravity. The quantities of débris at the bases of many
cliffs, forming slopes of talus (Fig. 94), testify to the importance of the action of
gravity in getting material from higher to lower levels. Another phase of gravity-
work is shown in Fig. 95. Here, under the influence of gravity and expansion and
contraction, due to freezing and thawing and wetting and drying, the surface
material is creeping down slope. In the process the rock is being broken. The
process illustrated by the figure involves weathering as well as other factors.
The foregoing are among the commoner processes of weathering, although they
do not exhaust the list. The more active and tangible processes by which surface
rocks are broken up, such as wave wear, river wear and glacier wear, are processes
of corrasion. The mechanical wear effected by wind-driven sand might be

considered either as corrasion or as weathering. It is more likely to be regarded as
corrasion if the amount of wear is considerable enough to be obvious. Rock is
sometimes decomposed by the chemical action of hot vapors, gases, and waters
rising to the surface from considerable depths. This is often seen in volcanic
regions. A conspicuous illustration is seen in the canyon of the Yellowstone in the
National Park. Decay of this sort is perhaps not properly weathering, but is not
always readily distinguished from it.

Fig. 95.—Shows the downward creep of soil and slaty rock under the influence of gravity.

The importance of weathering in the general processes of erosion is shown in
many ways. In regions where the mantle rock is the product of the decay of the
solid rock beneath, and such regions constitute a large portion of the earth’s
surface, the soil and subsoil represent the excess of weathering over transportation.
Since most of the earth’s surface is covered with soil to a greater or less depth, it is
clear that, on the whole, weathering keeps ahead of transportation. Again, it is clear
that the loosening of rock by weathering greatly increases the erosion which a
given amount of moving water can accomplish. Not only this, but weathering plays
a much more important rôle in the development of valleys than is commonly
realized. This is best illustrated by the valleys of young swift streams. The valley

which is not at its top ten times as wide as its stream is rare. The stream which has
such a canyon has been cutting chiefly at its bottom. Ignoring its lateral corrasion,
which is slight, the valley which it would cut would have a width equal to its own.
This is illustrated by Fig. 96. Weathering in its broadest sense is largely responsible
for the width of such a valley, in so far as it exceeds the width of the stream. The
work of weathering, slope wash, etc., has been to get the material which originally
lay between a, b, and c down to the stream. The stream has then carried it away.
The above illustration would not apply to old and sluggish streams, for they, by
their meandering, widen their valleys independently of weathering.

Fig. 96.—Diagram of a valley the top of which is ten times the width of the stream.

Weathering is a part of erosion, but only a part. In so far as it is effected by
solution the process involves the transportation of that which is dissolved to some
other point. Transportation is also involved to some extent in the other processes of
weathering, but the central idea of the processes embraced under this term is the
loosening and disrupting of rock by which it is prepared for transportation.

Transportation.
The second element of erosion is transportation. The transportation of
mechanical sediment is to be distinguished from the transportation of materials in
solution. In so far as mineral matter is dissolved it becomes, so far as flowage is
concerned, a part of the stream. If the quantity dissolved were large it might
influence the mobility of the water, but the amount is usually too slight to influence
the flow sensibly.
The sediment transported by a stream is either rolled along its bottom or carried
in suspension at some higher level. The coarser materials (gravel and sand) are
carried chiefly in the former position, and the finer (silt and mud) largely in the
latter.
Transporting power and velocity.—The transporting power of running water
depends on its velocity. The formula expressing the relations between them is as
follows: Transporting power, t, varies as the sixth power of velocity, v, (tαv6); that
is, doubling the velocity of the stream increases its transporting power 64-fold.
Strictly speaking, this means that if a stream of given velocity is just able to move a
stone of a given size, a stream with double that velocity will be just able to move a
stone of the same shape 64 times as large as the first. This may be graphically
illustrated as follows: Let a current be supposed just able to move the cube a (Fig.

97). If the current be doubled, twice as much water will strike the same surface
with twice the force in the same time; that is, the force exerted on the cube a will be
quadrupled. It will, therefore, be able not only to move the one cube, but it will be
able to move three other cubes (b, c, and d) besides (Fig. 98). The same current
against any other equal surface would also be able to move four small cubes, and
there are sixteen such surfaces on the face of the large cube (Fig. 99). It follows that
the dimension of the cube which the stream with the doubled velocity can move is
four times as great as that of the cube which the original current could move, and
the cubical contents of such a cube is 64 times as great as that of the first (64 = 26)
(Fig. 99). Swift streams, therefore, have enormously greater power of
transportation than sluggish ones. It does not necessarily follow that transportation
keeps pace with transporting power; that depends on the accessibility of materials
suitable for transportation. A stream of great transporting power, like the Niagara at
its rapids, may carry little sediment, because there is little to be had.
The velocity of a stream depends chiefly on three elements—its gradient, its
volume, and its load, (i.e., the sediment it is moving). The higher the gradient the
greater the volume, and the less the load the greater the velocity. The relation
between gradient and velocity is evident; that between volume and velocity is
illustrated by every stream in time of flood, when its rate of flow is greatly
increased. The relation between velocity and load is less obvious, but none the less
definite. Every particle of sediment carried by a stream makes a draught on its
energy, and energy expended in this way reduces the velocity. The draught on a
stream’s energy of a particle carried in suspension is measured by its mass into the
distance it would fall in a unit of time in still water. It follows that a large particle
makes a stronger draught on a stream’s energy than the same amount of material in
smaller pieces. It follows also that the comminution of sediment facilitates
transportation in much more than a simple ratio, for not only can a given amount of
energy carry more fine material than coarse, but a larger proportion of a stream’s
energy can be utilized in the transportation of the fine.

Fig. 97–99.—Diagrammatic representation of the effect of increased velocity on transporting power.

How sediment is carried.—Coarse materials, such as gravel stones, are rolled
along the bottoms of the swift streams which carry them. Their movement is
effected by the impact of water. The same is true to a large extent of sand grains,
especially if they be coarse. So far as concerns the material rolled along the bottom
it is to be noted that a stream’s transporting power is dependent on the velocity of
the water at its bottom. This is much less than the surface, or even the average
velocity. The particles of fine sediments, such as silt and mud, are frequently
carried by streams quite above their bottoms, as shown by the roiliness of many
streams. A particle of mud is usually a small bit of mineral matter, the specific
gravity of which is two or three times that of water. Why does it not sink through
the water and come to rest at the bottom of the stream, or suffer transportation as
the gravel does?
A particle of sediment in running water
is obviously subject to two forces, that of
the current which tends to move it nearly
horizontally down-stream, and that of
gravity which tends to carry it to the bed
of the stream. In Fig. 100, the arrows ab
and ac represent respectively the relative
force of gravity and a current of 5 miles
per hour. As a result of these two forces Fig. 100.—Diagram to illustrate the relative
the particle would tend to descend in the strength of the two forces acting on a
particle in suspension. The arrows represent
general direction of ad, a line which the relative strength of the two forces when
represents the resultant of these forces, the stream’s velocity is 5 miles per hour. No
though not the exact path which a particle account is taken in the diagram of the
viscosity of the water, or of the acceleration
acted on by them would take in water. If a of velocity of fall.
river were the simple straightforward
current which it is popularly thought to be, a particle in suspension would reach its
bottom in the time it would take to sink through an equal depth of still water, for
the descent would be none the less certain and none the less prompt because of the
forward movement of the water. The current would simply be a factor in
determining the position of the particle when it reached the bottom, not the time of
reaching it. Very fine particles, like those of clay, though having the same specific
gravity as grains of sand, would sink less readily than coarser ones, because they
expose larger surfaces, relative to their mass, to the water through which they sink.
But even such particles, unless of extraordinary fineness, would presently reach the
bottom if acted on only by a horizontal current and gravity. Since even sediment
which is not of exceeding fineness is kept in suspension it is clear that some other
factor is involved. This is found, in part at least, in the subordinate upward currents
in a stream.

Where a bowlder occurs in the bed of a stream (Fig. 101) the water which strikes
it is in part forced up over it. If there be many bowlders the process is frequently
repeated, and the number of upward currents is great. Any roughness will serve the
same purpose, and every stream’s bed is rough to a greater or less extent. Where
there are roughnesses at the sides of a channel, currents are started which flow from
them toward the center. The varying velocities of the different parts of a stream
serve a similar purpose. The curves in a river tend to give the water a rotatory
movement. A river is therefore to be looked upon not as a single straightforward
current, but as a multitude of currents, some rising from the bottom toward the top,
some descending from top to bottom, some diverging from the center toward the
sides, and some converging from the sides toward the center. The existence of these
subordinate currents is often evident from the boiling and eddying readily seen in
many streams. It is, of course, true that the sum of the upward currents is always
less than the sum of the downward, so that the aggregate motion of the water is
down slope; but it is also true that minor upward currents are common. Sediment in
suspension is held up chiefly by such currents, which, locally and temporarily,
overcome the effect of gravity. The particles in suspension are constantly tending to
fall, and frequently falling; but before they reach the bottom many of them are
seized and carried upward by the subordinate currents, only to sink and be carried
up again. Even if they reach the bottom, as they frequently do, they may be picked
up again. It is probable that every particle of sediment of such size that it would
sink readily in still water is dropped and picked up many times in the course of any
long river journey, and its periods of rest often exceed its periods of movement.

Fig. 101.—Diagram to illustrate the effect of bowlders, a and b, in a stream’s bed on the currents of
water impinging against them.

Independently of the subordinate currents, the different velocities of the different
parts of a stream tend to keep materials in suspension by exerting different
pressures on the different sides of suspended particles.[46]
River ice sometimes facilitates the transportation of débris which the water alone
could not carry. The ice freezes to bowlders in the banks of the streams, to those
which are partially submerged, and sometimes to those altogether submerged
beneath slight depths of water. When the ice breaks up in the spring such bowlders,
buoyed up by the ice, may be floated far down the stream. The influence of ice in
this connection is most considerable in high latitudes, but it is of consequence as far
south as Virginia, where the river deposits sometimes contain bowlders which the

unaided streams could not have carried. Ground ice sometimes forms about
bowlders in the bottoms of streams, especially in the quiet pools of turbulent rivers,
and floats them to the surface before the surface itself is frozen.[47] In the floods of
spring rivers often spread their ice widely over their flood-plains. It is sometimes
massed in constricted portions of valleys so as to form great dams, the breaking of
which is attended with great destruction.

Corrasion.
Abrasion.—The wear effected by running water is corrasion. So long as the
materials to be carried away are incoherent it is easy to see how running water
picks them up and carries them forward. The water which gathers in the
depressions on the slope of a cultivated field gathers earthy matter from the surface
over which it passes, even before it is concentrated into rills, and the rills continue
the process. Thus the loose materials of the surface are gathered at the very sources
of the streams, and the amount of sediment in the water after a heavy shower, even
at the head of the stream, may be great. The run-off from the slopes of any valley in
any part of its course likewise brings sediment to the stream, which gathers more
from its bed whereever it flows with sufficient velocity over incoherent material.
Streams also undercut their banks, and receive new load from the fall of the
overhanging material.
By far the larger part of the sediment acquired by a normal stream is made up of
material loosened in advance by the processes of weathering. The stream, or the
waters which get together to make the stream, find them ready-made; but rivers
frequently wear rock which is not weathered, for the principal valleys of the earth’s
surface are cut in solid rock, and many of them in rock of exceeding hardness. How
does the stream wear the solid rock?
When a stream flows over a rock bed, the wear which it accomplishes depends
chiefly on the character of the rock, the velocity of the stream, and the load it
carries. If the rock be stratified and in thin layers, and if these thin layers be broken
by numerous joints at high angles to the stratification planes, the impact of the
water of a clear stream of even moderate strength may be effective in dislodging
bits of the rock. This condition of things is often seen where streams run on beds of
shale or slate. If the rock be hard and without bedding-planes and joints, or if its
layers be thick and its joints few, clear water will be much less effective. If the
surface of the rock be rough, the mechanical action of a swift stream of clear water
might still produce some effect on it; but if massive hard rock presents a smooth
surface to a clear stream, the mechanical effect of even a swift current is slight.
This general principle is illustrated by the Niagara River. Just above the falls the
current is swift. When the river is essentially free from sediment, the surface of the

limestone near the bank beneath it is sometimes distinctly green from the presence
of the one-celled plants (fresh-water algæ) which grow upon it. The whole force of
the mighty torrent is not able to sweep them from their moorings. Were the stream
supplied with a tithe of the sand which it is capable of carrying, it would not take
many hours, and perhaps not many minutes, to remove the last trace of vegetation.
This illustration furnishes a clue to the method by which the erosion of solid rock in
a stream’s bed is effected.
It has been seen that the ingathering waters which make a stream often have
abundant sediment before they reach well-defined stream channels, and that the
streams continue to gather sediment whereever their beds are composed of material
which is readily detached. The sediments which the stream carries are the tools
with which it works. Without them it is relatively impotent, so far as the abrasion of
solid rock is concerned; with them, it may wear any rock over which it passes (Fig.
102).
We have next to inquire the methods by which running water uses its tools in the
excavation of valleys. When gravel is rolled along in the channel of a stream there
is friction between it and the bed over which it moves. If the pebbles be as hard as
the bed over which they are rolled their movement must result in its wear, and even
if they be softer more or less wear takes place. As the moving stones wear the rock
of the stream’s bed they are themselves worn by impact with it and with one
another. In all cases the softer material suffers the more rapid wear. The first effect
of wear on materials in transportation is the reduction of their rugosities of surface.
The projecting points and sharp angles are worn off, and the stones are reduced to
rounded water-worn forms. The particles broken off make grains of sand, or, if very
fine, particles of silt or mud. Even after a stone has been rounded it is subject to
further wear and reduction, and in the course of time may be literally worn out.
The sediment carried in suspension, as well as that rolled along the bottom, may
wear the rock bed of a stream. When a grain of sand in suspension escapes from an
upward moving current it may not sink quietly. If it be caught by a downward
current it may be made to strike a blow on the bed of the stream, and the effect of
the blow is to wear the surface which receives it. The larger the grain and the
stronger the current the greater the wear.

Fig. 102.—Some of the tools with which a stream works. The cobbles and bowlders have been shifted
by the stream in its flow. Other stones and bowlders now in transit cause the ripples in the stream.
The Chelan River, Wash., just above its junction with the Columbia. (Willis, U. S. Geol. Surv.)

The ceaseless repetition of the blows struck by the material in suspension, or
rolled on its bottom, hour after hour, day after day, and year after year, will
accomplish sensible results. In the long course of the ages this process has
excavated deep valleys. Concomitant processes are largely concerned in making
valleys wide, but the depth of valleys cut in solid rock is chiefly the result of the
impact and friction of the sediment in transportation.
The wear effected in this way is not proportional to the number of blows struck.
Since every pebble and every grain of sand carried diminishes the velocity of a
stream, and since with diminished velocity the force of the blows struck is
diminished, it follows that the blows may become so weak, as the result of their
multiplication, as to be ineffective. The larger the load, therefore, which the stream
carries, the more the tools with which it has to work, but the less effectively can it
use them; and the load may be so far increased as to destroy its corrasive power
altogether. On the other hand, the smaller the load of the stream the greater its
velocity and the more effectively will its tools be used; but their number may be so
far reduced that their aggregate effect is slight. To accomplish the greatest results
on a bed of solid rock a stream must have tools to work with, but must not be so
heavily burdened as to interfere with its effective use of them.
Whatever the cause of their unequal velocities swift and slow streams corrade
their valleys differently. The erosion of a swift stream is chiefly at the bottom of its

channel. The sluggish stream lowers its channel less rapidly, while lateral erosion is
relatively more important. The result is that slow streams increase the width of their
valleys more than the depth, while swift streams increase the depth more than the
width. It follows that slow streams develop flats, while swift ones do not. Not only
is a slow stream more likely to have a flat, and therefore a better chance to
meander, but it is more likely to take advantage of opportunities in this line, for a
slow stream gets out of the way for such obstacles as it may encounter, while a
swift stream is much more likely to get obstacles out of its way.
Special phases of corrasion are introduced where waterfalls and other
peculiarities dependent on inequalities of rock resistance occur.
Solution.—In most cases the solution effected by a stream is much less
important than its mechanical work. Only when conditions are unfavorable to the
latter, is solution the chief factor in the excavation of a valley. This may be the case
where a stream’s bed is over soluble rock, such as limestone, and where the stream
is clear, or its gradient so low that its current is sluggish. The solvent power of
water is not influenced by the presence of sediment, though the presence of
sediment offers the water a greater surface on which to work.

CONDITIONS AFFECTING THE RATE OF EROSION.

In considering the rate of erosion, both the work of the stream in its
valley and that of the general run-off are to be considered. The conditions
which favor the most rapid erosion in a stream’s channel are not necessarily
those which determine most rapid degradation in the basin outside of the
valley.

The Influence of Declivity.
In general the greater the declivity the more rapid the rate of erosion,
whether in the stream’s channel or on the slopes above it. The truth of this
conclusion is illustrated by the great erosive power of swift streams as
compared with slow ones.
It does not follow, however, that high declivity favors each element of
erosion. The effect of declivity on weathering is far from simple. For
example, great declivity, by allowing more of the rainfall to flow off over
the surface, and by causing it to flow off more promptly, restricts the work
of solution, and therefore of decomposition, both at the surface and beneath
it. High declivity is also unfavorable to the growth of vegetation, and so to
the wedge-work of roots. On the other hand, a given amount of wedge-work
of roots and ice is more effective where the slope is steep than where it is
gentle, for such materials as are loosened descend the slopes more readily.
The prompt removal of weathered materials, by exposing fresh surfaces of
rock, accelerates weathering. The total amount of weathering may therefore
not be diminished by the increase of slope, even though certain of its
processes are hindered.
The effect of high declivity on transportation, the second element of
erosion, is too patent to need explanation.
Corrasion likewise is favored by high declivity, for the abrasive power of
a stream increases as the square of its velocity. With corrasive power
increased, corrasion will also be increased if the water has tools to work
with. Since high declivity greatly increases both the transporting and the
corrasive power of running water, and favors certain elements of
weathering, it is clear that the aggregate effect of high declivity is to favor

erosion, whether in the channel of the stream or on the general surface of its
drainage basin.

The Influence of Rock.
The physical constitution, the chemical composition, and the stratigraphy
of a rock formation, influence the rate at which it may be broken up and
carried away. Clastic or fragmental rocks are usually stratified and made up
of cemented pebbles (conglomerate), sand grains (sandstone), or particles of
mud (shale). Igneous rocks, such as granite, are massive instead of
stratified, and are usually made up of great numbers of interlocking crystals
which bind one another together. Some crystalline rocks, such as schists,
though not stratified, possess cleavage, which has much the effect of
stratification, so far as erosion is concerned. All rocks are affected by
systems of more or less nearly vertical cracks called joints. All these
structures have their influence upon the rate of degradation.
Physical constitution.—Clastic rocks may be firmly cemented, or their
constituents may be loosely bound together. The less the coherence the
more ready the disintegration, and the finer the particles the more easily are
they carried away. When the particles in transportation are angular they
effect more wear on the bed over which they move, and on one another,
than when they are round. The difference is great where the particles are
large, and little where they are very small. If the materials carried be harder
than the bed over which they pass, corrasion of the latter is favored.
Chemical composition.—Something also depends on the chemical
composition of the rock, since this affects its solubility, and therefore its
rate of decomposition. The more soluble the rock the larger the proportion
of it which will be taken away in solution; but it does not follow that the
most soluble rock will be most rapidly eroded, since the rate of erosion
depends on abrasion as well as solution, and a rock which is readily soluble,
as rocks go, may be less easily abraded than a rock which is made of
discrete and insoluble particles bound together by a soluble cement. In such
rocks, for example a sandstone in which the grains are cemented together
by lime carbonate, the solution of the cement sets free a considerable
quantity of sand, so that a small amount of solution prepares a large amount
of sediment for removal. A stream might cut its valley much more rapidly in
such a sandstone than in a compact limestone, though the latter is, as a

whole, the more soluble. The constituent minerals of crystalline rocks resist
solution and decay unequally, and when any one is dissolved or
decomposed the rock crumbles and the less soluble constituents are ready
for removal by mechanical means. So long as the material loosened by
disintegration is removed, chemical heterogeneity favors erosion; but if the
loosened débris is not removed erosion is not favored by chemical
heterogeneity. In such a case erosion would be most rapid where the rock
was most soluble.
Structure.—The structure of the rock has much to do with the rate of its
erosion. Other things being equal, stratified rock is more readily eroded
than massive rock, since stratification-planes are planes of cleavage, and
therefore of weakness. Taking advantage of these planes the water has less
breaking to perform to reduce the material to a transportable condition. For
the same reason a thin-bedded formation is more easily eroded than a thick-
bedded one.

Fig. 103 and 104.—Diagrams to illustrate the fact that a stream crosses many more
cleavage-planes when the beds of rock are inclined than when they are horizontal.

The beds of stratified rock may be horizontal, vertical, or inclined, and
inclined strata may stand at any angle between horizontality and verticality.
In indurated formations the rate of erosion is influenced both by the
position of the strata and by the relation of the direction of the flowing
water to their dip and strike. On the whole the strata which are horizontal,
or but slightly inclined, are probably less favorable for rapid erosion than
those which are vertical or inclined at considerable angles. This is at least
true where the layers are of uniform hardness and the joints infrequent.
Horizontal strata expose fewer cleavage planes to the water flowing over
them than strata in any other position. In Fig. 103 the stream which has the
profile ad crosses bedding-planes at b and c. In Fig. 104, where the beds dip
up-stream, many more division-planes are crossed in the same distance.
Since bedding-planes are planes of weakness, it follows that horizontal and

nearly horizontal strata are not, under ordinary conditions of erosion, in a
position favorable for most rapid wear. When strata are horizontal, it makes
no difference which way the stream runs, for the current sustains the same
relation to the cleavage-planes whatever its course.
In the case of incoherent material the position of the beds, or even their
existence, has little influence on the rate of erosion. Such formations are
weak in all directions, not simply along bedding-planes.
When the strata are vertical,
three distinct cases may arise (Fig.
105). The stream may flow (1)
with the strike (aa); (2) at right
angles to the strike (bb); or (3)
oblique to it (cc) at any angle
whatsoever. It is perhaps not
possible to say which of these
positions is most favorable for
erosion, for the character of the
rock, the thickness of its layers, its
Fig. 105.—Diagram to illustrate the various
relations a stream may sustain to the outcrop
ability to stand with steep slopes,
of vertical layers of rock. and the strength of the currents
concerned, would influence the
result. A stream which flows at right angles to the strike (bb, Fig. 105)
would cross more cleavage-planes in a given distance than a stream flowing
in any other direction, and would strike the outcropping edges of layers at
the angle of greatest advantage. A stream flowing along the strike (aa), on
the other hand, has better opportunity to sink its channel on cleavage-
planes, and the current oblique to the strike (cc), has some of the advantages
of each of the others.
When the strata are inclined five cases may arise. (1) The stream may be
parallel to the strike (aa, Fig. 106), when it makes no difference which way
the current flows; it may be at right angles to the strike (bb′), and (2)
flowing with the dip (toward b′), or (3) against it (toward b); it may be
oblique to the strike, and flowing (4) in the general direction of dip (toward
c′); or (5) in the opposite direction (toward c). As before, the stream flowing
at right angles to the strike would cross the largest number of layers in a
given distance, and so have an opportunity to take advantage of more

cleavage-planes than a stream in
any other position. But in the case
of inclined strata a new element
enters into the problem. When the
stream flows parallel to the strike,
the valley which is in process of
deepening is not sunk vertically,
but is shifted more or less in the
direction of the dip (Fig. 107).
This is called monoclinal shifting.
The result is that there is a
constant tendency to undermine Fig. 106.—Diagram to illustrate the various
relations a stream may sustain to the
(sap) the valley bluff on the down- outcrops of inclined layers of rock.
dip side, and this process of
sapping will, according to its rate, accelerate the growth of the valley,
especially in width. Monoclinal shifting is favored by the presence of a hard
layer (H), as shown in Fig. 107, if this stratum is the bed of the stream.
In the second and third cases
mentioned above, the only difference is
in the angle at which the current strikes
the outcropping edges of layers and
laminæ. The mechanical advantage is
with the stream which flows with the
Fig. 107.—Diagram to illustrate
monoclinal shifting. The valley abc, as
dip. In the fourth and fifth cases
seen in cross-section, becomes deb, as something will depend on the angle
the stream lowers its channel. which the stream’s course makes with
the strike. In all these cases, as in those where the strata are vertical, much
will depend on the thickness and resistance of the layers and on the strength
of the currents concerned.

The Influence of Climate.
Climate has both a direct and an indirect effect on erosion. Its direct
influence is through precipitation, evaporation, changes of temperature, and
wind; its indirect, through vegetation. Like declivity and rock structure,
climate does not affect all elements of erosion equally.

The chief elements of climate are temperature, moisture, and atmospheric
movements; the principal factors which influence it are latitude, altitude,
distance from the sea, direction of prevailing winds, and topographic
relations.
The effects of variations in temperature on rock weathering have already
been discussed (p. 43). They are chiefly mechanical, and are seen at their
best where the daily range is great.
High temperature favors chemical action, and the weathering of rock by
decomposition is at its best in the presence of abundant moisture in regions
where the temperature is uniformly high. Furthermore, a warm moist
climate favors the growth of vegetation, the decay of which supplies the
water with organic acids which greatly increase its solvent power. The
climatic conditions favoring mechanical weathering are therefore different
from those favoring chemical weathering. High temperature and abundant
moisture and vegetation are found in many tropical regions, and here the
rock is often decomposed to greater depths, on the whole, than in high
latitudes. How far this is the result of rapid weathering, and how far of slow
removal, due in part to the protective influence of the plants, cannot be
affirmed. If the weathered material is not removed, it will presently become
a mantle thick enough to retard the processes which brought it into
existence.
So long as the water of the surface and that in the soil remains unfrozen,
temperature affects neither corrasion nor transportation. But in middle and
high latitudes the surface is frozen for some part of each year. During this
time corrasion is at a minimum, for although the streams continue to flow
there is relatively little water running over the surface outside the drainage
channels, and that little is relatively ineffective. Under some conditions,
therefore, temperature affects both corrasion and transportation.
The humidity of the atmosphere has an influence even more important
than that of temperature on the rate of erosion, and its influence is exerted
on each of the elements of that complex process. A moist atmosphere favors
oxidation, carbonation, hydration, and the growth of vegetation, all of
which promote certain phases of rock weathering. On the other hand,
humidity tends to prevent sudden and considerable variations in
temperature, thus checking the weathering effected by this means.

Precipitation, the most important single factor in determining the rate of
erosion, is dependent on atmospheric humidity. Its amount, its kind (rain or
snow), and its distribution in time, are the elements which determine its
effectiveness in any given place.
Other things being equal the greater the amount of precipitation the more
rapid the corrasion and transportation. Much, however, depends on its
distribution in time. A given amount of rainfall may be distributed equally
through the year, or it may fall during a wet season only. The maximum
inequality of distribution would occur if all the rainfall of a given period
were concentrated in a single shower. With such concentration the volume
of water flowing off over the surface immediately after the down-pour
would be greater than under any other conditions of precipitation, and since
velocity is increased with volume, and erosive power with velocity, it
follows that the erosive power of a given amount of water would be greater
under these circumstances than under any other. Furthermore, a larger
proportion of the precipitation would run off over the surface under these
circumstances than under any other, for less of it would sink beneath the
surface and less would be evaporated. If erosive power and rate of erosion
were equal terms, this would therefore be the condition for greatest erosion;
but erosive power and rate of erosion do not always correspond. If the water
falling in this way could get hold of all the material it could carry, extreme
concentration of precipitation would be the condition favorable for most
rapid erosion. But if the amount of available material for transportation is
slight, a large part of the force of the water could not be utilized in erosion.
It follows that if there were a large amount of disintegrated material on the
surface, erosion would be greater the greater the concentration of
precipitation. If, on the other hand, there were but little disintegrated
material on the surface, frequent showers, with intervening periods when
conditions were favorable for weathering, that is, for preparing material for
transportation, might be more favorable for rapid erosion. While the total
energy of running water available for erosion under these conditions would
be less than before, there might in the long run be more material for
transport; for weathering in the presence of moisture, and all that goes with
it, might be more effective in preparing material for transportation, than
weathering during the long periods of drought which would occur if the
precipitation were concentrated to its maximum. Temperature favoring, the
uniform distribution of moisture through the year would allow the growth

of vegetation, which, although favoring some processes of weathering,
retards erosion in general. While therefore it is not possible to say what
distribution of rainfall favors most rapid erosion without knowing the
nature of the surface on which it is to fall, enough has been said to show
that the problem is by no means a simple one. Some of the most striking
phases of topography developed by erosion, such as those of the Bad Lands
(Figs. 75 to 78, and 108), are developed where the rainfall is unequally
distributed in time, and too slight or too infrequent to support abundant
vegetation.

Fig. 108.—Bad-land topography developed under conditions of aridity and unequal
distribution of rainfall. Slope of Pinal Mountains, Ariz. (Ransome, U. S. Geol. Surv.)

During its fall, and immediately after, rain is more effective than an equal
amount of snow; but the snow may be accumulated through a considerable
period of the year, and then melted rapidly, when it has an effect
comparable to that which would be produced by the concentration of the
rainfall into a limited period of the year. If the ground beneath be frozen
when the snow melts (and this is often the case) the erosion accomplished
by the resulting water will be diminished.
Except in dry regions, where wind-work sometimes exceeds water-work,
the movements of the atmosphere are of less importance directly than
precipitation in determining the rate of erosion. But even in regions which
are not arid the winds have much to do with the rate of evaporation and the
distribution of rainfall, so that their indirect effect is great. Even their direct
effects in moist climates are not to be lost sight of, for even here the surface

is sometimes dry enough to yield dust and sand, and the uprooting of trees
so disturbs the surface as to make earthy débris more accessible to wind and
water. Where trees gain precarious footholds on steep slopes, as they often
do, they are likely to be overturned as soon as they are large enough to offer
considerable resistance to the wind, and in the overturning, large quantities
of rock are sometimes loosened and carried down the slope by gravity. This
phase of destructive work is seen at its best on the walls of gorges, where
trees often flourish until their tops project above the rim of the valley.
Through vegetation, climate influences erosion in ways which are easily
defined qualitatively, but not quantitatively. Both by its growth (wedge-
work of roots) and by its decay (supplying CO2, etc., to descending waters)
it favors certain phases of weathering; but, on the other hand, it retards
corrasion and transportation both by wind and water. This is well shown
along the banks of streams and on the faces of cliffs, in clay, sand, etc. Its
aggregate effect is probably unfavorable to erosion by mechanical means,
and favorable to that by chemical processes.

Fig. 109.—Characteristic cliffs of high arid regions. Right wall of Snake River canyon,
nearly opposite the mouth of Salmon River, Id. Two spring-formed coves, with “Castle
Rock” between. (Russell, U. S. Geol. Surv.)

Erosion in high arid regions differs from that in regions of abundant
rainfall in several ways. It is obvious that the valleys will develop more
slowly in the former, that they will remain young longer, that the period
necessary for the dissection of the surface is greater, that the watercourses
will be less numerous, and that fewer of them will have permanent streams.
There are certain other differences which are less obvious. If the arid region

be high and composed of heterogeneous strata, the topography which
erosion develops is more angular (Fig. 83) than that of the humid region.
This is because there is less rock decay, and less vegetation to hold the
products of decay. The more resistant beds of rock therefore come into
greater prominence, especially on slopes, where they develop cliffs (Figs.
109 and 110). These general principles find abundant illustration in the
plateaus of the western part of the United States,[48] where the cliffs are by
no means confined to the immediate valleys of the streams (Fig. 1, Pl. XII).

Fig. 110.—A Butte. A characteristic feature of the arid plateau region of the West. (Dutton,
Mono. II, U. S. Geol. Surv.)

EFFECTS OF UNEQUAL HARDNESS.

In the preceding pages incidental reference has been made to the results
of inequalities of rock resistance. This topic will now be considered more
fully.
Rapids and falls.—Returning for a moment to the hypothetical island
with which our study of erosion began, let a horizontal layer of hard rock be

assumed to run through it (H, Fig. 111). As the rain falls on the land and
runs off over it, wear will be less rapid where the hard layer comes to the
surface than at the higher or lower levels. As a result, the slope will become
steeper at and below the outcrop of the hard layer, and less steep
immediately above it, as shown by ab in Fig. 111. Under these conditions
the water passing over the hard ledge constitutes rapids. The increased
erosion which accompanies the increased velocity makes the rapids more
rapid. The process may continue until the water falls, rather than flows over
the hard layer (cd, Fig. 111). With continued rainfall the edges of the hard
layer, together with the slopes above and below, would continue to recede
toward the center of the island. Under conditions of absolute homogeneity
of material, save for the hard layer specified, no valley would be developed,
and therefore no stream.
If the surface was so changed as to allow of the development of a valley
(p. 63) the same principles would be applicable. As an active stream passes
from a hard layer to one less resistant, the greater wear on the latter gives
origin to rapids. At first the rapids would be slight (a, Fig. 112), but would
become more considerable (b) as time and erosion go on. When the bed of
the rapids becomes sufficiently steep, the rapids become falls[49] (cd). When
the water falls rather than flows over the rock surface below the hard layer,
erosion assumes a new phase. The hard layer is then undermined, and the
undermining causes the falls to recede. This phase of erosion is sometimes
called sapping.

Fig. 111.—Diagram representing a horizontal layer of hard rock in an island, and its effects
on erosion.

Fig. 112.—Diagram illustrating the development of a fall where the hard layer dips gently
up-stream.

If the hard layer which occasions a
fall dips up-stream (Fig. 112), its
outcrop in the stream’s bed becomes
lower as the fall recedes (e). When it
has become so low that the water
passing over it no longer reacts
effectively against the less resistant
material beneath (f), sapping ceases,
and the point of greatest erosion may be
shifted from the soft material beneath
the fall to the hard layer itself. The Fig. 113.—Diagram illustrating the
actual rate of erosion at this point may conditions which exist at Niagara
Falls. (Gilbert.)
be no greater than before, though the
relative rate is. Under these circumstances the vertical edge of the hard
layer will presently be converted into an incline (f), and as this takes place
the fall becomes rapids. The conversion of the falls into the rapids begins
about the time the lower edge of the hard stratum in the channel reaches
grade. By continuation of the process which transformed the falls into
rapids, the rapids become less rapid, and when the upper edge of the hard
layer has been brought to grade, the rapids disappear (h, Fig. 112). The
history of rapids which succeed falls is the reverse of that which preceded.
The later rapids are steepest at the beginning of their history, the earlier at
their end. Stated in other terms, rapids are steepest when nearest falls in
time. Slight differences in hardness in successive layers often occasion
successive falls or rapids (Fig. 114).
If the hard layer which occasions the falls be horizontal, instead of
dipping up-stream, the general result would be the same; but, other things
being equal, the duration of the falls developed under these conditions
would be greater, since they must recede farther before becoming rapids.
If the layers of unequal hardness in a stream’s bed be vertical and the
course of the stream at right angles to the strike, rapids, and perhaps falls,
will develop (Fig. 115). The chances for falls are greater, the greater the
difference in hardness. Falls developed under these conditions, as well as
the rapids preceding and following, would remain constant in position until
the resistant layer was brought to grade, but they would ultimately
disappear as in the preceding cases. Falls are not likely to develop where

the strata of the stream’s bed dip down-stream, though they may develop
even under these conditions if the gradient of the stream is greater than the
dip of the strata (Fig. 116).

Fig. 114.—Falls in Utica shale, Canajoharie, N. Y. (Darton, U. S. Geol. Surv.)

The inequality of resistance in the rock which occasions a fall may be
original or secondary. In the case of Niagara Falls[50] (Fig. 113) relatively
resistant limestone overlies relatively weak shale. At the Falls of St.
Anthony (Minneapolis) limestone overlies friable sandstone. The falls of
the Yellowstone and the Shoshone Falls of the Snake River (Idaho), are in
igneous rock. In the former case the unequal resistance is occasioned by
unequal decay of the rock, due perhaps to the rise of hot vapors which have
decomposed the rock along the lines of their ascent; in the latter, a more
resistant sort of igneous rock overlies a less resistant.

Structural features, such as jointing, sometimes give rise to falls, or
determine their distinctive features (Fig. 117), even where the formations
involved are of uniform hardness. A joint plane has the effect of a weak
vertical or highly inclined bed. If an open joint is discovered in a stream’s
bed, the water enters it. If it finds an outlet below, a channel is worn along
the new line of flow, with rapids or falls where the water descends. Rock
originally homogeneous may be much fractured in some parts, while it
remains unbroken in others. Where a stream passes from the solid to the
broken portion rapids, or even falls, may develop.

Fig. 115.—Diagram illustrating the development of falls over a vertical hard layer.

Fig. 116.—Diagram illustrating the possibility of falls where the beds dip down-stream.

Falls may originate in still other ways. If for any reason a stream is
forced out of its valley, it may in its flow find entrance to another valley, or
to another part of its own valley, over a steep slope. If the structure of the
slope favors, a fall may speedily develop. The Falls of St. Anthony are an
example, the Mississippi having been turned out of its earlier course by
deposits of glacial drift. Again, if an obstruction of any sort, such as a flow
of lava, dams a stream, rapids or falls are developed where the water
overflows the dam. When a main valley is notably deepened by glaciation
the drainage from tributary valleys may fall into it, if the tributaries were
not equally deepened. Falls which originated in this way are common in the
western mountains of the United States, as well as in most mountain regions
recently affected by local glaciers (Fig. 118).
One waterfall often breeds others. Thus where a fall recedes beyond the
mouth of a tributary stream, the tributary falls. The Falls of Minnehaha, on
a small tributary to the Mississippi, near Minneapolis, may serve as an
illustration. In such cases the falls may not develop from rapids. Once in

existence, the fall of a tributary follows the same history as that of a main
stream.

Fig. 117.—Kepler’s Cascade, in the Yellowstone Park. The jointed and fractured character
of the igneous rocks occasions a series of falls and rapids. (Iddings, U. S. Geol. Surv.)

Streams which have falls are relatively clear.[51] If a stream favorably
situated for the development of a fall carried a heavy load, deposition would
take place below the rapids, and the tendency would be to aggrade the
channel at that point and so to prevent the development of the fall. Falls
occur only on streams which have relatively high gradients. This means that
the streams which have falls are well above base-level, and streams well
above base-level are young. Falls therefore are a mark of topographic
youth.

Fig. 118.—The Upper Yosemite Falls.

The fall of the Niagara[52] (Pl. IX) is one of the most remarkable known,
both because of its large volume of water and its great descent, between 160
and 170 feet. The rate at which the fall is receding is a matter of interest not
only in itself, but because, once determined, it may be made to serve as a
unit of measurement for certain important events in geological history. It
was formerly conjectured that this fall was receding at the rate of one to
three feet per century, but it was not until recent years that its actual rate of
recession was approximately fixed. By surveys executed in 1842 and 1890
it has been determined that its average rate of recession between those dates

was something like 4½ feet per year, or about 150 times as great as the
highest estimate stated above. It is to be noted that this is the average rate
of recession, for all parts of the ledge over which the water falls are not
receding at the same rate. The point of the “Horseshoe” has, during the
same time, gone back at more than twice this rate.[53]

Fig. 119.—A group of pot-holes. (Turner, U. S. Geol. Surv.)

Rapids and falls sometimes occasion the development of pot-holes (Fig.
119), a peculiar rather than important erosion feature. The holes are
excavated in part by the falling and eddying of silt-charged water, but
chiefly by stones which the eddies move. Pot-holes which are not now in

immediate association with rapids or falls often point to the former
existence of rapids or falls.
Rock terraces.—The tendency to sapping shown in many waterfalls is
also shown in the weathering and erosion of the sides of a valley where a
hard layer outcrops above the bottom, and the profile of the side slopes of
the valley simulates that of the stream; that is, the slope becomes gentle just
above the hard layer, and steep, or even vertical, at and below its outcrop.
This is illustrated by Fig. 120, where the hard layer through which the
stream has sunk its valley stands out as a rock terrace on either side of the
valley. Such terraces are not rare and are popularly believed to be old
“water-lines”; that is, to represent the height at which the water once stood.
In one sense this interpretation is correct, since a river has stood at all levels
between that of the surface in which its valley started, and its present
channel, but the shelf of hard rock does not mean that the river, after
attaining its present channel, was ever so large as to fill the valley to the
level of the terrace. Rock terraces may also result from changes of level.

Fig. 120.—Diagram to illustrate the development of rock terraces.

Narrows.—Inequalities in hardness occasion another peculiarity
common to valleys. If a stream crosses vertical or highly inclined strata of
unequal hardness, its valley is usually constricted at the crossing of the
harder layers. If such a constriction be notable it is called a narrows, or
sometimes a water-gap (Figs. 121, 159, and Fig. 2, Pl. XII). The
Appalachian Mountains afford numerous examples. The constriction arises
because the processes which widen the valley are less effective on the hard
layer than on the less resistant ones on either hand. Though most narrows
are due to the superior resistance of the rock where they occur, they are
sometimes the result of other causes.

Fig. 121.—Lower narrows of the Baraboo River, Wis. The even-crested ridge is Huronian
quartzite. The surroundings are of Cambrian sandstone. (Atwood.)

Fig. 122.—A hog-back, Jura-Trias. Colorado City, Colo. (Russell, U. S. Geol. Surv.)

Narrows are much more conspicuous in certain stages of erosion than in
others. While a valley is still so young as to be narrow at all points, no
narrows will be conspicuous; but at a later stage in its history, when the
valley is otherwise wide, narrows are more pronounced. At a still later
stage, when the hard strata themselves approach base-level, the narrows
again become inconspicuous.
From what has preceded it is clear that rapids or falls are likely to occur
at narrows, especially in the early part of their history.
Other effects on topography.—Inequalities in the hardness of rock
develop certain peculiarities of topography other than those of valleys. The

less resistant portions of a land area more or less distant from streams are
worn down more readily than those which are more resistant. If great areas
of high land be capped with hard rock they are likely to remain as plateaus
after surrounding areas of less resistance are brought low. If the hard
capping affects a small area instead of a large one, the elevation is a butte, a
hill, or a mountain, instead of a plateau (Fig. 110). Many buttes and small
mesas are but remnants of former plateaus (Mesa Lauriano, N. M., Fig. 1,
Pl. XII). A feature of buttes and mesas capped by hard rock is the steep
slope or cliff corresponding to the edge of the hard bed (Figs. 78 and 109).

Fig. 123.—A ridge due to the outcropping edge of hard Jurassic rock. Wyoming.

If the rock of a region be stratified and the layers tilted, the removal of
the softer beds leaves the harder ones projecting above the general level in
the form of ridges or “hog-backs” (Figs. 122 and 123). Dikes of igneous
rock, harder than the beds which they intersect, likewise become ridges
after the degradation of their surroundings. The plugs of old volcanic vents
and other igneous intrusions of limited area often constitute conspicuous
hills or mountains after erosion has removed their less resistant
surroundings (Fig. 124). Inequalities of hardness are therefore responsible
for many hills and ridges. In the isolation of the hills and ridges picturesque
coves are developed, where the attitude and distribution of the weak and
strong rocks are propitious. The bottoms of the coves are located on the
weak rocks, and above them rise the precipitous slopes of the resistant ones.

Round valley (Fig. 1, Pl. XVII, High Bridge, N. J., quadrangle, U. S. Geol.
Surv.) and the coves about the head of Hiawassee River (Dahlonega, Ga.,
quadrangle) are examples.

Fig. 124.—Matteo tepee, Wyo. Mass of igneous rock exposed by erosion, and preserved
because of its superior resistance. (Detroit Photo. Co.)

Ridges and hills resulting from the unequal degradation of unequally
resistant terranes are not equally prominent at all stages in an erosion cycle.
In early youth the material surrounding the hard bodies of rock has not been
removed; in early maturity considerable portions of their surroundings still
remain about them; but in late maturity or early old age the outcropping
masses of hard rock have been more perfectly isolated and are most
conspicuous. Most of the even-crested ridges of the Appalachian system, as
well as many others which might be mentioned, became ridges in this way.
In the final stages of an erosion cycle the ridges of hard rock are themselves
brought low. Isolated remnants of hard rock which remain distinctly above
their surroundings in the late stages of an erosion cycle (Fig. 124) are
known as Monadnocks, the name being derived from Mount Monadnock,
N. H., an elevation of this sort developed in a cycle antedating the present.

Fig. 125–27.—Diagrams illustrating piracy, where the stream which does not flow over rock
of superior hardness captures those which do. Fig. 126 represents a further development
of the drainage shown in Fig. 125, and Fig. 127 represents a still later stage.

Fig. 128–30.—Diagrams to illustrate piracy, where the competing streams all cross a hard
layer. The diagrams represent successive stages of development.

Adjustment of streams to rock structures.—Valleys (gullies) locate
themselves at the outset without immediate regard to the hardness and
softness of their beds. It is primarily the slope about the head of a gully
which determines its line of growth, though relative hardness often
determines the details of slope, even in the early stages of an erosion cycle.
Once established, streams tend to hold their courses, even if this involves
the crossing of resistant layers.

While a region where more and less resistant layers of rock come to the
surface is in a youthful stage of erosion, some of the valleys (and therefore
the streams) are likely to be located on the less resistant rock, some on the
more resistant, and some partly on the one and partly on the other. The
streams on the weaker rock will deepen their valleys more rapidly than the
others, and those which flow across stronger and weaker rocks alternately
will deepen their valleys more rapidly than those which run on hard rock all
the time. The former conclusion is self-evident. The latter appears from the
fact that rapids will be likely to develop at the crossing of each hard layer,
thus accelerating erosion at those points. Such a stream therefore not only
has less hard rock to erode than one which flows on resistant rock all the
time, but it erodes that which it does cross much faster.

Fig. 131, 132.—The capture of the head of Beaverdam Creek by the Shenandoah Va.-W. Va.
(After Willis.)

Streams which do not cross hard layers therefore have an advantage over
those which do, and the tributaries to such streams, since they join deeper
mains, have an advantage over the tributaries to the others. The valleys of
the former may lengthen until their heads reach the latter, and capture their
streams. This sequence of events is illustrated in the accompanying

diagrams (Figs. 125–27). Even where several streams cross the same
resistant bed, piracy is likely to take place among them, for some are sure to
deepen their valleys faster than others, because of inequalities of volume,
load, or hardness. This is illustrated by Figs. 128–30. An actual case is
shown in Figs. 131, 132. Though piracy may take place when streams do
not flow over rock of unequal hardness (p. 103), it is much more common
where unequal resistance of the rock puts one stream at a disadvantage as
compared with another.
The changes in the courses of streams, by means of which they come to
sustain definite and stable relations to the rock structure beneath, are known
as processes of adjustment.[54] Since streams and valleys adjust themselves
to other conditions as well, this phase of adjustment may be called
structural adjustment. Structural adjustment is not uncommon among rivers
flowing over strata which are vertical or highly inclined, since in these
positions the hard and soft strata are most likely to come to the surface in
frequent alternation. The smaller streams suffer capture and adjustment
first, since, as a rule, they have shallower valleys. It often happens that main
streams, because of their deeper valleys, hold courses not in adjustment
with structure (the Delaware, the Susquehanna, etc.), while tributary
streams are captured, diverted, and adjusted. The capture of a tributary,
however, leads both to the diminution of its main and to the increase of its
captor, and the weakened stream may ultimately fall a prey to the one which
is strengthened.
The processes of adjustment go on until the streams flow as much as
possible on the weaker beds, and as little as possible on the stronger, when
adjustment is complete. This amounts to the same thing as saying that the
outcrops of the hard layers tend to become divides. In many cases an area is
so situated that there is no escape for its drainage except across resistant
rock. In this case its drainage is completely adjusted when as few streams as
possible cross the resistant rock, and these by the shortest routes.
Adjustment has been carried to a high degree of perfection in most parts
of the Appalachian system. Here, as in all other mountains of similar
structure, strata of unequal hardness were folded into ridges. In this case,
the folds have been truncated by erosion, exposing the more and the less
resistant beds (H and S respectively) in alternate belts along the flanks of
the truncated folds (ab and cd, Fig. 133). The streams, especially the lesser

ones, now flow along the strike of the softer beds much more commonly
than elsewhere, and where they cross the hard layers it is usually at right
angles to the strike. This is shown in Fig. 134, where the arrows indicate the
direction of strike. In the history of these rivers, however, a factor is
involved which has not yet been considered, and these streams will be
referred to later.

Fig. 133.—Diagram showing the outcrops of hard layers on the flanks of a truncated fold.
cd represents th/e present surface; dotted lines above, earlier surfaces.

Fig. 134.—Example of adjusted drainage in a region of folded rocks, Va.-W. Va.

Fig. 135.—Diagram to illustrate readjustment of drainage, as base-level is approached.

Fig. 136.—Diagram to illustrate superimposition. The consequent stream on the upper
formation is superimposed on the underlying structures when the upper bed has been cut
through.

As base-level is approached, the outcrops of hard rock are brought low.
When they have been reduced to the level of their surroundings, the streams
may flow without regard to the resistance of the rock beneath, for
downward cutting has ceased. As this stage of erosion is approached, a
readjustment of the drainage may take place, and the waters which had

taken long and circuitous courses to avoid hard rock, may change their
courses to more direct ones (compare Figs. 130 and 135). Adjustment is,
therefore, a relative term, and streams which are adjusted at one stage of
erosion, are not necessarily adjusted at another.
It sometimes happens that rocks of unequal resistance are covered by
beds of uniform hardness. A consequent stream developed on the latter may
find itself out of structural adjustment when it has cut its channel down to
the level of the heterogeneous beds below. Such a stream is said to be
superimposed (Fig. 136) on the underlying structure. Structural adjustment
is likely to follow.

INFLUENCE OF JOINTS AND FOLDS.

Joints.—Various structural features of rock other than hardness influence
its erosion. Apart from the stratification planes, most rock formations are
affected by joints or fissures. The joints are often, but not always, nearly
vertical. Two sets are generally present, and sometimes more. If but two,
they usually meet at a large angle; if more than two, two are likely to be
nearly perpendicular to each other, while the third and fourth sets have such
directions as to cut the others at large angles. These joints allow the ingress
of water, roots, etc., which help to weather and disrupt rocks. Occasionally
there is notable sag of the beds of rock along joint planes, but this effect is
usually superficial only (Fig. 137). Where the jointage planes are frequent
and open, the columns bounded by them sometimes topple over on cliff
faces, either by undercutting, or by the wedge-work of roots or ice.
The effect of joints on erosion may often be seen along a stream which
flows in a rock gorge. In such situations, the outlines of the banks are
sometimes angular, and sometimes crenate (Fig. 138), the reëntrants being
located at the joints. By working into and widening joints, running water
sometimes isolates masses of rock as islands (Fig. 139). In a region free
from mantle rock, or where the mantle rock is meagre, joints often
determine the courses of valleys by directing the course of surface drainage.
This is shown in many parts of the arid west. In regions where the rocks are
notably faulted, the courses of the streams are sometimes controlled by the
courses of the fault planes. This is the case, for example, in central
Washington.[55]

Fig. 137.—Shows the sagging of beds along joints. The disturbance does not extend far
below the surface. Cook’s quarry (Niagara limestone) near La Salle, Niagara Co., N. Y.
(Gilbert, U. S. Geol. Surv.)

The jointing of rocks often shows itself distinctly in the weathered faces
of cliffs (Figs. 140 and 141), especially in arid and semi-arid regions, or
where the slope is too steep for the accumulation of soil and rock-waste on
its surface.
If a stream flowing over jointed rock has falls, the conditions are
sometimes afforded for the development of an exceptional and striking
scenic feature. If above Niagara Falls, for example, there were an open joint
in the bed of the stream (as at b, Fig. 142), some portion of the water would
descend through it. After reaching a lower level it might find or make a
passage through the rock to the river below the falls. If even a little water
took such a course, the flow would enlarge its channel, making a
passageway between the joint through which the water descended and the
valley below the falls (bcde, Fig. 142). This passageway might become
large enough to accommodate all the water of the river. In this case, the
entire fall would be transferred from the position which it previously

occupied (f) to the position of the enlarged joint (b). The fall would then
recede. The underground channel between the old falls and the new would
be bridged by rock (bf″ and f‴, Fig. 143), making a natural bridge. The
natural bridge near Lexington, Va. (Fig. 144), almost 200 feet above the
stream which flows beneath it, is believed to have been developed in this
way. A similar bridge is now in process of development in Two Medicine
River in northwestern Montana (Fig. 145). Once in existence, a natural
bridge will slowly weather away.

Fig. 138.—Figure showing crenate river bank, the reëntrants being determined by joints.
Dells of the Wisconsin River, near Kilbourn, Wis. (Atwood.)

Fig. 139.—Lone Rock. An island isolated by the notable widening of a series of joints. The
joints in the rock of the island have themselves been so widened that a rowboat may be
taken through it in two directions. Lower Dells of the Wisconsin. (Meyers.)

Fig. 140.—Effect of columnar structure on weathering. Material unconsolidated. Spur of
south end of Sheep Mountain. (Lippincott, U. S. Geol. Surv.)

It is not to be understood that all natural bridges have had this history.
They are sometimes developed from underground caves when parts of their
roofs are destroyed, as well as in various other ways.

Fig. 141.—Effect of columnar structure on weathering. Big Bad Lands, S. D. (Darton, U. S.
Geol. Surv.)

Folds.—The erosion of folded strata (anticlines and synclines) leads to
the development of distinctive topographic features. So soon as a fold
begins to be lifted, it is, by reason of its position, subject to more rapid
erosion than its surroundings. For the same reason the crest of the fold is
likely to be degraded more rapidly than its lower slopes, and must suffer
more degradation before it is brought to base-level. Folds are usually
composed of beds of unequal resistance, and as the degradation of a fold
proceeds, successive layers are worn from the top, and the alternating hard
and soft layers composing it are exposed. So soon as this is accomplished,
adjustment of the streams is likely to begin, and the watercourses, and later
the valley plains, come to be located on the outcrops of the less resistant
layers, while the outcrops of the harder beds become ridges.
If the axis of an eroded anticline were horizontal, a given hard layer, the
arch of which has been cut off, would, after erosion, outcrop on both sides
of the axis. When the topography was mature these outcrops would

constitute parallel ridges, or parallel lines of hills; when the region had been
base-leveled, the outcrops would be in parallel belts, though no longer
ridges or hills. The lower the plane of truncation, the farther apart would the
outcrops be in the anticline, and the nearer together in the syncline
(compare ab and cd, Fig. 133).

Fig. 142.—A natural bridge in process of development; longitudinal section at the left;
transverse section, looking toward e, at the right.

Fig. 143.—The same as Fig. 142 at a later stage of development.

If, on the other hand, the axis of the anticline or syncline to be eroded
was not horizontal, that is, if it plunged, the topographic result would be
somewhat different. Suppose a plunging anticline to be truncated at base-
level. If either end of the fold plunged below the plane of truncation, the
outcrops of a given layer on opposite sides of the axis would converge in
the direction of plunge, and come together at the end. At a stage of erosion
antedating planation (say late maturity) there would have been a ridge, or a
succession of hills, in the position corresponding to the outcrop of a hard
layer, with a canoe-shaped valley within. If two hard layers were involved,
instead of one, there would be two encircling ridges, with a curved valley
between them, and a canoe-shaped valley within the innermost (Fig. 146).
If the anticline plunged both ways, the valley enclosed by the hard-layer
ridge would be canoe-shaped at both ends (Fig. 147). In such a case there
would be likely to be a low gap (water-gap) in the rim of the valley through
which the drainage which degraded the surface escaped, but there would be
likely to be but one, for if two or more streams had drained the area of the
valley at an early stage of erosion, one would be likely to have captured the
others (see p. 138) before late maturity. A succession of doubly-plunging
anticlines and synclines might give rise to a very complex series of ridges

and valleys. Illustrations of the above phenomena are found at various
points in the Appalachian Mountains, especially in eastern Pennsylvania.[56]

Fig. 144.—The Natural Bridge of Virginia, from the southeast (Walcott, U. S. Geol. Surv.)

Fig. 145.—A natural bridge in development. Two Medicine River, Mont. Corresponds to
the stage represented by Fig. 142, and the view corresponds to that shown
diagrammatically at the right-hand end of the figure. (Whitney.)

In the structural adjustment which goes with the erosion of folds, it often
happens that the valleys come to be located on the anticlines, while the
outcrops of the hard layers on the flanks of the anticlines, or even in the
original synclines, become the mountains. The adjustments by which
valleys come to be located on anticlines are somewhat as follows:[57] Fig.
148 represents two doubly-plunging anticlines with a syncline between, the
relative elevations being shown by contour lines. At the outset, the drainage
of such a region must have followed the structural valley, and its initial
course, consequent on the slope, must have been down the axial trough.
Drainage from the anticlines into the synclines would have promptly
developed valleys, and the valleys would soon have acquired streams.

Fig. 146.—A canoe-shaped valley bordered by a ridge formed by the outcrop of a hard
layer in a plunging syncline. The ridge bounding the canoe-valley is separated from an
outer ridge by a curved valley underlain by relatively weak rock. (After Willis.)

Fig. 147.—A diagram to illustrate the effects of erosion on a doubly-plunging anticline
made up of beds of unequal hardness.

Fig. 148–51.—Diagrams to illustrate the shifting of rivers from a synclinal to an anticlinal
position. (After Davis.)

The anticlines and synclines under consideration are assumed to have a
thick hard layer at the surface, and softer beds below. This is shown in the
cross-section introduced in the figure, the upper hard stratum (m) being
indicated by the dots, while the softer one (n) is white. The line oo
represents base-level, which is below the hard layer both in the syncline and
anticline, but much farther below in the latter position than in the former.
Because of their higher gradients, and because of the greater fracturing to
which the region they drain was presumably subject at the time of folding,
the tributary streams might cut through the hard layer sooner than the main
stream which they join. This done, they would enlarge their valleys rapidly
in the softer rock beneath, and secondary tributaries would be developed
(Fig. 149). When the condition of things represented in Fig. 149 is reached,
the streams c and d, tributary to the synclinal stream, come into
competition. The former has the advantage over the latter, because it joins
the main stream at a lower level. Stream c will therefore be likely to capture
d. The incipient stages of the capture are stealthy, and the later bold. At first
the divide between their head waters is shifted northward inch by inch,

because the gradient toward g is higher than that toward e. The capture of
the head waters of e is as slow as the migration of the divide, until the
divide reaches the point where e joins f. The stream f is then diverted
promptly into the valley of g, and is at once led away to c (see Fig. 150).
Strengthened by its increased volume, the stream c (Fig. 150) lowers its
valley across the hard layer more rapidly than before, and so holds the
advantage it has gained. Not only this, but the beheaded stream d (Fig. 150),
because of its diminished volume, sinks its valley into the hard layer less
rapidly than before, and its decrease in power also works to the advantage
of the stream leading to c. The result is that the divide between fg and d
does not remain constant, but is driven back step by step toward a.
Similarly a tributary to the main stream at b (Fig. 150), may by means of
its tributary h, capture the waters of fg, and lead them to the synclinal valley
at b (compare Figs. 150 and 151). Deprived of its main source of supply (at
c) the synclinal stream is greatly diminished above b, and cuts more and
more slowly, while the stream fgh (Fig. 151), having greater volume and
working mainly in softer rock, sinks its channel faster than the stream in the
synclinal axis. Under these circumstances, the stream at f may cut its valley
below the valley in the synclinal axis a (Fig. 150). In this event, the divide
between f and a (Fig. 150) may be pushed back until the synclinal stream is
beheaded at a and carried out of the syncline and over into the anticlinal
valley (Fig. 151). Thus, the old anticlinal axis comes to be the course of the
main stream. Similarly the stream entering the syncline at b (Fig. 151)
might later be captured by i, thus lengthening its anticlinal course.
It is not to be understood that this sequence of events will take place in
the degradation of every anticline, but the principles here set forth will
always be operative. The result specified will be accomplished wherever
hard and soft layers have the relations indicated in the diagrams; that is,
where the stream in the syncline finds itself on a resistant layer as it
approaches base-level, while at the same time the (original) tributary
streams are working in softer beds. It is not to be understood, therefore, that
streams migrate from synclines to anticlines for the sake of getting out of
the former positions into the latter. If they shift their courses it is to find
easier ones.
That these changes are not fanciful is shown by the fact that the
adjustment described corresponds with that shown in many parts of the

Appalachian Mountains, and in other mountains of similar structure.
If in a later stage of its history, the new main stream, fh, were to cut its
bed down to a lower hard layer, while the original stream, ab, reached a
softer bed beneath the hard one above, the latter would again have an
advantage, and a new series of adjustments would be inaugurated which
might result in re-establishing the main stream in its original synclinal
position.

EFFECT OF CHANGES OF LEVEL.

Rise.—If after being base-leveled, or notably reduced by erosion, a
region is uplifted so as to increase the gradients and therefore the velocities
of the streams which drain it, the streams are said to be rejuvenated, and a
new cycle of erosion is begun. If the rise of the area were equal everywhere,
while the coast line remained constant in position, there would be an
immediate increase in velocity only at the debouchures of the streams
flowing directly into the sea. At the debouchures of such streams there
would be rapids or falls. Each rapids or falls would promptly recede, and
with the recession, the acceleration of velocity resulting from the uplift
would be felt farther and farther up-stream, and ultimately to its source. The
rejuvenated streams would cut new valleys in the bottoms of their old ones
(Figs. 152 and 153). The new valleys would begin where the increase in
velocity was first felt, and they would be lengthened by head erosion just as
valleys of the first cycle were lengthened.

Fig. 152.—Cross-section of a wide valley, ab, in the bottom of which a younger valley, cd,
has been excavated as the result of uplift.

Fig. 153.—Diagram to illustrate in ground plan an ideal case of rejuvenation as the result
of uplift.

When the head of the new part of a valley of a rejuvenated stream
recedes past the mouth of a tributary adjusted[58] to the gradient of the main
stream before rejuvenation, the velocity of the tributary is accelerated at its
debouchure, and it begins to excavate a new valley in the bottom of its old
one. The new valley commences at the lower end of the old one, and
develops headward (a and b, Fig. 153). Good illustrations are furnished by
the streams in the west central part of New Jersey. The Delaware has here a
sharply defined valley, and its tributaries are essentially as deep as their
main at the point of junction. Above this point they have high gradients for
a short distance (three to six miles), beyond which they wind sluggishly in
wide valleys with low gradients across a relatively high plateau. Their

profiles are illustrated by Fig. 154. The flat, though high, surface in which
their upper courses lie, appears to have been nearly base-leveled in an
earlier cycle, and then to have been elevated. The date of the elevation is
fixed, in terms of erosion, by the time necessary for the excavation of the
Delaware gorge, and the narrow gorges along the lower courses of its
tributaries. It was so recent that the effects of rejuvenation, proceeding from
the debouchures of the tributaries toward their heads, have not yet advanced
far from the Delaware. Similar relations are found elsewhere (Fig. 1, Pl.
XIII, s. c. Col.). Another peculiarity of rejuvenated drainage is shown in
Fig. 2, Plate XIII (s. Kan.). Here Elm Creek flows at a level 200 feet below
that of Sand Creek, 4 miles distant. The valley of the former appears to have
entered upon a new cycle as the result of uplift, while that of the latter, in
the area shown on the map, is still unrejuvenated. Farther down-stream, the
valley of Sand Creek shows signs of rejuvenation. It may be noted that a
tributary of Amber Creek has good opportunity to capture Sand Creek, for
the latter flows about 25 miles before reaching the level of Amber Creek at
its junction with Elm Creek.

PLATE XIII.

U. S. Geol. Surv. Fig. 1. COLORADO. Scale, 2+ mile per inch.

U. S. Geol. Surv. Fig. 2. KANSAS. Scale, 2+ mile per inch.

PLATE XIV.

U. S. Geol. Surv. Fig. 1. PENNSYLVANIA. Scale, 1+ mile per inch.

U. S. Geol. Surv. Fig. 2. CALIFORNIA. Scale, 1+ mile per inch.

Fig. 154.—Profile of a rejuvenated stream. The Lockatong River (N. J.) to the head of Mud
Run.

Should the lower end of a tributary valley fail to be degraded as fast as
the valley of the main at the point of junction, the tributary is out of
topographic adjustment with its main. Falls or rapids may result. When the
lower end of a tributary valley is distinctly above the level of its main, the
former is called a hanging valley. Hanging valleys developed by stream
erosion alone are not common except just after the recession of a falls past
the mouth of a tributary. Hanging valleys, as well as the characters and
relations illustrated by Figs. 152–154 are criteria of rejuvenation, but they

must be applied with discretion. Such profiles, for example, as that shown
in Fig. 154 may be developed when the rock of a stream’s bed is unequally
resistant, and hanging valleys are generally a result of glaciation (see
Chapter V).
Rejuvenated streams sometimes inherit certain peculiarities from their
aged ancestors. Thus a rejuvenated stream may intrench the meanders
possessed by the old stream which preceded (see Fig. 1, Pl. XIV, near
Harrisburg, Pa.), and intrenched meanders are one of the marks of
rejuvenated streams. They are not uncommon in the Appalachian Mountain
regions, and are known in other parts of the world. The Seine and the
Moselle furnish further illustrations.[59]
The history of the new cycle of erosion inaugurated by the uplift would
differ from that of the preceding cycle in that the new one would begin with
a drainage system already developed. Other things being equal, therefore,
the reduction of the land would proceed more rapidly in a subsequent cycle
than in the first.
The recognition of different cycles of erosion, separated by uplifts, is
often easy. The principles involved are illustrated by Fig. 155 which
represents an ideal profile of considerable length (say 50 miles). The points
a, a′, and a″ reach a common level. Below them there are areas b, b′, and b″
which have a nearly common elevation, below which are the sharp valleys
d, d′, and d″. The points a, a′, and a″ represent the cross-sections of ridges
formed by the outcrops of layers of hard rock. If the crests of the ridges are
level, the points a, a′, and a″ must represent remnants of an old base-level,
since at no time after a ridge of hard rock becomes deeply notched does it
acquire an even crest, until it is base-leveled.[60] At all earlier stages its crest
is uneven. After the cycle represented by the remnants a, a′, and a″ was
completed, the region suffered uplift. A new cycle represented by the plain
b, b′, and b″ was well advanced, though not completed, when the region
was again elevated, and the rejuvenated streams began to cut their valleys d,
d′, and d″ in the plain of the previous incomplete cycle. The elevations, c
and c′ (intermediate in elevation between a, a′, and a″, and b, b′, and b″)
may represent either remnants of the first base-level plain which were
lowered, but not obliterated, while the plane b, b′, b″ was developing; or
they may represent a cycle intermediate between that during which a, a′, a″
and b, b′, b″ were developed. If the intermediate elevations (c, c′) have a

common height and level crests, the presumption would be in favor of the
latter interpretation. If they be numerous and of varying heights, as is
possible, they may in the field obscure the planes (a, a′, a″ and b, b′, b″)
developed in the different cycles, which, in the figure, are distinct.

Fig. 155.—Diagram to illustrate cycles of erosion where the beds are tilted.

If the strata involved be horizontal the determination of cycles is
sometimes less easy. Thus in Fig. 156, it is not possible to say whether a
and a′ represent remnants of an old base-level, or whether they represent the
original surface from which degradation started. So, too, the various
benches below a, such as b, b′, and b″ may readily be the result of the
superior hardness of the beds at this level. For the determination of
successive uplifts in the field it is necessary to consider areas of
considerable size, and to eliminate the topographic effects of inequalities of
hardness, and of certain other factors to be mentioned presently.

Fig. 156.—Diagram to illustrate cycles of erosion where the beds are horizontal.

The inequalities in the depths of the young valleys in Figs. 155 and 156
may be explained on the supposition that the deeper ones belong to main
streams, and the shallower ones to tributaries. Such a valley as that shown
at e, Fig. 155, suggests rejuvenation at this point; but farther up the stream
which occupies this valley, rejuvenation might not be apparent. In this case,
the main streams might be flowing in new valleys, d, d′, etc., while the
heads of their tributaries are still flowing in the older valleys of the
preceding cycle (compare Fig. 154 and Fig. 1, Pl. XIII).
It is by the application of the preceding principles that it is known that
the Appalachian Mountains, after being folded, were reduced to a peneplain
(p. 76), throughout their whole extent from the Hudson River to Alabama.
The peneplain level is indicated by the level crests of the Appalachian
ridges, shown in cross profile by the high points of Fig. 157. The system

was then uplifted, and in the cycle of erosion which followed, broad plains
were developed at a new and lower level, corresponding in a general way to
the plains b, b′, and b″ of Fig. 155. The plains were located, for the most
part, where the less resistant strata come to the surface. Above them rose
even-crested ridges, the outcrops of the resistant layers, which had been
isolated by the degradation of the softer beds between. They constitute the
present mountain ridges (the high points of Fig. 157). The evenness of their
crests, testifying to the completeness of the first peneplanation, is shown in
Fig. 158, which represents, diagrammatically, a longitudinal profile of an
Appalachian Mountain ridge. The evenness of the crest is interrupted by (1)
notches (b, c, etc., Fig. 158) cut by the streams in later cycles, and (2) by
occasional elevations above the common level (monadnocks, a, a′, Fig.
158). The monadnocks are generally rather inconspicuous, but there is a
notable group of them in North Carolina and Tennessee. Mount Mitchell
and Roane Mountain are examples. When long distances are considered, the
ridge crests depart somewhat from horizontality. This is believed to be due,
in part at least, to deformations of the old peneplain during the uplift which
inaugurated the second cycle of erosion.

Fig. 157.—Cross-section of a portion of the Appalachian Mountains to illustrate the
phenomena of erosion cycles. (After Rogers.)

Fig. 158.—A diagrammatic longitudinal profile of an Appalachian Mountain ridge.

The extent to which the second cycle of erosion recorded in the present
topography had proceeded before its interruption by uplift, is indicated by
the extent of the valley plains (Fig. 157) below the mountain ridges. While
these plains were being developed on the weak rocks, narrow valleys only
(Fig. 158) were cut in the resistant rocks which now stood out as ridges. In
Fig. 158 some of these valleys are shallow (c, c′, c″, etc.), and but one of
them deep. The former may be either (1) the valleys of streams which
crossed the hard layer at the beginning of the cycle, and which were
diverted before their valleys became deep; or (2) they may represent the
heads of valleys now working back into the ridges. The deep valley (b)
represents the work of a stream which has held its course across the hard

layer while the latter was being isolated as a mountain ridge (compare Figs.
131 and 132). Deep narrows of this sort are often called water-gaps. Similar
valleys, whether shallow or deep, from which drainage has been diverted,
are sometimes called wind-gaps. The second cycle of erosion, while still far
from complete, was interrupted by uplift (relative or absolute), and a new
cycle inaugurated. This event was so recent that the new (third) cycle has
not yet advanced far.

Fig. 159.—The Kittatinny Mountains and Delaware Water-Gap from Manunka Chunk.
(N. J. Geol. Surv.)

Recently it has been urged that another cycle, intermediate between the
first and second, is to be recognized.[61]
Some of the features just described are illustrated by Fig. 159. The even
mountain crest in the background is the Kittatinny Mountain of New Jersey
and its continuation in Pennsylvania. In common with other corresponding
crests it represents the oldest recorded base-level (or peneplain) of the
region. The great gap in the mountain is the Delaware Water-Gap. Below
the mountain crest there is another plain, developed in a subsequent cycle of
erosion, while the valley plain in the foreground represents the work of a
still later cycle.

Fig. 160.—Showing certain peculiarities of Appalachian drainage. 1 = the Susquehanna; 2
= the Potomac; 3 = the James; 4 = the Roanoke; 5 = the Coosa; 6 = the Tennessee; 7 = the
Kanawha; 8 = head of New River; 9 = head of the French Broad.

The oldest erosion plain of the Appalachian Mountains, the results of
which are seen in the even-crested ridges so characteristic of the system, is
sometimes called the Kittatinny base-level.[62] It was completed early in the
Cretaceous period, and hence is sometimes known as the Cretaceous base-
level. The next lower plain, imperfectly developed, has been called the
Shenandoah Plain,[62a] from the Shenandoah Valley where it is well seen
(Fig. 132 and Fig. 2, Pl. XII). It is to be noted that the terms base-level and
peneplain have both been used in connection with these old plains. Graded
plain is equally applicable. The truth is that the topographic types

represented by these three terms grade into one another. It may be
questioned whether definitions should be insisted on which differentiate
these types more sharply than Nature has.
Many of the peculiarities of the drainage of the Appalachian Mountain
system are intimately connected with the history just outlined. Thus three
great rivers, the Delaware, the Susquehanna, and the Potomac, have their
sources west of the Appalachians proper, cross the system in apparent
disregard of the structure, and flow into the Atlantic. The James and
Roanoke head far to the west, although not beyond the mountain system,
and flow eastward, while the New River (leading to the Kanawha) farther
south, heads east of the mountain-folds, and flows northwestward across the
alternating hard and soft beds of the whole Appalachian system, to the Ohio
(Fig. 160). The French Broad, a tributary to the Tennessee, has a similar
course. Such streams are clearly not in structural adjustment, and afford
good opportunities for piracy. Their courses were apparently assumed
during the time of the Kittatinny base-level, when the streams had so low a
gradient as not to be affected by the structure (p. 150). Elevation
rejuvenated them, and they have held their courses in succeeding cycles
across beds of unequal resistance, though smaller streams have become
somewhat thoroughly adjusted. Crustal deformations have also helped them
to hold their courses, for the Cretaceous peneplain seems to have been tilted
to the southeast at its northern end, and to the southwest at its southern,
when the succeeding cycle began.
Streams which hold their early courses in spite of changes which have
taken place since their courses were assumed are said to be antecedent.
They antedate the crustal movements which, but for pre-existent streams,
would have given origin to a different arrangement of river courses. As a
result of crustal movements, therefore, a consequent stream may become
antecedent. Master streams are more likely to hold their courses, and
therefore to become antecedent, than subordinate ones.
The uplift of base-leveled beds, especially if the beds are tilted so as to
bring layers of unequal resistance to the surface at frequent intervals,
affords conditions favorable for extensive adjustment. The numerous wind-
gaps in the mountain ridges, representing the abandoned courses of minor
streams, and the less numerous water-gaps, which indicate the resistance of
large streams to structural adjustment, are instructive witnesses of the extent

to which adjustment has gone. So extensive has been the adjustment among
the streams of the Appalachian Mountains that there is probably no
considerable stream in the whole system which has not gained or lost
through its own or its neighbors’ piracy. The history of the rivers of the
Appalachian Mountains has been further complicated by a considerable
amount of warping during the periods of uplift.[63]

Fig. 161, 162.—Diagrams to illustrate the effect of crustal warping on stream erosion. The
dotted lines represent the profiles of the streams before deformation; the full lines, after.
Erosion will be stimulated between a and b in each case, and between c and d in Fig. 162.
Below b, Fig. 161, the stream will be drowned, and erosion therefore stopped. Erosion
will also be stopped or retarded above a, between b and c, and below d in Fig. 162.

Sinking.—The land on which a river system is developed may be
depressed relative to sea-level. In this case the sea would occupy the lower
ends of valleys, converting them into bays and estuaries. A stream in this
condition is said to be drowned. Of drowned rivers there are many
examples along the Atlantic coast. Thus the St. Lawrence River is drowned
up to Montreal, and the Hudson up to Albany. If the drowned portion of the
latter valley were not so narrow, it would be a bay. Delaware and
Chesapeake Bays, as well as many smaller ones, both north and south, are
likewise the drowned ends of river valleys (see figures, Chapter VI). If all
parts of a drainage basin sank equally, the velocities of the streams above
the limit of drowning would not be changed, for the gradients would remain
the same as before. The fact that a river’s channel is below sea-level is not
to be taken as proof that the valley is drowned. Thus the bottom of the
channel of the Mississippi is as much as 100 feet below the level of the
Gulf, some 20 miles above New Orleans.[64]

Differential movement. Warping.—Where a land surface on which a
river system is established suffers warping, some parts going up and others
down, the opposite movements being either absolute or relative, various
phenomena would result. This may be illustrated by the accompanying
diagrams (Figs. 161 and 162), where the profiles of the streams are
represented as warped from the positions represented by the dotted lines, to
the positions shown by the full lines. The velocity will be accelerated below
the points of differential elevation (between a and b, Fig. 161, and between
a and b, and c and d, Fig. 162), but checked above (above a, and between b
and c, Fig. 162). Above an elevation which notably checks its flow, a stream
is ponded. If the ponding is slight, a marsh may develop above the
obstruction; if more considerable, a lake is formed. Lakes of this class are
likely to be short-lived, since the ponded waters are likely to soon overflow
and lower their outlet so as to drain the lake. The elevation which ponds the
stream may be great enough and rapid enough so that the resulting lake
finds an outlet by some course other than that originally followed by the
stream. Where a stream holds its course across an uplift athwart its valley,
either with or without ponding, it becomes an antecedent stream (see p.
169), since it has a course assumed before the latest deformation of the
crust and in apparent disregard of present surface configuration. Thus the
Columbia River holds its antecedent course across areas which have been
uplifted (differentially) hundreds and even thousands of feet.[65] Some of the
striking scenic features of this noble valley are the result of these changes in
the country through which it flows. A lesser stream would have been
diverted, as many of its tributaries have been. Even its course across the
Cascade ranges is believed to be antecedent.[66]

Fig. 163, 164.—Piracy stimulated by warping. Uplift along axis 1–2.

Another peculiarity of valleys and streams resulting from changes of
level is illustrated in Fig. 2, Pl. XIV (southern California). The main valleys
of this part of the coast were developed when the land stood considerably
higher than now. Later the subsidence of the coast converted the lower ends
of the valleys into bays or fiords. The bays were then transformed into
lagoons by deposition. Subsequent rise of the land or depression of the sea
allowed the drainage from the old lagoons to cut across the deposits which
had converted the bays into lagoons. The result is an old, wide valley above,
suggested by a young one below.

If the warpings were considerable, much more decisive changes in
drainage would result. Suppose the drainage of a given region to be
represented by the streams in Fig. 163. If there is uplift along the axis 1–2,
that part of ac above the axis of uplift would be ponded, or at least have its
velocity checked, while the flow of some of the tributaries of d would be
accelerated, and might work back and capture the other stream (Fig. 164).
Crustal warping was one of the conditions under which the Tennessee
achieved its present anomalous course, and its history[67] is illustrative of the
complex changes which drainage suffers when warping affects the area
where the rock structures are of unequal resistance. At the close of the
Cretaceous cycle of erosion, when the Appalachian Mountains had been
reduced to a peneplain, the waters falling in the area now drained by the
upper course of the Tennessee flowed south-south-west to the Gulf in a
stream (the Appalachian River, a, Fig. 165) the lower part of which had the
general position of the Coosa and the Alabama.
To the west of the Appalachian River, shorter streams flowed west and
southwest into the Mississippi embayment (Fig. 165) by courses which are
not now definitely known. The succeeding cycle of erosion was inaugurated
by uplift and deformation of the peneplain. The axis of greatest elevation
(AB, Fig. 166) was nearly parallel to the Appalachian River, and the effect
of the differential uplift was to impose a greater task on this river (a, Fig.
166), which flowed along the axis of uplift, than upon the rivers which
flowed westward and southwestward to the Mississippi embayment. The
result was that the strongest of the southwesterly flowing streams worked
its head back into the drainage basin of the Appalachian River, and
captured, one by one, the head-waters of its westerly tributaries,
establishing some such drainage relations as are shown in Fig. 166. Still
later, after the land area of the region had been considerably extended by
the withdrawal of the sea, the Appalachian River itself was reached by the
invading stream, and its waters carried away to the Mississippi Bay by a
course the lower part of which is thought to have corresponded
approximately with the course of the present Black River (b, Fig. 167).

Fig. 165.—Shows the general position of the main drainage lines in the southern
Appalachians at the close of the Cretaceous cycle of erosion. The lower part of stream b is
made to follow the course of a portion of the present Tennessee.

Fig. 166.—Shows the general position of the main drainage lines in the southern
Appalachians, after the capture of the westerly tributaries of the Appalachian River by
stream b. Compare Fig. 165.

Fig. 167.—A stage later than that shown in Fig. 166. The sea is represented as having
withdrawn from a considerable area which was submerged at earlier stages (Figs. 165,
166).

Fig. 168.—Shows the final change which resulted in the present course of the Tennessee.
The land is represented as somewhat higher than now.[68]

Still later there was further deformation which caused additional changes
in the drainage. The whole region was uplifted, relatively if not absolutely,
but the uplift was differential, being greatest along the axis represented by
AB, Fig. 167. The effect of the deformation was to stimulate the tributaries
of the Ohio flowing north from this axis. Their growth was further
accelerated by the weakness of the strata over which they ran. At the same
time, the uplift to the south led the southwesterly flowing stream (b, Fig.
167) to discover relatively hard beds of rock in its lower course, and these

beds retarded its down-cutting. The result was that a tributary of the Ohio
(a, Fig. 167) finally tapped the main stream flowing to the southwest (b,
Fig. 167) and carried its upper part over to the Ohio (Fig. 168). This was the
beginning of the present Tennessee.

THE AGGRADATIONAL WORK OF RUNNING WATER.

Principles involved.—Since deposition results from the failure of
transportation, the factors which control transportation also influence
deposition. Transportation by streams is determined largely by velocity, and
the most important factors influencing velocity are slope, volume, and load
(p. 115). Of these the first two are usually of greater importance than the
third.
A stream is said to be loaded when it has all the sediment it can carry; it
is loaded with fine material when it has all the fine material it can carry, and
with coarse material when it has all the coarse it can transport. A stream
loaded with coarse material flows more swiftly than one loaded with fine,
for a larger percentage of a stream’s energy can be utilized in carrying fine
material than coarse, and hence a larger percentage of the energy of a
stream which carries a load of the latter will express itself in velocity.
Deposition takes place whenever a stream finds itself with more load
than it can carry, and is an expression of the stream’s refusal to remain
overloaded. A stream may become overloaded in various ways. It might at
first seem unnecessary to inquire whether a stream may be overloaded at its
source, but the question is not necessarily to be answered in the negative.
The source of a stream is not always a definite point. In a general way it
may be said that the source of the normal stream is at that point in its valley
where the bottom is as low as the ground-water level of the region. But
since the ground-water level is not constant (p. 71) the source of a stream is
likely to be farther up its valley in a wet season than in a dry one (p. 72).
After a heavy shower, the run-off descends to the axis of the valley from the
slopes on all sides, and temporarily the stream begins above the point which
marks even its wet-season source. If under such circumstances the slopes
about the head of the valley are notably steeper than the slope of the valley
itself, as they frequently are, the water flowing down them may gather an
amount of material which it cannot carry after it reaches the bottom of the

valley. This may be the case at, or even above, the point which marks the
source of the permanent stream. It is, therefore, possible for a stream to be
overloaded at its source, if we take the source to be the point whence the
water permanently flows. Deposition may, therefore, be taking place in a
valley at the head of its permanent stream, or temporarily even in the valley
above it.
Streams issuing from glaciers sometimes have more load than they can
carry after they escape from the ice. If the stream be regarded as beginning
at the point where it issues from beneath the ice it may be overloaded at its
source.[69]
Under certain circumstances, a stream may overload itself. Thus if a
stream loaded with coarse detritus reaches a portion of its valley where fine
material is accessible in abundance, some of the velocity which is helping
to carry the coarse may be used in picking up and carrying the fine. This
reduces the velocity, and since the stream already had all the coarse material
it could carry, reduction of velocity must result in deposition. It follows that
when a stream fully loaded with coarse material picks up fine, it becomes
overloaded, so far as the coarse material is concerned.
Again, tributaries may overload their mains. While tributaries are usually
smaller than their mains, they frequently have higher gradients, and the
smaller stream of higher gradient may bring to the larger stream of lower
gradient more material than the latter can carry away. Thus deposition may
take place at the point of junction of tributaries with their mains. This may
go so far as to pond the latter enough to cause its expansion into a river-
lake. Lake Pepin, in the Mississippi River at the mouth of the Chippewa (in
Wis.), is an example.
Streams may become overloaded by losing velocity or volume, or both.
Decrease in velocity is brought about either by decrease in declivity or in
volume. In general, streams have lower gradients and greater volumes in
their lower courses than in their upper, and these two elements affect
velocity in different ways. If the increase in volume be not enough to
counterbalance the decrease in declivity, as is often the case, a stream which
is loaded in its upper course will deposit in its lower. The decrease of
velocity at the debouchure of a stream almost always leads to deposition.

Decrease in velocity as the result of decrease in volume is less common.
When decrease in volume occurs, it may be the result of (1) evaporation, (2)
the absorption of water into the bed of the stream, or (3) branching—the
giving off of distributaries. While evaporation is going on everywhere, the
diminution of a stream by this means is usually more than balanced by the
increase from tributaries, rainfall, and springs; but in arid regions a very
different condition of things sometimes exists. If mountains in an arid
region be capped with snow, its melting supplies the streams during the
melting season. As the streams flow out from the mountains through dry
regions, they receive little or no increment from rainfall, tributaries, or
springs, and evaporation reduces the volume of water, or even dissipates it
altogether. Absorption of water into the bed of the stream often
accompanies evaporation. Reduction of volume by evaporation and by
absorption is especially common in arid regions. Wherever loaded streams
are reduced in volume, whether by evaporation or absorption, deposition
takes place.
The third way by which velocity is decreased as the result of decreasing
volume is illustrated at the debouchures of many streams. Near the Gulf, for
example, the Mississippi branches repeatedly (see Fig. 190). The same
phenomena are often seen where one stream joins another (Fig. 169).
Individually the distributaries are much smaller than the main stream before
they separated from it, and because they are smaller their combined
surfaces are greater, and the amount of energy consumed in the friction of
flow is increased. The velocity of the water and its carrying power are,
therefore, reduced. Thus the branching of streams gives rise to deposition,
and where deposition takes place the gradient of the stream is reduced, and
this occasions still further deposition. The sediment which fills up the
channel and checks the flow finally compels the stream, or some part of it,
to transgress its banks. Deposition, therefore, favors the development of
distributaries, and the development of distributaries in turn favors
deposition.

Fig. 169.—Delta of the Chelan River at its junction with the Columbia. Shows the tendency
of streams to distribute where active deposition is in progress. (Willis, U. S. Geol. Surv.

The foregoing statements make it clear that a stream may be eroding in
one part of its valley while it is depositing in another, and that erosion may
alternate with deposition in the same place, on account of fluctuations in
volume, and, therefore, in velocity of the stream. It will be seen in the
sequel that erosion and deposition may be taking place at the same time in
the same part of the valley. The activities of a river are so nicely balanced
that slight disturbance at one point causes disturbance at all points below.

The deposits.
Types.—Turning from the principles which underlie river deposition to
the deposits themselves, they are found to occur in various situations.
Running water usually descends from steeper slopes above to gentler slopes
below, and ends at the sea, or in a lake or inland basin. Wherever there is a
sudden decrease in its gradient, as at the base of a hill, ridge, or mountain,
running water is likely to leave a large part of its load, building an alluvial
fan or cone (Figs. 67, 68, and Pl. VI). Even where there is no sudden
decrease in the gradient of a stream, there is likely to be a gradual one, and
in spite of the fact that the increased volume of a stream in its lower course
tends to overcome the effect of diminished gradient on velocity, deposition
is likely to take place as the gradient is reduced. Deposits occasioned by the
gradual reduction of a stream’s velocity often have great extent in the

direction of a stream’s flow. They cover the flood plains of streams, making
them alluvial plains (Fig. 73). When a stream reaches the sea or a lake its
current is destroyed and its load dropped, unless taken in charge by the
waves and currents of the standing water. Sediment accumulated in quantity
at the debouchures of streams gives rise to deltas (Figs. 169, 187). Alluvial
cones and fans, alluvial plains, and deltas, are the principal types of river
deposits. Apart from these well-defined types there are bars in the channels
of depositing streams, and much ill-defined alluvium which does not allow
of ready classification.
Alluvial fans and cones.—The only distinction between the alluvial fan
and the alluvial cone is one of slope, the cones (they are but half-cones at
best) being steeper than the fans. Alluvial fans and cones have their most
striking development where temporary torrents, occasioned by showers or
the rapid melting of snow, issue from mountain ravines. Such streams
usually carry heavy loads of detritus, the coarser part of which is likely to
be deposited at the base of the mountain slope. Cones and fans built by such
streams have a periodic rather than a steady growth.
At the beginning of its development the material of the alluvial cone is
deposited much as in a talus cone (compare Fig. 170 with Figs. 67 and 68).
Its deposition chokes the channel of the stream, and some of the water then
seeks new courses to right and left of the apex of the deposit. This expands
the area of deposition to right and left, while the water which flows over it
lengthens it in the direction of flow.
The course and behavior of the water after reaching an alluvial cone is
instructive. As its velocity is checked, deposition often takes place in the
channel, diminishing its capacity. As the channel is filled up, the water
tends to overflow on either side. The overflowing water, being shallow, has
so little velocity that much of its load is dropped on either margin of the
channel, building up levees. The water ever and anon breaks through the
levees, giving rise to distributary streams, each of which aggrades its
channel and builds its own miniature levees (Fig. 171). Not rarely this
process of channel-filling and levee-building goes on until the channels of
the little rivulets are above the general level of the cone on which they rest.
The rivulet then runs in a groove on the crest of a little ridge. The channels
on the surfaces of fans and cones are fewest and deepest at their heads, and
more numerous and shallower below. In some cases the surface-water

disappears altogether before the outer border of the fan is reached, by
sinking into the débris.

Fig. 170.—A talus cone. North Greenland Coast. The talus cone reaches the sea-level.
Drawn from photograph.

Alluvial fans and cones have various forms, and often attain considerable
dimensions. Their angles of slope depend on the amount of reduction of
velocity which the depositing water suffers, and the amount and kind of
load which it carries. The maximum slope of the cone is the angle at which
the loose material involved will lie. The minimum slope of the fan, on the
other hand, approaches horizontality. If many alluvial fans develop in
proximity to one another, as at the base of a mountain range, they may
expand laterally until they merge. A long succession of them may thus give
rise to an extensive alluvial piedmont plain, or a compound alluvial fan.
The lower edge of such a fan is often somewhat lobate. Such plains exist
along the bases of many mountain ranges (Pl. VI), and may be seen in
miniature even along low ridges.

Fig. 171.—Miniature levees on an alluvial cone. Slope of Gray Peak, Colo. (R. T.
Chamberlin.)

A permanent stream, as well as a temporary one, may develop an alluvial
fan at the base of a mountain slope; but since the mountain course of the
former is likely to be less steep than that of the latter, its waters suffer a
correspondingly less reduction of velocity at any one point. The fan of the
permanent stream is therefore likely to be relatively flat, and to stretch far
down the valley. Such fans grade into valley plains. From the general
principles already discussed, it is clear that well-developed fans go with
relatively youthful stages of erosion, and belong normally to the upper parts
of drainage lines.
Ill-defined alluvium.—There is a widespread mantle of alluvial material
deposited by running water which was not organized into distinct streams.
The water which runs down smooth slopes in sheets during showers carries
fine earthy matter, as well as some that is coarser. These materials are
largely deposited at the bases of the slopes, forming basal accumulations of
greater or less extent, comparable in origin to alluvial fans. A relatively
small amount of the slope wash is carried far out from the base of the
declivities. It is not easy to realize the extent to which this process is taking
place. There is hardly a slope without loose material, and there is hardly an
acre of low land below a slope on which running water has not deposited

sediment washed down from above. When it is remembered that this is as
true of gentle slopes and their surroundings as of steep slopes, though
perhaps not to the same extent, and that a very large part of the earth’s
surface is made up of sensible slopes, or of flats at their bases, some idea of
the aggregate effect may be gained.
There is another way of looking at the same question. Earthy matter is
being continually transferred from land to sea, and chiefly from high land.
Rarely does it start from any point distant from the shore and move
uninterruptedly to it. It is transported a short distance and lodged, to be
again picked up, carried forward another step in its journey, and lodged
again. For a very large part of the earth’s surface it would be true to say that
its mantle rock is material in transit from higher land to the sea.
Alluvial plains.—Most streams, whether heading in mountains or not,
have gentler gradients in their lower courses than in their upper, and in spite
of increasing volume are usually unable to carry to their debouchures all the
material gathered above. The excess of load is dropped chiefly on the flood-
plains of the streams and constitutes them alluvial plains.
The making of an alluvial plain usually involves both erosion and
deposition. When a stream has cut its channel to grade, downward erosion
ceases, or more exactly, downward cutting is, on the average,
counterbalanced by deposition. So long as a stream is cutting downward
rapidly, it carries away whatever débris descends the side slopes. When it
approaches grade, the débris which descends the side slopes tends to
accumulate at their bases, and the V-shaped cross-section of the valley
becomes U-shaped (see Fig. 172). At about the same time the stream begins
to meander, for, having lost something of its former velocity, it is more
easily turned from side to side. As it begins to meander, it widens the
bottom of its valley. This is the initial stage in the development of the valley
flat (2 and 3, Fig. 172). In its meandering the stream encroaches on the talus
accumulations at the bases of its valley’s slopes. The side-cutting may
remove all the loose débris and even undercut the bluff as at a, Fig. 173.
The stream’s meanders shift their positions from time to time so that the
valley flat is successively widened at different points. By lateral planation,
therefore, a stream tends to develop a flat as soon as it reaches grade. This
is the initial part of erosion in the making of a river flat, but a flat developed
by erosion alone is not an alluvial plain.

So soon as the flat developed by a stream exceeds the width of its
channel, the water (except in times of flood) does not cover it all at the
same time. On any part which it temporarily abandons, some débris
(alluvium) is likely to be left. This deposit of alluvium constitutes the valley
flat an alluvial plain (Fig. 174). It will be seen that the valley flat is
commonly an alluvial plain from the beginning.

Fig. 172.—Diagram illustrating the transformation of a V-shaped valley into a U-shaped
valley.

Fig. 173.—Diagram to illustrate the widening of a valley flat by erosion. Compare 3, Fig.
172.

Once the valley flat and alluvial plain are begun, their further
development is easily followed. The stream in flood overflows the banks of
its channel. The velocity of the overflowing water is reduced, and if it has
much load a part of it will be dropped and the plain aggraded. Meantime
meandering and lateral planation continue. Thus the flood-plain is widened
by erosion, and aggraded by alluviation, the two processes going on
simultaneously.

Fig. 174.—An alluvial plain. The diagram suggests the relative importance of lateral
planation and alluviation in the development of the flat.

Flood-plains, chiefly the result of planation, but partly of aggradation, are
a normal feature of river valleys, after a certain stage of development has
been reached. This stage is that at which downward erosion becomes slight
in comparison with lateral erosion. It follows that an alluvial plain normally
begins its development where the valley is first brought to grade, that is, in

its lower course. As the development of the valley goes on, the head of the
flood-plain advances up-stream, and at the same time its older parts become
wider.
Flood-plains due to alluviation
only.—Exceptionally, an alluvial plain
is developed by deposition only. Thus if
a stream becomes overloaded while its
valley is still narrow, as sometimes
happens, deposition follows, and, as
aggradation proceeds, the narrow valley Fig. 175.—Diagrammatic representation
acquires a progressively wider bottom of a flood plain developed by
(Fig. 175). Wide valley plains are alluviation only.
sometimes developed in this way. Flood plains developed wholly by
alluviation are sometimes formed under conditions which are independent
of the stage of a valley’s development. Thus if a stream suddenly acquires
an exceptional supply of detritus in its upper course, the development of an
alluvial plain begins immediately below the point of overloading.
The overload might be acquired in various ways. (1) If a stream taps
another (piracy) which carries a large quantity of sediment, carrying off
both water and sediment to a channel with a lower gradient, deposition may
take place where, under the earlier conditions, there was none. (2) Again,
when a stream cuts through a barrier near its head waters, its velocity, and,
therefore, its eroding power, may be so increased in its upper course that
sediment enough is acquired to occasion deposition below, where none took
place before. (3) In working back through formations of varying degrees of
resistance, a stream’s head may presently reach a formation or a region
which yields abundant sediment, even though there was no especial barrier
below. (4) If an advancing glacier should reach the head waters of a stream,
its discharge to the stream would greatly increase the load of the latter, and,
although its volume would be augmented at the same time, deposition
might result. As a matter of fact, streams carrying glacial drainage are
usually aggrading streams. In general, anything which greatly increases the
load of a stream near its head is likely to cause deposition, and so the
development of a flood plain, at some point farther down the valley.

Fig. 176.—Anastomosing of a depositing stream. Yahtse River, Alaska. (Russell, U. S. Geol.
Surv.)

Streams which are actively aggrading their valleys are likely to
anastomose (Figs. 176, 177). This results from the filling of the channels
until they are too small to accommodate all the water. The latter then breaks
out of the channel at few or many points. The new channels thus established
suffer the same fate.

Fig. 177.—Anastomosing of the Platte River, Dawson Co., Neb. (U. S. Geol. Surv.)

Flood-plains due to obstructions.—Again, any obstacle in a stream’s
course is likely to cause deposition above. Thus dams built across rivers
entail the deposition of sediment above. Where a stream flows over the
outcropping edges of strata of different strength, the more resistant serve, in
some sense, as dams. Above them the stream cuts its bed to a low gradient,
and, becoming sluggish, drops more or less of the detritus brought down
from above. Obstacles of any sort across a stream’s channel, therefore,
favor the development of alluvial plains.

Fig. 178.—The levees of the Mississippi in cross-section, 4 miles north of Donaldsonville,
⨉
La. Vertical scale 50. The horizontal line in the diagram represents sea-level. The
bottom of the channel at this point is far below sea-level.

Levees.—As the stream in flood escapes its channel and overspreads its
plain, its immediate banks are the site of active deposition, for it is here that
the velocity of the overflowing water is first notably checked. On the banks
of the channel, therefore, low alluvial ridges, called natural levees, are built
up (Fig. 178, and Pl. XV). They may be narrow, or hundreds of feet in
width, and are often several feet above the plains behind them, giving the
latter a slope away from the channel of the stream. They are sometimes high
enough to control the courses of tributary streams, as shown by numerous
tributaries to the Mississippi below the Ohio. The Yazoo, for example,
flows some 200 miles on the flood-plain of the Mississippi before it joins
that river near Vicksburg. The levees even become divides, directing
drainage away from the streams they guard (Pl. XV). Streams sometimes
build levees faster than their tributaries aggrade their channels. The latter
are then ponded, giving rise to lakes. The lakes on the lower courses of the
tributaries to the Red River of Louisiana are examples.[70] They are
sometimes built up above their natural level and kept in repair by human
agency so as to confine the streams in time of flood. This is a source of
danger unless they be steadily maintained, for the breaking of such levees
often occasions great destruction. A case in point is the breaking of the
levees of the Mississippi near New Orleans in 1890. The water broke
through the levees at the Nita and Martinez crevasses (Fig. 187) and flowed
eastward (from the former) with a current of 15 miles per hour, spreading
destruction in its path. The water flowed eastward through Lakes
Pontchartrain and Borgne, and entered Mobile Bay with such volume,

velocity, and load of mud, as to destroy for a time the oyster and fish
industries of that locality.[71]
PLATE XV.

U. S. Geol. Surv. NEAR HAHNVILLE, Scale, 1+ mile per inch.
LOUISIANA.

PLATE XVI.

U. S. Geol. Surv. Fig. 1. Scale, 2+ miles per inch.
MISSOURI U. S. Geol. Surv. Scale, 2 miles per inch.
. Fig. 2. MISSOURI.

U. S. Geol. Surv. Fig. 3. MISSOURI. Scale, 2+ miles per inch.

Fig. 179.—Flood-plain of the Mississippi River south of the mouth of the Ohio. (From
charts of the Miss. Riv. Commission.)

Flood-plain meanders. Cut-and-fill.—A stream with an alluvial plain is
likely to meander widely (Pl. XVI). In general terms this may be said to be
the result of low velocity, which allows it to be easily turned aside. Were the
course of such a stream made straight, it would soon become crooked again.
The manner of change is illustrated by Figs. 180 and 181. If the banks be
less resistant at some points than at others, as is always the case, the stream
will cut in at those points. If the configuration of the channel is such as to
direct a current against a given point, a (Fig. 180), the result is the same,
even without inequality of material. Once a curve in the bank is started, it is

increased by the current which is directed
into it. Furthermore, as the current issues
from the curve, it impinges against the
opposite bank and develops a curve at that
point. The water issuing from this curve
develops another, and so on.
Once started, the curves or meanders tend
to become more and more pronounced
(compare Figs. 180 and 181). In the case
represented by Fig. 1, Plate XVI (Missouri
River near Brunswick, Mo.) the narrow neck
of land between curves is almost cut through.
When this is accomplished, the stream will
abandon its wide curve. A later stage in the
process is shown in Fig. 2, Plate XVI (the
Osage River near Schell, Mo.).
The straightening of the channel is often
accomplished in another way. Even before
the meanders reach the stage represented by
Fig. 1, Plate XVI, the position of the channel
becomes unstable. In time of flood, the
whole flat is covered with flowing water.
Fig. 180.—Diagram illustrating an
early stage in the development The greater depth of water in the channel
of meanders. The shaded part tends to give it a velocity greater than that of
represents the area over which the water on the flat outside. But the distance
the stream has worked.
from a to c via b (Fig. 181) is much greater
than that in a direct line. It follows that the slope from a to c direct is
greater than that by way of b. If the current between a and c in time of flood
be strong enough to erode, it may deepen its bed, and thereby increase the
volume of water following this course. The increased volume gives
increased velocity, and the result may be the opening of a channel between
a and c direct. The channel may be worn so deep that when the flood
subsides, the stream will follow it. So long as the abandoned channel-curve
remains unfilled with sediment, it is often called a cut-off. If it contains
standing water and has the proper form, it is called an ox-bow lake (Fig.
182), or sometimes a bayou. The water-filled portions are not always bows

(Fig. 183, Osage River, near Butler, Mo.). Cut-offs, with or without
standing water, are of common occurrence along most rivers with wide
plains. Meandering is not confined to streams which are near sea-level.
Even small creeks at high altitudes may meander, if so situated as to have
slight velocity. Trout Creek in the Yellowstone Park (Fig. 184) is an
example.

Fig. 181.—Diagram illustrating later stages in the development of meanders.

Fig. 182.—Meanders and cut-offs (ox-bow lakes) in the Mississippi Valley a little below
Vicksburg. The figure also shows the migration of meanders down-stream, and their
tendency to increase themselves. (From charts Nos. 18 and 19, Mississippi River
Commission.)

There seems to be some relation between the width of the belt within
which a stream meanders, and the width of the stream itself. Recently it has
been estimated that the ratio between them is 18:1.[72]
During the development of the meanders it is to be noted that lateral
planation on the one side of a stream is accompanied by deposition on the
other. This is cut-and-fill. The sediment eroded from the curve which is
concave toward the stream is shifted down-stream, while that deposited in

the curve which is convex toward the
stream is brought down from above.
Thus even in the development of
meanders, the material which is
dislodged is shifted down-stream.
Since the current directed against the
down-stream side of a growing
meander is on the average stronger
than that directed against the opposite
side, the meander itself has a tendency
to migrate down-stream (Fig. 182).
In their evolution, the curves of a
stream’s channel often reach and
undermine the valley bluff (Pl. VII).
Since the meanders are, on the
average, shifted down-stream
individually, and since meanders are
frequently developed in new places, it Fig. 183.—Bayou Lakes, Osage River, near
Butler, Mo.
follows that a meandering stream
tends to widen its valley throughout. Widening is also effected in other
ways, for a stream with a flood-plain sometimes abandons its channel
altogether for miles at a stretch, and the new course chosen may be against
one of the bluffs of the valley. Such changes are most likely to take place
where deposition along channel and levees has brought the part of the
flood-plain (though not necessarily the bottom of the channel) adjacent to
the stream above the level of that farther from it (Fig. 178). The change is
likely to be effected in time of flood.
Flood-plains often attain great size. That of the Mississippi below the
Ohio (Fig. 179) has a width ranging from rather more than 20 miles at
Helena (Ark.), to something like 80 miles in the latitude of Greenville
(Miss.).[73] Below the Ohio its area is something like 30,000 square miles,
and its entire area has been estimated at about 50,000 square miles.[74]
Theoretically, the rotation of the earth should affect the erosion of
streams, increasing it on the one bank (the right in the northern hemisphere
and the left in the southern) and decreasing it on the other.[75] The streams
doubtless accommodate themselves to the rotation of the earth in the

original development of their gradation-plains and flood-plains, and the
later effects of rotation are usually inconspicuous.

Fig. 184.—Meanders of Trout Creek, Yellowstone Park. (Walcott, U. S. Geol. Surv.)

Scour-and-fill.—It has already been shown that aggrading streams cut
laterally at the same time that they build up their plains. It is now to be
added that they periodically deepen their channels to a notable extent, and
that the deepening of the channel takes place at the very time when the
flood-plain is being aggraded. In other words, the stream in flood aggrades
its plain, and degrades its channel. This follows from the fact that the
current is sluggish in the former position, where the water is shallow, and
rapid in the latter, where it is deep. When the flood subsides, the channel,
deepened while the current was torrential, is filled again by the feebler
current which follows. This alternate deepening and filling is known as
scour-and-fill. It is well illustrated by the Missouri River. At Nebraska City,
scour is believed to occasionally reach depths of 70 to 90 feet.[76] At Blair,
about 25 miles above Omaha, the same river is believed to cut to bed-rock
(about 40 feet below the bottom of the channel in low water) twice a year,
that is, during floods.[77] Fig. 185 shows the changes recorded in the channel
of the river at this point during the year 1883. It shows that the scour-and-

fill during this year amounted to almost 40 feet. All streams similarly
situated do a like work. The material thus eroded is shifted down-stream,
some of it for short distances only, and some of it to the sea. Even an
aggrading stream therefore is not without erosive activity; it is a stream
whose fill exceeds its scour, not one which has ceased to erode.

Fig. 185.—Diagram illustrating scour and fill in the Missouri River. A record of soundings
at Blair Bridge (near Omaha), 1883. Shows also the cross-sections of the river at various
rates. (Todd, Bull. 158, U. S. Geol. Surv.)

Materials of the flood-plain.—As a result of its varying velocities in
flood and low water, a stream may deposit coarse material at one time and
fine at another. A similar sequence of deposits takes place in the flood-plain
of a meandering stream, irrespective of floods. Flood-plain deposits are
often therefore very heterogeneous, as shown in Fig. 186, which represents
the constitution of the alluvium of the Missouri River at Omaha. The
deposits of the streams range from the finest clay, through sand to gravel,
and even bowlders. In general they become finer down-stream. In a given
plain, they are usually coarser below and finer above.

Fig. 186.—Diagram to show the heterogeneous character of alluvial deposits. (Todd, Bull.
158, U. S. Geol. Surv.)

Topography of the flood-plain.—The flood-plain is nearly, but not
altogether, flat. It has a gentle slope down-stream, and often for a distance
from the sides toward the center (Fig. 174). This latter slope is the result of
deposition by waters descending to the plain from the sides. It is destroyed
wherever a meandering stream reaches its bluffs. When levees are well
developed, there is a slope from them toward the sides of the valley (Fig.
178), but it rarely continues to the limiting bluffs. Since a stream with a
well-developed flat frequently shifts its course, old levees and abandoned
channels lend variety to the topography of the flood-plain.
The topographic adjustment of tributaries.[78]—The meandering and
shifting of a main stream affects its tributaries. If a main stream swings
against the bluff through which a tributary enters, the latter brings its
channel into topographic adjustment by lowering its end to the level of the
main. If now the main stream opposite the tributary swings to the other side
of its valley, the tributary must make its way across the flat with a very low
gradient. Not only this, but the flat of the main valley through which the
tributary must flow is likely to be aggraded by the main in time of flood.
The result is that the tributary stream becomes an aggrading stream at its
debouchure, and topographic adjustment is not established until it has filled
up the lower end of its valley to some notable extent. The filling of the
lower end of the tributary likewise affects the lower ends of its lower
tributaries.

Fig. 187.—A general view of the Mississippi delta.

If the main stream again swings over to the point where the tributary
issues from its valley, the tributary stream and all its affected tributaries
again become eroding streams. Thus scour-and-fill are not confined to the
valley of the main stream.
River-lakes.—While rivers are in general hostile to lakes, they
sometimes give origin to them. Oxbow lakes (Fig. 182 and Pl. XVI), due to
the cut-offs of meandering streams, have already been referred to. Lakes
formed in the same way have other forms (Pl. XVI and Fig. 183). Rivers
also give rise to lakes through the deposits they make. If a main stream
obstructs its tributaries by deposition at their debouchures, their lower
courses are ponded and converted into lakes. The lakes along the tributaries
to the Red River of Louisiana have already been cited as examples. If a
tributary brings more load to its main than the latter can carry away, the
detritus constitutes a partial dam, ponding the river and causing it to expand
into a lake above. Such is the origin of Lake Pepin already referred to. In
mountain regions, the alluvial cones of tributary valleys sometimes pond
their mains.

Fig. 188.—Longitudinal section of an incipient delta made of coarse material.

Rivers may be dammed in other ways, as by lava flows, by landslides, by
glacial drift, etc. In all such cases, lakes may come into existence, but they
are not due primarily to the activity of the river itself.
Deltas.[79]—Where a stream enters standing water, or a slower stream, a
special form of plain, the delta, is sometimes built up (Figs. 169, 187, and
188). Deltas and alluvial fans have much in common, and their only notable
differences are those imposed by the differences in the conditions of
deposition. The current of the stream is checked, but not altogether stopped,
at its immediate debouchure. If it carries abundant sediment, much of it will
be promptly dropped where the decrease in velocity is first felt. Such flow
as there is beyond the debouchure is not confined to a definite channel, and
the deposits made are therefore spread more or less on either side of the line
which represents the continuation of the stream’s course.
As the depth of the water into which the stream flows increases, the
current diminishes. Out to the point where the depth of the standing water is
less than the depth of the current, the latter affects the bottom, and the
surface of the deposits made slopes gently seaward; but where the depth of
the standing water is such that the projected stream current is ineffective at
the bottom, all the load rolled along the bottom is dropped, and a
depositional slope is established (Fig. 188), its upper edge being below sea-
level by an amount corresponding roughly to the depth of the current which
brings the detritus. The outer slope is relatively steep and well-defined
where the detritus is coarse, and relatively gentle and ill-defined where it is
fine. Thus the stream tends to construct a sort of platform in the water just
beyond its debouchure. The successive deposits on the outer abrupt slope
will dip conformably with its surface (Fig. 189). The finest sediment will be
carried beyond the steep slope, and conform to the topography of the
bottom beyond (c, Fig. 189).

At the beginning, the top of the delta platform is at the level of the
bottom of the stream’s channel at the point of debouchure, but it is
gradually aggraded as water continues to flow over it. Its landward margin
is presently built up to sea-level and then above it, and as the delta grows
the delta-land is extended seaward (compare Figs. 188 and 189). At the
same time the channel of the stream above the original head of the delta is
aggraded, for the current there is checked by the aggradation of the delta.
Thus alluvial deposits continuous with the delta are extended landward.

Fig. 189.—Longitudinal section of the delta at a later stage of development.

The projection of the direction of the lower end of the stream may be said
to be the axis of the extra-debouchure current. From this axis, where the
flow is strongest, the movement diverges more or less to right and left.
Since the velocity of the diverging water is reduced more rapidly than that
of the water which follows the axis of flow, deposition is likely to take
place faster on either side of the axis than along the axis itself. The result is
that the extra-debouchure current tends to build up levee-like ridges on
either side, making a sort of sluice-way for itself. This sluice-way is
gradually extended seaward, and at the same time gradually filled. As its
capacity is reduced, more and more of the water flows over its sides.
Presently the escape of the water over the little side-levees will develop a
break at some point, and a line of distinct flow then diverges from the main
current. This distributary repeats the history of its main. Thus the processes
of levee-building, channel-filling, and levee-breaking follow one another,
until some such system of currents as shown in Fig. 190 is developed. The
result is that a delta’s growth is not simply in the line of extension of the
main stream, but in a more or less semicircular area, the center of the circle
being a point slightly below the position of the debouchure of the stream
when the delta began. At any stage in its development the margin of the
delta is more or less crenate (Fig. 191), or characterized by delta fingers
(Fig. 190), the projections corresponding to the positions of the
debouchures of the latest streams flowing across it. The extreme ends of the

delta lobes (Fig. 190), and of groups of the delta fingers, often have
something of the shape of the Greek letter from which the name originated,
but the resemblance in form between a well-developed delta and the Greek
letter is not striking. Deltas are sometimes built in bays, and in such cases
their forms are predetermined on all sides but one. The head of a delta is
sometimes arbitrarily located at the point where the first distributaries are
given off. Since this point shifts widely with time, the definition can hardly
be accepted. On this basis the head of the Mississippi delta is about 200
miles above its lower end. In reality it is much farther north.

Fig. 190.—The terminus of the delta of the Mississippi. (C. and G. Survey.)

Fig. 191.—A miniature delta.

The structure of a delta, shown in Fig. 189, shows its history. At any
stage in its growth the river discharges its sediment across that part of the
platform already built. The sediment rolled at the bottom of the current is
dumped on reaching the steep slope, and constitutes the inclined fore-set
beds shown in Fig. 189. The material in suspension is carried farther, settles
more gradually, and constitutes the bottom-set beds (c, Fig. 189). In time
the bottom-set beds, originally deposited some distance beyond the
debouchure, may come to be overlain by the fore-set beds, deposited at a
later time. While the fore-set beds are being deposited on the steep slopes of
the delta, and the bottom-set beds beyond, deposition is also taking place on
the top of the delta. These top-set beds are laid down in a nearly horizontal
position, and their seaward margin is gradually extended. Thus the delta
comes to have the threefold structure shown in Fig. 189.
That part of the delta which is above the abrupt slope of its front
corresponds in all essentials to an alluvial fan; but the delta as a whole
differs from the fan in its abrupt and crenate or digitate margin.
It is to be noted that the delta is not wholly the product of a stream’s
activity. The stream supplies the material, but the lake or sea renders at least
passive assistance in its disposition. Not all rivers opening into the sea build
deltas, and their failure is often the result of waves or shore currents which
carry off the river sediment. Deltas are, however, sometimes formed in tidal
seas, as at the debouchures of the Yukon; the Mackenzie, where the tidal

range is three feet; the Niger, where the range is four feet; the Hoang-Ho,
where the range is eight feet; and the Brahmaputra and Ganges, where the
range is sixteen feet.[80] Since lakes, bays, gulfs, and inland seas have
weaker waves and currents than the open sea, they are more favorable than
the latter for the growth of deltas. Hence occur such deltas as those of the
Mississippi, the Nile, the Po, and the Danube.
Deltas are likely to be absent, or confined to the heads of bays, on coasts
which have recently sunk. Their general absence on the Atlantic coast of the
United States is a case in point.
The following figures give some idea of the extent of deltas, and of their
importance in land building. The Mississippi delta is advancing into the
Gulf at the rate of about 100 yards per year, or a mile in 16 or 17 years. Its
length is more than 200 miles, its area more than 12,000 square miles, and
its depth at New Orleans has been estimated at 700[81] to 1000[82] feet. This
great depth is believed to be the result of subsidence, and so of the
superposition of one delta on another.[83] The delta of the Yukon has a sea
margin of 70 miles, and extends more than 100 miles inland. The delta of
the Rhône has also had a remarkable growth, considering the size and the
history of the stream. Arles, near the debouchure of the stream, was 14 to 16
miles inland in the fourth century b.c., and is now 30 miles inland.[84] The
Rhône has also built a great delta in Lake Geneva, and its lower delta is
built of sediment gathered below the lake. The Po has built a delta 14 miles
beyond Adria, the port which gave its name to the Adriatic Sea. The
extension of this delta has been at the average rate of about 50 feet per year,
but recently, on account of artificial embankments, the rate has been much
more rapid.[85] The Ganges and Brahmaputra together have made a delta of
great size. Its area is sometimes estimated to be as high as 50,000 or 60,000
square miles, and its head is more than 200 miles from the sea.[86] The head
of the Nile delta is 90 miles from the sea, and it has a coastal border of 180
miles. The head of the delta of the Hoang-Ho is about 300 miles from the
coast, and its seaward border has a length of about 400 miles, though with
some highland interruptions.[87]
After a delta has been built into a lake, the lake may disappear, leaving
the delta out of water. Such “fossil” deltas, if so recently exposed that
erosion has not destroyed their distinctive features, are readily recognized
by their flat tops, their abrupt and lobate fronts, and their characteristic

structure. They are often a means of determining the former existence of
extinct lakes,[88] or the former higher levels of lakes which still exist.[89]
Elevated deltas on seashores show either a rise of the land or a depression
of the sea-level.
The material which is carried along the coasts or shores from the mouths
of rivers may take on various and peculiar forms, according to the strength,
direction, and relations of waves and currents. The consideration of these
forms belongs more properly to the work of the sea than to that of rivers,
since rivers are not concerned in their construction except in supplying
material.
Delta lakes.—Delta-building streams sometimes help to form lakes by
throwing their deposits around an area which fails to be aggraded to sea-
level. Lake Pontchartrain, and other lakes in the delta of the Mississippi are
examples (Fig. 187).

Fig. 192.—Terraces of the Frazier River at Lillooet, B. C. (Calvin.)

STREAM TERRACES.
Stream terraces[90] are bench-like flats or narrow plains along the sides of
valleys (Fig. 192). They are usually narrow, but sometimes have great

length in the direction of the axis of the valley. They originate in various
ways.
Due to inequalities of hardness.—Reference has already been made (p.
140) to the effect of hard horizontal layers in the development of terraces
and terraciform projections on the sides of valleys (Fig. 120). Such terraces
are the result of differential degradation, and the upper surface of the hard
layer marks the lower limit of the terrace, which commonly has a distinct
slope toward the stream. Except where interrupted by tributary valleys, such
terraces are likely to be continuous in a valley so long as the structure
remains the same and the stream sustains the same relation to it. Such
terraces would first show themselves in the older part of the valley. The
effect of inclination of the hard stratum on the development of such terraces
will be readily inferred. Terraces and benches of this sort are not equally
distinct at all stages of a valley’s history. For great distinctness, the hard
layer should have been exposed long enough to allow the general processes
of erosion to have effected considerable differential wear, but not long
enough to allow the topographic effects of unequal resistance to be
obliterated.

Fig. 193.—Diagram illustrating a distinct terrace and a “second bottom (b),” which may be
regarded as a low terrace.

Normal flood-plain terraces.—It has been seen that deposition in a river
valley stands in more or less definite relationship to the stage of its
development, and that the deposition which leads to the development of an
alluvial plain is likely to take place where the higher gradient of the upper
course gives place to the gentler gradient of the lower. It has also been seen
that as a stream’s history advances, the stretch where the gradient is high
recedes up-stream, and that the point which marks the head of active
deposition follows. It follows that a river flat or flood-plain normally begins
in the lower part of a valley, and works progressively headward, its upper
end following, at some considerable distance, the head of the valley itself.
The commoner river terraces are remnants of former flood-plains, below
which the streams which made them have cut their channels. It has already

been pointed out (p. 184) that processes of erosion and deposition work
together in the development of flood-plains, and that some flood-plains
have but little alluvium (Fig. 174), while others owe their origin wholly to
stream deposits (Fig. 175). It follows that terraces developed from flood-
plains may be of rock, of alluvium, or of rock covered with alluvium.
The amount which a river channel must be deepened in order to change
the remnants of its flood-plain to terraces cannot be definitely stated. When
a channel is so deep that the remnants of a former flood-plain are no longer
flooded, they would be called terraces, especially if a lower flood-plain has
been developed. Even though not above the reach of floods, they are often
called terraces if they are notably above the channel and separated from it
by a lower plain. Thus the flat at b, Fig. 193, would be called a terrace, even
though covered by water in exceptional floods; but the flat at c, but slightly
above the channel, would hardly be called a terrace.

Fig. 194.—Diagram illustrating the beginning of the development of a terrace from a flood-
plain.

The question now arises why a stream, having once developed a flood-
plain, should sink its channel to a lower level, leaving parts of the old flood-
plain as terraces. This may be brought about by the operation of various
causes.
(1) In the first place, the head of the valley-plain where the first notable
deposition takes place normally advances up-stream. After the advance has
been considerable, the descending stream may, on reaching the head of its
valley-plain, lose so much of its load as to be able to sink its channel into
the flood-plain farther down the valley (Fig. 194).

(2) Ordinarily a stream does not drop all its load at the head of its plain,
but only its excess; but it will always drop coarse sediment to take fine, if
fine be available. For a relatively small amount of coarse material dropped,
a relatively large amount of fine may be taken up (p. 179). Other things
being equal, it follows that when a stream drops coarse material to take fine,
its channel is degraded unless there is at the same time a great reduction in
the stream’s energy. Such reduction is likely to go with the decreasing
declivity down-stream; but this is partly, or sometimes wholly,
counterbalanced by the increasing volume of water. By the exchange of
load, therefore, a stream may ultimately sink its channel below the flood-
plain which the earlier and perhaps smaller stream had developed.

Fig. 195.—Diagram to illustrate the development of river terraces by the widening of a
channel or meander belt. The valley flat above might not be called a terrace; but the
same plain below, where the meander belt has some width, would be called a terrace.

(3) Again, so long as a stream is actively eroding at its head, there is
likely to be some aggradation below. At a later stage in the stream’s history,
when active erosion at the head has ceased because of the reduction of the
surface, less material will be carried from the upper part of the valley, and
the stream on the flood-plain below, formerly loaded with material from up
the valley, is now free to take up and carry away material temporarily left
on the flood-plain. The result is a deepening of the channel.
(4) Any stream which has reached the flood-plain stage is likely to
meander. After the flood-plain has become wide, the width of the belt
within which the stream meanders is less than the width of its plain. In the
Lower Mississippi, for example, the meander belt is often no more than a
third to a tenth of the width of the flood-plain. It has already been pointed

out that the meanders migrate down the valley. In so doing they depress the
meander belt, the tendency being to reduce it to the level of the channel,
and, therefore, below the level of the flood-plain. As the meander belt
widens, the depression which it develops becomes more and more
capacious. Presently it may attain such dimensions as to hold the water of
ordinary floods. At this stage, or even before, such parts of the earlier flood-
plain as remain, are terraces.
These several tendencies conspire to partially destroy river flood-plains,
and to transform such parts as remain into terraces in the normal course of a
river’s history. They appear first in the lower part of the valley, and migrate
headward, following the course of nearly every other phase of activity in a
stream’s history. The heads of the terraces follow, at a respectful distance,
the head of the flood-plain, just as the head of the flood-plain follows at a
distance the head of the valley. The second and subsequent flood-plains and
the terraces to which they give origin follow the same course.
Terraces developed by the normal activities of a stream are always low,
and it is improbable that they would ordinarily be conspicuous. The vertical
distance between the first (highest) and second is greater than that between
the second and third. The principles developed on page 65 et seq., in
connection with the erosion of the hypothetical island, are applicable here.
Flood-plain terraces due to other causes.—Certain other causes,
accidental rather than normal to a stream, result in the development of
terraces from flood-plains. (1) If there be uplift in a region where the rivers
have flats, the streams are rejuvenated, and the remnants of their former
flood-plains become terraces. (2) If an alluvial flood-plain has been built as
the result of an excessive supply of sediment (p. 186), the exhaustion or
withdrawal of the excessive supply would leave the stream again relatively
clear, and free to erode where it had been depositing. It would forthwith set
to work to carry away the material which it had temporarily unloaded on the
plain. The plains built up in many valleys in the northern part of our
continent during the glacial period, when the drainage from the ice coursed
through them, have subsequently been partially destroyed by erosion, and
their remnants have become terraces. A notable reduction in the amount of
available sediment, even when the earlier supply was not excessive,
produces a similar result. (3) A notable increase in the volume of a stream,
without corresponding increase in load, as when one stream captures

another, may occasion the development of terraces by allowing the stream
to deepen its channel. (4) Above any barrier which dams a stream, a flood-
plain is likely to be developed. When the barrier is removed the stream will
cut more or less deeply into the plain above, leaving terraces. (5) The
recession of a falls through a flood-plain converts such parts of it as remain,
into terraces.
In conclusion, it may be stated that many river terraces, mostly very low,
are normal features of valley development, coming into existence at definite
stages in a valley’s history. They are generally composed, in large part, of
river alluvium. Others result from more or less accidental causes, working
singly or in conjunction, and to this class belong all of the more
conspicuous terraces developed from flood-plains. The structure of a terrace
often affords some clue to its origin (Fig. 196).

Fig. 196.—Terraces partly of rock and partly of alluvium. Such terraces indicate successive
uplifts, or some other change which had a similar effect on the stream which made the
valley.

Discontinuity of terraces.—When a stream sinks its channel into its
flood-plain, it does not follow that a terrace remains on each side. Where
the stream’s deepened channel is in the middle of its flood-plain, there is,
temporarily, a terrace on either side; but wherever the deepened channel is
at one margin of its flood-plain, a terrace remains on the other side only.
Even where continuous at the outset, terraces soon become discontinuous,
for all processes of subaërial erosion conspire to destroy them. A stream is
likely to meander on its second and later flood-plains, as on its first and
highest one. Wherever the meanders on its second flood-plain reach the
borders of the first flood-plain, the terrace at that point disappears, and
since the meanders are continually migrating, terraces are continually
disappearing. The same would be true of the second terrace, if a second
were developed. The removal of portions of a terrace by the sweep of
meanders is likely to leave the remnants cuspate toward the stream.[91]
Again, tributary streams, in bringing their channels into topographic
adjustment with their mains, cut through the terraces of the latter. New
gullies develop on the faces of the terraces and their heads work back across
them, dissecting them still further. At the same time, sheet erosion and other

phases of slope wash tend to drive the scarps of the terraces back toward the
bluff beyond. By the time a second series of terraces is well developed, no
more than meagre remnants of the first may remain.
From the foregoing considerations it is clear that the extent to which river
terraces once developed, now remain, is dependent in part on the length of
time which has elapsed since the river sank its channel below them. Other
things being equal, the greater their age the more meagre their remnants.
Terraces developed from river plains formed chiefly by alluviation stand
a better chance of long life than most other alluvial terraces. This is because
of the configuration of the original valley, the aggradation of which gave
origin to the plain. The principle involved is illustrated by Fig. 197. In
developing the second flood-plain the river encounters the rock wall of the
valley. This greatly retards lateral erosion, and the terrace above,
defended[92] by the rock, is likely to be long-lived.

Fig. 197.—Diagram to show why certain terraces are longer lived than others.

Alluvial terraces, like rock shelves, are popularly thought to mark “old
levels of the river.” In one sense this is true, but not in the sense in which
the expression is commonly used. Every level, from the crest of the
bounding bluffs to the bottom of a valley, is a level at which water ran for a
longer or shorter time; but the terrace does not mean that the river was once
so much larger than now as to fill the valley from its present channel to the
level of the terraces.
Termini of terraces.—From the mode of development of terraces it will
be seen that, traced up-stream, each terrace should theoretically grade into a
flood-plain at its upper end (Fig. 194), and that the upper end of the second
(from the top) terrace, where there are two, would not be so far up-stream
as the upper end of the first (highest). This is represented diagrammatically
in Fig. 198.
The down-stream termini of terraces are rarely distinct. This is partly
because the notable meandering of the streams in their lower courses is
antagonistic to the preservation of terraces. If all terraces once developed
remained, and if delta-building proceeded without interruption from waves,

the relations should be somewhat as follows: Traced down-stream, the cliff
between the oldest (highest) terrace and the next younger becomes
gradually lower until it finally disappears, and the continuation of the two is
found in a common plain. The cliff between the second and third terraces
should disappear in the same way, and below its disappearance the plain
representing their continuation is continuous with that representing the
continuation of the first and second. The cliff between the second and third
terraces may or may not continue farther down-stream than that between the
first and second. The plains below the terraces finally become continuous
with the lowest flood-plain and with the delta. These relations can rarely be
seen because of the destruction of the older terraces, and because of the
erosion by waves along shore.

Fig. 198.—Diagram looking up the valley, showing two terraces below, one in the middle,
and none above. The relations are purely diagrammatic.

The topography of terraces is similar to that of flood-plains, except in so
far as modified by erosion. While flat in general, the terrace may slope
either toward or from the valley bluff, and its surface may be marked by all
the minor irregularities which characterize a flood-plain.

CHAPTER IV.
THE WORK OF GROUND- (UNDERGROUND) WATER.

Many familiar facts demonstrate the general presence of abundant water beneath the surface of the land. The
thousands of wells in regions peopled by civilized races, and the countless springs which issue from the sides of
mountains and valleys are a sufficient proof both of the wide distribution of ground-water and of its great
abundance.
Certain well-known facts make it clear that ground-water is intimately connected with rainfall. In a dry season
the level of the water in wells commonly sinks, and after a heavy rain it rises (p. 71); and the amount of sinking
is greater when the drought is long, and the rise is most notable when the rainfall is heavy. Many springs which
discharge large quantities of water during a wet season flow with reduced volume, or cease to flow altogether in
periods of drought. Furthermore, the water of springs and wells has the properties which rain-water would
possess after sinking beneath the surface and dissolving mineral substances. Rain-water is seen to sink beneath
the surface with every shower, and since this source seems altogether adequate for ground-water, and since no
other source is known whence any considerable amount of ground-water might come, it is concluded that
atmospheric precipitation is its chief source.
Water gets beneath the surface by processes which are readily seen. Wherever the soil is porous some of the
rain which falls upon it is absorbed. Sinking through the soil to the solid rock it finds cracks and pores through
which it descends to great depths. Nowhere are the rocks beneath the mantle rock so compact and so free from
cracks, when any considerable area is considered, as to prevent the percolation of water through them.
Conditions influencing descent of rain-water.—There are several conditions which influence not only the
amount of water which sinks beneath the surface in a given area, but the proportion of the precipitation which
follows this course. These are as follows: (1) Amount of precipitation.—In a general way it is true that the
greater the amount of precipitation the greater the amount of water which will sink beneath the surface. Other
things being equal, a region of heavy precipitation is a region where wells are easily obtained and springs
common. (2) Rate of precipitation.—A given amount of precipitation may be concentrated in a short interval, or
distributed through a considerable period of time. In the latter case more of the water sinks beneath the surface;
in the former, a larger proportion runs off over the surface. The reason is readily seen. Water passes through
small spaces, such as those of soil, slowly, and its rate of passage decreases rapidly with decreasing size of the
passageways. When rain falls rapidly on a surface of even moderately close texture, the uppermost layer of soil
is promptly filled with water, and since the water passes downward slowly, the uppermost saturated part of the
soil becomes virtually impervious. While in this condition, the water which falls on it will run off if there be
slope, and stand if there be none. In the latter case it will sink slowly as the water in the soil passes down to
lower levels. If precipitation takes place no faster than the water can sink through the soil, all the water may
become ground-water. (3) The topography of the surface has much to do with determining the proportion of
rainfall which becomes ground-water. If the surface be flat, more will sink in; if it be sloping, more of it will run
off before it has time to sink. Other things being equal, the steeper the slope the larger the proportion of the
rainfall which will run off over it. (4) The texture of the soil, or other material on which the rain falls, helps to
determine what proportion of it sinks beneath the surface. If the surface materials be porous, the water sinks
readily; if of close texture, it finds less ready ingress. Other things being equal, the closer the texture of the soil
the less the proportion of the rainfall which will enter it. (5) The texture and structure of the rock beneath the
surface have some influence on the amount of ground-water. The rock may be stratified or massive; it may be
abundantly or sparsely jointed; it may be porous or compact. On the whole, stratified rock is more favorable for
the entrance of water than unstratified, partly because of its greater average porosity, and partly because the
planes of division between beds often allow the passage of water. If the beds of stratified rock are vertical or
inclined, water finds its way into them more readily than if they are horizontal, in so far as it descends along

stratification planes. Horizontally bedded rock, or rock which is not bedded at all, may be so much jointed, and
the joints so open, as to allow the water to enter readily.
The conditions favorable to the sinking of abundant water below the surface are therefore heavy precipitation,
falling slowly on a surface with little relief, a soil of open texture underlain by rock which is porous, or affected
by vertical or highly inclined planes of cleavage. The annual discharge of water by rivers is estimated to be
about 22 percent. of the rainfall on the land.[93]
Supply of ground-water not altogether dependent on local rainfall.—The amount of ground-water in a
given region is not always entirely dependent on the local rainfall. Ground-water is in constant movement, and
entering the soil or rock at one point it may, after a long subterranean journey, reach a point far from that at
which it entered. Thus beneath the Great Plains of the West there is much subterranean water which fell on the
eastern slopes of the mountains to the west. It has flowed beneath the surface to the Plains, where some of it is
now withdrawn for the purposes of irrigation in regions where rainfall is deficient. The accompanying diagram
(Fig. 199) illustrates the flow here described.

Fig. 199.—Diagram illustrating the general point that ground-water is not dependent entirely upon local supply.

Fig. 200.—Diagram illustrating the position of the ground-water surface (the dotted line) in a region of undulatory topography.

The ground-water surface. Water table.—The water table has already been defined (p. 71) as the upper
surface of the ground-water. In a flat region of uniform structure the ground-water surface is essentially level,
but rises and falls with the rainfall. Where the topography of a region is not flat, the ground-water surface is not
level. As a rule it is higher, though farther below the surface, under an elevation than under surrounding
lowlands, as illustrated by Fig. 200. The explanation is not far to seek. If a hill of sand be exposed to rainfall,
most of the water falling on its porous surface will sink into it. If the precipitation continues long enough, as in a
protracted rain, the hill of sand will be filled with water, the water occupying the interstices between the grains.
If the sand of the hill could be removed, leaving the water which it contains on the same area, it would constitute
a mound perhaps a third or a fourth as high as the hill itself. If unsupported, this mound of water would spread
promptly in all directions until its surface was level. While the sand remains, the water in it constitutes a mound,
and has a tendency to spread. It does in fact spread, but since the process involves great friction the spreading is
slow. With the spreading the surface of the water in the sand sinks, and sinks fastest at the center where it is
highest (b, Fig. 201). If the process were not interrupted the surface of the water in the hill would, in time, sink
approximately to the level of the water in the surrounding land (d, Fig. 201); but at every stage preceding the
last, the surface of the water would be higher beneath the summit of the hill than elsewhere, though farther from
the surface. In regions of even moderate precipitation the water surface beneath the hills rarely sinks to the level
of that in the lowlands adjacent, before being raised by further rains.

Fig. 201.—Diagram to illustrate the relations of ground-water to the surface.

The water-level beneath the lowlands also sinks. Some of it finds its way into valleys, some of it sinks to
greater depths, and some of it evaporates; but since the water surface beneath the elevation sinks more rapidly

than that beneath the lowland, the two approach a common level. Their difference will be least at the end of a
drought, and greatest just after heavy rains.
Depth to which ground-water sinks.—The depth to which ground-water penetrates has not been determined
empirically. No borings or excavations of any sort have been made to such depths as to indicate that its limit was
being approached, though some of them are a mile or more deep. There is a popular belief that water sinks until
it reaches a temperature sufficient to convert it into steam, but except for special localities where hot lava lies
near the surface, this belief is not well founded. In the first place, it is not known at what temperature water
below the surface would be converted into steam. While water boils at sea-level at a temperature of 212° (Fahr.)
a higher temperature would be necessary below that level.
Assuming the temperature of water sinking beneath the surface to be 50° Fahr., its temperature must be raised
162° to bring it to the temperature at which it would boil at sea-level. On the above assumption of initial
temperature, the following table shows the depths at which water would reach a temperature of 212° Fahr. under
various assumptions as to the rate of increase of temperature. It shows also the pressure in atmospheres which
would exist at these several depths if the overlying rock were full of water.
Depth at which
Rate of Increase Equivalent Pressure
Temperature of 212°
of Temperature. in Atmospheres.
would be reached.
1° for 50 feet 8,100 feet 238 (approximately)
1° for 60 “ 9,720 “ 285 “
1° for 70 “ 11,340 “ 333 “

With an initial temperature of 80°, corresponding to that of the warmer parts of the earth’s surface, instead of
50°, the table would be as follows:
1° for 50 feet 6,600 feet 194 (approximately)
1° for 60 “ 7,920 “ 214 “
1° for 70 “ 9,240 “ 272 “

The temperature at which water boils increases with the pressure. A pressure of about 200 atmospheres is the
critical pressure for water; that is, the pressure which, if increased, will prevent boiling altogether. The depth at
which a pressure of 200 atmospheres would be reached, supposing the upper rock to be full of water, is about
6800 feet. The temperature of the water at this depth, under various assumptions as to initial temperature and
rate of increase of heat, is shown in the following table:
Rate of Increase Temperature at a
Initial Temperature.
of Temperature. Depth of 6,800 Feet.
50° 1° for 50 feet 186° Fahr.
50° 1° for 60 “ 163° “
50° 1° for 70 “ 147° “
80° 1° for 50 “ 216° “
80° 1° for 60 “ 193° “
80° 1° for 70 “ 177° “

Only one of these temperatures reaches the boiling-point of water at sea-level. It is therefore clear that at this
depth water has not even closely approached the boiling temperature for this depth, and since this is the depth of
the critical pressure, it is clear that it cannot boil at any greater depth. The descent of water is therefore not
stopped, under normal conditions of crustal temperature, because it reaches its boiling-point. Locally, as in the
vicinity of active or recently extinct volcanoes, the case may be different.
It is conceivable that water may descend until it reaches its critical temperature (somewhere between 610° and
635° Fahr.). The depth at which the critical temperature would be reached, under various assumptions, is shown
in the following table:
Rate of Increase
Initial Temperature. Depth of Critical Temperature.
of Temperature.
50° 1° for 50 feet 28,000 to 29,250 feet
50° 1° for 60 “ 33,600 to 35,100 “
50° 1° for 70 “ 39,200 to 40,950 “
80° 1° for 50 “ 26,500 to 27,750 “
80° 1° for 60 “ 31,800 to 33,300 “
80° 1° for 70 “ 37,100 to 38,850 “

There is good reason, in the increasing density beneath the surface, for believing that the rate of increase of
temperature decreases with depth, and therefore that the rate of 1° for 50 feet for the depths concerned is too
high. The greater depths of the table above are therefore believed to more nearly represent the truth than the
lesser ones. (See discussion of underground temperature in Chapter XI.)
If descending water attained its critical temperature, the extent to which the resulting water-gas might be
absorbed is not known. So far as limited by temperature, therefore, it is not possible to assign a limit to the
descent of water under average conditions of crustal temperature.
Other considerations seem to place a limit to the descent of water. Rock, solid and unyielding as it seems, is
yet plastic when under sufficiently great pressure. The cracks and cavities affecting it are believed to descend a
distance which is but slight in comparison with the radius of the earth. Even if openings were once formed at
greater depths, they could not persist, for the adjacent rock, under the pressure which there exists, would “flow”
in, in effect (though perhaps not in principle) much as stiff liquid might, and close them. The outer zone of the
earth where cavities may exist is known as the zone of fracture.[94] The depth of the zone of fracture differs for
different rocks, but is not believed to extend below some such depth as five or six miles, even for the most
resistant.[95] It is to be noted that these depths are less than those at which the critical temperature of water would
be reached under most of the conditions, including all the more probable ones, specified in the above table.
Let it be assumed that water descends through openings in the rock to a depth of six miles. At this depth it
would, under the various assumptions specified in the first and second columns of the following table, have the
temperature indicated in the third column:
Rate of Increase Temperature at
Initial Temperature.
of Temperature. Depth of Six Miles.
50° 1° for 50 feet 683° Fahr.
50° 1° for 60 “ 578° “
50° 1° for 70 “ 502° “
80° 1° for 50 “ 713° “
80° 1° for 60 “ 608° “
80° 1° for 70 “ 532° “

In two of these cases, namely, those in which the assumed rate of increase of temperature is highest, the
temperature of the water at the assumed lower limit of the zone of fracture is above the critical temperature of
water. If the assumptions involved in these two cases be correct, water might descend to the point where it
would be converted into water-gas, and in this condition it might be occluded by the hot rock. In the other cases,
involving the more probable assumptions, the critical temperature is not reached at a depth of six miles. If pores
and cracks do not extend to greater depths, liquid water could not; and since the water at this depth has probably
not reached its critical temperature, it cannot exist as water-gas. If it does not exist in the form of water-gas, its
occlusion by the hot rock substance would not be probable. It would seem, therefore, that the descent of water
under ordinary conditions is much more likely to be limited by the zone of fracture, than by temperature.
Movement of ground-water.[96]—Ground-water is in more or less continual movement. If all the water be
pumped out of a well it soon fills up again to its normal level by inflow from all sides. Springs and flowing
wells also demonstrate the movement of ground-water. Near the surface the movement of ground-water is
primarily downward if the medium through which it passes is equally permeable in all directions; but so soon as
the descending water reaches the water surface, its descent is checked and its movement is partly lateral.
The commonest sort of movement of ground-water is that exemplified as the water sinks beneath the surface,
namely, slow percolation through the pores and cracks of the soil and rock. Ground-water is not generally
organized into definite streams, though underground streams, mostly small, are sometimes seen in caves and
crevices, and sometimes issue as springs. Most underground streams which issue as springs probably have
definite channels for short distances only before they issue. It is probable that ground-water frequently flows in
considerable quantity along somewhat definite planes, without having open channels. Thus every porous bed of
rock is likely to serve as the pathway along which subterranean drainage passes. This is especially true where
the porous bed is underlain by an impervious one. The “reservoirs” from which artesian wells draw their supply
are not usually streams or lakes, but porous beds of rock through which abundant water passes. As the supply is
drawn off at one point, it is renewed by water entering elsewhere. Since the freedom of movement of ground-
water is notably influenced by the porosity of the rock, and since the rock is, on the average, most porous and
the pores largest near the surface, the movement of ground-water is, on the average, greatest near the surface,

and least at its lower limit. In general the decrease of movement is much more rapid than the decrease in the size
of the pores. It follows that while the upper part of the ground-water, especially that above ground-water level,
moves somewhat freely, the lower part moves much more slowly. It is probable, indeed, that the movement in
the lower part of the subterranean hydrosphere is extremely slight.
The amount of ground-water.—The porosity of surface rocks varies widely, and the porosity of but few has
been determined.[97] Such determinations as have been made are chiefly on building stones, in which the range of
porosity varies from a fraction of one percent., in the case of granite, to nearly 30 percent. in the case of some
sandstones. Building stone is perhaps more dense than the average surface rock. Furthermore, such tests as have
been made do not take account of the larger cracks and openings of rock, for these would not appear in the
specimens tested; nor of the mantle rock, which generally contains a large amount of water. From such
determinations as have been made it is estimated that the average porosity of the outer part of the lithosphere is
somewhere between 5 and 10 percent. If the porosity diminishes regularly to a depth of six miles, where it
becomes zero, the average porosity to this depth would be half the surface porosity.[98] An average porosity of
two and one-half percent. would mean that the rock contains enough water to form a layer nearly 800 feet deep.
With an average porosity of 5 percent., this figure would be doubled.[99] While these figures are not to be
regarded as measurements, they perhaps give some idea of the amount of ground-water. It is this sphere of
ground-water which justifies the term hydrosphere, as applied to the waters of the earth.
Fate of ground-water.—Most of the water which sinks into the earth reaches the surface again after a longer
or shorter journey. Some of it is evaporated from the surface directly; some of it is taken up by plants and is
passed by them into the atmosphere; some of it issues in the form of springs; some of it seeps out; some of it is
drawn out through wells; and much of the remainder finds its way underground to the sea or to lakes, issuing as
springs beneath them. A small portion of the descending waters enters into permanent combination with mineral
matter. Many minerals are known to take up water, being changed thereby from an anhydrous to a hydrous
condition. It does not necessarily follow, however, that the total supply of water is thereby decreasing. Minerals
once hydrated may be dehydrated subsequently, the water being set free. Furthermore, considerable quantities of
water in the form of vapor issue from volcanoes, and volcanic vents often continue to steam long after volcanic
action proper has ceased. It is probable that some, and perhaps much of the water issuing from these vents has
never been at the surface before, and it is not now possible to affirm that the supply from this source does not
offset, or even surpass, the depletion of the hydrosphere resulting from mineral hydration.

THE WORK OF GROUND-WATER.

Ground-water effects very considerable results in the course of its history. These results are partly chemical
and partly mechanical, the former being far more important than the latter.

Chemical Work.
The results of the chemical and chemico-physical action of water may be grouped in several more or less
distinct categories.
1. The simplest effect is the subtraction of soluble mineral matter. Pure water is in itself a solvent of certain
minerals; but the carbonic-acid gas extracted from the atmosphere, and the products of organic decay extracted
from the soil make ground-water a much more efficient solvent. Something of the results which it achieves is
shown by its composition. All ground-water, whether issuing as springs or drawn out through wells, contains
much more mineral matter than the water which falls as rain, and the excess is acquired in its underground
course.
The subtraction of soluble matter from rock renders it porous. The amount of material dissolved from a given
place may be trivial or considerable, according to the character of the rock, the readiness with which water has
access to it, and the character of the water. Locally, the subtraction of mineral matter may be the chief, or even
the only appreciable, effect of the ground-water.
2. It sometimes happens that ground-water with certain mineral substances in solution exchanges them for
other substances extracted from the rock. Thus the process of substitution is effected. By this process the lime
carbonate of a shell imbedded in rock may be removed, molecule by molecule, and some other substance, such

as silica, left in its place. When the process is complete, the substance of the shell has been completely removed,
though its form and structure are still preserved in the new material which has taken the place of the old. Buried
logs are sometimes converted into stone by the substitution of mineral matter for the vegetable tissue. This is
petrification. Petrification is altogether distinct from incrustation, which simply means the coating of an object
with mineral matter. A bird’s nest may be incrusted with lime carbonate, but it is not thereby petrified. Solution
is a necessary antecedent of substitution.
3. The materials which are subtracted from the rock at one point may be added to other rock elsewhere. Thus
a third type of change, addition, is effected. Rock may at one time and place be rendered porous by the
subtraction of some of its substance, and the openings thus formed may subsequently become the receptacles of
deposits from solution. This is exemplified in the stalactitic deposits of many caves. Not uncommonly cracks
and fissures are filled with mineral matter deposited by the waters which pass through them. Thus arise veins
which, for the most part, are nothing more than cracks and crevices filled by mineral matter brought to them in
solution, and precipitated on their walls. Most veins of metallic ores have originated in this way.
4. A further series of changes is effected by ground-water when it, or the mineral matter it contains, enters into
combination with the mineral matter through which it passes. One of the commonest processes of this sort,
hydration, has already been referred to (pp. 43 and 428); but in the development of many of the commoner
hydrous minerals changes other than hydration are involved. These changes result in new mineral combinations,
the new minerals being developed out of the old, usually with some additions or subtractions. In the long course
of time changes of this sort may be very great, so great indeed that large bodies of rock are radically changed,
both chemically and physically. Much of the old substance may remain, but it has entered into new and more
stable combinations with the materials which the water has brought to it.
Quantitative importance of solution.—In general, solution is probably most effective at a relatively slight
distance below the surface. In the outermost zone of mantle rock the materials are usually less soluble than
below, for they often represent the residuum after the soluble parts of the formation from which they originated
were dissolved out. Below this zone the rock contains more soluble matter, and the water, charged with organic
matter in its descent through the soil, is in condition to dissolve it. At greater depths the water has become
saturated to some extent, and, so far forth, less active. Here, too, the movement is less free. The increased
pressure at considerable depths, on the other hand, facilitates solution, which must be understood to take place
under proper circumstances in any zone reached by the water.
Calculations have been made which illustrate the quantitative importance of the solution effected by ground-
water. The springs of Leuk (Switzerland) bring to the surface annually more than 2000 tons of calcium sulphate
(gypsum) in solution, and in the same time the springs of Bath (England) bring up an amount of mineral matter
in solution sufficient to make a column 9 feet in diameter, and 140 feet high.[100]
The amount of mineral matter in solution in streams is also significant, for while stream-water is not all
derived from ground-water, much of it had such an origin. In the case of several streams, among them the
Thames and the Elbe, careful estimates of the amount of dissolved mineral matter have been made. Though the
Thames drains an area only about one-tenth as large as the State of New York, it is estimated to carry about 1500
tons of mineral matter in solution to the sea daily.[101]
From the uppermost 20,000 square miles of its drainage basin the Elbe is estimated to carry yearly about
1,370,000 tons of mineral matter in solution. Estimates of the amounts of material carried to the sea in solution
by several rivers are given on pp. 102 and 103. Much of this matter was brought to the rivers by waters which
had been underground before reaching the streams.
From these figures it is clear that we have to reckon here with a very considerable factor in the lowering of
land surfaces. From the amount of lime carbonate carried by the Thames it has been estimated that the average
amount of this material dissolved from the limestone area drained by this stream is 143[102] tons per square mile
per year. It is estimated that, on the average, something like one-third as much matter is carried to the sea in
solution as in the form of sediment, and that by this process alone land areas would be lowered something like
one foot in 13,000 years.[103]
Deposition of mineral matter from solution.—The deposition of material from solution is effected in
several ways. (1) It is sometimes deposited by evaporation. This is well shown where water seeps out on arid
lands. The same process is illustrated when water is boiled. (2) Reduction of temperature often occasions

deposition. In general, hot water is a better solvent of mineral matter than cold,[104] and if it issues with abundant
mineral matter in solution the precipitation of some of it is likely to take place. (3) Plants sometimes cause the
precipitation of mineral matter from solution. About some hot springs, even where the temperature of the water
is very high small plants of low type (algæ) grow in profusion. In ways which are not perfectly understood these
algæ extract the mineral matter from the hot water. They are now thought to be a chief factor in the deposits
about the hot springs of the Yellowstone Park.[105] The influence of organisms on precipitation from solution is
not confined to the waters of hot springs. (4) A fourth factor involved in the deposition of mineral matter from
solution is pressure. Pressure increases the solvent power of water with respect to minerals directly; it produces
the same effect indirectly by its effect on the solution of gases. As water charged with gas comes to the surface,
the pressure is relieved and some of the gas escapes. Such mineral matter as was held in solution by the help of
the gas which escapes is then precipitated. (5) Precipitation is also sometimes effected by the mingling of waters
containing different mineral substances in solution. Such mingling of solutions would be most common along
lines of ready subterranean flow, and while each portion of the water entering a crevice or porous bed may be
able to keep its own mineral matter in solution, their mingling may involve chemical changes resulting in the
formation of insoluble compounds, and therefore in deposition. This principle has probably been involved in the
filling of many fissures and crevices, converting them into veins. (6) The escape of gases from water, whether
from increase of temperature or by the disturbance of water, sometimes causes the deposition of mineral matter
held in solution.
The deposition of material held in solution is most notable at two zones, one below that of most active
solution, and the other at the surface, where evaporation is active. Under proper conditions, however, deposition
may take place at any level reached by water.

Mechanical Work.
The mechanical work of ground-water is relatively unimportant. Wherever it is organized into definite
streams, the channels through which it flows are likely to be increased by mechanical erosion as well as by
solution. Either beneath the surface, or after the streams issue, the mechanical sediment carried will be
deposited.

Fig. 202.—Diagram to illustrate the general form and relations of caves developed by solution. The black portions represent the cavern
spaces. Some limestone sinks are represented on the surface above.

RESULTS OF THE WORK OF GROUND-WATER.

Weathering.—Where the solution effected by ground-water in any locality is slight and equally distributed,
the result is to make the rock porous. If, for example, some of the cement of sandstone is dissolved, the texture
of the rock becomes more open; but if all the cement be removed the rock is changed from sandstone to sand. If
a complex crystalline rock contains among its many minerals some one which is more soluble than the others,
that one may be dissolved. This has the effect of breaking up the rock, since each mineral acts as a binder for the
rest. It might happen that no one of the minerals is dissolved completely, but that some one of them is
decomposed by water, and certain of its constituents removed. Such change would be likely to cause the mineral
so affected to crumble, and with its crumbling, if it be an important constituent of the rock, the integrity of the

rock is destroyed. Where considerable chemical changes, especially
subtractions, are going on, the rock is likely to crumble. The increase in
volume attendant on hydration, etc., sometimes leads to the disruption of
rock. These are phases of weathering. (For other phases of weathering see pp.
54 and 110.)
Caverns.[106]—Where local solution is very great results of another sort
may be effected. In formations like limestone, which are relatively soluble,
considerable quantities of material are frequently dissolved from a given
place. Instead of making the rock porous, in the usual sense of the term, large
caverns may be developed (Fig. 202). In their production, solution may be
abetted by the mechanical action of the water passing through the openings
which solution has developed. Considerable caves are found chiefly in
limestone. They were probably developed when the surface relief was slight,
and surface drainage therefore poor. Regions where caves were developed
under these conditions may subsequently acquire relief, so that caves are not
now confined to flat regions.
One of the best known regions of caves is in the basin of the Ohio in
Kentucky and southern Indiana, where the number of caves is large, and the
size of some of them, such as Mammoth and Wyandotte, very great. A
ground-plan of Wyandotte (Ind.) Cave is shown in Fig. 203. The aggregate
length of the passageways is about 23½ miles.

Fig. 203.—Ground-plan of Wyandotte
Cave. The unshaded areas represent
the passageways. (21st Ann. Rept.,
Ind. Geol. Surv.)

Fig. 204.—Deposits in Wyandotte (Ind.) Cave. (Hains.)

Fig. 205.—Deposits in Wyandotte Cave. (Hains.)

Deposition often takes place in caves after they are formed (Figs. 204 and 205). It may even go on at the same
time that the cave is being excavated. Here are formed the well-known stalactites and stalagmites. A stalactite
may start from a drop of water leaking through the roof of the cave. Evaporation, or the escape of some of the
carbonic gas in solution, results in the deposition of some of the lime carbonate about the margin of the drop, in
the form of a ring. Successive drops make successive deposits on the lower edge of the ring, which grows
downward into a hollow tube through which descending water passes, making its chief deposits at the end.
Deposition in the tube may ultimately close it, while deposition on the outside, due to water trickling down in
that position, may greatly enlarge it.

Fig. 206.—A limestone sink-hole, east-northeast of Cambria, Wyo., exceptional for its steep sides. Minnekahta limestone. (Darton, U. S.
Geol. Surv.)

Underground caves sometimes give rise to topographic features which are of local importance. When the
solution of material in a cavern has gone so far that its roof becomes thin and weak, it may collapse, giving rise
to a sink or depression in the surface over the site of the original cave. This is so common that regions of
limestone caves are often affected by frequent sinks formed in this way. They are a conspicuous feature of the
landscape in the cave region of Kentucky, and are well known in many other limestone districts. They are known
as limestone sinks. (Fig. 206 and Fig. 2, Pl. XVII.)

Fig. 207.—A fresh landslide near Medicine Lake, Mont. The bare space shows the position from which the slide started. (Whitney.)

Fig. 208.—Landslide topography. The protruding mass on the right has slumped down from the mountain to the left. South face of
Landslip Mountain, Colo., seen from the west; Rico quadrangle. (Cross, U. S. Geol. Surv.)

Under certain circumstances caves may give rise to striking features of another sort. If for any reason the roof
is destroyed at the two ends of a cave, remaining intact over the middle, the latter part constitutes a natural
bridge. Natural bridges also originate in other ways (pp. 151, 153).

Fig. 209.—Landslide topography on Badger Mountain, Washington. The slumping material in this case is basalt.

Creep, slumps, and landslides.—When the soil and subsoil on a slope become charged with water they tend
to move downward. When the movement is too slow to be sensible it is called creep. The common downward
inclination of trees growing in such situations, the result of the more rapid creep of the surface as compared with
the deeper part of the soil, is both an expression of the movement and of its slowness. Other factors besides
ground-water are involved in creep (see p. 112).
When the movement is rapid enough to be sensible the material is said to slump or slide. This may happen
when the slope on which water-charged mantle rock lies is steep (Fig. 207). Great landslides of this sort have
been recorded, and some of them have done great damage. Where a stream’s banks are high, and of unindurated
material, such as clay, considerable masses sometimes slump from the bank or bluff into the river, or settle away
slowly from their former positions. This is a common phenomenon along streams which have cut valleys in
drift, and along shores on which waves are encroaching. The same phenomenon is common on a larger scale on
the slopes of steep mountains.[107] Considerable terraces are sometimes developed on their slopes in this way, but
they are usually irregular and discontinuous (Figs. 208, and 209). The loose débris on steep slopes sometimes
assumes a sort of flowing motion and descends the slope with some such form and at some such rate as a glacier.
Such bodies of débris are sometimes called “talus glaciers” (Fig. 210). In many such cases, snow and ice have
had some part in their development.
In creep and in landslides gravity is the force involved, and the ground-water only a condition which makes
gravity effective. Gravity alone accomplishes similar results, as illustrated by Fig. 211.

Summary.
All in all, ground-water is to be looked upon as a most important geological agent. When it is remembered
that a very large part of all the water which falls on the surface of the earth, either in the form of rain or snow,
sinks beneath the surface; that much of it sinks to a great depth; that much of it has a long underground course
before it reappears at the surface; that it is everywhere and always active, either in subtracting from the rock
through which it passes, in adding to it, in effecting the substitution of one mineral substance for another, or in

bringing about new chemical combinations; and when it is remembered that this process has been going on for
untold millions of years, it will be seen that the total result accomplished must be stupendous. The rock
formations of the earth to the depths to which ground-water penetrates are to be looked upon as a sort of
chemical laboratory through which waters are circulating in all directions, charged with all sorts of mineral
substances. Some of the substances in solution are deposited beneath the surface, and some are brought to the
surface where the waters issue. Much of the material brought to the surface in solution is carried to the sea and
utilized by marine organisms in the making of shells. Without the mineral matter brought to the sea by springs
and rivers, many shell-bearing animals of great importance, geologically, would perish. Biologically, therefore,
as well as geologically, ground-water is of great importance.
PLATE XVII.

U. S. Geol. Surv. Fig. 1. HUNTERDON COUNTY, NEW JERSEY. Scale, 1+ miles per inch.

U. S. Geol. Surv. Fig. 2. NEAR PIKEVILLE, TENNESSEE. Scale, 1+ miles per inch.

PLATE XVIII.

U. S. Geol. Surv. Fig. 1. WASHINGTON. Scale, 2+ miles per inch.

U. S. Geol. Surv. Fig. 2. CALIFORNIA. Scale, 2+ miles per inch.

Fig. 210.—A “talus glacier,” in Silver Basin, near Silverton, Colo. (Cross, U. S. Geol. Surv.)

Fig. 211.—A huge mass of rock settling into the Canyon of the Colorado. A result of gravity action. (Atwood.)

SPRINGS AND FLOWING WELLS.

The term spring is applied to any water which issues from beneath the surface with sufficient volume to cause
a distinct current. If the water issues so slowly as to merely keep the surface moist, it is not called a spring, but
seepage. The spring from which water issues with a strong current, especially if it be upward, is comparable to a
flowing well, while the spring from which water issues with little force, and without upward movement, is
comparable to the flow of water into a common well.
Springs often issue from the sides of valleys (Fig. 212), the bottoms of which are below ground-water level.
They are especially likely to issue at the surface of relatively impervious layers, and where the valley slopes cut
joints, porous beds, or other structures which allow free flow of ground-water.

Fig. 212.—Diagram illustrating positions, a and b, favorable for springs.

Springs are classified in various ways, and these several classifications suggest characteristics worthy of note.
They are sometimes said to be deep and shallow. The “deep” spring, as the term is ordinarily used, is one which
issues with great force, and with something of upward movement, and the “shallow” spring, one which issues
with little force, and without upward movement; but the spring which issues with force is not necessarily deep,
nor is the one which issues with little force necessarily shallow. The idea involved in this grouping would be
better expressed by strong and feeble. Springs are also classified as cold and thermal, the latter term meaning
simply that the temperature is such as to make the springs seem warm or hot. The temperature of thermal springs
ranges up to the boiling-point of water. Between deep springs and shallow ones, and between cold springs and
thermal, respectively, there is no sharp line of demarkation. Again, some springs are continuous in their flow,
while others are intermittent. Most intermittent springs flow after periods of precipitation, but dry up during
droughts (see p. 202). Springs are also classified as mineral and common. Mineral springs, in the popular sense
of the term, are of two types: (1) Those which contain an unusual amount of mineral matter, and (2) those which
contain some unusual mineral. Springs are especially likely to be called mineral if the substances which they
contain, have, or are supposed to have, some medicinal property. All springs which are not “mineral” are
“common.” This classification is not altogether rational, for all springs contain more or less mineral matter, and
many springs which are “common,” contain more mineral matter than some springs that are “mineral.” Mineral
springs are themselves classified according to the kind and amount of mineral matter they contain. Thus saline
springs contain salt; sulphur springs contain compounds (especially gaseous) of sulphur; chalybeate springs
contain iron compounds, especially the sulphate; calcareous springs contain abundant lime carbonate, etc. These
various mineral substances are extracted from the rock, sometimes by simple solution, and sometimes by
solution resulting from other chemical change. The salt of saline springs is usually extracted from beds of salt
beneath the surface. Lime carbonate, one of the commonest substances in solution in ground-water, is dissolved
from limestone, or derived by chemical change from rocks containing other calcium compounds. Thus lime
feldspars, by carbonation, give rise to lime carbonate. The chalybeate waters often arise from the oxidation of
iron sulphide, a mineral which is common in many sedimentary rocks. The iron sulphate is itself subject to
change in the presence of the ubiquitous lime carbonate. From this change iron carbonate results, and this is
usually quickly altered to iron oxide, which, being relatively insoluble, is precipitated. About chalybeate springs,
therefore, iron oxide is frequently being deposited. Medicinal springs are those which contain some substance or
substances which have, or are supposed to have, curative properties.
Mineral matter in solution.—The number and variety of mineral substances in spring water is very great,
and the amount of solid matter in solution varies widely. Some of the hot springs of the Yellowstone Park
contain nearly three grams (2.8733) of mineral matter per kilogram.[108]
The composition of various spring and well waters is shown in the accompanying table, which gives some
idea of the range of mineral substances commonly in solution in ground-water.

Geysers.—Geysers are intermittently eruptive hot springs. They occur only in volcanic regions (past or
present) and in but few of them. Active geysers are virtually confined to the Yellowstone Park and Iceland,
though they formerly existed at other places. Those of New Zealand have but recently become extinct. The great
geyser region of the world is the Yellowstone Park, where there are said to be more than sixty active geysers.
The cause of the eruption is steam. The surface-water sinks down until, at some unknown depth, it comes into
contact with rock sufficiently hot to boil it. The source of the heat is not open to inspection, but it is believed to
be the uncooled part of an extrusive lava flow, or of an intrusive lava mass. From what was said on pp. 216 and
217 it is clear that geysers do not have their origin in water which sinks down to the zone of great heat, where
the increment of heat is normal.
The water of a geyser issues through a tube of unknown length. Whether the tube is open down to the source
of the heat is not determinable, but water from such a source finds its way to the tube. Water may enter the tube
from all sides and at various levels from top to bottom. The heating may precede or follow its entrance into the
tube, or both. So far as the water is heated after it enters the tube, the point of most rapid heating may be at the
bottom of the tube or at some point above. If the temperature of the source of heat were high enough to convert
the descending water into steam as fast as it enters the tube, the steam would escape continuously, though there
would be no geyser; but if the rock is only hot enough to bring the water to the boiling-point after some lapse of
time, and after some water has accumulated, an eruption is possible.
ANALYSES OF AMERICAN SPRING-WATERS.[109]
[Reduced to Parts per 1000 by Dr H. J. Van Hoesen.]
Waters Artesian Artesian Artesian Manitou Opal Spring Sulphur Spring Hot Hot Boiling Warm
well well well[110] Spring Spring Spring Spring Spring
“Glacier
Spouting
Spring”
Location Lexington, Saratoga, Sheboygan, Manitou, Col. Yellowstone Los Angeles, Hot Sp. Ward’s Shaffer’s Warm
Ky. N. Y. Wis. National Cal. Station, Ranch, Ranch, Spring
Park C. P. R. base of Honey Sta. B.
R. Granite Lake & B. R.
Mts., Valley, Mono
Nev. Cal. Basin
Date ...... 1872 Feb. 1876 ...... ...... ...... ...... ...... ...... ......
Analyst R. Peters F. A. C. F. Oscar Loew H. Leffman Oscar Loew T. M. T. M. T. M. T. M.
Cairns Chandler Chatard Chatard Chatard Chatard
and C. F.
Chandler
References Ky. Geol. Am. Am. U. S. G. S. W., U. S. Geol. An. Rep. Ante, p. Ante, p. Ante, p. Bulletin
Surv., N. Chemist, Chemist, 100th M.. & Geog. U. S. G. S. W., 49 53 51 No. 9,
S., Vol. Nov. 1876, p. Vol. III, p. Surv. Id., 100th M., U. S.
V, p. 189 1872, p. 370 618 Wyo. 1876, p. 195 Geol.
164 Ter., Survey,
1878, p. p. 27
393
Sodium, Na .09227 4.72640 2.0398 .45164 .4615 .10424 .7743 .3554 .3040 .6116
Potassium,
.00919 .35806 .1285 .05980 ...... Trace .0669 .0191 .0094 .0630
K
Calcium, Ca .02136 .94050 1.0739 .44400 .0344 .0305 .0367 .0121 .0589
Magnesium, .50600[111]
.01805 .53470 .2352 .05860 ...... .0010 .0034 .0004 .0604
Mg
Barium, Ba ...... .01848 Trace
Strontium,
Sr Trace[112] .00057
Lithium, Li Trace .01078 .0003 .00039 ...... Trace
Iron, Fe Trace[113] .00341 .0027 Trace[114] ...... Trace
Manganese,
...... ...... .0009 ...... ...... Trace
Mn
Chlorine, Cl .07465 7.47400 4.2730 .24850 .7496 Trace[115] .9697 .2396 .2070 .2272
Bromine, Br .04661 .0025
Iodine, I .00060 Trace
Fluorine, Fl ...... Trace[116]
Carbonic
acid, CO2 .12160 5.82603 .1792 1.11001 ...... .03516 ...... Trace ...... .5787

Sulphuric
acid, H2SO4 .03218 .00234 2.0318 .20696 .0325 .16140 .3555 .3901 .3492 .3131

Phosphoric
acid, H3PO4 Trace .00005 .0004 ...... ...... Trace

Boracic acid,
H3BO3 Trace Trace

Alumina,
Al2O3 ...... .00770 .0022 ...... ...... Trace .0010 ...... ...... .0018

Silica, SiO2 .00940 .01174 .0080 .02010 .7680 Trace .2788 .1136 .1310 .1220
Hydrogen in
bicarbonates, ...... .09713 .0030
H
Organic
Trace Trace Trace ...... ...... Trace
substances
Oxygen, O ...... ...... ...... ...... ...... ...... .0194[117] .0255[117] .0080[117] .0325[117]
.37870 20.05910 9.9814 2.60000 2.0460 .80680 2.4953 1.1834 1.0211 2.0692
Carbonic
acid, CO2 ...... 2.015[118] ...... ...... ...... In excess Trace
Sulphuretted
hydrogen, ...... ...... ...... ...... ...... 0.5000
H2S

The exact sequence of events which leads up to an
eruption is not known, but a definite conception of the
principles involved may perhaps be secured by a definite
case. Suppose a geyser-tube filled with water, and heated at
its lower end. As the water is heated below, convection
tends to distribute the heat throughout the column of water
above. If convection were free, and the tube short, the result
would be a boiling spring; but if the tube is long, and
especially if convection is impeded, the water at some level
below the surface may be brought to the boiling-point
earlier than that at the top. Under these circumstances if
even a little water in the lower part of the tube is converted
into steam, the steam will raise the column of water above,
and it will overflow. The overflow relieves the pressure on
all parts of the column of water below the surface. If before
the overflow there was any considerable volume of water
essentially ready to boil, the relief of pressure following the
overflow might allow it to be converted into steam
suddenly, and the sudden conversion of any considerable
quantity of water into steam would cause the eruption of all
the water above it (Figs. 213 and 214). The height to which
the water would be thrown would depend upon the amount
of steam, the size and straightness of the tube, etc.
It is clear that everything which impedes convection in
the geyser tube will hasten the period of eruption, since
impeded circulation will have the effect of holding the heat
down, and so of bringing the water at some level below the
top more quickly to the boiling-point. It follows that
anything which chokes up the tube, or which increases the
viscosity of the water, or its surface tension, would hasten
an eruption.[119] Fig. 213.—“Old Faithful” in eruption.
Geysers often build up crater-like basins or cones (Figs. 214 to 217) about themselves, the cone being of
material deposited from solution. In the Yellowstone Park the precipitation of the matter in solution (chiefly
silica) is partly due to cooling and partly to the algæ which abound even in the boiling water, and the brilliant
colors of the deposits about the springs are attributable to these plants. When the water from any geyser or hot

spring ceases to flow the plants die and the colors disappear. The details of the surface of the deposits about
geysers and hot springs are often complicated, and frequently very beautiful (Fig. 218).

Fig. 214.—“Giant” Geyser, Yellowstone Park, in eruption. Shows also the cone. (Wineman.)

Fig. 215.—Cone (or crater) of Castle Geyser, Yellowstone Park. (Detroit Photo. Co.)

Fig. 216.—Cone (or crater) of Grotto Geyser, Yellowstone Park. (Detroit Photo. Co.)

The heating of geyser and hot-spring water must cool the lava or other source of heat below. As this takes
place, the time between eruptions becomes longer and longer. In the course of time, therefore, the geyser must
cease to be eruptive, and when this change is brought about the geyser becomes a hot spring. Within historic
times several geysers have ceased to erupt and new ones have been developed. In the Yellowstone Park, where

there are said to be something like 3000 vents of all sorts,
hot springs which are not eruptive greatly outnumber the
geysers. From many of the vents but little steam issues, and
from some, little else.

Fig. 217.—Cone of Giant Geyser, Yellowstone Park.
(Wineman.)

Fig. 218.—Hot springs deposits. Terraces of Mammoth Hot Springs, Yellowstone Park.

A few geysers have somewhat definite periods of eruption. Of such “Old Faithful” is the type; but even this
geyser, which formerly erupted at regular intervals of about an hour, is losing the reputation on which its name is
based. Not only is its period of eruption lengthening, but it is becoming irregular, and the irregularity appears to
be increasing. In the short time during which this geyser has been under observation its period has changed from
a regular one of sixty minutes, or a little less, to an irregular one of seventy to ninety minutes. In the case of
some geysers years elapse between eruptions, and in some the date of the last eruption is so distant that it is
uncertain whether the vent should be looked upon as a geyser or merely a hot spring.
In the Yellowstone Park[120] the geysers are mainly in the bottoms of valleys (Fig. 219), but the deposits
characteristic of geysers are found in not a few places well above the present bottoms. These deposits record the
fact that in earlier times the geysers were at higher levels than now. It is probable that they have been, at all
stages in their history, near the bottoms of the valleys, and that, as the valleys have been deepened the ground-
water has found lower and lower points of issue. In this respect the geysers have probably had the same history
as other springs.

Fig. 219.—Hot springs and geysers. Norris Geyser basin, Yellowstone Park.

Unless new intrusions of lava occur, or unless heat is otherwise renewed at the proper points, it is probable
that all existing geysers will become extinct within a time which is, geologically, short. New geyser regions may,
however, develop as old ones disappear.
Artesian wells.—Originally the terms artesian wells and flowing wells were synonymous; but at the present
time any notably deep well is called artesian, especially if it descends to considerable depths below the mantle
rock. The artesian well which does not flow, does not differ from common wells in principle; but being deeper,
the water which it affords is often more thoroughly filtered and frequently more highly mineralized than that of
other wells. The flowing well is really a gushing spring, the opening of which was made by man.
Flowing wells[121] depend upon certain relations of rock structure, water supply, and elevation. Generally
speaking a flowing well is possible in any place underlain by any considerable bed of porous rock, if such rock
outcrops at a sufficiently higher level in a region of adequate rainfall, and is covered by a layer or bed of
impervious, or relatively impervious rock. This statement involves four conditions, all of which are illustrated
by Fig. 199, where a is the bed of porous rock. It is not necessary that the beds of rock form a structural basin,
nor is it usually necessary to take account of the character of the rock beneath the porous bed which contains the
water.
The bed of porous rock is the “reservoir” of the flowing well. Formations of sand or sandstone, and of gravel
or conglomerate, most commonly serve as the reservoirs. In order that it may contain abundant water it must
have some thickness, and its outcropping edge must be so situated that the water may enter freely and be
replenished, chiefly by rain, as the water flows out at the well.
A relatively impervious layer of rock above the reservoir (b, Fig. 199) is most important; otherwise the water
in the reservoir will leak out, and there will be little or no “head” at the well site. Thus if the rock overlying
stratum a (Fig. 199) were badly broken, the fractures extending up to the surface, the conditions would be
unfavorable for flowing wells. Under such conditions, wells in the positions of those shown in Fig. 199 might

get abundant water, but they would not be likely to flow. If the stratum next below the reservoir is not
impervious, some lower one probably is. No layer of rock is more impervious than one which is full of water,
and the substructure of any bed which might serve as a reservoir is usually full of water, even if the rock be
porous.
If the outcrop of the reservoir be notably above the site of the well, and if it be kept full by frequent rains, the
“head” will be strong, though the water at the well will not rise to the level of the outcrop of the reservoir.
Experience has shown that an allowance of about one foot per mile of subterranean flow should be made. Thus
if the site of the well be 100 miles from the outcrop of the water-bearing stratum, and 200 feet below it, the
water will rise something like 100 feet above the surface at the well. This rule is, however, not applicable
everywhere. The failure of the water to rise to the level of its head is due to the adhesion and the friction of flow
through the rock. The more porous the rock the less the reduction of head by friction. The height of the flow is
also influenced by the number of wells drawing on the same reservoir, on the degree of imperviousness of the
confining bed above, etc.
Flowing wells, often relatively shallow, are frequently obtained from unconsolidated drift. Some such
relations as suggested by Fig. 220 would afford the conditions for flowing wells in such a formation.

Fig. 220.—Figure illustrating the principle of artesian wells in drift.

CHAPTER V.
THE WORK OF SNOW AND ICE.

A part of the atmospheric precipitation falls as snow, and this, like the
rain, does its appropriate work in degrading the land. Over the larger part of
the land surface the snow of the winter does not endure through the
succeeding summer, and when it melts it follows the same course as the
precipitation which falls as rain; but in cold regions where the fall of snow
is heavy some of it remains unmelted and constitutes perennial snow-fields.

SNOW- AND ICE-FIELDS.

Snow-fields.—Mountain heights and polar lands are the most common
habitats of snow-fields, though they are not confined to these situations. In
North America there are numerous small snow-fields in the western
mountains, from Mexico on the south to Alaska on the north, their number
and size increasing in the latter direction. In the United States there are few
snow-fields south of the parallel of 36° 30′, and most of the many hundreds
north of that latitude (excluding Alaska) are small (Pl. XVIII, Fig. 1,
Washington, Lat. 48° 5′, Long. 121° 5′; Fig. 2, Lat. 41° 25′, Long. 122° 12′.
From Glacier Peak and Shasta Special Quadrangles, U. S. Geol. Surv.).
Farther north, especially in Alaska, the snow-fields of the western
mountains attain much greater size. In Europe snow-fields comparable to
those of the northwestern part of the United States and British Columbia
occur in the Alps (Fig. 221), the Pyrenees, the Caucasus, and the
Scandinavian mountains. In Asia snow-fields occur in the Himalayas and in
many of the high mountains farther north, from Turkestan on the southwest
nearly to the coast on the northeast. In South America there are snow-fields
of small size even in equatorial latitudes, and farther south in the Chilean
Andes there are some of considerable size. Small snow-fields occur on the
highest peaks of tropical Africa, and in the mountains of New Zealand. For

reasons which will appear later, much of every considerable snow-field is
really ice.
In addition to these limited fields of snow in mountain regions, there are
fields of much greater extent covering wide expanses of plain and plateau in
the polar regions. The greater part of the island of Greenland is covered
with a single field of ice and snow, the size of which is variously estimated
at 300,000 to 400,000 square miles (Fig. 222)—an area 400 to 600 times as
large as the snow-and-ice-covered area of Switzerland. Numerous islands to
the west of North Greenland are also partly covered with snow, the areas of
the snow-fields far exceeding those of most mountain regions. In Antarctica
there is believed to be a still larger field, the largest of the earth. Its area is
not even approximately known, but such data as are at hand indicate that it
may have an extent of 3,000,000 or 4,000,000 square miles.

Fig. 221.—An alpine snow-field.

The only condition necessary
for a snow-field is an excess of
snowfall over snow waste. The
lower edge of a snow-field, the
snow-line, is dependent chiefly on
temperature and snowfall. In
general it does not depart much
from the summer isotherm of 32°,
though it may be well above this
isotherm where the snowfall is
light. That the snow-line is not a
function of temperature only is
shown by its position in various
places. In the equatorial portion of
the Andes, for example, the snow-
line has an altitude of about
16,000 feet on the east side of the
mountains, where the precipitation
is heavier, and of about 18,500
feet on the west side, where it is
lighter. For the same reason the
snow-line in the Himalayas is
3000 or 4000 feet lower on the
south side than on the north.
While in equatorial regions the
Fig. 222.—Map of Greenland. The borders snow-line has an altitude of
only are free from ice. (Stieler.) 15,000 to 18,000 feet, it
approaches or even reaches sea-level in the latitude of Antarctica and North
Greenland. In intermediate latitudes it has an intermediate position.
While temperature and snowfall are the most important factors
controlling the position of the snow-line, both humidity and the movements
of the air are of some importance, since both affect the rate of evaporation
of snow and ice.
The passage of snow into névé and ice.—The snow does not lie on the
surface long before it undergoes obvious change. The light flakes soon
begin to be transformed into granules, and the snow becomes “coarse-

grained.” The granular character, so pronounced in the snow of the last
banks which remain in the spring in temperate latitudes, is even more
distinct in perennial snow-fields, either at the surface or just beneath it. This
granular snow is called névé, or firn. Still deeper beneath the surface, where
the thickness of the snow is great, the névé becomes more compact and
finally coherent, and grades into porous ice. This gradation is accomplished
at no great depth, though the thicknesses of snow and névé are by no means
constant.
Structure of the ice.—Ice formed beneath a snow-field is in some sense
stratified. It is made up of successive falls of snow which tend to retain the
form of layers. This follows from two or three conditions. The snow of one
season, or of one period of precipitation, may have been considerably
changed before the succeeding fall of snow. So also the surface of the
snow-field at the end of the melting season is often covered with a visible
amount of earthy matter, some of which was blown up and dropped on the
surface during the melting season, and some of which was concentrated in
that position by the melting of the snow in which it was originally
imbedded. The amount of earthy matter is often sufficient to define snows
of successive years, or perhaps of minor periods of precipitation, and makes
distinct the stratification which would otherwise remain obscure. The
snowfall of successive years has been estimated by this means[122] where the
snow is exposed in crevices in the snow-field.
In addition to its rude stratification, the ice of the deeper portions often
acquires a stratiform structure which may perhaps best be called foliation to
distinguish it from the stratification which arises from deposition. The
foliation appears to result mainly from the shearing of one part over another
in the course of the movements to which the ice is subjected, as will be
illustrated presently.
Texture.—The ice derived from the snow is formed of interlocking
crystalline grains. The crystalline character is present from the beginning,
for it is assumed by the snowflakes when they form, and the subsequent
changes seem only to modify the original crystals by building up some and
destroying others. By the time the snow is converted into névé, the granules
have become coarse, and wherever the ice derived from the névé has been
examined, the granular crystalline texture is present. The individual crystals
in the ice are usually larger than those of the névé, and more closely grown

together. In the fresh unexposed ice the crystals are so intimately
interlocked that they are not readily seen except under a polarizing
microscope, but when the ice has been honeycombed by partial melting, the
granules become partially separated and may be easily seen. Fig. 223 shows
quantities of them which have been washed down from the surface, and
disposed as cones at its base. While a given mass of snow in a great snow-
and ice-field cannot be followed consecutively through its whole history,
yet since (1) the granular texture is pronounced in the névé stage where the
granules show evidences of growth, and since (2) the same texture is also
pronounced in the last stages of the ice when it is undergoing dissolution, as
well as at all observed intermediate stages, and since (3) the crystals are, on
the average, larger in proportion to the lateness of the stage of their history,
while (4) experiment has shown that granules grow under the conditions
which exist in snow-fields, and (5) that they persist under very considerable
pressure, it is legitimate to assume that a granular crystalline condition
persists throughout all stages, and is a feature of progressive growth.

Fig. 223.—Figure showing cones of granules of ice which have been washed down the front
of the glacier by streamlets, and accumulated after the manner of talus or alluvial cones.
North Greenland.

Inauguration of movement.—Eventually the increase in depth of snow
and ice in a snow-field gives rise to motion. The exact nature of the motion
has not yet been demonstrated to the satisfaction of all investigators. Brittle

and resistant as ice seems, it exhibits, under proper conditions, some of the
outward characteristics of a plastic substance. Thus it may be made to
change its form, and may even be moulded into almost any desired shape if
carefully subjected to sufficient pressure, steadily applied through long
intervals of time.[123] These changes may be brought about without visible
fracture, and have been thought to point to a viscous condition of the ice.
There is much reason, however, as will be seen later, to question this
interpretation of the ultimate nature of the movement. Whatever this may
be, the mass result of the movement in a field of ice is comparable, in a
superficial way at least, to that which would be brought about if the ice
were capable of moving like a viscous liquid, the motion taking place with
extreme slowness. This slow motion of ice in an ice-field is glacier motion,
and ice thus moving is glacier ice.

Fig. 224.—Ice-caps of small size. The figure also shows some valley glaciers extending out
from the main ice-sheet and from the local ice-caps. A portion of the North Greenland
coast, north of Inglefield Gulf. Lat. about 78°. (Peary.)

Fig. 225.—Small ice-caps in the northwestern part of Iceland. (Thoraddsen’s geological
map of Iceland.)

Fig. 226.—A glacial lobe, midway between an ice-cap and a valley glacier. A protrusion
from a local ice-cap east of Cape York, Greenland.

If both the surface on which the ice-sheet develops and its surroundings
be essentially plane, as may happen in high latitudes, and if the snow- and
ice-field be symmetrical in shape, the outward movement will be
approximately equal in all directions, and the area covered by the spreading
ice-field will remain more or less circular. If the ice-field rests on a steeply
inclined surface, like a mountain slope, the movement becomes one-sided

in conformity to the slope. If the surface, otherwise plane, be affected by
valleys parallel to the direction of movement, the ice in the valleys will be
deeper than that on the divides between them, and its movement stronger. In
the valleys, therefore, the ice will advance farther than elsewhere before
being melted, and the outline of the ice will become lobate, the lobes
occupying the depressions. These general relations are shown in Figs. 224
and 225. If the depressions be wide and shallow, the lobes will be broad and
short (Fig. 226); if the depressions be narrow and deep, the lobes will be
relatively narrow and Long. If the snow and ice rest on a surface consisting
chiefly of steep valleys and sharp ridges, as is common on mountain slopes,
the snow and ice are chiefly gathered in the valleys, and take a linear form.

TYPES OF GLACIERS.

These different forms give rise to different terms. The ice which spreads
with some approach to equality in all directions from a center is a glacier, is
indeed the type of the greatest glaciers, but is commonly called an ice-cap.
The same name is applied to any glacier in which there is movement in all
directions from the center, even though its shape departs widely from a
circle. The glacier covering the larger part of Greenland (Fig. 222) is a good
example of a large ice-cap, and the glaciers on some of the flat-topped
peninsular promontories of the same island are good examples of small
ones (Fig. 224). If ice-caps cover a large part of a continent, as some of
those of the past have done, they are often called continental glaciers.

Fig. 227.—Characteristic end of a North Greenland (Bryant) glacier.

Fig. 228.—The Rhône glacier. (Reid.)

Fig. 229.—Characteristic end of a North Greenland glacier. North side of Herbert Island,
Inglefield Gulf.

Fig. 230.—The end of an alpine (Forno, Switzerland) glacier. (Reid.)

Fig. 231.—Deploying end of a North Greenland glacier.

Where ice-caps are developed on plateaus whose borders are trenched by
valleys, ice-tongues from the edge of the ice-cap often extend down into the
valleys and give rise to one type of valley glacier (Figs. 224 and 227). A
second and more familiar type of valley glacier occupies mountain valleys,
and is the offspring of mountain snow-fields (Fig. 228). The former are
confined chiefly to high latitudes, and are distinguished as polar or high-
latitude glaciers (Figs. 227 and 229); the latter are known as alpine glaciers
(Figs. 228 and 230). The end and side slopes of polar glaciers are, as a rule,
much steeper than those of alpine glaciers. When a valley glacier descends
through its valley to the plain beyond, its end deploys, forming a fan (Fig.
231). The deploying ends of adjacent glaciers sometimes merge, and the
resulting body of ice constitutes a piedmont glacier (Fig. 232). At the
present time, piedmont glaciers are confined to high latitudes. In some cases
the snow-field that gives rise to a glacier is restricted to a relatively small
depression in the side of a mountain, or in the escarpment of a plateau. In
such cases the snow-field and glacier are hardly distinguishable, and the
latter descends but little below the snow-line. In many cases it does not
even enter the narrow valley which leads out from the depression occupied
by the snow-field. Such a glacier is nestled in the face of a cliff, and may
therefore be called a cliff glacier[124] (Figs. 233 and 234). The snow-field of
a cliff glacier is sometimes no more than a great snowdrift, accumulated

through successive years. Cliff glaciers are often as wide as long, and are
always small, and between them and valley glaciers there are all gradations
(Fig. 235). Occasionally the end of a valley glacier, or the edge of an ice-
sheet reaches a precipitous cliff, and the end or edge of the ice breaks off
and accumulates like talus below. The ice fragments may then again
become a coherent mass by regelation, and the whole may resume motion.
Such a glacier is called a reconstructed glacier. The precipitous cliffs of the
Greenland coast furnish illustrations.

Fig. 232.—The Malaspina glacier, Alaska; the best known example of a piedmont glacier.
(Russell.)

Fig. 233.—A cliff glacier. North Greenland type. North side of Herbert Island, Inglefield
Gulf. The lower half of the white area is snow, and snow talus. So also are the white
patches to the right. The height of the cliff is perhaps 2000 feet. The water in the
foreground is the sea.

Fig. 234.—Chancy glacier; a cliff glacier of the Montana type. (Shepard.)

Of the foregoing types of glaciers, the ice-caps far exceed all others both
in size and importance, while the valley glaciers out-rank, in the same
respects, the other types; but since the valley glaciers are the most familiar
type, the general phenomena of glaciers will be discussed with primary
reference to them.

THE GENERAL PHENOMENA OF GLACIERS.[125]

Dimensions.—Glaciers which occupy valleys leading down from snow-
fields sometimes reach the upper parts of the valleys only, sometimes
extend through them, and sometimes push out on the plain beyond. In
length they range from a fraction of a mile to many miles, and though their
width is usually much less than their length, the reverse is sometimes the
case (Figs. 233, 234, and 235). Their thickness is usually measured by

hundreds of feet rather than by denominations of other orders, but the
variation is great, and exact measurements are almost wholly wanting. The
minimum thickness is that necessary to cause movement, and this varies
with the slope, the temperature, and other conditions. There is also much
variation in the thickness in different parts of a glacier. As a rule, it is
thinnest in its terminal portion, and thickest at some point intermediate
between this and its source, but nearer the latter than the former. Cliff and
reconstructed glaciers are comparable in size to the smaller valley glaciers.
Piedmont glaciers may attain greater size.

Fig. 235.—A glacier in the Cascades near Cascade Pass, Wash. A glacier intermediate
between a cliff glacier and a valley glacier. (Willis, U. S. Geol. Surv.)

An ice-cap is theoretically thickest at its center and thins away to its
borders, but its actual dimensions are influenced by the topography on
which it is developed. The Greenland ice-cap is known to rise about 9000
feet above the sea, and it probably reaches considerably higher than this in
the unexplored center of its broad dome. The height of the land surface
beneath is unknown, but it is unlikely that it averages half this amount, and
hence the ice is probably 5000 feet or more thick in the center. There is
reason to think that it is much thicker in Antarctica.

Limits.—The ice of a glacier is always moving forward (neglecting
temporary halts), but the end of a glacier may be retreating, advancing or
remaining stationary, according as the rate of wastage is greater, less, or just
equal to the forward movement of the ice. The position of the lower end of
the glacier is therefore determined by the ratio of movement to wastage. Its
upper end is generally ill-defined. In a superficial sense, it is the point
where the ice emerges from the snow-field; but the lower limit of the snow-
field is often ill-defined, and in any case is not the true upper limit of the
glacier, since there must be movement from the granular mass of ice
beneath the snow to make up for the waste below, and the moving ice
beneath the snow-field which feeds the tongue of ice in the valley is just as
really a part of the glacier as the more consolidated portion in the valley
below. If a definite upper limit for an alpine glacier is to be named, it should
probably be the Bergschrund, a gaping crevasse, or series of crevasses
which sometimes open near the precipitous slope of the peak or cliff where
the snow-field lies. The Bergschrund is formed by the moving of the lower
part of the snow-field away from the portion above.
The lower end of a glacier is usually free from snow and névé in summer,
but, traced toward its source, it first becomes covered with névé, then with
snow, and finally merges into the snow-field without having ceased to be a
glacier. The term glacier is, however, commonly used to mean merely the
more solid portion outside (below) the névé.
Movement.—The fact of glacier movement is established in various
ways, the most obvious being by the advance of its lower end. Such
advance is too slow to be seen from day to day, and is only detected when
the lower end of the glacier overrides or overturns objects in front of it, or
moves out over ground previously unoccupied. But even when the end of a
glacier is not advancing, the movement of the ice may be established by
means of stakes or other marks set on the surface. If the positions of these
marks relative to fixed points on the sides of the valley be determined, they
are found after a time to have moved down the valley. Rows of stakes or
lines of stones set across a glacier in the upper, middle and lower portions
have revealed many facts concerning the movement of the ice.
Generally speaking, the middle of a valley glacier moves more rapidly
than its sides (Fig. 236), but in some cases, especially in large glaciers,
there are found to be two or more main lines of movement, with belts of