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Title: The Elements of Geology

Author: William Harmon Norton

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*** START OF THE PROJECT GUTENBERG EBOOK THE ELEMENTS OF
GEOLOGY ***

A Valley with Rocky Ledges cut in Horizontal Strata, Scotland

THE
ELEMENTS OF GEOLOGY

BY

WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE

GINN & COMPANY

BOSTON * NEW YORK * CHICAGO * LONDON

Copyright, 1905, by

WILLIAM HARMON NORTON

ALL RIGHTS RESERVED

5511

T h e At h en eu m P ress
GINN & COMPANY PROPRIETORS
BOSTON * U.S.A.

PREFACE
Geology is a science of such rapid growth that no apology is expected when from
time to time a new text-book is added to those already in the field. The present work,
however, is the outcome of the need of a text-book of very simple outline, in which
causes and their consequences should be knit together as closely as possible,—a need
long felt by the author in his teaching, and perhaps by other teachers also. The author
has ventured, therefore, to depart from the common usage which subdivides geology
into a number of departments,—dynamical, structural, physiographic, and historical,
—and to treat in immediate connection with each geological process the land forms
and the rock structures which it has produced.

It is hoped that the facts of geology and the inferences drawn from them have
been so presented as to afford an efficient discipline in inductive reasoning. Typical
examples have been used to introduce many topics, and it has been the author’s aim
to give due proportion to both the wide generalizations of our science and to the
concrete facts on which they rest.

There have been added a number of practical exercises such as the author has
used for several years in the class room. These are not made so numerous as to

displace the problems which no doubt many teachers prefer to have their pupils solve
impromptu during the recitation, but may, it is hoped, suggest their use.

In historical geology a broad view is given of the development of the North
American continent and the evolution of life upon the planet. Only the leading types
of plants and animals are mentioned, and special attention is given to those which
mark the lines of descent of forms now living.

By omitting much technical detail of a mineralogical and paleontological nature,
and by confining the field of view almost wholly to our own continent, space has
been obtained to give to what are deemed for beginners the essentials of the science a
fuller treatment than perhaps is common.

It is assumed that field work will be introduced with the commencement of the
study. The common rocks are therefore briefly described in the opening chapters. The
drift also receives early mention, and teachers in the northern states who begin
geology in the fall may prefer to take up the chapter on the Pleistocene immediately
after the chapter on glaciers.

Simple diagrams have been used freely, not only because they are often clearer
than any verbal statement, but also because they readily lend themselves to
reproduction on the blackboard by the pupil. The text will suggest others which the
pupil may invent. It is hoped that the photographic views may also be used for
exercises in the class room.

The generous aid of many friends is recognized with special pleasure. To
Professor W. M. Davis of Harvard University there is owing a large obligation for the
broad conceptions and luminous statements of geologic facts and principles with
which he has enriched the literature of our science, and for his stimulating influence
in education. It is hoped that both in subject-matter and in method the book itself
makes evident this debt. But besides a general obligation shared by geologists
everywhere, and in varying degrees by perhaps all authors of recent American text-
books in earth science, there is owing a debt direct and personal. The plan of the
book, with its use of problems and treatment of land forms and rock structures in
immediate connection with the processes which produce them, was submitted to
Professor Davis, and, receiving his approval, was carried into effect, although
without the sanction of precedent at the time. Professor Davis also kindly consented
to read the manuscript throughout, and his many helpful criticisms and suggestions
are acknowledged with sincere gratitude.

Parts of the manuscript have been reviewed by Dr. Samuel Calvin and Dr. Frank
M. Wilder of the State University of Iowa; Dr. S. W. Beyer of the Iowa College of

Agriculture and Mechanic Arts; Dr. U. S. Grant of Northwestern University;
Professor J. A. Udden of Augustana College, Illinois; Dr. C. H. Gordon of the New
Mexico State School of Mines; Principal Maurice Ricker of the High School,
Burlington, Iowa; and the following former students of the author who are engaged
in the earth sciences: Dr. W. C. Alden of the United States Geological Survey and the
University of Chicago; Mr. Joseph Sniffen, instructor in the Academy of the
University of Chicago, Morgan Park; Professor Martin Iorns, Fort Worth University,
Texas; Professor A. M. Jayne, Dakota University; Professor G. H. Bretnall,
Monmouth College, Illinois; Professor Howard E. Simpson, Colby College, Maine;
Mr. E. J. Cable, instructor in the Iowa State Normal College; Principal C. C. Gray of
the High School, Fargo, North Dakota; and Mr. Charles Persons of the High School,
Hannibal, Missouri. A large number of the diagrams of the book were drawn by Mr.
W. W. White of the Art School of Cornell College. To all these friends, and to the
many who have kindly supplied the illustrations of the text, whose names are
mentioned in an appended list, the writer returns his heartfelt thanks.

WILLIAM HARMON NORTON
Cornell College, Mount Vernon, Iowa
July, 1905

INTRODUCTORY NOTE
During the preparation of this book Professor Norton has frequently discussed its
plan with me by correspondence, and we have considered together the matters of
scope, arrangement, and presentation.

As to scope, the needs of the young student and not of the expert have been our
guide; the book is therefore a text-book, not a reference volume.

In arrangement, the twofold division of the subject was chosen because of its
simplicity and effectiveness. The principles of physical geology come first; the
several chapters are arranged in what is believed to be a natural order, appropriate to
the greatest part of our country, so that from a simple beginning a logical sequence of
topics leads through the whole subject. The historical view of the science comes
second, with many specific illustrations of the physical processes previously studied,
but now set forth as part of the story of the earth, with its many changes of aspect
and its succession of inhabitants. Special attention is here given to North America,
and care is taken to avoid overloading with details.

With respect to method of presentation, it must not be forgotten that the text-book
is only one factor in good teaching, and that in geology, as in other sciences, the

teacher, the laboratory, and the local field are other factors, each of which should
play an appropriate part. The text suggests observational methods, but it cannot
replace observation in field or laboratory; it offers certain exercises, but space cannot
be taken to make it a laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the illustrative
descriptions that a good and experienced teacher should supply. Frequent use is made
of induction and inference in order that the student may come to see how reasonable
a science is geology, and that he may avoid the too common error of thinking that the
opinions of “authorities” are reached by a private road that is closed to him. The
further extension of this method of presentation is urged upon the teacher, so that the
young geologist may always learn the evidence that leads to a conclusion, and not
only the conclusion itself.

W. M. DAVIS
Harvard University, Cambridge, Mass.
July, 1905

ACKNOWLEDGMENT OF ILLUSTRATIONS
Adams, Professor F. D., McGill University, Canada, 241.

Alden, Dr. W. C., Washington, D.C., 353.

American Museum of Natural History, New York, 344.

Ash, H. C., Galesburg, Ill., 133.

Beyer, Dr. S. W., Iowa College of Agriculture, 363.

Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371.

Carney, Frank, Ithaca, N.Y., 356.

Clark, Dr. Wm. B., Maryland Geological Survey, 43.

Borne, Dr. Georg v. d., Jena, Germany, 5, 6.

Daly, Dr. R. A., Ottawa, Canada, 164.

Defieux, C. A., Liverpool, England, 154.

* Detroit Photographic Co., 235, 236.

* Ellis, W. M., Edna, Kan., 13.

Fairchild, Professor H. L., University of Rochester, 141, 357.

Field Columbian Museum, Chicago, 87.

Forster, Dr. A. E., University of Vienna, 32.

Gardner, J. L., Boston, 12, 140, 352.

Geological Survey of Canada, 256.

Gilbert, Dr. G. K., by courtesy of the American Book Company, 39.

* Haines, Ben, New Albany, Ind., 33.

* Haynes, F. J., St. Paul, Minn., 52, 95, 233.

Henderson, Judge Julius, Boulder, Col., 94.

James, George Wharton, Pasadena, Cal., 16, 127, 215, 229.

Johnston-Lavis, Professor H. J., Beaulieu, France, 216.

King, J. Harding, Stourbridge, England, 119.

Lawson, Dr. Andrew C., University of California, 113.

Le Conte, Professor J. N., University of California, 8.

Libbey, Dr. William, Princeton University, 92.

* McAllister, T. H., New York, 242.

* Meyers, H. C., Boise, Id., 19.

Mills, Professor H. A., Cornell College, 208, 304.

Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128, 183, 226,
234, 255, 340, 364, 367.

* Notman, Wm. & Son, Montreal, Canada, 98, 181.

Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73.

Oldham, Dr. R. D., Geological Survey of India, 120.

* Peabody, H. C., Pasadena, Cal., 54.

* Pierce, C. C. & Co., Los Angeles, Cal., 15.

Pillsbury, Arthur, San Francisco, Cal., 115.

* Rau, Wm., Philadelphia, 18, 21, 122, 123, 218.

Reusch, Dr. Hans, Geological Survey of Norway, 112.

Reynolds, Professor S. H., University College, Bristol, England, 202.

Ricker, Principal Maurice Burlington, Iowa, 48, 89.

* Shepard, E. A., Minneapolis, Minn., 105.

Smith, W. S. Tangier, Los Gatos, Cal., 186.

* Soule Photographic Co., Boston, 131.

U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79, 80, 110, 111,
114, 125, 126, 129, 130, 142, 151, 153, 169, 172, 177, 178, 188, 211,
212, 214, 228, 237, 238, 239, 243, 244, 254, 257, 340, 341, 353,
355.

U. S. National Museum, 149, 220, 221, 222, 225, 332.

* Valentine & Sons, Dundee, Scotland, 40, 136, 227.

Vroman, A. C., Pasadena, Cal., 17.

* Ward’s Natural Science Establishment, Rochester, N.Y., 152.

* Welch, R., Belfast, Ireland, 1, 37.

* Westgate, Dr. L. G., Ohio Wesleyan University, 66.

Whymper, Edward, London, England, 106.

* Wilcox, W. D., Washington, D.C., 20.

* Wilson, Dr. A. W. G., McGill University, Canada, 68.

* Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213.

* Worsley-Benison, F. H., Cheapstow, England, 170.

* Dealer in photographs or lantern slides.

CONTENTS
Page
Introduction.—The Scope And Aim Of Geology 1

PART I

EXTERNAL GEOLOGICAL AGENCIES
Chapter
I. The Work Of The Weather 5
II. The Work Of Ground Water 39
III. Rivers And Valleys 54
IV. River Deposits 93
V. The Work Of Glaciers 113
VI. The Work Of The Wind 144
VII. The Sea And Its Shores 155
VIII. Offshore And Deep-Sea Deposits 174

PART II

INTERNAL GEOLOGICAL AGENCIES
IX. Movements Of The Earth’s Crust 195
X. Earthquakes 233
XI. Volcanoes 238
XII. Underground Structures Of Igneous Origin 265
XIII. Metamorphism And Mineral Veins 281

PART III

HISTORICAL GEOLOGY
XIV. The Geological Record 291
XV. The Pre-Cambrian Systems 304
XVI. The Cambrian 315
XVII. The Ordovician And Silurian 327
XVIII. The Devonian 341
XIX. The Carboniferous 350
XX. The Mesozoic 368
XXI. The Tertiary 394
XXII. The Quaternary 416

INDEX 451

THE ELEMENTS OF GEOLOGY

INTRODUCTION

THE SCOPE AND AIM OF GEOLOGY
Geology deals with the rocks of the earth’s crust. It learns from their composition
and structure how the rocks were made and how they have been modified. It
ascertains how they have been brought to their present places and wrought to their
various topographic forms, such as hills and valleys, plains and mountains. It studies
the vestiges which the rocks preserve of ancient organisms which once inhabited our
planet. Geology is the history of the earth and its inhabitants, as read in the rocks of
the earth’s crust.

To obtain a general idea of the nature and method of our science before
beginning its study in detail, we may visit some valley, such as that illustrated in the
frontispiece, on whose sides are rocky ledges. Here the rocks lie in horizontal layers.
Although only their edges are exposed, we may infer that these layers run into the
upland on either side and underlie the entire district; they are part of the foundation
of solid rock which everywhere is found beneath the loose materials of the surface.

The ledges of the valley of our illustration are of sandstone. Looking closely at
the rock we see that it is composed of myriads of grains of sand cemented together.
These grains have been worn and rounded. They are sorted also, those of each layer
being about of a size. By some means they have been brought hither from some more
ancient source. Surely these grains have had a history before they here found a
resting place,—a history which we are to learn to read.

The successive layers of the rock suggest that they were built one after another
from the bottom upward. We may be as sure that each layer was formed before those
above it as that the bottom courses of stone in a wall were laid before the courses
which rest upon them.

We have no reason to believe that the lowest layers which we see here were the
earliest ever formed. Indeed, some deep boring in the vicinity may prove that the
ledges rest upon other layers of rock which extend downward for many hundreds of
feet below the valley floor. Nor may we conclude that the highest layers here were
the latest ever laid; for elsewhere we may find still later layers lying upon them.

A short search may find in the rock relics of animals, such as the imprints of
shells, which lived when it was deposited; and as these are of kinds whose nearest
living relatives now have their home in the sea, we infer that it was on the flat sea
floor that the sandstone was laid. Its present position hundreds of feet above sea level
proves that it has since emerged to form part of the land; while the flatness of the
beds shows that the movement was so uniform and gentle as not to break or strongly
bend them from their original attitude.

The surface of some of these layers is ripple-marked. Hence the sand must once
have been as loose as that of shallow sea bottoms and sea beaches to-day, which is
thrown into similar ripples by movements of the water. In some way the grains have
since become cemented into firm rock.

Note that the layers on one side of the valley agree with those on the other, each
matching the one opposite at the same level. Once they were continuous across the
valley. Where the valley now is was once a continuous upland built of horizontal

layers; the layers now show their edges, or outcrop, on the valley sides because they
have been cut by the valley trench.

The rock of the ledges is crumbling away. At the foot of each step of rock lie
fragments which have fallen. Thus the valley is slowly widening. It has been
narrower in the past; it will be wider in the future.

Through the valley runs a stream. The waters of rains which have fallen on the
upper parts of the stream’s basin are now on their way to the river and the sea. Rock
fragments and grains of sand creeping down the valley slopes come within reach of
the stream and are washed along by the running water. Here and there they lodge for
a time in banks of sand and gravel, but sooner or later they are taken up again and
carried on. The grains of sand which were brought from some ancient source to form
these rocks are on their way to some new goal. As they are washed along the rocky
bed of the stream they slowly rasp and wear it deeper. The valley will be deeper in
the future; it has been less deep in the past.

In this little valley we see slow changes now in progress. We find also in the
composition, the structure, and the attitude of the rocks, and the land forms to which
they have been sculptured, the record of a long succession of past changes involving
the origin of sand grains and their gathering and deposit upon the bottom of some
ancient sea, the cementation of their layers into solid rock, the uplift of the rocks to
form a land surface, and, last of all, the carving of a valley in the upland.

Everywhere, in the fields, along the river, among the mountains, by the seashore,
and in the desert, we may discover slow changes now in progress and the record of
similar changes in the past. Everywhere we may catch glimpses of a process of
gradual change, which stretches backward into the past and forward into the future,
by which the forms and structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long process is geology.
Geology treats of the natural changes now taking place upon the earth and within it,
the agencies which produce them, and the land forms and rock structures which
result. It studies the changes of the present in order to be able to read the history of
the earth’s changes in the past.

The various agencies which have fashioned the face of the earth may. be divided
into two general classes. In Part I we shall consider those which work upon the earth
from without, such as the weather, running water, glaciers, the wind, and the sea. In
Part II we shall treat of those agencies whose sources are within the earth, and among
whose manifestations are volcanoes and earthquakes and the various movements of
the earth’s crust. As we study each agency we shall notice not only how it does its
work, but also the records which it leaves in the rock structures and the land forms

which it produces. With this preparation we shall be able in Part III to read in the
records of the rocks the history of our planet and the successive forms of life which
have dwelt upon it.

Part I

EXTERNAL GEOLOGICAL AGENCIES

CHAPTER I

THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed a process
which is going forward in all lands. Everywhere the rocks are crumbling away; their
fragments are creeping down hillsides to the stream ways and are carried by the
streams to the sea, where they are rebuilt into rocky layers. When again the rocks are
lifted to form land the process will begin anew; again they will crumble and creep
down slopes and be washed by streams to the sea. Let us begin our study of this long
cycle of change at the point where rocks disintegrate and decay under the action of
the weather. In studying now a few outcrops and quarries we shall learn a little of
some common rocks and how they weather away.

Stratification and jointing. At the sandstone ledges we saw that the rock was
divided into parallel layers. The thicker layers are known as strata, and the thin
leaves into which each stratum may sometimes be split are termed laminæ. To a
greater or less degree these layers differ from each other in fineness of grain,
showing that the material has been sorted. The planes which divide them are called
bedding planes.

Besides the bedding planes there are other division planes, which cut across the
strata from top to bottom. These are found in all rocks and are known as joints

(Fig. 1). Two sets of joints, running at about right angles to each other, together with
the bedding planes, divide the sandstone into quadrangular blocks.

Sandstone. Examining a piece of
sandstone we find it composed of
grains quite like those of river sand or
of sea beaches. Most of the grains are
of a clear glassy mineral called
quartz. These quartz grains are very
hard and will scratch the steel of a
knife blade. They are not affected by
acid, and their broken surfaces are
irregular like those of broken glass.

The grains of sandstone are held
together by some cement. This may
be calcareous, consisting of soluble
carbonate of lime. In brown
sandstones the cement is commonly
ferruginous,—hydrated iron oxide, or
iron rust, forming the bond, somewhat
as in the case of iron nails which have
rusted together. The strongest and
most lasting cement is siliceous, and
sand rocks whose grains are closely
Fig. 1. Cliff of Sandstone, Ireland cemented by silica, the chemical
substance of which quartz is made,
are known as quartzites.

We are now prepared to understand how sandstone is affected by the action of the
weather. On ledges where the rock is exposed to view its surface is more or less
discolored and the grains are loose and may be rubbed off with the finger. On gentle
slopes the rock is covered with a soil composed of sand, which evidently is crumbled
sandstone, and dark carbonaceous matter derived from the decay of vegetation.
Clearly it is by the dissolving of the cement that the rock thus breaks down to loose
sand. A piece of sandstone with calcareous cement, or a bit of old mortar, which is
really an artificial stone also made of sand cemented by lime, may be treated in a test
tube with hydrochloric acid to illustrate the process.

A limestone quarry. Here also we find the rock stratified and jointed (Fig. 2). On
the quarry face the rock is distinctly seen to be altered for some distance from its
upper surface. Below the altered zone the rock is sound and is quarried for building;

but the altered upper layers are too soft and
broken to be used for this purpose. If the
limestone is laminated, the laminae here have
split apart, although below they hold fast
together. Near the surface the stone has
become rotten and crumbles at the touch, while
on the top it has completely broken down to a
thin layer of limestone meal, on which rests a
fine reddish clay. Fig. 2. Section of Limestone
Quarry
Limestone is made of minute grains of
carbonate of lime all firmly held together by a Scale, 1 in. = 30 ft. a, red residual clay;
calcareous cement. A piece of the stone placed mn, pitted surface of rotted limestone;
in a test tube with hydrochloric acid dissolves bb, limestone divided into thin layers; c,
with brisk effervescence, leaving the insoluble thick layers of laminated limestone, the
impurities, which were disseminated through laminæ being firmly cemented together;
it, at the bottom of the tube as a little clay. j, j, j, joints. Is bb thin-layered because
originally so laid, or because it has been
We can now understand the changes in the broken up by weathering, although once
upper layers of the quarry. At the surface of the like c thick-layered?
rock the limestone has completely dissolved,
leaving the insoluble residue as a layer of reddish clay. Immediately below the clay
the rock has disintegrated into meal where the cement between the limestone grains
has been removed, while beneath this the laminae are split apart where the cement
has been dissolved only along the planes of lamination where the stone is more
porous. As these changes in the rock are greatest at the surface and diminish
downward, we infer that they have been caused by agents working downward from
the surface.

At certain points these agencies have been more effective than elsewhere. The
upper rock surface is pitted. Joints are widened as they approach the surface, and
along these seams we may find that the rock is altered even down to the quarry floor.

A shale pit. Let us now visit some pit where shale—a laminated and somewhat
hardened clay—is quarried for the manufacture of brick. The laminae of this fine-
grained rock may be as thin as cardboard in places, and close joints may break the
rock into small rhombic blocks. On the upper surface we note that the shale has
weathered to a clayey soil in which all traces of structure have been destroyed. The
clay and the upper layers of the shale beneath it are reddish or yellow, while in many
cases the color of the unaltered rock beneath is blue.

The sedimentary rocks. The three kinds of layered rocks whose acquaintance
we have made—sandstone, limestone, and shale—are the leading types of the great
group of stratified, or sedimentary, rocks. This group includes all rocks made of
sediments, their materials having settled either in water upon the bottoms of rivers,
lakes, or seas, or on dry land, as in the case of deposits made by the wind and by
glaciers. Sedimentary rocks are divided into the fragmental rocks—which are made
of fragments, either coarse or fine—and the far less common rocks which are
constituted of chemical precipitates.

The sedimentary rocks are divided
according to their composition into the
following classes:

1. The arenaceous, or quartz rocks,
including beds of loose sand and
gravel, sandstone, quartzite, and
conglomerate (a rock made of
cemented rounded gravel or pebbles).

2. The calcareous, or lime rocks,
including limestone and a soft white
rock formed of calcareous powder
known as chalk.

3. The argillaceous, or clay rocks,
including muds, clays, and shales.
These three classes pass by mixture
into one another. Thus there are limy
and clayey sandstones, sandy and
clayey limestones, and sandy and limy
shales. Fig. 3. Conglomerate

Granite. This familiar rock may be studied as an example of the second great
group of rocks,—the unstratified, or igneous rocks. These are not made of cemented
sedimentary grains, but of interlocking crystals which have crystallized from a
molten mass. Examining a piece of granite, the most conspicuous crystals which
meet the eye are those of feldspar. They are commonly pink, white, or yellow, and
break along smooth cleavage planes which reflect the light like tiny panes of glass.
Mica may be recognized by its glittering plates, which split into thin elastic scales. A
third mineral, harder than steel, breaking along irregular surfaces like broken glass,
we identify as quartz.

How granite alters under the action of the weather may be seen in outcrops where
it forms the bed rock, or country rock, underlying the loose formations of the surface,
and in many parts of the northern states where granite bowlders and pebbles more or
less decayed may be found in a surface sheet of stony clay called the drift. Of the
different minerals composing granite, quartz alone remains unaltered. Mica weathers
to detached flakes which have lost their elasticity. The feldspar crystals have lost
their luster and hardness, and even have decayed to clay. Where long- weathered
granite forms the country rock, it often may be cut with spade or trowel for several
feet from the surface, so rotten is the feldspar, and here the rock is seen to break
down to a clayey soil containing grains of quartz and flakes of mica.

These are a few simple illustrations of the surface changes which some of the
common kinds of rocks undergo. The agencies by which these changes are brought
about we will now take up under two divisions,—chemical agencies producing rock
decay and mechanical agencies producing rock disintegration.

The Chemical Work Of Water
As water falls on the earth in rain it has already absorbed from the air carbon
dioxide (carbonic acid gas) and oxygen. As it sinks into the ground and becomes
what is termed ground water, it takes into solution from the soil humus acids and
carbon dioxide, both of which are constantly being generated there by the decay of
organic matter. So both rain and ground water are charged with active chemical
agents, by the help of which they corrode and rust and decompose all rocks to a
greater or less degree. We notice now three of the chief chemical processes
concerned in weathering,—solution, the formation of carbonates, and oxidation.

Solution. Limestone, although so little affected by pure water that five thousand
gallons would be needed to dissolve a single pound, is easily dissolved in water
charged with carbon dioxide. In limestone regions well water is therefore “hard.” On
boiling the water for some time the carbon dioxide gas is expelled, the whole of the
lime carbonate can no longer be held in solution, and much of it is thrown down to
form a crust or “scale” in the kettle or in the tubes of the steam boiler. All waters
which flow over limestone rocks or soak through them are constantly engaged in
dissolving them away, and in the course of time destroy beds of vast extent and great
thickness.

Fig. 4. Surface of Limestone furrowed by Weathering, Montana

The upper surface of limestone rocks becomes deeply pitted, as we saw in the
limestone quarry, and where the mantle of waste has been removed it may be found
so intricately furrowed that it is difficult to traverse (Fig. 4).

Beds of rock salt buried among the strata are dissolved by seeping water, which
issues in salt springs. Gypsum, a mineral composed of hydrated sulphate of lime, and
so soft that it may be scratched with the finger nail, is readily taken up by water,
giving to the water of wells and springs a peculiar hardness difficult to remove.

The dissolving action of moisture may be noted on marble tombstones of some
age, marble being a limestone altered by heat and pressure and composed of
crystalline grains. By assuming that the date on each monument marks the year of its
erection, one may estimate how many years on the average it has taken for
weathering to loosen fine grains on the polished surface, so that they may be rubbed
off with the finger, to destroy the polish, to round the sharp edges of tool marks in
the lettering, and at last to open cracks and seams and break down the stone. We may
notice also whether the gravestones weather more rapidly on the sunny or the shady
side, and on the sides or on the top.

The weathered surface of granular limestone containing shells shows them
standing in relief. As the shells are made of crystalline carbonate of lime, we may

infer whether the carbonate of lime is less soluble in its granular or in its crystalline
condition.

The formation of carbonates. In attacking minerals water does more than
merely take them into solution. It decomposes them, forming new chemical
compounds of which the carbonates are among the most important. Thus feldspar
consists of the insoluble silicate of alumina, together with certain alkaline silicates
which are broken up by the action of water containing carbon dioxide, forming
alkaline carbonates. These carbonates are freely soluble and contribute potash and
soda to soils and river waters. By the removal of the soluble ingredients of feldspar
there is left the silicate of alumina, united with water or hydrated, in the condition of
a fine plastic clay which, when white and pure, is known as kaolin and is used in the
manufacture of porcelain. Feldspathic rocks which contain no iron compounds thus
weather to whitish crusts, and even apparently sound crystals of feldspar, when
ground to thin slices and placed under the microscope, may be seen to be milky in
color throughout because an internal change to kaolin has begun.

Oxidation. Rocks containing
compounds of iron weather to reddish
crusts, and the seams of these rocks are
often lined with rusty films. Oxygen and
water have here united with the iron,
forming hydrated iron oxide. The effects of
oxidation may be seen in the alteration of
many kinds of rocks and in red and yellow
colors of soils and subsoils.

Pyrite is a very hard mineral of a pale
brass color, found in scattered crystals in
many rocks, and is composed of iron and Fig. 5. Bowlder split by Heat and
sulphur (iron sulphide). Under the attack of Cold, Western Texas
the weather it takes up oxygen, forming iron
sulphate (green vitriol), a soluble compound, and insoluble hydrated iron oxide,
which as a mineral is known as limonite. Several large masses of iron sulphide were
placed some years ago on the lawn in front of the National Museum at Washington.
The mineral changed so rapidly to green vitriol that enough of this poisonous
compound was washed into the ground to kill the roots of the surrounding grass.

Agents Of Mechanical Disintegration
Heat and cold. Rocks exposed to the direct rays of the sun become strongly
heated by day and expand. After sunset they rapidly cool and contract. When the

difference in temperature between day and night is considerable, the repeated strains
of sudden expansion and contraction at last become greater than the rocks can bear,
and they break, for the same reason that a glass cracks when plunged into boiling
water (Fig. 5).

Rocks are poor conductors of heat, and hence their surfaces may become
painfully hot under the full blaze of the sun, while the interior remains comparatively
cool. By day the surface shell expands and tends to break loose from the mass of the
stone. In cooling in the evening the surface shell suddenly contracts on the
unyielding interior and in time is forced off in scales (Fig. 6).

Fig. 6. Bowlders scaling off under Heat and Cold, Western Texas

Many rocks, such as granite, are made up of grains of various minerals which
differ in color and in their capacity to absorb heat, and which therefore contract and
expand in different ratios. In heating and cooling these grains crowd against their
neighbors and tear loose from them, so that finally the rock disintegrates into sand.

The conditions for the destructive action of heat and cold are most fully met in
arid regions when vegetation is wanting for lack of sufficient rain. The soil not being
held together by the roots of plants is blown away over large areas, leaving the rocks
bare to the blazing sun in a cloudless sky. The air is dry, and the heat received by the
earth by day is therefore rapidly radiated at night into space. There is a sharp and

sudden fall of temperature after sunset, and the rocks, strongly heated by day, are
now chilled perhaps even to the freezing point.
In the Sahara the thermometer has been known to fall 131° F. within a few hours. In the light air of the Pamir
plateau in central Asia a rise of 90° F. has been recorded from seven o’clock in the morning to one o’clock in the
afternoon. On the mountains of southwestern Texas there are frequently heard crackling noises as the rocks of
that arid region throw off scales from a fraction of an inch to four inches in thickness, and loud reports are made
as huge bowlders split apart. Desert pebbles weakened by long exposure to heat and cold have been shivered to
fine sharp-pointed fragments on being placed in sand heated to 180 degrees F. Beds half a foot thick, forming the
floor of limestone quarries in Wisconsin, have been known to buckle and arch and break to fragments under the
heat of the summer sun.

Frost. By this term is meant the freezing and thawing of water contained in the
pores and crevices of rocks. All rocks are more or less porous and all contain more or
less water in their pores. Workers in stone call this “quarry water,” and speak of a
stone as “green” before the quarry water has dried out. Water also seeps along joints
and bedding planes and gathers in all seams and crevices. Water expands in freezing,
ten cubic inches of water freezing to about eleven cubic inches of ice. As water
freezes in the rifts and pores of rocks it expands with the irresistible force illustrated
in the freezing and breaking of water pipes in winter. The first rift in the rock,
perhaps too narrow to be seen, is widened little by little by the wedges of successive
frosts, and finally the rock is broken into detached blocks, and these into angular
chip-stone by the same process.

It is on mountain tops and in high latitudes that the effects of frost are most
plainly seen. “Every summit” says Whymper, “amongst the rock summits upon
which I have stood has been nothing but a piled-up heap of fragments” (Fig. 7). In
Iceland, in Spitzbergen, in Kamchatka, and in other frigid lands large areas are
thickly strewn with sharp-edged fragments into which the rock has been shattered by
frost.

Organic agents. We must
reckon the roots of plants and
trees among the agents which
break rocks into pieces. The tiny
rootlet in its search for food and
moisture inserts itself into some
minute rift, and as it grows slowly
wedges the rock apart. Moreover,
the acids of the root corrode the
rocks with which they are in
contact. One may sometimes find
in the soil a block of limestone
wrapped in a mesh of roots, each

Fig. 7. Rocks broken by Frost, Summit of the of which lies in a little furrow
Eggischhorn, Switzerland where it has eaten into the stone.

Rootless plants called lichens often cover and corrode rocks as yet bare of soil;
but where lichens are destroying the rock less rapidly than does the weather, they
serve in a way as a protection.

Conditions favoring disintegration and decay. The disintegration of rocks
under frost and temperature changes goes on most rapidly in cold and arid climates,
and where vegetation is scant or absent. On the contrary, the decay of rocks under the
chemical action of water is favored by a warm, moist climate and abundant
vegetation. Frost and heat and cold can only act within the few feet from the surface
to which the necessary temperature changes are limited, while water penetrates and
alters the rocks to great depths.

The pupil may explain.

In what ways the presence of joints and bedding planes assists in the breaking up
and decay of rocks under the action of the weather.

Why it is a good rule of stone masons never to lay stones on edge, but always on
their natural bedding planes.

Why stones fresh from the quarry sometimes go to pieces in early winter, when
stones which have been quarried for some months remain uninjured.

Why quarrymen in the northern states often keep their quarry floors flooded
during winter.

Why laminated limestone should not be used for curbstone.

Why rocks composed of layers differing in fineness of grain and in ratios of
expansion do not make good building stone.

Fine-grained rocks with pores so small that capillary attraction keeps the water
which they contain from readily draining away are more apt to hold their pores ten
elevenths full of water than are rocks whose pores are larger. Which, therefore, are
more likely to be injured by frost?

Which is subject to greater temperature changes, a dark rock or one of a light
color? the north side or the south side of a valley?

The Mantle of Rock Waste

We have seen that rocks are everywhere slowly wasting away. They are broken in
pieces by frost, by tree roots, and by heat and cold. They dissolve and decompose
under the chemical action of water and the various corrosive substances which it
contains, leaving their insoluble residues as residual clays and sands upon the
surface. As a result there is everywhere forming a mantle of rock waste which covers
the land. It is well to imagine how the country would appear were this mantle with its
soil and vegetation all scraped away or had it never been formed. The surface of the
land would then be everywhere of bare rock as unbroken as a quarry floor.

The thickness of the mantle. In any locality the thickness of the mantle of rock
waste depends as much on the rate at which it is constantly being removed as on the
rate at which it is forming. On the face of cliffs it is absent, for here waste is removed
as fast as it is made. Where waste is carried away more slowly than it is produced, it
accumulates in time to great depth.
The granite of Pikes Peak is disintegrated to a depth of twenty feet. In the city of Washington granite rock is
so softened to a depth of eighty feet that it can be removed with pick and shovel. About Atlanta, Georgia, the
rocks are completely rotted for one hundred feet from the surface, while the beginnings of decay may be noticed
at thrice that depth. In places in southern Brazil the rock is decomposed to a depth of four hundred feet.

In southwestern Wisconsin a reddish residual clay has an average depth of thirteen feet on broad uplands,
where it has been removed to the least extent. The country rock on which it rests is a limestone with about ten per
cent of insoluble impurities. At least how thick, then, was that portion of the limestone which has rotted down to
the clay?

Distinguishing characteristics of residual waste. We must learn to distinguish
waste formed in place by the action of the weather from the products of other
geological agencies. Residual waste is unstratified. It contains no substances which
have not been derived from the weathering of the parent rock. There is a gradual
transition from residual waste into the unweathered rock beneath. Waste resting on
sound rock evidently has been shifted and was not formed in place.
In certain regions of southern Missouri the land is covered with a layer of broken flints and red clay, while the
country rock is limestone. The limestone contains nodules of flint, and we may infer that it has been by the decay
and removal of thick masses of limestone that the residual layer of clay and flints has been left upon the surface.
Flint is a form of quartz, dull-lustered, usually gray or blackish in color, and opaque except on thinnest edges,
where it is translucent.

Over much of the northern states there is spread an unstratified stony clay called
the drift. It often rests on sound rocks. It contains grains of sand, pebbles, and
bowlders composed of many different minerals and rocks that the country rock
cannot furnish. Hence the drift cannot have been formed by the decay of the rock of
the region. A shale or limestone, for example, cannot waste to a clay containing
granite pebbles. The origin of the drift will be explained in subsequent chapters.

The differences in rocks are due more to their soluble than to their insoluble
constituents. The latter are few in number and are much the same in rocks of widely
different nature, being chiefly quartz, silicate of alumina, and iron oxide. By the
removal of their soluble parts very many and widely different rocks rot down to a
residual clay gritty with particles of quartz and colored red or yellow with iron oxide.

In a broad way the changes which rocks undergo in weathering are an adaptation
to the environment in which they find themselves at the earth’s surface,—an
environment different from that in which they were formed under sea or under
ground. In open air, where they are attacked by various destructive agents, few of the
rock- making minerals are stable compounds except quartz, the iron oxides, and the
silicate of alumina; and so it is to one or more of these comparatively insoluble
substances that most rocks are reduced by long decay.
Which produces a mantle of finer waste, frost or chemical decay? which a thicker mantle? In what respects
would you expect that the mantle of waste would differ in warm humid lands like India, in frozen countries like
Alaska, and in deserts such as the Sahara?

The soil. The same agencies which produce the mantle of waste are continually
at work upon it, breaking it up into finer and finer particles and causing its more
complete decay. Thus on the surface, where the waste has weathered longest, it is
gradually made fine enough to support the growth of plants, and is then known as
soil. The coarser waste beneath is sometimes spoken of as subsoil. Soil usually
contains more or less dark, carbonaceous, decaying organic matter, called humus,
and is then often termed the humus layer. Soil forms not only on waste produced in
place from the rock beneath, but also on materials which have been transported, such
as sheets of glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather is more rapid and effective on the fragments
of the mantle of waste than on the rocks from which waste is being formed. Why?

Any fresh excavation of cellar or cistern, or cut for road or railway, will show the
characteristics of the humus layer. It may form only a gray film on the surface, or we
may find it a layer a foot or more thick, dark, or even black, above, and growing
gradually lighter in color as it passes by insensible gradations into the subsoil. In
some way the decaying vegetable matter continually forming on the surface has
become mingled with the material beneath it.

How humus and the subsoil are mingled. The mingling of humus and the
subsoil is brought about by several means. The roots of plants penetrate the waste,
and when they die leave their decaying substance to fertilize it. Leaves and stems
falling on the surface are turned under by several agents. Earthworms and other
animals whose home is in the waste drag them into their burrows either for food or to
line their nests. Trees overthrown by the wind, roots and all, turn over the soil and

subsoil and mingle them together. Bacteria also work in the waste and contribute to
its enrichment. The animals living in the mantle do much in other ways toward the
making of soil. They bring the coarser fragments from beneath to the surface, where
the waste weathers more rapidly. Their burrows allow air and water to penetrate the
waste more freely and to affect it to greater depths.
Ants. In the tropics the mantle of waste is worked over chiefly by ants. They excavate underground galleries
and chambers, extending sometimes as much as fourteen feet below the surface, and build mounds which may
reach as high above it. In some parts of Paraguay and southern Brazil these mounds, like gigantic potato hills,
cover tracts of considerable area.

In search for its food—the dead wood of trees—the so-called white ant constructs
runways of earth about the size of gas pipes, reaching from the base of the tree to the
topmost branches. On the plateaus of central Africa explorers have walked for miles
through forests every tree of which was plastered with these galleries of mud. Each
grain of earth used in their construction is moistened and cemented by slime as it is
laid in place by the ant, and is thus acted on by organic chemical agents. Sooner or
later these galleries are beaten down by heavy rains, and their fertilizing substances
are scattered widely by the winds.
Earthworms. In temperate regions the waste is worked over largely by earthworms. In making their burrows
worms swallow earth in order to extract from it any nutritive organic matter which it may contain. They treat it
with their digestive acids, grind it in their stony gizzards, and void it in castings on the surface of the ground. It
was estimated by Darwin that in many parts of England each year, on every acre, more than ten tons of earth pass
through the bodies of earthworms and are brought to the surface, and that every few years the entire soil layer is
thus worked over by them.

In all these ways the waste is made fine and stirred and enriched. Grain by grain
the subsoil with its fresh mineral ingredients is brought to the surface, and the rich
organic matter which plants and animals have taken from the atmosphere is plowed
under. Thus Nature plows and harrows on “the great world’s farm” to make ready
and ever to renew a soil fit for the endless succession of her crops.

The world processes by which rocks are continually wasting away are thus
indispensable to the life of plants and animals. The organic world is built on the ruins
of the inorganic, and because the solid rocks have been broken down into soil men
are able to live upon the earth.

Solar energy. The source of the energy which accomplishes all this necessary
work is the sun. It is the radiant energy of the sun which causes the disintegration of
rocks, which lifts vapor into the atmosphere to fall as rain, which gives life to plants
and animals. Considering the earth in a broad way, we may view it as a globe of solid
rock,—the lithosphere,—surrounded by two mobile envelopes: the envelope of air,—
the atmosphere; and the envelope of water,—the hydrosphere. Under the action of
solar energy these envelopes are in constant motion. Water from the hydrosphere is

continually rising in vapor into the atmosphere, the air of the atmosphere penetrates
the hydrosphere,—for its gases are dissolved in all waters,—and both air and water
enter and work upon the solid earth. By their action upon the lithosphere they have
produced a third envelope,—the mantle of rock waste.

This envelope also is in movement, not indeed as a whole, but particle by
particle. The causes which set its particles in motion, and the different forms which
the mantle comes to assume, we will now proceed to study.

Movements of the Mantle of Rock Waste
At the sandstone ledges which we first visited we saw not only that the rocks
were crumbling away, but also that grains and fragments of them were creeping
down the slopes of the valley to the stream and were carried by it onward toward the
sea. This process is going on everywhere. Slowly it may be, and with many
interruptions, but surely, the waste of the land moves downward to the sea. We may
divide its course into two parts,—the path to the stream, which we will now consider,
and its carriage onward by the stream, which we will defer to a later chapter.

Gravity. The chief agent concerned in the movement of waste is gravity. Each
particle of waste feels the unceasing downward pull of the earth’s mass and follows it
when free to do so. All agencies which produce waste tend to set its particles free and
in motion, and therefore cooperate with gravity. On cliffs, rocks fall when wedged
off by frost or by roots of trees, and when detached by any other agency. On slopes
of waste, water freezes in chinks between stones, and in pores between particles of
soil, and wedges them apart. Animals and plants stir the waste, heat expands it, cold
contracts it, the strokes of the raindrops drive loose particles down the slope and the
wind lifts and lets them fall. Of all these movements, gravity assists those which are
downhill and retards those which are uphill. On the whole, therefore, the downhill
movements prevail, and the mantle of waste, block by block and grain by grain,
creeps along the downhill path.
A slab of sandstone laid on another of the same kind at an angle of 17° and left in the open air was found to
creep down the slope at the rate of a little more than a millimeter a month. Explain why it did so.

Rain. The most efficient agent in the carriage of waste to the streams is the rain.
It moves particles of soil by the force of the blows of the falling drops, and washes
them down all slopes to within reach of permanent streams. On surfaces unprotected
by vegetation, as on plowed fields and in arid regions, the rain wears furrows and
gullies both in the mantle of waste and in exposures of unaltered rock (Fig. 17).
At the foot of a hill we may find that the soil has accumulated by creep and wash to the depth of several feet;
while where the hillside is steepest the soil may be exceedingly thin, or quite absent, because removed about as
fast as formed. Against the walls of an abbey built on a slope in Wales seven hundred years ago, the creeping

waste has gathered on the uphill side to a depth of seven feet. The slow-flowing sheet of waste is often dammed
by fences and walls, whose uphill side gathers waste in a few years so as to show a distinctly higher surface than
the downhill side, especially in plowed fields where the movement is least checked by vegetation.

Talus. At the foot of cliffs there is usually to be found a slope of rock fragments
which clearly have fallen from above (Fig. 8). Such a heap of waste is known as
talus. The amount of talus in any place depends both on the rate of its formation and
the rate of its removal. Talus forms rapidly in climates where mechanical
disintegration is most effective, where rocks are readily broken into blocks because
closely jointed and thinly bedded rather than massive, and where they are firm
enough to be detached in fragments of some size instead of in fine grains. Talus is
removed slowly where it decays slowly, either because of the climate or the
resistance of the rock. It may be rapidly removed by a stream flowing along its base.

Fig. 8. Talus at Foot of Granite Cliffs, Sierra Nevada Mountains

In a moist climate a soluble rock, such as massive limestone, may form talus little
if any faster than the talus weathers away. A loose-textured sandstone breaks down
into incoherent sand grains, which in dry climates, where unprotected by vegetation,
may be blown away as fast as they fall, leaving the cliff bare to the base. Cliffs of

such slow-decaying rocks as quartzite and granite when closely jointed accumulate
talus in large amounts.

Talus slopes may be so steep as to reach the angle of repose,
i.e. the steepest angle at which the material will lie. This angle
varies with different materials, being greater with coarse and
angular fragments than with fine rounded grains. Sooner or later
a talus reaches that equilibrium where the amount removed
from its surface just equals that supplied from the cliff above. Fig. 9. Diagram
As the talus is removed and weathers away its slope retreats Illustrating
together with the retreat of the cliff, as seen in Figure 9. Retreat of
Cliff, c, and
Graded slopes. Where rocks weather faster than their waste Talus, t
is carried away, the waste comes at last to cover all rocky
ledges. On the steeper slopes it is coarser and in more rapid movement than on slopes
more gentle, but mountain sides and hills and plains alike come to be mantled with
sheets of waste which everywhere is creeping toward the streams. Such unbroken
slopes, worn or built to the least inclination at which the waste supplied by
weathering can be urged onward, are known as graded slopes.

Of far less importance than the silent, gradual creep of waste, which is going on
at all times everywhere about us, are the startling local and spasmodic movements
which we are now to describe.

Avalanches. On steep mountain sides the accumulated snows of winter often slip
and slide in avalanches to the valleys below. These rushing torrents of snow sweep
their tracks clean of waste and are one of Nature’s normal methods of moving it
along the downhill path.

Fig. 10. A Landslide, Quebec

Landslides. Another common and abrupt method of delivering waste to streams
is by slips of the waste mantle in large masses. After long rains and after winter frosts
the cohesion between the waste and the sound rock beneath is loosened by seeping
water underground. The waste slips on the rock surface thus lubricated and plunges
down the mountain side in a swift roaring torrent of mud and stones.

We may conveniently mention here a second type of
landslide, where masses of solid rock as well as the mantle
of waste are involved in the sudden movement. Such slips
occur when valleys have been rapidly deepened by streams
or glaciers and their sides have not yet been graded. A
favorable condition is where the strata dip (i.e. incline
downwards) towards the valley (Fig. 11), or are broken by Fig. 11. Diagram
joint planes dipping in the same direction. The upper Illustrating
layers, including perhaps the entire mountain side, have Conditions
been cut across by the valley trench and are left supported favorable to a
only on the inclined surface of the underlying rocks. Water Landslide
lm, limestone dipping

may percolate underground along this surface and loosen toward valley of river,
the cohesion between the upper and the underlying strata r; sh, shale
by converting the upper surface of a shale to soft wet clay,
by dissolving layers of a limestone, or by removing the cement of a sandstone and
converting it into loose sand. When the inclined surface is thus lubricated the
overlying masses may be launched into the valley below. The solid rocks are broken
and crushed in sliding and converted into waste consisting, like that of talus, of
angular unsorted fragments, blocks of all sizes being mingled pell-mell with rock
meal and dust. The principal effects of landslides may be gathered from the
following examples.
At Gohna, India, in 1893, the face of a spur four thousand feet high, of the lower ranges of the Himalayas,
slipped into the gorge of the headwaters of the Ganges River in successive rock falls which lasted for three days.
Blocks of stone were projected for a mile, and clouds of limestone dust were spread over the surrounding country.
The débris formed a dam one thousand feet high, extending for two miles along the valley. A lake gathered
behind this barrier, gradually rising until it overtopped it in a little less than a year. The upper portion of the dam
then broke, and a terrific rush of water swept down the valley in a wave which, twenty miles away, rose one
hundred and sixty feet in height. A narrow lake is still held by the strong base of the dam.

In 1896, after forty days of incessant rain, a cliff of sandstone slipped into the Yangtse River in China,
reducing the width of the channel to eighty yards and causing formidable rapids.

At Flims, in Switzerland, a prehistoric landslip
flung a dam eighteen hundred feet high across the
headwaters of the Rhine. If spread evenly over a
surface of twenty-eight square miles, the material
would cover it to a depth of six hundred and sixty
feet. The barrier is not yet entirely cut away, and
several lakes are held in shallow basins on its
hummocky surface.

A slide from the precipitous river front of the
citadel hill of Quebec, in 1889, dashed across
Champlain Street, wrecking a number of houses
and causing the death of forty-five persons. The
strata here are composed of steeply dipping slate.

In lofty mountain ranges there may not be a
single valley without its traces of landslides, so
Fig. 12. Bowlders of Weathering, Granite common there is this method of the movement of
waste, and of building to grade over-steepened
Quarry, Cape Ann, Massachusetts slopes.

Rock Sculpture By Weathering
We are now to consider a few of the forms into which rock masses are carved by
the weather.

Bowlders of weathering. In many quarries and outcrops we may see that the
blocks into which one or more of the uppermost layers have been broken along their
joints and bedding planes are no longer angular, as are those of the layers below. The

edges and corners of these blocks have been worn
away by the weather. Such rounded cores, known as
bowlders of weathering, are often left to strew the
surface.

Differential weathering. This term covers all
cases in which a rock mass weathers differently in
different portions. Any weaker spots or layers are
etched out on the surface, leaving the more resistant
in relief. Thus massive limestones become pitted
where the weather drills out the weaker portions. In
these pits, when once they are formed, moisture
gathers, a little soil collects, vegetation takes root, and Fig. 13. Differential
thus they are further enlarged until the limestone may Weathering on a Monument,
be deeply honeycombed. Colorado

Fig. 14. Honeycombed Limestone, Iowa

Fig. 15. Cliffs and Slopes on North Wall of the Grand Canyon of the Colorado
River, Arizona

On the sides of canyons, and elsewhere where the edges of strata are exposed, the
harder layers project as cliffs, while the softer weather back to slopes covered with
the talus of the harder layers above them. It is convenient to call the former cliff
makers and the latter slope makers.

Differential weathering plays a large part in the sculpture of the land. Areas of
weak rock are wasted to plains, while areas of hard rock adjacent are still left as hills
and mountain ridges, as in the valleys and mountains of eastern Pennsylvania. But in
such instances the lowering of the surface of the weaker rock is also due to the wear
of streams, and especially to the removal by them from the land of the waste which
covers and protects the rocks beneath.
Rocks owe their weakness to several different causes. Some, such as beds of loose sand, are soft and easily
worn by rains; some, as limestone and gypsum for example, are soluble. Even hard insoluble rocks are weak
under the attack of the weather when they are closely divided by joints and bedding planes and are thus readily
broken up into blocks by mechanical agencies.

Fig. 16. Taverlone Mesa, New Mexico

Fig. 17. Monuments, Arizona
Note the rain furrows on the slope at the foot of the monuments. In the foreground are seen fragments of
petrified trunks of trees, composed of silica and extremely resistant to the weather. On the removal of the
rock layers in which these fragments were imbedded they are left to strew the surface in the same way as
are the residual flints of southern Missouri.

Outliers and monuments. As cliffs retreat under the attack of the weather,
portions are left behind where the rock is more resistant or where the attack for any
reason is less severe. Such remnant masses, if large, are known as outliers. When
flat-topped, because of the protection of a resistant horizontal capping layer, they are

termed mesas (Fig. 16),—a term applied also to the flat-topped portions of dissected
plateaus (Fig. 129). Retreating cliffs may fall back a number of miles behind their
outliers before the latter are finally consumed.

Monuments are smaller masses and may be but
partially detached from the cliff face. In the breaking
down of sheets of horizontal strata, outliers grow
smaller and smaller and are reduced to massive
rectangular monuments resembling castles (Fig. 17).
The rock castle falls into ruin, leaving here and there an
isolated tower; the tower crumbles to a lonely pillar,
soon to be overthrown. The various and often
picturesque shapes of monuments depend on the kind
of rock, the attitude of the strata, and the agent by
which they are chiefly carved. Thus pillars may have a
capital formed of a resistant stratum. Monuments may
be undercut and come to rest on narrow pedestals, Fig. 18. Undercut
wherever they weather more rapidly near the ground, Monuments, Colorado
either because of the greater moisture there, or—in arid
climates—because worn at their base by drifting sands.

Stony clays disintegrating under the rain often contain bowlders which protect
the softer material beneath from the vertical blows of raindrops, and thus come to
stand on pedestals of some height. One may sometimes see on the ground beneath
dripping eaves pebbles left in the same way, protecting tiny pedestals of sand.

Mountain peaks and ridges. Most mountains have been carved out of great
broadly uplifted folds and blocks of the earth’s crust. Running water and glacier ice
have cut these folds and blocks into masses divided by deep valleys; but it is by the
weather, for the most part, that the masses thus separated have been sculptured to the
present forms of the individual peaks and ridges.

Frost and heat and cold sculpture high mountains to sharp, tusklike peaks and
ragged, serrate crests, where their waste is readily removed (Fig. 8).

The Matterhorn of the Alps is a famous example of a mountain peak whose
carving by the frost and other agents is in active progress. On its face “scarcely a
rock anywhere is firmly attached,” and the fall of loosened stones is incessant.
Mountain climbers who have camped at its base tell how huge rocks from time to
time come leaping down its precipices, followed by trains of dislodged smaller
fragments and rock dust; and how at night one may trace the course of the bowlders
by the sparks which they strike from the mountain walls. Mount Assiniboine, Canada

(Fig. 20), resembles the Matterhorn in form and
has been carved by the same agencies.

“The Needles” of Arizona are examples of
sharp mountain peaks in a warm arid region
sculptured chiefly by temperature changes.

Chemical decay, especially when carried on
beneath a cover of waste and vegetation, favors
the production of rounded knobs and dome-
shaped mountains.

The weather curve. We have seen that
weathering reduces the angular block quarried
by the frost to a rounded bowlder by chipping
off its corners and smoothing away its edges. In
much the same way weathering at last reduces
to rounded hills the earth blocks cut by streams
or formed in any other way. High mountains
may at first be sculptured by the weather to
savage peaks (Fig. 181), but toward the end of
their life history they wear down to rounded
hills (Fig. 182). The weather curve, which may
be seen on the summits of low hills (Fig. 21), is
Fig. 19. Roosevelt Column, convex upward.
Idaho
An erosion pillar 70 feet high. How
was it produced? Why quadrangular?
What does it show as to the recent
height of the hillside surface?

Fig. 20. Mount Assiniboine, Canada

Fig. 21. Big Round Top and Little Round Top, Gettysburg,
Pennsylvania

In Figure 22, representing a cubic block of stone whose faces are a yard square, how many square feet of
surface are exposed to the weather by a cubic foot at a corner a; by one situated in the middle of an edge b; by
one in the center of a side c? How much faster will a and b weather than c, and what will be the effect on the
shape of the block?

The cooperation of various agencies in rock sculpture. For the sake of
clearness it is necessary to describe the work of each geological agent separately. We
must not forget, however, that in Nature no agent works independently and alone;
that every result is the outcome of a long chain of causes. Thus, in order that the
mountain peak may be carved by the agents of disintegration, the waste must be

rapidly removed,—a work done by many agents, including
some which we are yet to study; and in order that the waste
may be removed as fast as formed, the region must first have
been raised well above the level of the sea, so that the agents of
transportation could do their work effectively. The sculpture of
the rocks is accomplished only by the cooperation of many
forces.
Fig. 22.
The constant removal of waste from the surface by creep
and wash and carriage by streams is of the highest importance, because it allows the
destruction of the land by means of weathering to go on as long as any land remains
above sea level. If waste were not removed, it would grow to be so thick as to protect
the rock beneath from further weathering, and the processes of destruction which we
have studied would be brought to an end. The very presence of the mantle of waste
over the land proves that on the whole rocks weather more rapidly than their waste is
removed. The destruction of the land is going on as fast as the waste can be carried
away.

We have now learned to see in the mantle of waste the record of the destructive
action of the agencies of weathering on the rocks of the land surface. Similar records
we shall find buried deeply among the rocks of the crust in old soils and in rocks
pitted and decayed, telling of old land surfaces long wasted by the weather. Ever
since the dry land appeared these agencies have been as now quietly and unceasingly
at work upon it, and have ever been the chief means of the destruction of its rocks.
The vast bulk of the stratified rocks of the earth’s crust is made up almost wholly of
the waste thus worn from ancient lands.

Fig. 23. Mount Sneffels, Colorado
Describe and account for what you see in this view. What changes may the mountain be
expected to undergo in the future from the agencies now at work upon it?

In studying the various geological agencies we must remember the almost
inconceivable times in which they work. The slowest process when multiplied by the
immense time in which it is carried on produces great results. The geologist looks
upon the land forms of the earth’s surface as monuments which record the slow
action of weathering and other agents during the ages of the past. The mountain
peak, the rounded hill, the wide plain which lies where hills and mountains once
stood, tell clearly of the great results which slow processes will reach when given
long time in which to do their work. We should accustom ourselves also to think of
the results which weathering will sooner or later bring to pass. The tombstone and
the bowlder of the field, which each year lose from their surfaces a few crystalline
grains, must in time be wholly destroyed. The hill whose rocks are slowly rotting
underneath a cover of waste must become lower and lower as the centuries and
millenniums come and go, and will finally disappear. Even the mountains are
crumbling away continually, and therefore are but fleeting features of the landscape.

CHAPTER II

THE WORK OF GROUND WATER
Land waters. We have seen how large is the part that water plays at and near the
surface of the land in the processes of weathering and in the slow movement of waste
down all slopes to the stream ways. We now take up the work of water as it descends
beneath the ground,—a corrosive agent still, and carrying in solution as its load the
invisible waste of rocks derived from their soluble parts.

Land waters have their immediate source in the rainfall. By the heat of the sun
water is evaporated from the reservoir of the ocean and from moist surfaces
everywhere. Mingled as vapor with the air, it is carried by the winds over sea and
land, and condensed it returns to the earth as rain or snow. That part of the rainfall
which descends on the ocean does not concern us, but that which falls on the land
accomplishes, as it returns to the sea, the most important work of all surface
geological agencies.

The rainfall may be divided into three parts: the first dries up, being discharged
into the air by evaporation either directly from the soil or through vegetation; the
second runs off over the surface to flood the streams; the third soaks in the ground
and is henceforth known as ground or underground water.

The descent of ground water. Seeping through the mantle of waste, ground
water soaks into the pores and crevices of the underlying rock. All rocks of the upper
crust of the earth are more or less porous, and all drink in water. Impervious rocks,
such as granite, clay, and shale, have pores so minute that the water which they take
in is held fast within them by capillary attraction, and none drains through. Pervious
rocks, on the other hand, such as many sandstones, have pore spaces so large that
water filters through them more or less freely. Besides its seepage through the pores
of pervious rocks, water passes to lower levels through the joints and cracks by
which all rocks, near the surface are broken.

Even the closest-grained granite has a pore space of 1 in 400, while sandstone
may have a pore space of 1 in 4. Sand is so porous that it may absorb a third of its
volume of water, and a loose loam even as much as one half.

The ground-water surface is the
name given the upper surface of
ground water, the level below which
all rocks are saturated. In dry seasons
the ground-water surface sinks. For
ground water is constantly seeping
Fig. 24. Diagram Illustrating the Relation of
downward under gravity, it is
the Ground-Water Surface to the
evaporated in the waste and its
Surface of the Ground
moisture is carried upward by
The dotted line represents the ground-water surface,
capillarity and the roots of plants to
and the arrows indicate the direction of the
the surface to be evaporated in the
movements of ground-water. m, marsh; w, well; r,
air. In wet seasons these constant
river
losses are more than made good by
fresh supplies from that part of the rainfall which soaks into the ground, and the
ground-water surface rises.

In moist climates the ground-water surface (Fig. 24) lies, as a rule, within a few
feet of the land surface and conforms to it in a general way, although with slopes of
less inclination than those of the hills and valleys. In dry climates permanent ground
water may be found only at depths of hundreds of feet. Ground water is held at its
height by the fact that its circulation is constantly impeded by capillarity and friction.
If it were as free to drain away as are surface streams, it would sink soon after a rain
to the level of the deepest valleys of the region.

Wells and springs. Excavations
made in permeable rocks below the
ground-water surface fill to its level
and are known as wells. Where valleys
cut this surface permanent streams are
formed, the water either oozing forth
along ill-defined areas or issuing at
definite points called springs, where it
is concentrated by the structure of the
rocks. A level tract where the ground-
water surface coincides with the surface
of the ground is a swamp or marsh.

By studying a spring one may learn
much of the ways and work of ground
water. Spring water differs from that of
the stream into which it flows in several Fig. 25. A Spring, Kansas
respects. If we test the spring with a Is the rock over which the spring discharges
thermometer during successive months, pervious or impervious?
we shall find that its temperature
remains much the same the year round. In summer it is markedly cooler than the
stream; in winter it is warmer and remains unfrozen while the latter perhaps is locked
in ice. This means that its underground path must lie at such a distance from the
surface that it is little affected by summer’s heat and winter’s cold.

While the stream is often turbid with surface waste washed into it by rains, the
spring remains clear; its water has been filtered during its slow movement through
many small underground passages and the pores of rocks. Commonly the spring
differs from the stream in that it carries a far larger load of dissolved rock. Chemical
analysis proves that streams contain various minerals in solution, but these are
usually in quantities so small that they are not perceptible to the taste or feel. But the
water of springs is often well charged with soluble minerals; in its slow, long journey
underground it has searched out the soluble parts of the rocks through which it seeps
and has dissolved as much of them as it could. When spring water is boiled away, the
invisible load which it has carried is left behind, and in composition is found to be
practically identical with that of the soluble ingredients of the country rock. Although
to some extent the soluble waste of rocks is washed down surface slopes by the rain,
by far the larger part is carried downward by ground water and is delivered to
streams by springs.
In limestone regions springs are charged with calcium carbonate (the carbonate of lime), and where the
limestone is magnesian they contain magnesium carbonate also. Such waters are “hard”; when used in washing,
the minerals which they contain combine with the fatty acids of soap to form insoluble curdy compounds. When

springs rise from rocks containing gypsum they are hard with calcium sulphate. In granite regions they contain
more or less soda and potash from the decay of feldspar.

The flow of springs varies much less during the different seasons of the year than
does that of surface streams. So slow is the movement of ground water through the
rocks that even during long droughts large amounts remain stored above the levels of
surface drainage.

Movements of ground water. Ground water is in constant movement toward its
outlets. Its rate varies according to many conditions, but always is extremely slow.
Even through loose sands beneath the beds of rivers it sometimes does not exceed a
fifth of a mile a year.

In any region two zones of flow
may be distinguished. The upper zone
of flow extends from the ground-
water surface downward through the
waste mantle and any permeable
rocks on which the mantle rests, as
far as the first impermeable layer, Fig. 26. Geological Conditions favorable to
where the descending movement of Strong Springs
the water is stopped. The deep zones a, limestone; b, shale; c, coarse sandstone; d,
of flow occupy any pervious rocks limestone; e, sandstone; f, fissure. The strata dip
which may be found below the toward the South, S. Redraw the diagram, marking
impervious layer which lies nearest to the points at which strong springs (ss) may be
the surface. The upper zone is a vast expected.
sheet of water saturating the soil and
rocks and slowly seeping downward through their pores and interstices along the
slopes to the valleys, where in part it discharges in springs and often unites also in a
wide underflowing stream which supports and feeds the river (Fig. 24).

A city in a region of copious rains,
built on the narrow flood plain of a
river, overlooked by hills, depends for
its water supply on driven wells, within
the city limits, sunk in the sand a few
yards from the edge of the stream. Are Fig. 27. Fig. 28.
these wells fed by water from the river
percolating through the sand, or by Fig. 27. Diagram of Well which goes dry
ground water on its way to the stream in Drought, a, and of of Unfailing
and possibly contaminated with the Well, b
sewage of the town? Redraw the diagram, showing by dotted line the

At what height does underground normal ground-water surface and by broken line
water stand in the wells of your region? the ground-water surface at times of drought
Does it vary with the season? Have you
ever known wells to go dry? It may be Fig. 28. Diagram of Wet Weather Stream,
possible to get data from different wells a, and of Permanent Stream, b
and to draw a diagram showing the Redraw the diagram, showing ground-water
ground-water surface as compared with surface by dotted line
the surface of the ground.

Fissure springs and artesian wells. The deeper zones of flow lie in pervious
strata which are overlain by some impervious stratum. Such layers are often carried
by their dip to great depths, and water may circulate in them to far below the level of
the surface streams and even of the sea. When a fissure crosses a water- bearing
stratum, or aquifer, water is forced upward by the pressure of the weight of the water
contained in the higher parts of the stratum, and may reach the surface as a fissure
spring. A boring which taps such an aquifer is known as an artesian well, a name
derived from a province in France where wells of this kind have been long in use.
The rise of the water in artesian wells, and in fissure springs also, depends on the
following conditions illustrated in Figure 29. The aquifer dips toward the region of
the wells from higher ground, where it outcrops and receives its water. It is inclosed
between an impervious layer above and water- tight or water-logged layers beneath.
The weight of the column of water thus inclosed in the aquifer causes water to rise in
the well, precisely as the weight of the water in a standpipe forces it in connected
pipes to the upper stories of buildings.

Fig. 29. Section across South Dakota from the Black Hills to
Sioux Falls (S), Illustrating the Conditions of Artesian
Wells
a, crystalline impervious rocks; b, sedimentary rocks, shales, limestones,
and sandstones; c, pervious sandstone, the aquifer; d, impervious shales; w,
w, w, artesian wells.

Which will supply the larger region with artesian wells, an aquifer whose dip is
steep or one whose dip is gentle? Which of the two aquifers, their thickness being
equal, will have the larger outcrop and therefore be able to draw upon the larger
amount of water from the rainfall? Illustrate with diagrams.

The zone of solution. Near the surface, where the circulation of ground water is
most active, it oxidizes, corrodes, and dissolves the rocks through which it passes. It
leaches soils and subsoils of their lime and other soluble minerals upon which plants
depend for their food. It takes away the soluble cements of rocks; it widens fissures
and joints and opens winding passages along the bedding planes; it may even remove
whole beds of soluble rocks, such as rock salt, limestone, or gypsum. The work of
ground water in producing landslides has already been noticed. The zone in which
the work of ground water is thus for the most part destructive we may call the zone
of solution.

Fig. 30. Diagram of Caverns and Sink Holes

Caves. In massive limestone rocks, ground water dissolves channels which
sometimes form large caves (Fig. 30). The necessary conditions for the excavation of
caves of great size are well shown in central Kentucky, where an upland is built
throughout of thick horizontal beds of limestone. The absence of layers of insoluble
or impervious rock in its structure allows a free circulation of ground water within it
by the way of all natural openings in the rock. These water ways have been gradually
enlarged by solution and wear until the upland is honeycombed with caves. Five
hundred open caverns are known in one county.
Mammoth Cave, the largest of these caverns, consists of a labyrinth of chambers and winding galleries whose
total length is said to be as much as thirty miles. One passage four miles long has an average width of about sixty
feet and an average height of forty feet. One of the great halls is three hundred feet in width and is overhung by a
solid arch of limestone one hundred feet above the floor. Galleries at different levels are connected by well-like
pits, some of which measure two hundred and twenty-five feet from top to bottom. Through some of the lowest of
these tunnels flows Echo River, still at work dissolving and wearing away the rock while on its dark way to
appear at the surface as a great spring.

Natural bridges. As a cavern enlarges and the surface of the land above it is
lowered by weathering, the roof at last breaks down and the cave becomes an open
ravine. A portion of the roof may for a while remain, forming a “natural bridge.”

Sink holes. In limestone regions channels under ground may become so well
developed that the water of rains rapidly drains away through them. Ground water
stands low and wells must be sunk deep to find it. Little or no surface water is left to
form brooks.

Fig. 31. Sink Holes in the Karst, Austria
Thus across the limestone upland of central Kentucky one meets but three surface streams in a hundred miles.
Between their valleys surface water finds its way underground by means of sink holes. These are pits, commonly
funnel shaped, formed by the enlargement of crevice or joint by percolating water, or by the breakdown of some
portion of the roof of a cave. By clogging of the outlet a sink hole may come to be filled by a pond.

Central Florida is a limestone region with its drainage largely subterranean and in part below the level even of
the sea. Sink holes are common, and many of them are occupied by lakelets. Great springs mark the point of issue
of underground streams, while some rise from beneath the sea. Silver Spring, one of the largest, discharges from a
basin eight hundred feet wide and thirty feet deep a little river navigable for small steamers to its source. About
the spring there are no surface streams for sixty miles.

The Karst. Along the eastern coast of the
Adriatic, as far south as Montenegro, lies a belt of
limestone mountains singularly worn and
honeycombed by the solvent action of water. Where
forests have been cut from the mountain sides and
the red soil has washed away, the surface of the
white limestone forms a pathless desert of rock
where each square rod has been corroded into an
intricate branch work of shallow furrows and sharp
ridges. Great sink holes, some of them six hundred
feet deep and more, pockmark the surface of the
land. The drainage is chiefly subterranean. Surface
streams are rare and a portion of their courses is
often under ground. Fragmentary valleys come
suddenly to an end at walls of rock where the rivers
which occupy the valleys plunge into dark tunnels to
reappear some miles away. Ground water stands so
far below the surface that it cannot be reached by
wells, and the inhabitants depend on rain water
stored for household uses. The finest cavern of
Europe, the Adelsberg Grotto, is in this region. Karst,
the name of a part of this country, is now used to
designate any region or landscape thus sculptured by
the chemical action of surface and ground water. We
must remember that Karst regions are rare, and
striking as is the work of their subterranean streams,
it is far less important than the work done by the

sheets of underground water slowly seeping through Fig. 32. Underground Stream Issuing from
all subsoils and porous rocks in other regions.
Base of Cliff, the Karst, Austria
Even when gathered into definite channels,
ground water does not have the erosive power of surface streams, since it carries with it little or no rock waste.
Regions whose underground drainage is so perfect that the development of surface streams has been retarded or
prevented escape to a large extent the leveling action of surface running waters, and may therefore stand higher
than the surrounding country. The hill honeycombed by Luray Cavern, Virginia, has been attributed to this cause.

Cavern deposits. Even
in the zone of solution water
may under certain
circumstances deposit as
well as erode. As it trickles
from the roof of caverns, the
lime carbonate which it has
taken into solution from the
layers of limestone above is
deposited by evaporation in
the air in icicle-like pendants
called stalactites. As the
drops splash on the floor
there are built up in the same
way thicker masses called
stalagmites, which may
Fig. 33. Stalactites and Stalagmites, Marengo
grow to join the stalactites
Cavern, Indiana
above, forming pillars. A
stalagmitic crust often seals with rock the earth which accumulates in caverns,
together with whatever relics of cave dwellers, either animals or men, it may contain.
Can you explain why slender stalactites formed by the drip of single drops are often hollow pipes?

The zone of cementation. With increasing depth subterranean water becomes
more and more sluggish in its movements and more and more highly charged with
minerals dissolved from the rocks above. At such depths it deposits these minerals in
the pores of rocks, cementing their grains together, and in crevices and fissures,
forming mineral veins. Thus below the zone of solution where the work of water is to
dissolve, lies the zone of cementation where its work is chemical deposit. A part of
the invisible load of waste is thus transferred from rocks near the surface to those at
greater depths.

As the land surface is gradually lowered by weathering and the work of rain and
streams, rocks which have lain deep within the zone of cementation are brought
within the zone of solution. Thus there are exposed to view limestones, whose cracks

were filled with calcite (crystallized carbonate of lime), with quartz or other
minerals, and sandstones whose grains were well cemented many feet below the
surface.
Cavity filling. Small cavities in the rocks are often found more or less completely filled with minerals
deposited from solution by water in its constant circulation underground. The process may be illustrated by the
deposit of salt crystals in a cup of evaporating brine, but in the latter instance the solution is not renewed as in the
case of cavities in the rocks. A cavity thus lined with inward-pointing crystals is called a geode.

Concretions. Ground water seeping through the pores of rocks may gather minerals disseminated throughout
them into nodular masses called concretions. Thus silica disseminated through limestone is gathered into nodules
of flint. While geodes grow from the outside inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays concretions may, as they grow, press the clay aside.
In many other rocks concretions are made by the process of replacement. Molecule by molecule the rock is
removed and the mineral of the concretion substituted in its place. The concretion may in this way preserve intact
the lamination lines or other structures of the rock (Fig. 34). Clays and shales often contain concretions of lime
carbonate, of iron carbonate, or of iron sulphide. Some fossil, such as a leaf or shell, frequently forms the nucleus
around which the concretion grows.

Why are building stones more easily worked when “green” than after their quarry water has dried out?

Deposits of ground water in arid
regions. In arid lands where ground
water is drawn by capillarity to the
surface and there evaporates, it leaves
as surface incrustations the minerals
held in solution. White limy
incrustations of this nature cover
considerable tracts in northern
Mexico. Evaporating beneath the
surface, ground water may deposit a
limy cement in beds of loose sand and
gravel. Such firmly cemented layers
are not uncommon in western Kansas
and Nebraska, where they are known
Fig. 34. Concretions in Sandstone, as “mortar beds.”
Wyoming
Thermal springs. While the lower
limit of surface drainage is sea level, subterranean water circulates much below that
depth, and is brought again to the surface by hydrostatic pressure. In many instances
springs have a higher temperature than the average annual temperature of the region,
and are then known as thermal springs. In regions of present or recent volcanic
activity, such as the Yellowstone National Park, we may believe that the heat of
thermal springs is derived from uncooled lavas, perhaps not far below the surface.
But when hot springs occur at a distance of hundreds of miles from any volcano, as
in the case of the hot springs of Bath, England, it is probable that their waters have

risen from the heated rocks of the earth’s interior. The springs of Bath have a
temperature of 120° F., 70° above the average annual temperature of the place. If we
assume that the rate of increase in the earth’s internal heat is here the average rate, 1°
F. to every sixty feet of descent, we may conclude that the springs of Bath rise from
at least a depth of forty-two hundred feet.

Water may descend to depths from which it can never be brought back by
hydrostatic pressure. It is absorbed by highly heated rocks deep below the surface.
From time to time some of this deep- seated water may be returned to open air in the
steam of volcanic eruptions.

Fig. 35. Calcareous Deposits from Hot Springs, Yellowstone
National Park

Surface deposits of springs. Where subterranean water returns to the surface
highly charged with minerals in solution, on exposure to the air it is commonly
compelled to lay down much of its invisible load in chemical deposits about the
spring. These are thrown down from solution either because of cooling, evaporation,
the loss of carbon dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide from
subterranean sources and are able therefore to take up large quantities of lime
carbonate from the limestone rocks through which they pass. On reaching the surface
the pressure is relieved, the gas escapes, and the lime carbonate is thrown down in
deposits called travertine. The gas is sometimes withdrawn and the deposit produced
in large part by the action of algae and other humble forms of plant life.

At the Mammoth Hot Springs in the valley of the Gardiner River, Yellowstone National Park, beautiful
terraces and basins of travertine are now building, chiefly by means of algae which cover the bottoms, rims, and
sides of the basins and deposit lime carbonate upon them in successive sheets. The rock, snow-white where dry, is
coated with red and orange gelatinous mats where the algae thrive in the over-flowing waters.

Similar terraces of travertine are found to a height of fourteen hundred feet up the valley side. We may infer
that the springs which formed these ancient deposits discharged near what was then the bottom of the valley, and
that as the valley has been deepened by the river the ground water of the region has found lower and lower points
of issue.

In many parts of the country calcareous springs occur which coat with lime carbonate mosses, twigs, and
other objects over which their waters flow. Such are popularly known as petrifying springs, although they merely
incrust the objects and do not convert them into stone.

Silica is soluble in alkaline waters, especially when these are hot. Hot springs
rising through alkaline siliceous rocks, such as lavas, often deposit silica in a white
spongy formation known as siliceous sinter, both by evaporation and by the action of
algae which secrete silica from the waters. It is in this way that the cones and
mounds of the geysers in the Yellowstone National Park and in Iceland have been
formed (Fig. 234).

Where water oozes from the earth one may sometimes see a rusty deposit on the
ground, and perhaps an iridescent scum upon the water. The scum is often mistaken
for oil, but at a touch it cracks and breaks, as oil would not do. It is a film of hydrated
iron oxide, or limonite, and the spring is an iron, or chalybeate, spring. Compounds
of iron have been taken into solution by ground water from soil and rocks, and are
now changed to the insoluble oxide on exposure to the oxygen of the air.

In wet ground iron compounds leached by ground water from the soil often
collect in reddish deposits a few feet below the surface, where their downward
progress is arrested by some impervious clay. At the bottom of bogs and shallow
lakes iron ores sometimes accumulate to a depth of several feet.
Decaying organic matter plays a large part in these changes. In its presence the insoluble iron oxides which
give color to most red and yellow rocks are decomposed, leaving the rocks of a gray or bluish color, and the
soluble iron compounds which result are readily leached out,—effects seen where red or yellow clays have been
bleached about some decaying tree root.

The iron thus dissolved is laid down as limonite when oxidized, as about a chalybeate spring; but out of
contact with the air and in the presence of carbon dioxide supplied by decaying vegetation, as in a peat bog, it
may be deposited as iron carbonate, or siderite.

Total amount of underground waters. In order to realize the vast work in
solution and cementation which underground waters are now doing and have done in
all geological ages, we must gain some conception of their amount. At a certain
depth, estimated at about six miles, the weight of the crust becomes greater than the
rocks can bear, and all cavities and pores in them must be completely closed by the
enormous pressure which they sustain. Below a depth, therefore, water cannot go.

Above it all rocks are water-soaked, up to the limit of their capacity, to within a few
feet of the surface. Estimating the average pore space of the rocks above a depth of
six miles at from two and a half per cent to five per cent of their volume, it is found
that the total amount of ground water may be great enough to cover the entire surface
of the earth to a depth of from eight hundred to sixteen hundred feet.

CHAPTER III

RIVERS AND VALLEYS
The run-off. We have traced the history of that portion of the rainfall which
soaks into the ground; let us now return to that part which washes along the surface
and is known as the run-off. Fed by rains and melting snows, the run-off gathers into
courses, perhaps but faintly marked at first, which join more definite and deeply cut
channels, as twigs their stems. In a humid climate the larger ravines through which
the run-off flows soon descend below the ground-water surface. Here springs
discharge along the sides of the little valleys and permanent streams begin. The water
supplied by the run-off here joins that part of the rainfall which had soaked into the
soil, and both now proceed together by way of the stream to the sea.

River floods. Streams vary greatly in volume during the year. At stages of flood
they fill their immediate banks, or overrun them and inundate any low lands adjacent
to the channel; at stages of low water they diminish to but a fraction of their volume
when at flood.

At times of flood, rivers are fed chiefly by the run-off; at times of low water,
largely or even wholly by springs.
How, then, will the water of streams differ at these times in turbidity and in the relative amount of solids
carried in solution?

In parts of England streams have been known to continue flowing after eighteen months of local drought, so
great is the volume of water which in humid climates is stored in the rocks above the drainage level, and so
slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter and a “spring freshet” follows the melting of the
winter’s snows. A “June rise” is produced by the heavy rains of early summer. Low water follows in July and
August, and streams are again swollen to a moderate degree under the rains of autumn.

The discharge of streams. The per cent of rainfall discharged by rivers varies
with the amount of rainfall, the slope of the drainage area, the texture of the rocks,
and other factors. With an annual rainfall of fifty inches in an open country, about
fifty per cent is discharged; while with a rainfall of twenty inches only fifteen per
cent is discharged, part of the remainder being evaporated and part passing
underground beyond the drainage area. Thus the Ohio discharges thirty per cent of

the rainfall of its basin, while the Missouri carries away but fifteen per cent. A
number of the streams of the semi-arid lands of the West do not discharge more than
five per cent of the rainfall.
Other things being equal, which will afford the larger proportion of run-off, a region underlain with granite
rock or with coarse sandstone? grass land or forest? steep slopes or level land? a well-drained region or one
abounding in marshes and ponds? frozen or unfrozen ground? Will there be a larger proportion of run-off after
long rains or after a season of drought? after long and gentle rains, or after the same amount of precipitation in a
violent rain? during the months of growing vegetation, from June to August, or during the autumn months?

Desert streams. In arid regions the ground-water surface
lies so low that for the most part stream ways do not intersect it.
Streams therefore are not fed by springs, but instead lose
volume as their waters soak into the thirsty rocks over which
they flow. They contribute to the ground water of the region Fig. 36. Rise of
instead of being increased by it. Being supplied chiefly by the Ground-Water
run-off, they wither at times of drought to a mere trickle of Surface (broken
water, to a chain of pools, or go wholly dry, while at long line) beneath
intervals rains fill their dusty beds with sudden raging torrents. Valley (V) in Arid
Desert rivers therefore periodically shorten and lengthen their Region
courses, withering back at times of drought for scores of miles,
or even for a hundred miles from the point reached by their waters during seasons of
rain.

The geological work of streams. The work of streams is of three kinds,—
transportation, erosion, and deposition. Streams transport the waste of the land; they
wear, or erode, their channels both on bed and banks; and they deposit portions of
their load from time to time along their courses, finally laying it down in the sea.
Most of the work of streams is done at times of flood.

Transportation
The invisible load of streams. Of the waste which a river transports we may
consider first the invisible load which it carries in solution, supplied chiefly by
springs but also in part by the run-off and from the solution of the rocks of its bed.
More than half the dissolved solids in the water of the average river consists of the
carbonates of lime and magnesia; other substances are gypsum, sodium sulphate
(Glauber’s salts), magnesium sulphate (Epsom salts), sodium chloride (common salt),
and even silica, the least soluble of the common rock-making minerals. The amount
of this invisible load is surprisingly large. The Mississippi, for example, transports
each year 113,000,000 tons of dissolved rock to the Gulf.

The visible load of streams. This consists of the silt which the stream carries in
suspension, and the sand and gravel and larger stones which it pushes along its bed.

Especially in times of flood one may note the muddy water, its silt being kept from
settling by the rolling, eddying currents; and often by placing his ear close to the
bottom of a boat one may hear the clatter of pebbles as they are hurried along. In
mountain torrents the rumble of bowlders as they clash together may be heard some
distance away. The amount of the load which a stream can transport depends on its
velocity. A current of two thirds of a mile per hour can move fine sand, while one of
four miles per hour sweeps along pebbles as large as hen’s eggs. The transporting
power of a stream varies as the sixth power of its velocity. If its velocity is multiplied
by two, its transporting power is multiplied by the sixth power of two: it can now
move stones sixty-four times as large as it could before.
Stones weigh from two to three times as much as water, and in water lose the weight of the volume of water
which they displace. What proportion, then, of their weight in air do stones lose when submerged?

Measurement of stream loads. To obtain the total amount of waste transported
by a river is an important but difficult matter. The amount of water discharged must
first be found by multiplying the number of square feet in the average cross section
of the stream by its velocity per second, giving the discharge per second in cubic feet.
The amount of silt to a cubic foot of water is found by filtering samples of the water
taken from different parts of the stream and at different times in the year, and drying
and weighing the residues. The average amount of silt to the cubic foot of water,
multiplied by the number of cubic feet of water discharged per year, gives the total
load carried in suspension during that time. Adding to this the estimated amount of
sand and gravel rolled along the bed, which in many swift rivers greatly exceeds the
lighter material held in suspension, and adding also the total amount of dissolved
solids, we reach the exceedingly important result of the total load of waste discharged
by the river. Dividing the volume of this load by the area of the river basin gives
another result of the greatest geological interest,— the rate at which the region is
being lowered by the combined action of weathering and erosion, or the rate of
denudation.

The rate of denudation of river basins. This rate varies widely. The Mississippi
basin may be taken as a representative land surface because of the varieties of
surface, altitude and slope, climate, and underlying rocks which are included in its
great extent. Careful measurements show that the Mississippi basin is now being
lowered at a rate of one four-thousandth of a foot a year, or one foot in four thousand
years. Taking this as the average rate of denudation for the land surfaces of the globe,
estimates have been made of the length of time required at this rate to wash and wear
the continents to the level of the sea. As the average elevation of the lands of the
globe is reckoned at 2411 feet, this result would occur in nine or ten million years, if
the present rate of denudation should remain unchanged. But even if no movements
of the earth’s crust should lift or depress the continents, the rate of wear and the
removal of waste from their surfaces will not remain the same. It must constantly

decrease as the lands are worn nearer to sea level and their slopes become more
gentle. The length of time required to wear them away is therefore far in excess of
that just stated.
The drainage area of the Potomac is 11,000 square miles. The silt brought down in suspension in a year would
cover a square mile to the depth of four feet. At what rate is the Potomac basin being lowered from this cause
alone?

It is estimated that the Upper Ganges is lowering its basin at the rate of one foot in 823 years, and the Po one
foot in 720 years. Why so much faster than the Potomac and the Mississippi?

How streams get their loads. The load of streams is derived from a number of
sources, the larger part being supplied by the weathering of valley slopes. We have
noticed how the mantle of waste creeps and washes to the stream ways. Watching the
run-off during a rain, as it hurries muddy with waste along the gutter or washes down
the hillside, we may see the beginning of the route by which the larger part of their
load is delivered to rivers. Streams also secure some of their load by wearing it from
their beds and banks,—a process called erosion.

Erosion
Streams erode their beds chiefly by means of their bottom load,— the stones of
various sizes and the sand and even the fine mud which they sweep along. With these
tools they smooth, grind, and rasp the rock of their beds, using them in much the
fashion of sandpaper or a file.

Weathering of river beds. The
erosion of stream beds is greatly
helped by the work of the weather.
Especially at low water more or
less of the bed is exposed to the
action of frost and heat and cold,
joints are opened, rocks are pried
loose and broken up and made
ready to be swept away by the
stream at time of flood.

Potholes. In rapids streams
also drill out their rocky beds.
Where some slight depression
gives rise to an eddy, the pebbles Fig. 37. Pothole in Bed of Stream, Ireland
which gather in it are whirled
round and round, and, acting like the bit of an auger, bore out a cylindrical pit called
a pothole. Potholes sometimes reach a depth of a score of feet. Where they are

numerous they aid materially in deepening the channel, as the walls between them
are worn away and they coalesce.

Waterfalls. One of the most effective means of erosion which the river possesses
is the waterfall. The plunging water dislodges stones from the face of the ledge over
which it pours, and often undermines it by excavating a deep pit at its base. Slice
after slice is thus thrown down from the front of the cliff, and the cataract cuts its
way upstream leaving a gorge behind it.

Niagara Falls. The Niagara River flows
from Lake Erie at Buffalo in a broad channel
which it has cut but a few feet below the level
of the region. Some thirteen miles from the
outlet it plunges over a ledge one hundred and
seventy feet high into the head of a narrow
gorge which extends for seven miles to the
escarpment of the upland in which the gorge is
cut. The strata which compose the upland dip
gently upstream and consist at top of a massive
limestone, at the Falls about eighty feet thick,
and below of soft and easily weathered shale.
Beneath the Falls the underlying shale is cut
and washed away by the descending water and
retreats also because of weathering, while the
overhanging limestone breaks down in huge
blocks from time to time.

Niagara is divided by Goat Island into the
Horseshoe Falls and the American Falls. The
former is supplied by the main current of the
river, and from the semicircular sweep of its
rim a sheet of water in places at least fifteen or
twenty feet deep plunges into a pool a little
less than two hundred feet in depth. Here the
force of the falling water is sufficient to move
about the fallen blocks of limestone and use
them in the excavation of the shale of the bed.
At the American Falls the lesser branch of the
river, which flows along the American side of
Goat Island, pours over the side of the gorge
and breaks upon a high talus of limestone

Fig. 38. Map of the Gorge of the blocks which its smaller volume of water is
Niagara River unable to grind to pieces and remove.

A series of surveys have determined that from 1842 to 1890 the Horseshoe Falls
retreated at the rate of 2.18 feet per year, while the American Falls retreated at the
rate of 0.64 feet in the same period. We cannot doubt that the same agency which is
now lengthening the gorge at this rapid rate has cut it back its entire length of seven
miles.

While Niagara Falls have been cutting back a gorge seven miles long and from
two hundred to three hundred feet deep, the river above the Falls has eroded its bed
scarcely below the level of the upland on which it flows. Like all streams which are
the outlets of lakes, the Niagara flows out of Lake Erie clear of sediment, as from a
settling basin, and carries no tools with which to abrade its bed. We may infer from
this instance how slight is the erosive power of clear water on hard rock.

Fig. 39. Longitudinal Section of Niagara Gorge
Black, water; F, falls; R, rapids; W, whirlpool; E, escarpment; N, north; S, south

Assuming that the rate of recession of the combined volumes of the American and Horseshoe Falls was three
feet a year below Goat Island, and assuming that this rate has been uniform in the past, how long is it since the
Niagara River fell over the edge of the escarpment where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39) shows alternating deeps and shallows which
cannot be accounted for, except in a single instance, by the relative hardness of the rocks of the river bed. The
deeps do not exceed that at the foot of the Horseshoe Falls at the present time. When the gorge was being cut
along the shallows, how did the Falls compare in excavating power, in force, and volume with the Niagara of to-
day? How did the rate of recession at those times compare with the present rate? Is the assumption made above
that the rate of recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous rapid impossible to sound. Its depth has been
estimated at thirty- five feet. From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat Island the sooner, the American or the
Horseshoe Falls? What will be the fate of the Falls left behind when the other has passed beyond the head of the
island?

The rate at which a stream erodes its bed depends in part upon the nature of the rocks over which it flows.
Will a stream deepen its channel more rapidly on massive or on thin-bedded and close- jointed rocks? on
horizontal strata or on strata steeply inclined?

Fig. 40. A Stream in Scotland
In what ways is the bed now being deepened?

Deposition
While the river carries its invisible load of dissolved rock on without stop to the
sea, its load of visible waste is subject to many delays en route. Now and again it is
laid aside, to be picked up later and carried some distance farther on its way. One of
the most striking features of the river therefore is the waste accumulated along its
course, in bars and islands in the channel, beneath its bed, and in flood plains along
its banks. All this alluvium, to use a general term for river deposits, with which the
valley is cumbered is really en route to the sea; it is only temporarily laid aside to
resume its journey later on. Constantly the river is destroying and rebuilding its
alluvial deposits, here cutting and there depositing along its banks, here eroding and
there building a bar, here excavating its bed and there filling it up, and at all times
carrying the material picked up at one point some distance on downstream before
depositing it at another.

Fig. 41. Sand Bar deposited by Stream, showing Cross Bedding

These deposits are laid down by slackening currents where the velocity of the
stream is checked, as on the inner side of curves, and where the slope of the bed is
diminished, and in the lee of islands, bridge piers and projecting points of land. How
slight is the check required to cause a current to drop a large part of its load may be
inferred from the law of the relation of the transporting power to the velocity. If the
velocity is decreased one half, the current can move fragments but one sixty-fourth
the size of those which it could move before, and must drop all those of larger size.
Will a river deposit more at low water or at flood? when rising or when falling?

Stratification. River deposits are stratified, as may be seen in any fresh cut in
banks or bars. The waste of which they are built has been sorted and deposited in
layers, one above another; some of finer and some of coarser material. The sorting
action of running water depends on the fact that its transporting power varies with the
velocity. A current whose diminishing velocity compels it to drop coarse gravel, for
example, is still able to move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower currents may deposit on
the gravel bed layers of sand, and, still later, slack water may leave on these a layer of
mud. In case of materials lighter than water the transporting power does not depend
on the velocity, and logs of wood, for instance, are floated on to the sea on the
slowest as well as on the most rapid currents.

Cross bedding. A section of a bar
exposed at low water may show that it
is formed of layers of sand, or coarser
stuff, inclined downstream as steeply Fig. 42. Longitudinal Section of a River
often as the angle of repose of the Bar
material. From a boat anchored over the
lower end of a submerged sand bar we may observe the way in which this structure,
called cross bedding, is produced. Sand is continually pushed over the edge of the bar
at b (Fig. 42) and comes to rest in successive layers on the sloping surface. At the
same time the bar may be worn away at the upper end, a, and thus slowly advance
down stream. While the deposit is thus cross bedded, it constitutes as a whole a
stratum whose upper and lower surfaces are about horizontal. In sections of river
banks one may often see a vertical succession of cross-bedded strata, each built in the
way described.

Water wear. The coarser material of river deposits, such as cobblestones, gravel,
and the larger grains of sand, are water worn, or rounded, except when near their
source. Rolling along the bottom they have been worn round by impact and friction
as they rubbed against one another and the rocky bed of the stream.
Experiments have shown that angular fragments of granite lose nearly half their weight and become well
rounded after traveling fifteen miles in rotating cylinders partly filled with water. Marbles are cheaply made in
Germany out of small limestone cubes set revolving in a current of water between a rotating bed of stone and a
block of oak, the process requiring but about fifteen minutes. It has been found that in the upper reaches of
mountain streams a descent of less than a mile is sufficient to round pebbles of granite.

Fig. 43. Water-Worn Pebbles, Upper Potomac River, Maryland

Land Forms Due To River Erosion
River valleys. In their courses to the sea, rivers follow valleys of various forms,
some shallow and some deep, some narrow and some wide. Since rivers are known to
erode their beds and banks, it is a fair presumption that, aided by the weather, they
have excavated the valleys in which they flow.

Moreover, a bird’s-eye view or a map of a region shows the significant fact that
the valleys of a system unite with one another in a branch work, as twigs meet their
stems and the branches of a tree its trunk. Each valley, from that of the smallest
rivulet to that of the master stream, is proportionate to the size of the stream which
occupies it. With a few explainable exceptions the valleys of tributaries join that of
the trunk stream at a level; there is no sudden descent or break in the bed at the point
of juncture. These are the natural consequences which must follow if the land has
long been worked upon by streams, and no other process has ever been suggested
which is competent to produce them. We must conclude that valley systems have
been formed by the river systems which drain them, aided by the work of the
weather; they are not gaping fissures in the earth’s crust, as early observers imagined,
but are the furrows which running water has drawn upon the land.

As valleys are made by the slow wear of streams and the action of the weather,
they pass in their development through successive stages, each of which has its own
characteristic features. We may therefore classify rivers and valleys according to the
stage which they have reached in their life history from infancy to old age.

Young River Valleys
Infancy. The Red River of the North. A region in northwestern Minnesota and the adjacent portions of North
Dakota and Manitoba was so recently covered by the waters of an extinct lake, known as Lake Agassiz, that the
surface remains much as it was left when the lake was drained away. The flat floor, spread smooth with lake-laid
silts, is still a plain, to the eye as level as the sea. Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not exceeding sixty feet in depth, their gradients differing
little from the general slopes of the region. The trunk streams have but few tributaries; the river system, like a
sapling with few limbs, is still undeveloped. Along the banks of the trunk streams short gullies are slowly
lengthening headwards, like growing twigs which are sometime to become large branches.

The flat interstream areas are as yet but little scored by drainage lines, and in wet weather water lingers in
ponds in any initial depressions on the plain.

Fig. 44. A Young Lacustrine Plain; the Red River of the North
Scale 5 inches = about 11 miles. Contour interval, 20 feet

Fig. 45. A Young River, Iowa
Note that it has hardly begun to cut in the plain of glacial drift on which it flows

Contours. In order to read the topographic maps of the text-book and the laboratory the student should know
that contours are lines drawn on maps to represent relief, all points on any given contour being of equal height
above sea level. The contour interval is the uniform vertical distance between two adjacent contours and varies on
different maps. To express regions of faint relief a contour interval of ten or twenty feet is commonly selected;

while in mountainous regions a contour interval of two hundred and fifty, five hundred, or even one thousand feet
may be necessary in order that the contours may not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example just cited, or
upon a coastal plain lifted from beneath the sea or on a spread of glacial drift left by
the retreat of continental ice sheets, such as covers much of Canada and the
northeastern parts of the United States, its infantile stage presents the same
characteristic features,—a narrow and shallow valley, with undeveloped tributaries
and undrained interstream areas. Ground water stands high, and, exuding in the
undrained initial depressions, forms marshes and lakes.

Fig. 46. A Young Drift Region in Wisconsin
Describe this area. How high are the hills? Are they such in form and position as would
be left by stream erosion? Consult a map of the entire state and notice that the Fox River
finds its way to Lake Michigan, while the Wisconsin empties into the Mississippi.
Describe that portion of the divide here shown between the Mississippi and the St.
Lawrence systems. Which is the larger river, the Wisconsin or the Fox? Other things
being equal, which may be expected to deepen its bed the more rapidly? What changes
are likely to occur when one of these rivers comes to flow at a lower level than the
other? Why have not these changes occurred already?

Lakes. Lakes are perhaps the most obvious of these fleeting features of infancy.
They are short-lived, for their destruction is soon accomplished by several means. As
a river system advances toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the undrained depressions of
the region. Lakes having outlets are drained away as their basin rims are cut down by
the outflowing streams,—a slow process where the rim is of hard rock, but a rapid
one where it is of soft material such as glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing streams and the
wash of rains bring in waste. Waves abrade the shore and strew the débris worn from
it over the lake bed. Shallow lakes are often filled with organic matter from decaying
vegetation.

Does the outflowing stream, from a lake carry sediment? How does this fact affect its erosive power on hard
rock? on loose material?

Lake Geneva is a well-known example of a lake in process of obliteration.
The inflowing Rhone has already displaced the waters of the lake for a length of
twenty miles with the waste brought down from the high Alps. For this distance
there extends up the Rhone Valley an alluvial plain, which has grown lakeward
at the rate of a mile and a half since Roman times, as proved by the distance
inland at which a Roman port now stands.

How rapidly a lake may be silted up under exceptionally favorable Fig. 47. A Small Lake
conditions is illustrated by the fact that over the bottom of the artificial lake, of being broadened
thirty-five square miles, formed behind the great dam across the Colorado River
at Austin, Texas, sediments thirty-nine feet deep gathered in seven years. and shoaled by
Wave Wear
Lake Mendota, one of the many beautiful lakes of southern Wisconsin, is
ls, lake surface; dotted
rapidly cutting back the soft glacial drift of its shores by means of the abrasion
of its waves. While the shallow basin is thus broadened, it is also being filled line, initial shore; b, fill
with the waste; and the time is brought nearer when it will be so shoaled that made of material taken
vegetation can complete the work of its effacement.
from a

Fig. 48. A Lake well-nigh effaced, Montana
By what means is the lake bed being filled?

Along the margin of a shallow lake mosses, water lilies, grasses, and other water-
loving plants grow luxuriantly. As their decaying remains accumulate on the bottom,
the ring of marsh broadens inwards, the lake narrows gradually to a small pond set in
the midst of a wide bog, and finally disappears. All stages in this process of
extinction may be seen among the countless lakelets which occupy sags in the recent
sheets of glacial drift in the northern states; and more numerous than the lakes which
still remain are those already thus filled with carbonaceous matter derived from the

carbon dioxide of the atmosphere. Such fossil lakes are marked by swamps or level
meadows underlain with muck.

Fig. 49. A Level Meadow, Scotland
Explain its origin. What will be its future?

The advance to maturity. The infantile stage is brief. As a river advances toward
maturity the initial depressions, the lake basins of its area, are gradually effaced. By
the furrowing action of the rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire area, and not an acre
is left on which the fallen raindrop does not find already cut for it an uninterrupted
downward path which leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is now wholly
destroyed; the region is all reduced to valley slopes.

Fig. 50. Drainage Maps
A, an area in its infancy, Buena Vista County, Iowa;
B, an area in its maturity, Ringgold County, Iowa

The longitudinal profile of a
stream. This at first corresponds with
the initial surface of the region on
which the stream begins to flow,
although its way may lead through
basins and down steep descents. The Fig. 51. Successive Longitudinal Profiles
successive profiles to which it reduces of a Stream
am, initial profile, with waterfall at w, and basins at
its bed are illustrated in Figure 51. As
l and l´, which at first are occupied by lakes and
the gradient, or rate of descent of its
later are filled or drained; b, c, d, and e, profiles
bed, is lowered, the velocity of the
established in succession as the stream advances
river is decreased until its lessening
from infancy toward old age. Note that these
energy is wholly consumed in carrying
profiles are concave toward the sky. This is the
its load and it can no longer erode its
erosion curve. What contrasting form has the
bed. The river is now at grade, and its
weather weather curve (p. 34)?
capacity is just equal to its load. If now
its load is increased the stream deposits, and thus builds up, or aggrades, its bed. On
the other hand, if its load is diminished it has energy to spare, and resuming its work
of erosion, degrades its bed. In either case the stream continues aggrading or
degrading until a new gradient is found where the velocity is just sufficient to move
the load, and here again it reaches grade.

Fig. 52. A V-Valley,—the Canyon of the Yellowstone

Note the steep sides. What processes are at work upon them? How wide is the valley at
the base compared with the width of the stream? Do you see any river deposits along the
banks? Is the stream flowing swiftly over a rock bed, or quietly over a bed which it has
built up? Is it graded or ungraded? Note that the canyon walls project in interlocking
spurs

V-Valleys. Vigorous rivers well armed with waste make short work of cutting
their beds to grade, and thus erode narrow, steep-sided gorges only wide enough at
the base to accommodate the stream. The steepness of the valley slopes depends on
the relative rates at which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream may saw out a gorge
whose sides are nearly or even quite vertical, but as a rule young valleys whose
streams have not yet reached grade are V-shaped; their sides flare at the top because
here the rocks have longest been opened up to the action of the weather. Some of the
deepest canyons may be found where a rising land mass, either mountain range or
plateau, has long maintained by its continued uplift the rivers of the region above
grade.
In the northern hemisphere the north sides of river valleys are sometimes of
more gentle slope than the south sides. Can you suggest a reason?

The Grand Canyon of the Colorado River in Arizona. The Colorado River
trenches the high plateau of northern Arizona with a colossal canyon two hundred
and eighteen miles long and more than a mile in greatest depth (Fig. 15). The
rocks in which the canyon is cut are for the most part flat-lying, massive beds of
limestones and sandstones, with some shales, beneath which in places harder Fig. 53. Section of
crystalline rocks are disclosed. Where the canyon is deepest its walls have been
profoundly dissected. Lateral ravines have widened into immense amphitheaters,
the Yellowstone
leaving between them long ridges of mountain height, buttressed and rebuttressed Canyon
with flanking spurs and carved into majestic architectural forms. From the This canyon is 100
extremity of one of these promontories it is two miles or more across the gulf to
feet deep, 2500 feet
the point of the one opposite, and the heads of the amphitheaters are thirteen miles
apart. wide at the top, and
about 250 feet wide
The lower portion of the canyon is much narrower (Fig. 54) and its walls of
dark crystalline rock sink steeply to the edge of the river, a swift, powerful stream at the bottom.
a few hundred feet wide, turbid with reddish silt, by means of which it continually Neglecting any
rasps its rocky bed as it hurries on. The Colorado is still deepening its gorge. In cutting of the river
the Grand Canyon its gradient is seven and one half feet to the mile, but, as in all
ungraded rivers, the descent is far from uniform. Graded reaches in soft rock against the banks,
alternate with steeper declivities in hard rock, forming rapids such as, for estimate what part of
example, a stretch of ten miles where the fall averages twenty-one feet to the mile.
the excavation of the
Because of these dangerous rapids the few exploring parties who have traversed
the Colorado canyon have done so at the hazard of their lives. canyon is due to the
vertical erosion of its
bed by the river and
what to weathering
and rain wash on the
canyon sides

Fig. 54. Grand Canyon of the Colorado River, Arizona
The canyon has been shaped by several agencies. Its depth is due to the river which has sawed its way far
toward the base of a lofty rising plateau. Acting alone this would have produced a slitlike gorge little wider than
the breadth of the stream. The impressive width of the canyon and the magnificent architectural masses which fill
it are owing to two causes. Running water has gulched the walls and weathering has everywhere attacked and
driven them back. The horizontal harder beds stand out in long lines of vertical cliffs, often hundreds of feet in
height, at whose feet talus slopes conceal the outcrop of the weaker strata (Fig. 15). As the upper cliffs have been
sapped and driven back by the weather, broad platforms are left at their bases and the sides of the canyon descend
to the river by gigantic steps. Far up and down the canyon the eye traces these horizontal layers, like the flutings
of an elaborate molding, distinguishing each by its contour as well as by its color and thickness.

Fig. 55. Diagrams illustrating Conditions which produce
Falls or Rapids
A, vertical succession of harder and softer rocks; B, horizontal succession
of the same. In A the stream ab in sinking its bed through a mass of strata
of different degrees of hardness has discovered the weak layer s beneath
the hard layer h. It rapidly cuts its way in s, while in h its work is delayed.
Thus the profile afb´ is soon reached, with falls at f. In B the initial profile
is shown by dotted line.

The Grand Canyon of the Colorado is often and rightly cited as an example of the stupendous erosion which
may be accomplished by a river. And yet the Colorado is a young stream and its work is no more than well begun.

It has not yet wholly reached grade, and the great task of the river and its tributaries—the task of leveling the lofty
plateau to a low plain and of transporting it grain by grain to the sea—still lies almost entirely in the future.

Fig. 57. Diagram illustrating Migration of a
Fig. 56. Longitudinal Section of
Fall due to a Hard Layer H, in the
Yellowstone River at Lower
Midst of Soft Layers S and S, all
Fall, F, and Upper Fall, F´,
dipping upstream
Yellowstone National Park
a, b, c, d, and e, successive positions of the fall; r,
la, lava deeply decayed through action of
rapid to which the fall is reduced. Draw diagram
thermal waters; m and m´, masses of
showing migration of fall in strata dipping
decayed lavas to whose hardness the falls
downstream. Under what conditions of inclination
are due. Which fall will be worn away
of the strata will a fall migrate the farthest and
the sooner? How far upstream will each
have the longest life? Under what conditions will it
fall migrate? Draw profile of the river
migrate the least distance and soonest be
when one fall has disappeared
destroyed?

Waterfalls and rapids. Before the bed of a stream is reduced to grade it may be broken by abrupt descents
which give rise to waterfalls and rapids. Such breaks in a river’s bed may belong to the initial surface over which
it began its course; still more commonly are they developed in the rock mass through which it is cutting its valley.
Thus, wherever a stream leaves harder rocks to flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or rapid, results (Fig. 55).

At time of flood young tributaries with steeper courses than that of the trunk stream may bring down stones
and finer waste, which the gentler current cannot move along, and throw them as a dam across its way. The rapids
thus formed are also ephemeral, for as the gradient of the tributaries is lowered the main stream becomes able to
handle the smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a young river are not able to keep pace with
their master stream in the erosion of their beds because of their smaller volume, and thus join it by plunging over
the side of its gorge. But as the river approaches grade and slackens its down cutting, the tributaries sooner or
later overtake it, and effacing their falls, unite with it on a level.

Click on image to view larger version.

Fig. 58. Maturely Dissected Plateau near
Charleston, West Virginia
Compare the number of streams in any given number of square
miles with the number on an area of the same size in the Red
River Valley (Fig. 44). What is the shape of the ridges? Are their
summits broad or narrow? Are their crests even or broken by
knobs and cols (the depressions on the crest line)? If the latter,
how deeply have the cols been worn beneath the summits of the
knobs?

Waterfalls and rapids of all kinds are evanescent features of a river’s youth. Like
lakes they are soon destroyed, and if any long time had already elapsed since their
formation they would have been obliterated already.

Local baselevels. That balanced condition called grade, where a river neither
degrades its bed by erosion nor aggrades it by deposition, is first attained along
reaches of soft rocks, ungraded outcrops of hard rocks remaining as barriers which

give rise to rapids or falls. Until these barriers are worn away they constitute local
baselevels, below which level the stream, up valley from them, cannot cut. They are
eroded to grade one after another, beginning with the least strong, or the one nearest
the mouth of the stream. In a similar way the surface of a lake in a river’s course
constitutes for all inflowing streams a local baselevel, which disappears when the
basin is filled or drained.

Fig. 59. A Maturity Dissected Region of Slight Relief, Iowa

Mature And Old Rivers
Maturity is the stage of a river’s complete development and most effective work.
The river system now has well under way its great task of wearing down the land
mass which it drains and carrying it particle by particle to the sea. The relief of the
land is now at its greatest; for the main channels have been sunk to grade, while the
divides remain but little worn below their initial altitudes. Ground water now stands
low. The run-off washes directly to the streams, with the least delay and loss by
evaporation in ponds and marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley slopes is now
largest and sheds to the streams a heavier load of waste than ever before. At maturity
the river system is doing its greatest amount of work both in erosion and in the
carriage of water and of waste to the sea.

Fig. 60. Successive Stages, A, B, C, and D, in Valley-Widening by Planation
Describe valley A. What changes have taken place in B, C, and D? Do the river bends remain stationary or move
up or down valley? With what effect on the projecting spurs of the valley sides? Draw diagrams showing a still
later stage than D

Lateral erosion. On reaching grade a river ceases to scour its bed, and it does not
again begin to do so until some change in load or volume enables it to find grade at a
lower level. On the other hand, a stream erodes its banks at all stages in its history,
and with graded rivers this process, called lateral erosion, or planation, is specially
important. The current of a stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and continually wears and
saps its banks. On the inner side deposition takes place in the more shallow and
slower-moving water. The inner bank of bends is thus built out while the outer bank
is worn away. By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is thus changed by
planation to a flat-floored valley with flaring sides which gradually become subdued
by the weather to gentle slopes. While widening their valleys streams maintain a
constant width of channel, so that a wide-floored valley does not signify that it ever
was occupied by a river of equal width.

The gradient. The gradients of graded rivers differ widely. A large river with a
light load reaches grade on a faint slope, while a smaller stream heavily burdened
with waste requires a steep slope to give it velocity sufficient to move the load.
The Platte, a graded river of Nebraska with its headwaters in the Rocky Mountains, is enfeebled by the semi-
arid climate of the Great Plains and surcharged with the waste brought down both by its branches in the
mountains and by those whose tracks lie over the soft rocks of the plains. It is compelled to maintain a gradient of
eight feet to the mile in western Nebraska. The Ohio reaches grade with a slope of less than four inches to the
mile from Cincinnati to its mouth, and the powerful Mississippi washes along its load with a fall of but three
inches per mile from Cairo to the Gulf.

Other things being equal, which of graded streams will have the steeper gradient, a trunk stream or its
tributaries? a stream supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the Platte should increase in volume? What
changes would occur if the load should be increased in amount or in coarseness?

Fig. 61. Successive Cross Sections of a Region as it advances from Infancy a, to Old
Age e

The old age of rivers. As rivers pass their prime, as denudation lowers the relief
of the region, less waste and finer is washed over the gentler slopes of the lowering
hills. With smaller loads to carry, the rivers now deepen their valleys and find grade
with fainter declivities nearer the level of the sea. This limit of the level of the sea
beneath which they cannot erode is known as baselevel. [1] As streams grow old they
approach more and more closely to baselevel, although they are never able to attain
it. Some slight slope is needed that water may flow and waste be transported over the
land. Meanwhile the relief of the land has ever lessened. The master streams and their
main tributaries now wander with sluggish currents over the broad valley floors
which they have planed away; while under the erosion of their innumerable branches
and the wear of the weather the divides everywhere are lowered and subdued to more
and more gentle slopes. Mountains and high plateaus are thus reduced to rolling hills,
and at last to plains, surmounted only by such hills as may still be unreduced to the
common level, because of the harder rocks of which they are composed or because of
their distance from the main erosion channels. Such regions of faint relief, worn
down to near base level by subaërial agencies, are known as peneplains (almost
plains). Any residual masses which rise above them are called monadnocks, from the
name of a conical peak of New Hampshire which overlooks the now uplifted
peneplain of southern New England.
[1] The term “baselevel” is also used to designate the close approximation to sea level
to which streams are able to subdue the land.

In its old age a region becomes mantled with thick sheets of fine and weathered
waste, slowly moving over the faint slopes toward the water ways and unbroken by
ledges of bare rock. In other words, the waste mantle also is now graded, and as
waterfalls have been effaced in the river beds, so now any ledges in the wide streams
of waste are worn away and covered beneath smooth slopes of fine soil. Ground
water stands high and may exude in areas of swamp. In youth the land mass was
roughhewn and cut deep by stream erosion. In old age the faint reliefs of the land
dissolve away, chiefly under the action of the weather, beneath their cloak of waste.

Fig. 62. Peneplain surrounded by Monadnocks, Piedmont Belt, Virginia
From Davis’ Elementary Physical Geography

The cycle of erosion. The successive stages through which a land mass passes
while it is being leveled to the sea constitute together a cycle of erosion. Each stage
of the cycle from infancy to old age leaves, as we have seen, its characteristic records
in the forms sculptured on the land, such as the shapes of valleys and the contours of
hills and plains. The geologist is thus able to determine by the land forms of any
region the stage in the erosion cycle to which it now belongs, and knowing what are
the earlier stages of the cycle, to read something of the geological history of the
region.

Interrupted cycles. So long a time is needed to reduce a land mass to baselevel
that the process is seldom if ever completed during a single uninterrupted cycle of
erosion. Of all the various interruptions which may occur the most important are
gradual movements of the earth’s crust, by which a region is either depressed or
elevated relative to sea level.

Fig. 63. Young Inner Gorge in Wide Older Valley, Alaska

The depression of a region hastens its old age by decreasing the gradient of
streams, by destroying their power to excavate their beds and carry their loads to a
degree corresponding to the amount of the depression, and by lessening the amount
of work they have to do. The slackened river currents deposit their waste in Hood
plains which increase in height as the subsidence continues. The lower courses of the
rivers are invaded by the sea and become estuaries, while the lower tributaries are cut
off from the trunk stream.

Elevation, on the other hand, increases the activity of all agencies of weathering,
erosion, and transportation, restores the region to its youth, and inaugurates a new
cycle of erosion. Streams are given a steeper gradient, greater velocity, and increased
energy to carry their loads and wear their beds. They cut through the alluvium of
their flood plains, leaving it on either bank as successive terraces, and intrench
themselves in the underlying rock. In their older and wider valleys they cut narrow,
steep-walled inner gorges, in which they flow swiftly over rocky floors, broken here
and there by falls and rapids where a harder layer of rock has been discovered.
Winding streams on plains may thus incise their meanders in solid rock as the plains
are gradually uplifted. Streams which are thus restored to their youth are said to be
revived.

As streams cut deeper and the valley slopes are steepened, the mantle of waste of
the region undergoing elevation is set in more rapid movement. It is now removed
particle by particle faster than it forms. As the waste mantle thins, weathering attacks
the rocks of the region more energetically until an equilibrium is reached again; the
rocks waste rapidly and their waste is as rapidly removed.

Dissected peneplains. When a rise of the land brings one cycle to an end and
begins another, the characteristic land forms of each cycle are found together and the
topography of the region is composite until the second cycle is so far advanced that
the land forms of the first cycle are entirely destroyed. The contrast between the land
surfaces of the later and the earlier cycles is most striking when the earlier had
advanced to age and the later is still in youth. Thus many peneplains which have been
elevated and dissected have been recognized by the remnants of their ancient erosion
surfaces, and the length of time which has elapsed since their uplift has been
measured by the stage to which the new cycle has advanced.
The piedmont belt. As an example of an ancient peneplain uplifted and dissected we may cite the Piedmont
Belt, a broad upland lying between the Appalachian Mountains and the Atlantic coastal plain. The surface of the
Piedmont is gently rolling. The divides, which are often smooth areas of considerable width, rise to a common
plane, and from them one sees in every direction an even sky line except where in places some lone hill or ridge
may lift itself above the general level (Fig. 62). The surface is an ancient one, for the mantle of residual waste lies
deep upon it, soils are reddened by long oxidation, and the rocks are rotted to a depth of scores of feet.

At present, however, the waste mantle is not forming so
rapidly as it is being removed. The streams of the upland
are actively engaged in its destruction. They flow swiftly in
narrow, rock- walled valleys over rocky beds. This contrast
between the young streams and the aged surface which they
are now so vigorously dissecting can only be explained by
the theory that the region once stood lower than at present
and has recently been upraised. If now we imagine the
valleys refilled with the waste which the streams have
swept away, and the upland lowered, we restore the
Piedmont region to the condition in which it stood before
its uplift and dissection,—a gently rolling plain,
surmounted here and there by isolated hills and ridges.

Fig. 64. Incised Meanders of Oneota
River, Iowa

Fig. 65.

Describe the valley of stream a. Is it young or old? How
does the valley of b differ from that of a? Compare as to
form and age the inner valley of b with the outer valley
and with the valley of a. Account for the inner valley.
Why does it not extend to the upper portion of the course
of b? Will it ever do so? Draw longitudinal profile of b,
showing the different gradient of upper and lower
portions of its course not here seen. As the inner valley of
tributary c extends headward it may invade the valley of
a before the inner valley of a has worked upstream to the
area seen in the diagram. With what results?

Fig. 66. Dissected Peneplain of Southern New England
The surface of the ancient Piedmont plain, as it may be restored from the remnants of it found on the divides,
is not in accordance with the structures of the country rocks. Where these are exposed to view they are seen to be
far from horizontal. On the walls of river gorges they dip steeply and in various directions and the streams flow
over their upturned edges. As shown in Figure 67, the rocks of the Piedmont have been folded and broken and
tilted.

Fig. 67. Section in Piedmont Belt
M, a monadnock

It is not reasonable to believe that when the rocks of the Piedmont were thus folded and otherwise deformed
the surface of the region was a plain. The upturned layers have not always stopped abruptly at the even surface of
the Piedmont plain which now cuts across them. They are the bases of great folds and tilted blocks which must
once have risen high in air. The complex and disorderly structures of the Piedmont rocks are those seen in great
mountain ranges, and there is every reason to believe that these rocks after their deformation rose to mountain
height.

Fig. 68. The area of the Laurentian Peneplain (shaded)
The ancient Piedmont plain cuts across these upturned rocks as independently of their structure as the even
surface of the sawed stump of some great tree is independent of the direction of its fibers. Hence the Piedmont
plain as it was before its uplift was not a coastal plain formed of strata spread in horizontal sheets beneath the sea
and then uplifted; nor was it a structural plain, due to the resistance to erosion of some hard, flat-lying layer of
rock. Even surfaces developed on rocks of discordant structure, such as the Piedmont shows, are produced by
long denudation, and we may consider the Piedmont as a peneplain formed by the wearing down of mountain
ranges, and recently uplifted.

The Laurentian peneplain. This is the name given to a denuded surface on very
ancient rocks which extends from the Arctic Ocean to the St. Lawrence River and
Lake Superior, with small areas also in northern Wisconsin and New York.
Throughout this U-shaped area, which incloses Hudson Bay within its arms, the
country rocks have the complicated and contorted structures which characterize
mountain ranges (see Fig. 179, P. 211). But the surface of the area is by no means
mountainous. The sky line when viewed from the divides is unbroken by mountain
peaks or rugged hills. The surface of the arm west of Hudson Bay is gently
undulating and that of the eastern arm has been roughened to low-rolling hills and
dissected in places by such deep river gorges as those of the Ottawa and Saguenay.
This immense area may be regarded as an ancient peneplain truncating the bases of
long-vanished mountains and dissected after elevation.

In the examples cited the uplift has been a broad one and to comparatively little
height. Where peneplains have been uplifted to great height and have since been well
dissected, and where they have been upfolded and broken and uptilted, their
recognition becomes more difficult. Yet recent observers have found evidences of
ancient lowland surfaces of erosion on the summits of the Allegheny ridges, the
Cascade Mountains (Fig. 69), and the western slope of the Sierra Nevadas.

Fig. 69. View in the Cascade Mountains, Washington
The general level to which these ridges rise may be accounted for by the uplift and dissection of a once low-lying
peneplain

The southern Appalachian region. We have here an example of an area the latter part of whose geological
history may be deciphered by means of its land forms. The generalized section of Figure 70, which passes from
west to east across a portion of the region in eastern Tennessee, shows on the west a part of the broad Cumberland
plateau. On the east is a roughened upland platform, from which rise in the distance the peaks of the Great Smoky
Mountains. The plateau, consisting of strata but little changed from their original flat-lying attitude, and the
platform, developed on rocks of disordered structure made crystalline by heat and pressure, both stand at the
common level of the line AB. They are separated by the Appalachian valley, forty miles wide, cut in strata which
have been folded and broken into long narrow blocks. The valley is traversed lengthwise by long, low ridges, the
outcropping edges of the harder strata, which rise to about the same level,—that of the line cd. Between these
ridges stretch valley lowlands at the level ef excavated in the weaker rocks, while somewhat below them lie the
channels of the present streams now busily engaged in deepening their beds.

The valley lowlands. Were they planed by graded or ungraded streams? Have the present streams reached
grade? Why did the streams cease widening the floors of the valley lowlands? How long since? When will they
begin anew the work of lateral planation? What effect will this have on the ridges if the present cycle of erosion
continues long uninterrupted?

Fig. 70. Generalized Section of the Southern
Appalachian Region in Eastern Tennessee
The ridges of the Appalachian valley. Why do they stand above the valley lowlands? Why do their summits lie
in about the same plane? Refilling the valleys intervening between these ridges with the material removed by the
streams, what is the nature of the surface thus restored? Does this surface cd accord with the rock structures on
which it has been developed? How may it have been made? At what height did the land stand then, compared
with its present height? What elevations stood above the surface cd? Why? What name may you use to designate
them? How does the length of time needed to develop the surface cd compare with that needed to develop the
valley lowlands?

The platform and plateau. Why do they stand at a common level ab? Of what surface may they be remnants?
Is it accordant with the rock structure? How was it produced? What unconsumed masses overlooked it? Did the
rocks of the Appalachian valley stand above this surface when it was produced? Did they then stand below it?
Compare the time needed to develop this surface with that needed to develop cd. Which surface is the older?

How many cycles of erosion are represented here? Give the erosion history of the region by cycles, beginning
with the oldest, the work done in each and the work left undone, what brought each cycle to a close, and how long
relatively it continued.

CHAPTER IV

RIVER DEPOSITS
The characteristic features of river deposits and the forms which they assume
may be treated under three heads: (1) valley deposits, (2) basin deposits, and (3)
deltas.

Valley Deposits
Flood plains. The deposits which streams build along their courses at times of
flood are known as flood plains. A swift current then sweeps along the channel, while
a shallow sheet of water moves slowly over the flood plain, spreading upon it a thin
layer of sediment. It has been estimated that each inundation of the Nile leaves a
layer of fertilizing silt three hundredths of an inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the flat rock floor of a
valley which is being widened by the lateral erosion of a graded stream (Fig. 60).

Flood-plain deposits of great thickness may be built by aggrading rivers even in
valleys whose rock floors have never been thus widened (Fig. 368).

A cross section of a flood plain (Fig. 71) shows that it is
highest next the river, sloping gradually thence to the valley
sides. These wide natural embankments are due to the fact
that the river deposit is heavier near the bank, where the
velocity of the silt-laden channel current is first checked by Fig. 71. Cross
contact with the slower-moving overflow. Section of a Flood
Plain
Thus banked
off from the stream, the outer portions of a
flood plain are often ill-drained and
swampy, and here vegetal deposits, such as
peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the
Mississippi or the Missouri (Fig. 77), shows that the
courses of the tributaries on entering it are deflected
downstream. Why?

The aggrading streams by which flood
plains are constructed gradually build their
immediate banks and beds to higher and
higher levels, and therefore find it easy at
times of great floods to break their natural
embankments and take new courses over
the plain. In this way they aggrade each
portion of it in turn by means of their
shifting channels.

Braided channels. A river actively
engaged in aggrading its valley with coarse
waste builds a flood plain of comparatively
steep gradient and often flows down it in a
fairly direct course and through a network
of braided channels. From time to time a
channel becomes choked with waste, and
the water no longer finding room in it
breaks out and cuts and builds itself a new
way which reunites down valley with the
other channels. Thus there becomes
established a network of ever-changing

Fig. 72. Waste-filled Valley and channels inclosing low islands of sand and
Braided Channels of the Upper gravel.
Mississippi

Fig. 73. Terraced Valley of River in Central Asia

Fig. 74. Terraces carved in Alluvial Deposits
Which is older, the rock floor of the valley or the river deposits which fill
it? What are the relative ages of terraces a, b, c, and e? It will be noted that
the remnants of the higher flood plains have not been swept away by the
meandering river, as it swung from side to side of the valley at lower levels,
because they have been defended by ledges of hard rock in the projecting
spurs of the initial valley. The stream has encountered such defending
ledges at the point marked d

Terraces. While aggrading streams thus tend to shift their channels, degrading
streams, on the contrary, become more and more deeply intrenched in their valleys. It
often occurs that a stream, after having built a flood plain, ceases to aggrade its bed

because of a lessened load or for other
reasons, such as an uplift of the region, and
begins instead to degrade it. It leaves the
original flood plain out of reach of even the
highest floods. When again it reaches grade
Fig. 75. River Terraces of Rock at a lower level it produces a new flood
covered with Alluvium plain by lateral erosion in the older deposits,
c, recent flood plain of the river. To what remnants of which stand as terraces on one
rocesses is it due? Account for the alluvium at a or both sides of the valley. In this way a
nd b and on the opposite side of the valley at the valley may be lined with a succession of
ame levels. Which is the older? Account for the terraces at different levels, each level
flat rock floors on which these deposits of representing an abandoned flood plain.
alluvium rest. Give the entire history which may
be read in the section

Meanders. Valleys aggraded with fine
waste form well-nigh level plains over
which streams wind from side to side of a
direct course in symmetric bends known as
meanders, from the name of a winding river Fig. 76. Development of a Meander
of Asia Minor. The giant Mississippi has The dotted line in a, b, and c shows the stage
developed meanders with a radius of one preceding that indicated by the unbroken line
and one half miles, but a little creek may
display on its meadow as perfect curves only a rod or so in radius. On the flood plain
of either river or creek we may find examples of the successive stages in the
development of the meander, from its beginning in the slight initial bend sufficient to
deflect the current against the outer side. Eroding here and depositing on the inner
side of the bend, it gradually reaches first the open bend (Fig. 76, a) whose width and
length are not far from equal, and later that of the horseshoe meander (Fig. 76, b)
whose diameter transverse to the course of the stream is much greater than that
parallel with it. Little by little the neck of land projecting into the bend is narrowed,
until at last it is cut through and a “cut-off” is established. The old channel is now
silted up at both ends and becomes a crescentic lagoon (Fig. 76, c), or oxbow lake,
which fills gradually to an arc-shaped shallow depression.

Flood plains characteristic of mature rivers. On reaching grade a stream planes
a flat floor for its continually widening valley. Ever cutting on the outer bank of its
curves, it deposits on the inner bank scroll-like flood-plain patches (Fig 60). For a
while the valley bluffs do not give its growing meanders room to develop to their
normal size, but as planation goes on, the bluffs are driven back to the full width of
the meander belt and still later to a width which gives room for broad stretches of
flood plain on either side (Fig. 77).

Usually a river first attains
grade near its mouth, and here first
sinks its bed to near baselevel.
Extending its graded course
upstream by cutting away barrier
after barrier, it comes to have a
widened and mature valley over its
lower course, while its young
headwaters are still busily eroding
their beds. Its ungraded branches
may thus bring down to its lower
course more waste than it is
competent to carry on to the sea,
and here it aggrades its bed and
builds a flood plain in order to gain
a steeper gradient and velocity
enough to transport its load.

As maturity is past and the
relief of the land is lessened, a
smaller and smaller load of waste is
delivered to the river. It now has
energy to spare and again degrades
its valley, excavating its former
flood plains and leaving them in
terraces on either side, and at last in
its old age sweeping them away.

Fig. 77. Map of a portion of the Flood Plain of
the Missouri River
Each small square represents one square mile. How wide is the
flood plain of the Missouri? How wide is the flood plain of the
Big Sioux? Why is the latter river deflected down valley on
entering the flood plain of the master stream? How do the
meanders of the two rivers compare in size? How does the
width of each flood plain compare with the width of the belt
occupied by the meanders of the river? Do you find traces of
any former channels?

Fig. 78. Alluvial Cones, Wyoming
Alluvial cones and fans. In hilly and mountainous countries one often sees on a
valley side a conical or fan-shaped deposit of waste at the mouth of a lateral stream.
The cause is obvious: the young branch has not been able as yet to wear its bed to
accordant level with the already deepened valley of the master stream. It therefore
builds its bed to grade at the point of juncture by depositing here its load of waste,—a
load too heavy to be carried along the more gentle profile of the trunk valley.
Where rivers descend from a mountainous region upon the plain they may build alluvial fans of exceedingly
gentle slope. Thus the rivers of the western side of the Sierra Nevada Mountains have spread fans with a radius of
as much as forty miles and a slope too slight to be detected without instruments, where they leave the rock-cut
canyons in the mountains and descend upon the broad central valley of California.

As a river flows over its fan it commonly divides into a branchwork of shifting
channels called distributaries, since they lead off the water from the main stream. In
this way each part of the fan is aggraded and its symmetric form is preserved.

Piedmont plains. Mountain streams may build their confluent fans into
widespread piedmont (foot of the mountain) alluvial plains. These are especially
characteristic of arid lands, where the streams wither as they flow out upon the thirsty
lowlands and are therefore compelled to lay down a large portion of their load. In
humid climates mountain-born streams are usually competent to carry their loads of
waste on to the sea, and have energy to spare to cut the lower mountain slopes into
foothills. In arid regions foothills are commonly absent and the ranges rise, as from
pedestals, above broad, sloping plains of stream-laid waste.

Fig. 80. Section from the
Rocky Mountains Eastward
River deposits dotted

The High Plains. The rivers which flow eastward from
the Rocky Mountains have united their fans in a continuous
sheet of waste which stretches forward from the base of the
mountains for hundreds of miles and in places is five
hundred feet thick (Fig. 80). That the deposit was made in
ancient times on land and not in the sea is proved by the
remains which it contains of land animals and plants of
species now extinct. That it was laid by rivers and not by
fresh-water lakes is shown by its structure. Wide stretches
of flat-lying, clays and sands are interrupted by long,
narrow belts of gravel which mark the channels of the
ancient streams. Gravels, and sands are often cross bedded,
and their well worn pebbles may be identified with the
rocks of the mountains. After building this sheet of waste
the streams ceased to aggrade and began the work of
destruction. Large uneroded remnants, their surfaces flat as
a floor, remain as the High Plains of western Kansas and
Nebraska.

River deposits in subsiding troughs.
To a geologist the most important river
deposits are those which gather in areas of
gradual subsidence; they are often of vast
extent and immense thickness, and such
deposits of past geological ages have not
infrequently been preserved, with all their
records of the times in which they were Fig. 79. Tributaries and Distributaries
built, by being carried below the level of the of a
sea, to be brought to light by a later uplift. Fan-Building Stream
On the other hand, river deposits which
remain above baselevels of erosion are swept away comparatively soon.
The Great Valley Of California is a monotonously level plain of great fertility, four hundred miles in length
and fifty miles in average width, built of waste swept down by streams from the mountain ranges which inclose it,
—the Sierra Nevada on the east and the Coast Range on the west. On the waste slopes at the foot of the bordering
hills coarse gravels and even bowlders are left, while over the interior the slow-flowing streams at times of flood
spread wide sheets of silt. Organic deposits are now forming by the decay of vegetation in swampy tule (reed)
lands and in shallow lakes which occupy depressions left by the aggrading streams.

Deep borings show that this great trough is filled to a depth of at least two thousand feet below sea level with
recent unconsolidated sands and silts containing logs of wood and fresh- water shells. These are land deposits, and
the absence of any marine deposits among them proves that the region has not been invaded by the sea since the
accumulation began. It has therefore been slowly subsiding and its streams, although continually carried below
grade, have yet been able to aggrade the surface as rapidly as the region sank, and have maintained it, as at
present, slightly above sea level.

The Indo-Gangetic Plain, spread by the Brahmaputra, the Ganges, and the Indus river systems, stretches for
sixteen hundred miles along the southern base of the Himalaya Mountains and occupies an area of three hundred
thousand square miles (Fig. 342). It consists of the flood plains of the master streams and the confluent fans of the
tributaries which issue from the mountains on the north. Large areas are subject to overflow each season of flood,
and still larger tracts mark abandoned flood plains below which the rivers have now cut their beds. The plain is
built of far- stretching beds of clay, penetrated by streaks of sand, and also of gravel near the mountains. Beds of
impure peat occur in it, and it contains fresh-water shells and the bones of land animals of species now living in
northern India. At Lucknow an artesian well was sunk to one thousand feet below sea level without reaching the
bottom of these river-laid sands and silts, proving a slow subsidence with which the aggrading rivers have kept
pace.

Warped valleys. It is not necessary that an area should sink below sea level in
order to be filled with stream-swept waste. High valleys among growing mountain
ranges may suffer warping, or may be blockaded by rising mountain folds athwart
them. Where the deformation is rapid enough, the river may be ponded and the valley
filled with lake-laid sediments. Even when the river is able to maintain its right of
way it may yet have its declivity so lessened that it is compelled to aggrade its course
continually, filling the valley with river deposits which may grow to an enormous
thickness.
Behind the outer ranges of the Himalaya Mountains lie several waste-filled valleys, the largest of which are
Kashmir and Nepal, the former being an alluvial plain about as large as the state of Delaware. The rivers which
drain these plains have already cut down their outlet gorges sufficiently to begin the task of the removal of the
broad accumulations which they have brought in from the surrounding mountains. Their present flood plains lie as
much as some hundreds of feet below wide alluvial terraces which mark their former levels. Indeed, the
horizontal beds of the Hundes Valley have been trenched to the depth of nearly three thousand feet by the Sutlej
River. These deposits are recent or subrecent, for there have been found at various levels the remains of land
plants and land and fresh-water shells, and in some the bones of such animals as the hyena and the goat, of
species or of genera now living. Such soft deposits cannot be expected to endure through any considerable length
of future time the rapid erosion to which their great height above the level of the sea will subject them.

Characteristics of river deposits. The
examples just cited teach clearly the
characteristic features of extensive river
deposits. These deposits consist of broad,
Fig. 81. Cross Section of Aggraded flat-lying sheets of clay and fine sand left
Valley, showing Structure of River by the overflow at time of flood, and
Deposits traversed here and there by long, narrow
strips of coarse, cross-bedded sands and
gravels thrown down by the swifter currents of the shifting channels. Occasional beds
of muck mark the sites of shallow lakelets or fresh-water swamps. The various strata
also contain some remains of the countless myriads of animals and plants which live
upon the surface of the plain as it is in process of building. River shells such as the
mussel, land shells such as those of snails, the bones of fishes and of such land
animals as suffer drowning at times of flood or are mired in swampy places, logs of
wood, and the stems and leaves of plants are examples of the variety of the remains
of land and fresh-water organisms which are entombed in river deposits and sealed

away as a record of the life of the time, and as proof that the deposits were laid by
streams and not beneath the sea.

Basin Deposits
Deposits in dry basins. On desert areas without outlet to the sea, as on the Great
Basin of the United States and the deserts of central Asia, stream-swept waste
accumulates indefinitely. The rivers of the surrounding mountains, fed by the rains
and melting snows of these comparatively moist elevations, dry and soak away as
they come down upon the arid plains. They are compelled to lay aside their entire
load of waste eroded from the mountain valleys, in fans which grow to enormous
size, reaching in some instances thousands of feet in thickness.
The monotonous levels of Turkestan include vast alluvial tracts now in process of building by the floods of the
frequently shifting channels of the Oxus and other rivers of the region. For about seven hundred miles from its
mouth in Aral Lake the Oxus receives no tributaries, since even the larger branches of its system are lost in a
network of distributaries and choked with desert sands before they reach their master stream. These aggrading
rivers, which have channels but no valleys, spread their muddy floods—which in the case of the Oxus sometimes
equal the average volume of the Mississippi—far and wide over the plain, washing the bases of the desert dunes.

Playas. In arid interior basins the central depressions may be occupied by playas,
—plains of fine mud washed forward from the margins. In the wet season the playa is
covered with a thin sheet of muddy water, a playa lake, supplied usually by some
stream at flood. In the dry season the lake evaporates, the river which fed it retreats,
and there is left to view a hard, smooth, level floor of sun-baked and sun-cracked
yellow clay utterly devoid of vegetation.
In the Black Rock desert of Nevada a playa lake spreads over an area fifty miles long and twenty miles wide.
In summer it disappears; the Quinn River, which feeds it, shrinks back one hundred miles toward its source,
leaving an absolutely barren floor of clay, level as the sea.

Lake deposits. Regarding lakes as parts of river systems, we may now notice the
characteristic features of the deposits in lake basins. Soundings in lakes of
considerable size and depth show that their bottoms are being covered with tine
clays. Sand and gravel are found along; their margins, being brought in by streams
and worn by waves from the shore, but there are no tidal or other strong currents to
sweep coarse waste out from shore to any considerable distance. Where fine clays are
now found on the land in even, horizontal layers containing the remains of fresh-
water animals and plants, uncut by channels tilled with cross-bedded gravels and
sands and bordered by beach deposits of coarse waste, we may safely infer the
existence of ancient lakes.
Marl. Marl is a soft, whitish deposit of carbonate of lime, mingled often with more or less of clay,
accumulated in small lakes whose feeding springs are charged with carbonate of lime and into which little waste
is washed from the land. Such lakelets are not infrequent on the surface of the younger drift sheets of Michigan
and northern Indiana, where their beds of marl—sometimes as much as forty feet thick—are utilized in the

manufacture of Portland cement. The deposit results from the decay of certain aquatic plants which secrete lime
carbonate from the water, from the decomposition of the calcareous shells of tiny mollusks which live in
countless numbers on the lake floor, and in some cases apparently from chemical precipitation.

Peat. We have seen how lakelets are extinguished by the decaying remains of the
vegetation which they support. A section of such a fossil lake shows that below the
growing mosses and other plants of the surface of the bog lies a spongy mass
composed of dead vegetable tissue, which passes downward gradually into peat,—a
dense, dark brown carbonaceous deposit in which, to the unaided eye, little or no
trace of vegetable structure remains. When dried, peat forms a fuel of some value and
is used either cut into slabs and dried or pressed into bricks by machinery.

Fig. 82. Digging Peat, Scotland

When vegetation decays in open air the carbon of its tissues, taken from the
atmosphere by the leaves, is oxidized and returned to it in its original form of carbon
dioxide. But decomposing in the presence of water, as in a bog, where the oxygen of
the air is excluded, the carbonaceous matter of plants accumulates in deposits of peat.
Peat bogs are numerous in regions lately abandoned by glacier ice, where river systems are so immature that
the initial depressions left in the sheet of drift spread over the country have not yet been drained. One tenth of the
surface of Ireland is said to be covered with peat, and small bogs abound in the drift-covered area of New
England and the states lying as far west as the Missouri River. In Massachusetts alone it has been reckoned that
there are fifteen billion cubic feet of peat, the largest bog occupying several thousand acres.

Much larger swamps occur on the young coastal plain of the Atlantic from New Jersey to Florida. The Dismal
Swamp, for example, in Virginia and North Carolina is forty miles across. It is covered with a dense growth of
water-loving trees such as the cypress and black gum. The center of the swamp is occupied by Lake Drummond, a

shallow lake seven miles in diameter, with banks of pure-peat, and still narrowing from the encroachment of
vegetation along its borders.

Salt lakes. In arid climates a lake rarely receives sufficient inflow to enable it to
rise to the basin rim and find an outlet. Before this height is reached its surface
becomes large enough to discharge by evaporation into the dry air the amount of
water that is supplied by streams. As such a lake has no outlet, the minerals in
solution brought into it by its streams cannot escape from the basin. The lake water
becomes more and more heavily charged with such substances as common salt and
the sulphates and carbonates of lime, of soda, and of potash, and these are thrown
down from solution one after another as the point of saturation for each mineral is
reached. Carbonate of lime, the least soluble and often the most abundant mineral
brought in, is the first to be precipitated. As concentration goes on, gypsum, which is
insoluble in a strong brine, is deposited, and afterwards common salt. As the saltness
of the lake varies with the seasons and with climatic changes, gypsum and salt are
laid in alternate beds and are interleaved with sedimentary clays spread from the
waste brought in by streams at times of flood. Few forms of life can live in bodies of
salt water so concentrated that chemical deposits take place, and hence the beds of
salt, gypsum, and silt of such lakes are quite barren of the remains of life. Similar
deposits are precipitated by the concentration of sea water in lagoons and arms of the
sea cut off from the ocean.

Fig. 83. Map of Lake Bonneville and Lahontan
From Davis’ Physical Geography

Lakes Bonneville and Lahontan. These names are given to extinct lakes which once occupied large areas in
the Great Basin, the former in Utah, the latter in northwestern Nevada. Their records remain in old horizontal
beach lines which they drew along their mountainous shores at the different levels at which they stood, and in the
deposits of their beds. At its highest stage Lake Bonneville, then one thousand feet deep, overflowed to the north

and was a fresh-water lake. As it shrank below the outlet it became more and more salty, and the Great Salt Lake,
its withered residue, is now depositing salt along its shores. In its strong brine lime carbonate is insoluble, and that
brought in by streams is thrown down at once in the form of travertine.

Lake Lahontan never had an outlet. The first chemical
deposits to be made along its shores were deposits of travertine, in
places eighty feet thick. Its floor is spread with fine clays, which
must have been laid in deep, still water, and which are charged
with the salts absorbed by them as the briny water of the lake
dried away. These sedimentary clays are in two divisions, the
upper and lower, each being about one hundred feet thick (a and
c, Fig. 84). They are separated by heavy deposits of well-rounded,
cross-bedded gravels and sands (b, Fig. 84), similar to those
Fig. 84. Section of Deposits in
spread at the present time by the intermittent streams of arid Beds of Lakes Bonneville and
regions. A similar record is shown in the old floors of Lake Lahontan
Bonneville. What conclusions do you draw from these facts as to
the history of these ancient lakes?

Deltas
In the river deposits which are left above sea level particles of waste are allowed
to linger only for a time. From alluvial fans and flood plains they are constantly being
taken up and swept farther on downstream. Although these land forms may long
persist, the particles which compose them are ever changing. We may therefore think
of the alluvial deposits of a valley as a stream of waste fed by the waste mantle as it
creeps and washes down the valley sides, and slowly moving onwards to the sea.

In basins waste finds a longer rest, but sooner or later lakes and dry basins are
drained or filled, and their deposits, if above sea level, resume their journey to their
final goal. It is only when carried below the level of the sea that they are indefinitely
preserved.

On reaching this terminus, rivers deliver their load to the ocean. In some cases the
ocean is able to take it up by means of strong tidal and other currents, and to dispose
of it in ways which we shall study later. But often the load is so large, or the tides are
so weak, that much of the waste which the river brings in settles at its mouth, there
building up a deposit called the delta, from the Greek letter (Δ) of that name, whose
shape it sometimes resembles.

Deltas and alluvial fans have many common characteristics. Both owe their origin
to a sudden check in the velocity of the river, compelling a deposit of the load; both
are triangular in outline, the apex pointing upstream; and both are traversed by
distributaries which build up all parts in turn.

In a delta we may distinguish deposits of two distinct kinds,— the submarine and
the subaërial. In part a delta is built of waste brought down by the river and
redistributed and spread by waves and tides over the sea bottom adjacent to the

river’s mouth. The origin of these deposits is recorded in the remains of marine
animals and plants which they contain.

As the submarine delta grows near
to the level of the sea the distributaries
of the river cover it with subaërial
deposits altogether similar to those of
the flood plain, of which indeed the
subaërial delta is the prolongation. Here
extended deposits of peat may
accumulate in swamps, and the remains
of land and fresh- water animals and
plants swept down by the stream are
imbedded in the silts laid at times of
flood.

Borings made in the deltas of great Fig. 85. Delta of the Mississippi River
rivers such as the Mississippi, the
Ganges, and the Nile, show that the subaërial portion often reaches a surprising
thickness. Layers of peat, old soils, and forest grounds with the stumps of trees are
discovered hundreds of feet below sea level. In the Nile delta some eight layers of
coarse gravel were found interbedded with river silts, and in the Ganges delta at
Calcutta a boring nearly five hundred feet in depth stopped in such a layer.
The Mississippi has built a delta of twelve thousand three hundred square miles, and is pushing the natural
embankments of its chief distributaries into the Gulf at a maximum rate of a mile in sixteen years. Muddy shoals
surround its front, shallow lakes, e.g. lakes Pontchartrain and Borgne, are formed between the growing delta and
the old shore line, and elongate lakes and swamps are inclosed between the natural embankments of the
distributaries.

The delta of the Indus River, India, lies so low along shore that a broad tract of country is overflowed by the
highest tides. The submarine portion of the delta has been built to near sea level over so wide a belt offshore that
in many places large vessels cannot come even within sight of land because of the shallow water.

A former arm of the sea, the Rann of Cutch, adjoining
the delta on the east has been silted up and is now an
immense barren flat of sandy mud two hundred miles in
length and one hundred miles in greatest breadth. Each
summer it is flooded with salt water when the sea is brought
in by strong southwesterly monsoon winds, and the climate
during the remainder of the year is hot and dry. By the
Fig. 86. Radial Section of a Delta evaporation of sea water the soil is thus left so salty that no
vegetation can grow upon it, and in places beds of salt
his section of a delta illustrates the structure of several feet in thickness have accumulated. Under like
he platform which swift streams well loaded conditions salt beds of great thickness have been formed in
with coarse waste build in the water bodies into the past and are now found buried among the deposits of
which they empty. Three members may be ancient deltas.
istinguished: the bottom set beds, a: the fore set
eds, b; and the top set beds, c. Account for the

lope of each of these. Why are the bottom setSubsidence of great deltas. As a rule
eds of the finer material and why do they
great deltas are slowly sinking. In some
xtend beyond the others? How does the profile
instances upbuilding by river deposits has
f this delta differ from that of an alluvial cone
nd why? gone on as rapidly as the region has
subsided. The entire thickness of the Ganges delta, for example, so far as it has been
sounded, consists of deposits laid in open air. In other cases interbedded limestones
and other sedimentary rocks containing marine fossils prove that at times subsidence
has gained on the upbuilding and the delta has been covered with the sea.

It is by gradual depression that delta deposits attain enormous thickness, and,
being lowered beneath the level of the sea, are safely preserved from erosion until a
movement of the earth’s crust in the opposite direction lifts them to form part of the
land. We shall read later in the hard rocks of our continent the records of such ancient
deltas, and we shall not be surprised to find them as thick as are those now building
at the mouths of great rivers.

Lake deltas. Deltas are also formed where streams lose their velocity on entering
the still waters of lakes. The shore lines of extinct lakes, such as Lake Agassiz and
Lakes Bonneville and Lahontan, may be traced by the heavy deposits at the mouths
of their tributary streams.

• • • • •

We have seen that the work of streams is to drain the lands of the water poured
upon them by the rainfall, to wear them down, and to carry their waste away to the
sea, there to be rebuilt by other agents into sedimentary rocks. The ancient strata of
which the continents are largely made are composed chiefly of material thus worn
from still more ancient lands—lands with their hills and valleys like those of to-day
—and carried by their rivers to the ocean. In all geological times, as at the present,
the work of streams has been to destroy the lands, and in so doing to furnish to the
ocean the materials from which the lands of future ages were to be made. Before we
consider how the waste of the land brought in by streams is rebuilt upon the ocean
floor, we must proceed to study the work of two agents, glacier ice and the wind,
which cooperate with rivers in the denudation of the land.

Fig. 87. Section of Undifferentiated Drift near Chicago

CHAPTER V

THE WORK OF GLACIERS
The drift. The surface of northeastern North America, as far south as the Ohio
and Missouri rivers, is generally covered by the drift,—a formation which is quite
unlike any which we have so far studied. A section of it, such as that illustrated in
Figure 87, shows that for the most part it is unstratified, consisting of clay, sand,
pebbles, and even large bowlders, all mingled pell- mell together. The agent which
laid the drift is one which can carry a load of material of all sizes, from the largest
bowlder to the finest clay, and deposit it without sorting.

Fig. 88. Characteristic Pebbles from the Drift
No. 1 has six facets; No. 4, originally a rounded river pebble, has been nibbled down to
one flat face; Nos. 3 and 5 are battered subangular fragments on one side only

The stones of the drift are of many kinds. The region from which it was gathered
may well have been large in order to supply these many different varieties of rocks.
Pebbles and bowlders have been left far from their original homes, as may be seen in
southern Iowa, where the drift contains nuggets of copper brought from the region
about Lake Superior. The agent which laid the drift is one able to gather its load over
a large area and carry it a long way.

The pebbles of the drift are
unlike those rounded by running
water or by waves. They are
marked with scratches. Some are
angular, many have had their
edges blunted, while others have
been ground flat and smooth on
one or more sides, like gems
which have been faceted by being
held firmly against the lapidary’s
wheel (Fig. 88). In many places
the upper surface of the country
rock beneath the drift has been
swept clean of residual clays and
other waste. All rock rotten has
been planed away, and the ledges
of sound rock to which the surface
has been cut down have been
rubbed smooth and scratched with
long, straight, parallel lines
(Fig. 89). The agent which laid

Fig. 89. Smoothed and Scored Rock Surface the drift can hold sand and
exposed to View by the Removal of Overlying pebbles firmly in its grasp and can
Drift, Iowa grind them against the rock
beneath, thus planing it down and
scoring it, while faceting the pebbles also.

Neither water nor wind can do these things. Indeed, nothing like the drift is being
formed by any process now at work anywhere in the eastern United States. To find
the agent which has laid this extensive formation we must go to a region of different
climatic conditions.

The inland ice of Greenland.
Greenland is about fifteen hundred miles
long and nearly seven hundred miles in
greatest width. With the exception of a
narrow fringe of mountainous coast land, it
is completely buried beneath a sheet of ice,
in shape like a vast white shield, whose
convex surface rises to a height of nine
thousand feet above the sea. The few
explorers who have crossed the ice cap
found it a trackless desert destitute of all life
save such lowly forms as the microscopic
plant which produces the so- called “red
snow.” On the smooth plain of the interior
no rock waste relieves the snow’s dazzling
whiteness; no streams of running water are
seen; the silence is broken only by howling
storm winds and the rustle of the surface
snow which they drive before them.
Sounding with long poles, explorers find
that below the powdery snow of the latest
snowfall lie successive layers of earlier
snows, which grow more and more compact
downward, and at last have altered to
impenetrable ice. The ice cap formed by the
accumulated snows of uncounted centuries Fig. 90. Map of Greenland
Glacier ice covers all but the areas shaded
may well be more than a mile in depth. Ice
thus formed by the compacting of snow is distinguished when in motion as glacier
ice.

The inland ice of Greenland moves. It flows with
imperceptible slowness under its own weight, like, a
mass of some viscous or plastic substance, such as
Fig. 91. Hypothetical pitch or molasses candy, in all directions outward
Cross Section of toward the sea. Near the edge it has so thinned that
Greenland mountain peaks are laid bare, these islands in the sea of
ice being known as nunataks. Down the valleys of the
coastal belt it drains in separate streams of ice, or glaciers. The largest of these reach
the sea at the head of inlets, and are therefore called tide glaciers. Their fronts stand
so deep in sea water that there is visible seldom more than three hundred feet of the
wall of ice, which in many glaciers must be two thousand and more feet high. From
the sea walls of tide glaciers great fragments break off and float away as icebergs.
Thus snows which fell in the interior of this northern land, perhaps many thousands
of years ago, are carried in the form of icebergs to melt at last in the North Atlantic.

Greenland, then, is being modeled over the vast extent of its interior not by
streams of running water, as are regions in warm and humid climates, nor by currents
of air, as are deserts to a large extent, but by a sheet of flowing ice. What the ice sheet
is doing in the interior we may infer from a study of the separate glaciers into which
it breaks at its edge.

The smaller Greenland glaciers. Many of the smaller glaciers of Greenland do
not reach the sea, but deploy on plains of sand and gravel. The edges of these ice
tongues are often as abrupt as if sliced away with a knife (Fig. 92), and their structure
is thus readily seen. They are stratified, their layers representing in part the
successive snowfalls of the interior of the country. The upper layers are commonly
white and free from stones; but the lower layers, to the height of a hundred feet or
more, are dark with débris which is being slowly carried on. So thickly studded with
stones is the base of the ice that it is sometimes difficult to distinguish it from the
rock waste which has been slowly dragged beneath the glacier or left about its edges.
The waste beneath and about the glacier is unsorted. The stones are of many kinds,
and numbers of them have been ground to flat faces. Where the front of the ice has
retreated the rock surface is seen to be planed and scored in places by the stones
frozen fast in the sole of the glacier.

Fig. 92. A Greenland Glacier

We have now found in glacier ice an agent able to produce the drift of North
America. The ice sheet of Greenland is now doing what we have seen was done in
the recent past in our own land. It is carrying for long distances rocks of many kinds
gathered, we may infer, over a large extent of country. It is laying down its load
without assortment in unstratified deposits. It grinds down and scores the rock over
which it moves, and in the process many of the pebbles of its load are themselves
also ground smooth and scratched. Since this work can be done by no other agent, we
must conclude that the northeastern part of our own continent was covered in the
recent past by glacier ice, as Greenland is to-day.

Valley Glaciers
The work of glacier ice can be most conveniently studied in the separate ice
streams which creep down mountain valleys in many regions such as Alaska, the
western mountains of the United States and Canada, the Himalayas, and the Alps. As
the glaciers of the Alps have been studied longer and more thoroughly than any
others, we shall describe them in some detail as examples of valley glaciers in all
parts of the world.

Conditions of glacier formation. The condition of the great accumulation of
snow to which glaciers are due—that more or less of each winter’s snow should be
left over unmelted and unevaporated to the next—is fully met in the Alps. There is
abundant moisture brought by the winds from neighboring seas. The currents of
moist air driven up the mountain slopes are cooled by their own expansion as they
rise, and the moisture which they contain is condensed at a temperature at or below
32° F., and therefore is precipitated in the form of snow. The summers are cool and
their heat does not suffice to completely melt the heavy snow of the preceding winter.
On the Alps the snow line—the lower limit of permanent snow—is drawn at about
eight thousand five hundred feet above sea level. Above the snow line on the slopes
and crests, where these are not too steep, the snow lies the year round and gathers in
valley heads to a depth of hundreds of feet.

Fig. 93. Glaciers heading in Snow-filled Amphitheaters, the Alps

This is but a small fraction of the thickness to which snow would be piled on the
Alps were it not constantly being drained away. Below the snow fields which mantle
the heights the mountain valleys are occupied by glaciers which extend as much as a
vertical mile below the snow line. The presence in the midst of forests and meadows
and cultivated fields of these tongues of ice, ever melting and yet from year to year
losing none of their bulk, proves that their loss is made good in the only possible
way. They are fed by snow fields above, whose surplus of snow they drain away in

the form of ice. The presence of glaciers
below the snow line is a clear proof that,
rigid and motionless as they appear,
glaciers really are in constant motion down
valley.

The névé field. The head of an Alpine
valley occupied by a glacier is commonly a
broad amphitheater deeply filled with snow
(Fig. 93). Great peaks tower above it, and
snowy slopes rise on either side on the
flanks of mountain spurs. From these
heights fierce winds drift the snows into the
amphitheater, and avalanches pour in their
torrents of snow and waste. The snow of
the amphitheater is like that of drifts in late
winter after many successive thaws and
freezings. It is made of hard grains and
pellets and is called névé. Beneath the
surface of the névé field and at its outlet the
Fig. 94. Bergschrund of a Glacier in
granular névé has been compacted to a
Colorado
mass of porous crystalline ice. Snow has
been changed to névé, and névé to glacial ice, both by pressure, which drives the air
from the interspaces of the snowflakes, and also by successive meltings and
freezings, much as a snowball is packed in the warm hand and becomes frozen to a
ball of ice.

Fig. 95. Sea Wall of the Muir Glacier,
Alaska

The bergschrund. The névé is in slow motion. It breaks itself loose from the
thinner snows about it, too shallow to share its motion, and from the rock rim which
surrounds it, forming a deep fissure called the bergschrund, sometimes a score and
more feet wide (Fig. 94).

Size of glaciers. The ice streams of the Alps vary in size according to the amount
of precipitation and the area of the névé fields which they drain. The largest of Alpine
glaciers, the Aletsch, is nearly ten miles long and has an average width of about a
mile. The thickness of some of the glaciers of the Alps is as much as a thousand feet.
Giant glaciers more than twice the length of the longest in the Alps occur on the
south slope of the Himalaya Mountains, which receive frequent precipitations of
snow from moist winds from the Indian Ocean. The best known of the many
immense glaciers of Alaska, the Muir, has an area of about eight hundred square
miles (Fig. 95).

Fig. 96. Diagram Fig. 97.
showing Diagram
Movement of showing
Row of Stakes Movement
a, set in a of Vertical
direct line Row of
across the Stakes a,
surface of a set on side
glacier; b, c, of glacier
and d,
successive later
positions of the
stakes

Glacier motion. The motion of the glaciers of the Alps seldom exceeds one or
two feet a day. Large glaciers, because of the enormous pressure of their weight and
because of less marginal resistance, move faster than small ones. The Muir advances
at the rate of seven feet a day, and some of the larger tide glaciers of Greenland are

reported to move at the exceptional rate of fifty feet and more in the same time.
Glaciers move faster by day than by night, and in summer than in winter. Other laws
of glacier motion may be discovered by a study of Figures 96 and 97. It is important
to remember that glaciers do not slide bodily over their beds, but urged by gravity
move slowly down valley in somewhat the same way as would a stream of thick
mud. Although small pieces of ice are brittle, the large mass of granular ice which
composes a glacier acts as a viscous substance.
Crevasses. Slight changes of slope in the
glacier bed, and the different rates of motion
in different parts, produce tensions under
which the ice cracks and opens in great
fissures called crevasses. At an abrupt descent
in the bed the ice is shattered into great
fragments, which unite again below the
icefall. Crevasses are opened on lines at right
angles to the direction of the tension.
Transverse crevasses are due to a convexity in
the bed which stretches the ice lengthwise
(Fig. 99). Marginal crevasses are directed
upstream and inwards; radial crevasses are
found where the ice stream deploys from
some narrow valley and spreads upon some
more open space. What is the direction of the
tension which causes each and to what is it
due? (Figs. 100 and 101.

Fig. 98. Crevasses of a Glacier, Canada

Fig. 99. Longitudinal Section of a Fig. 100. Map
Portion of a Glacier, showing view of
Traverse Crevasses Marginal
Crevasses

Fig. 101. The Rhone Glacier, showing Radial Crevasses, the Alps

Lateral and medial moraines. The surface of a glacier is
striped lengthwise by long dark bands of rock débris. Those in the
center are called the medial moraines. The one on either margin is
a lateral moraine, and is clearly formed of waste which has fallen
on the edge of the ice from the valley slopes. A medial moraine
cannot be formed in this way, since no rock fragments can fall so
far out from the sides. But following it up the glacial stream, one
finds that a medial moraine takes its beginning at the junction of
the glacier and some tributary and is formed by the union of their
Fig. 102. Map
two adjacent lateral moraines (Fig. 102). Each branch thus adds a
View of the
medial moraine, and by counting the number of medial moraines
Junction of
of a trunk stream one may learn of how many branches it is
Two Branches
composed.
of a Glacier
he moraines are
epresented by
Surface moraines appear in the lower
roken lines course of the glacier as ridges, which may
reach the exceptional height of one

hundred feet. The bulk of such a ridge is ice. It has been Fig. 103. Cross
protected from the sun by the veneer of moraine stuff; while Section of a Glacier
the glacier surface on either side has melted down at least the showing Lateral
distance of the height of the ridge. In summer the lowering of Moraines l, l, and
the glacial surface by melting goes on rapidly. In Swiss Medial Moraines m,
glaciers it has been estimated that the average lowering of the m
surface by melting and evaporation amounts to ten feet a
year. As a moraine ridge grows higher and more steep by the lowering of the surface
of the surrounding ice, the stones of its cover tend to slip down its sides. Thus
moraines broaden, until near the terminus of a glacier they may coalesce in a wide
field of stony waste.

Fig. 104. Glacier with Medial Moraines, the Alps
Is the ice moving from or towards the observer?

Englacial drift. This name is applied to whatever débris is carried within the
glacier. It consists of rock waste fallen on the névé and there buried by
accumulations of snow, and of that engulfed in the glacier where crevasses have
opened beneath a surface moraine. As the surface of the glacier is lowered by
melting, more or less englacial drift is brought again to open air, and near the
terminus it may help to bury the ice from view beneath a sheet of débris.

The ground moraine. The drift dragged along at the glacier’s base and
lodged beneath it is known as the ground moraine. Part of the material of it has
fallen down deep crevasses and part has been torn and worn from the glacier’s
bed and banks. While the stones of the surface moraines remain as angular as
when they lodged on the ice, many of those of the ground moraine have been
blunted on the edges and faceted and scratched by being ground against one
another and the rocky bed.
In glaciers such as those of Greenland, whose basal layers are well loaded with drift and whose surface
layers are nearly clean, different layers have different rates of motion, according to the amount of drift with
which they are clogged. One layer glides over another, and the stones inset in each are ground and smoothed
and scratched. Usually the sides of glaciated pebbles are more worn than the ends, and the scratches upon
them run with the longer axis of the stone. Why?

The terminal moraine. As a glacier is in constant motion, it brings to its end
all of its load except such parts of the ground moraine as may find permanent
lodgment beneath the ice. Where the glacier front remains for some time at one
place, there is formed an accumulation of drift known as the terminal moraine. In
valley glaciers it is shaped by the ice front to a crescent whose convex side is
downstream. Some of the pebbles of the terminal moraine are angular, and some
are faceted and scored, the latter having come by the hard road of the ground
moraine. The material of the dump is for the most part unsorted, though the water
of the melting ice may find opportunity to leave patches of stratified sands and
gravels in the midst of the unstratified mass of drift, and the finer material is in
places washed away.

Fig. 105. Terminal Moraine of a Glacier in Montana
The ice has melted back from the morainic ridge on the left and is building another on the right. The hollow
between the ridges is occupied by a lakelet.

Glacier drainage. The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel whose portal
has been enlarged to a beautiful archway by melting in the sun and the warm air
(Fig. 107). The stream is gray with silt and loaded with sand and gravel washed
from the ground moraine. “Glacier milk” the Swiss call this muddy water, the

gray color of whose silt proves it rock flour freshly ground by the ice from the
unoxidized sound rock of its bed, the mud of streams being yellowish when it is
washed from the oxidized mantle of waste. Since glacial streams are well loaded
with waste due to vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits are known as
valley drift.

Fig. 106. Heavy Moraine about the Terminus of a Glacier in the
Rocky Mountains of Canada
Account for the fact that the morainic ridge rises considerably above the surface of the
ice

The sand brought out by streams from beneath a glacier differs from river sand in that it consists of
freshly broken angular grains. Why?

The stream derives its water chiefly from the surface melting of the glacier. As the ice is touched by the
rays of the morning sun in summer, water gathers in pools, and rills trickle and unite in brooklets which melt
and cut shallow channels in the blue ice. The course of these streams is short. Soon they plunge into deep
wells cut by their whirling waters where some crevasse has begun to open across their path. These wells lead
into chambers and tunnels by which sooner or later their waters find way to the rock floor of the valley and
there unite in a subglacial stream.

The lower limit of glaciers. The glaciers of a region do not by any means
end at a uniform height above sea level. Each terminates where its supply is

balanced by melting. Those therefore
which are fed by the largest and deepest
névés and those also which are best
protected from the sun by a northward
exposure or by the depth of their
inclosing valleys flow to lower levels
than those whose supply is less and
whose exposure to the sun is greater.

A series of cold, moist years, with an
abundant snowfall, causes glaciers to
thicken and advance; a series of warm,
dry years causes them to wither and melt
back. The variation in glaciers is now
carefully observed in many parts of the Fig. 107. Subglacial Stream Issuing
world. The Muir glacier has retreated from Tunnel in the Ice, Norway
two miles in twenty years. The glaciers
of the Swiss Alps are now for the most part melting back, although a well-known
glacier of the eastern Alps, the Vernagt, advanced five hundred feet in the year
1900, and was then plowing up its terminal moraine.
How soon would you expect a glacier to advance after its névé fields have been swollen with unusually
heavy snows, as compared with the time needed for the flood of a large river to reach its mouth after heavy
rains upon its headwaters?

On the surface of glaciers in summer time one may often see large stones
supported by pillars of ice several feet in height (Fig. 108). These “glacier
tables” commonly slope more or less strongly to the south, and thus may be
used to indicate roughly the points of the compass. Can you explain their
formation and the direction of their slope? On the other hand, a small and thin
stone, or a patch of dust, lying on the ice, tends to sink a few inches into it.
Why?

In what respects is a valley glacier like a mountain stream which flows out
Fig. 108. A Glacier upon desert plains?
Table Two confluent glaciers do not mingle their currents as do two confluent
rivers. What characteristics of surface moraines prove this fact?

What effect would you expect the laws of glacier motion to have on the slant of the sides of transverse
crevasses?

Fig. 109. Map of Malaspina Glacier, Alaska
A trunk glacier has four medial moraines. Of how many tributaries is it composed? Illustrate by diagram.

State all the evidences which you have found that glaciers move.

If a glacier melts back with occasional pauses up a valley, what records are left of its retreat?

Piedmont Glaciers
The Malaspina glacier. Piedmont (foot of the mountain) glaciers are, as the
name implies, ice fields formed at the foot of mountains by the confluence of
valley glaciers. The Malaspina glacier of Alaska, the typical glacier of this kind,
is seventy miles wide and stretches for thirty miles from the foot of the Mount
Saint Elias range to the shore of the Pacific Ocean. The valley glaciers which
unite and spread to form this lake of ice lie above the snow line and their
moraines are concealed beneath névé. The central area of the Malaspina is also
free from débris; but on the outer edge large quantities of englacial drift are
exposed by surface melting and form a belt of morainic waste a few feet thick
and several miles wide, covered in part with a luxuriant forest, beneath which the
ice is in places one thousand feet in depth. The glacier here is practically
stagnant, and lakes a few hundred yards across, which could not exist were the
ice in motion and broken with crevasses, gather on their beds sorted waste from
the moraine. The streams which drain the glacier have cut their courses in
englacial and subglacial tunnels; none flow for any distance on the surface. The

largest, the Yahtse River, issues from a high archway in the ice,—a muddy torrent
one hundred feet wide and twenty feet deep, loaded with sand and stones which it
deposits in a broad outwash plain (Fig. 110). Where the ice has retreated from the
sea there is left a hummocky drift sheet with hollows filled with lakelets. These
deposits help to explain similar hummocky regions of drift and similar plains of
coarse, water-laid material often found in the drift-covered area of the
northeastern United States.

The Geological
Work Of Glacier Ice
The sluggish glacier must
do its work in a different way
from the agile river. The
mountain stream is swift and
small, and its channel occupies
but a small portion of the
valley. The glacier is slow and
big; its rate of motion may be
less than a millionth of that of
running water over the same
declivity, and its bulk is
Fig. 110. Outwash Plain, the Delta of the
proportionately large and fills Yahtse River, Alaska
the valley to great depth.
Moreover, glacier ice is a solid body plastic under slowly applied stresses, while
the water of rivers is a nimble fluid.

Transportation. Valley glaciers differ from rivers as carriers in that they float
the major part of their load upon their surface, transporting the heaviest bowlder
as easily as a grain of sand; while streams push and roll much of their load along
their beds, and their power of transporting waste depends solely upon their
velocity. The amount of the surface load of glaciers is limited only by the amount
of waste received from the mountain slopes above them. The moving floor of ice
stretched high across a valley sweeps along as lateral moraines much of the waste
which a mountain stream would let accumulate in talus and alluvial cones.

While a valley glacier carries much of its load on top, an ice sheet, such as
that of Greenland, is free from surface débris, except where moraines trail away
from some nunatak. If at its edge it breaks into separate glaciers which drain

down mountain valleys, these tongues of ice will carry the selvages of waste
common to valley glaciers. Both ice sheets and valley glaciers drag on large
quantities of rock waste in their ground moraines.

Stones transported by glaciers are sometimes called erratics. Such are the
bowlders of the drift of our northern states. Erratics may be set down in an
insecure position on the melting of the ice.

Deposit. Little need be added here to what has already been said of ground
and terminal moraines. All strictly glacial deposits are unstratified. The load laid
down at the end of a glacier in the terminal moraine is loose in texture, while the
drift lodged beneath the glacier as ground moraine is often an extremely dense,
stony clay, having been compacted under the pressure of the overriding ice.

Erosion. A glacier erodes its bed and banks in two ways,—by abrasion and
by plucking.

The rock bed over which a glacier has moved is seen in places to have been
abraded, or ground away, to smooth surfaces which are marked by long, straight,
parallel scorings aligned with the line of movement of the ice and varying in size
from hair lines and coarse scratches to exceptional furrows several feet deep.
Clearly this work has been accomplished by means of the sharp sand, the
pebbles, and the larger stones with which the base of the glacier is inset, and
which it holds in a firm grasp as running water cannot. Hard and fine-grained
rocks, such as granite and quartzite, are often not only ground down to a smooth
surface but are also highly polished by means of fine rock flour worn from the
glacier bed.

In other places the bed of the glacier is rough and torn. The rocks have been
disrupted and their fragments have been carried away,—a process known as
plucking. Moving under immense pressure the ice shatters the rock, breaks off
projections, presses into crevices and wedges the rocks apart, dislodges the
blocks into which the rock is divided by joints and bedding planes, and freezing
fast to the fragments drags them on. In this work the freezing and thawing of
subglacial waters in any cracks and crevices of the rock no doubt play an
important part. Plucking occurs especially where the bed rock is weak because of
close jointing. The product of plucking is bowlders, while the product of abrasion
is fine rock flour and sand.

Is the ground moraine of Figure 87 due chiefly to abrasion or to plucking?

Roches moutonnées and
rounded hills. The prominences
left between the hollows due to
plucking are commonly ground
down and rounded on the stoss
side,—the side from which the ice
advances,—and sometimes on the
opposite, the lee side, as well. In
this way the bed rock often comes
to have a billowy surface known as
roches moutonnées (sheep rocks).
Hills overridden by an ice sheet
often have similarly rounded
Fig. 111. Roches Mountonnés, Bronx contours on the stoss side, while on
Park, New York the lee side they may be craggy,
either because of plucking or
because here they have been less worn from their initial profile (Fig. 112).

The direction of glacier movement. The direction of the flow of vanished
glaciers and ice sheets is recorded both in the differences just mentioned in the
profiles of overridden hills and also in the minute details of the glacier trail.

Flint nodules or other small prominences in the bed rock are found more
worn on the stoss than on the lee side, where indeed they may have a low cone of
rock protected by them from abrasion. Cavities, on the other hand, have their
edges worn on the lee side and left sharp upon the stoss.

Surfaces worn and torn in the ways which we have mentioned are said to be
glaciated. But it must not be supposed that a glacier everywhere glaciates its bed.
Although in places it acts as a rasp or as a pick, in others, and especially where
its pressure is least, as near the terminus, it moves over its bed in the manner of a
sled. Instances are known where glaciers have advanced over deposits of sand
and gravel without disturbing them to any notable degree. Like a river, a glacier
does not everywhere erode. In places it leaves its bed undisturbed and in places
aggrades it by deposits of the ground moraine.

Cirques. Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now destitute of
glaciers, but whose glaciation shows that they have supported glaciers in the past,
there are found similar crescentic hollows with high, precipitous walls and

glaciated floors. Their floors
are often basined and hold
lakelets whose deep and quiet
waters reflect the sheltering
ramparts of rugged rock which
tower far above them. Such
mountain hollows are termed
cirques. As a powerful spring
wears back a recess in the
valley side where it discharges,
so the fountain head of a glacier Fig. 112. A Glaciated Hill, Norway.
gradually wears back a cirque. Sharp Weathered Mountain Peaks in the Distance
In its slow movement the névé field broadly scours its bed to a flat or basined
floor. Meanwhile the sides of the valley head are steepened and driven back to
precipitous walls. For in winter the crevasse of the bergschrund which surrounds
the névé field is filled with snow and the névé is frozen fast to the rocky sides of
the valley. In early summer the névé tears itself free, dislodging and removing
any loosened blocks, and the open fissure of the bergschrund allows frost and
other agencies of weathering to attack the unprotected rock. As cirques are thus
formed and enlarged the peaks beneath which they lie are sharpened, and the
mountain crests are scalloped and cut back from either side to knife-edged ridges
(Figs. 113 and 93).

Fig. 113. Cirques, Sierra Nevada Mountains

In the western mountains of the United States many cirques, now empty of
névé and glacier ice, and known locally as “basins,” testify to the fact that in
recent times the snow line stood beneath the levels of their floors, and thus far
below its present altitude.

Fig. 114. A Glacier Trough, Montana

Glacier troughs. The channel worn to accommodate the big and clumsy
glacier differs markedly from the river valley cut as with a saw by the narrow and
flexible stream and widened by the weather and the wash of rains. The valley
glacier may easily be from one thousand to three thousand feet deep and from
one to three miles wide. Such a ponderous bulk of slowly moving ice does not
readily adapt itself to sharp turns and a narrow bed. By scouring and plucking all
resisting edges it develops a fitting channel with a wide, flat floor, and steep,
smooth sides, above which are seen the weathered slopes of stream-worn
mountain valleys. Since the trunk glacier requires a deeper channel than do its
branches, the bed of a branch glacier enters the main trough at some distance
above the floor of the latter, although the surface of the two ice streams may be
accordant. Glacier troughs can be studied best where large glaciers have recently
melted completely away, as is the case in many valleys of the mountains of the
western United States and of central and northern Europe (Fig. 114). The typical
glacier trough, as shown in such examples, is U-shaped, with a broad, flat floor,
and high, steep walls. Its walls are little broken by projecting spurs and lateral
ravines. It is as if a V-valley cut by a river had afterwards been gouged deeper
with a gigantic chisel, widening the floor to the width of the chisel blade, cutting
back the spurs, and smoothing and steepening the sides. A river valley could only
be as wide-floored as this after it had long been worn down to grade.

The floor of a glacier trough may not be graded; it is often interrupted by
irregular steps perhaps hundreds and even a thousand feet in height, over which
the stream that now drains the valley tumbles in waterfalls. Reaches between the
steps are often basined. Lakelets may occupy hollows excavated in solid rock,
and other lakes may be held behind terminal moraines left as dams across the
valley at pauses in the retreat of the glacier.

Fig. 115 Lynn Canal, Alaska, a Fjord
Fjords are glacier troughs now occupied in part or wholly by the sea, either because they were
excavated by a tide glacier to their present depth below sea level, or because of a submergence of the land.
Their characteristic form is that of a long, deep, narrow bay with steep rock walls and basined floor
(Fig. 115). Fjords are found only in regions which have suffered glaciation, such as Norway and Alaska.

Hanging valleys. These are lateral valleys which open on their main valley
some distance above its floor. They are conspicuous features of glacier troughs
from which the ice has vanished; for the trunk glacier in widening and deepening
its channel cut its bed below the bottoms of the lateral valleys (Fig. 116).

Since the mouths of hanging valleys are suspended on the walls of the glacier
trough, their streams are compelled to plunge down its steep, high sides in
waterfalls. Some of the loftiest and most beautiful waterfalls of the world leap
from hanging valleys,—among them the celebrated Staubbach of the
Lauterbrunnen valley of Switzerland, and those of the fjords of Norway and
Alaska (Fig. 117).

Hanging valleys are found also in river gorges where the smaller tributaries
have not been able to keep pace with a strong master stream in cutting down their

beds. In this case, however, they are a mark
of extreme youth; for, as the trunk stream
approaches grade and its velocity and power
to erode its bed decrease, the side streams
soon cut back their falls and wear their beds
at their mouths to a common level with that
of the main river. The Grand Canyon of the
Colorado must be reckoned a young valley.
At its base it narrows to scarcely more than
the width of the river, and yet its tributaries,
except the very smallest, enter it at a common
level.
Why could not a wide-floored valley, such as a glacier
trough, with hanging valleys opening upon it, be produced in
the normal development of a river valley?

The troughs of young and of mature glaciers. The
features of a glacier trough depend much on the length of
time the preexisting valley was occupied with ice. During
the infancy of a glacier, we may believe, the spurs of the
valley which it fills are but little blunted and its bed is but Fig. 116. A, V-River Valley, with
little broken by steps. In youth the glacier develops icefalls,
as a river in youth develops waterfalls, and its bed becomes Valley of Tributary joining
terraced with great stairs. The mature glacier, like the mature it a Accordant Level; B, the
river, has effaced its falls and smoothed its bed to grade. It
has also worn back the projecting spurs of its valley, making
Same changed after Long
itself a wide channel with smooth sides. The bed of a mature Glaciation to a Trough with
glacier may form a long basin, since it abrades most in its Hanging Valley
upper and middle course, where its weight and
motion are the greatest. Near the terminus,
where weight and motion are the least, it
erodes least, and may instead deposit a sheet of
ground moraine, much as a river builds a flood
plain in the same part of its course as it
approaches maturity. The bed of a mature
glacier thus tends to take the form of a long,
relatively narrow basin, across whose lower
end may be stretched the dam of the terminal
moraine. On the disappearance of the ice the
basin is rilled with a long, narrow lake, such as
Lake Chelan in Washington and many of the
lakes in the Highlands of Scotland.

Piedmont glaciers apparently erode but
little. Beneath their lake- like expanse of
sluggish or stagnant ice a broad sheet of
ground moraine is probably being deposited.

Fig. 117. Hanging Valley on the Wall of a Cirques and glaciated valleys
Fjord, Norway rapidly lose their characteristic
forms after the ice has withdrawn.
The weather destroys all smoothed, polished, and scored surfaces which are not
protected beneath glacial deposits. The oversteepened sides of the trough are
graded by landslips, by talus slopes, and by alluvial cones. Morainic heaps of
drift are dissected and carried away. Hanging valleys and the irregular bed of the
trough are both worn down to grade by the streams which now occupy them. The
length of time since the retreat of the ice from a mountain valley may thus be
estimated by the degree to which the destruction of the characteristic features of
the glacier trough has been carried.
In Figure 104 what characteristics of a glacier trough do you notice? What inference do you draw as to
the former thickness of the glacier?

Name all the evidences you would expect to find to prove the fact that in the recent geological past the
valleys of the Alps contained far larger glaciers than at present, and that on the north of the Alps the ice
streams united in a piedmont glacier which extended across the plains of Switzerland to the sides of the Jura
Mountains.

The relative importance of glaciers and of rivers. Powerful as glaciers are,
and marked as are the land forms which they produce, it is easy to exaggerate
their geological importance as compared with rivers. Under present climatic
conditions they are confined to lofty mountains or polar lands. Polar ice sheets
are permanent only so long as the lands remain on which they rest. Mountain
glaciers can stay only the brief time during which the ranges continue young and
high. As lofty mountains, such as the Selkirks and the Alps, are lowered by frost
and glacier ice, the snowfall will decrease, the line of permanent snow will rise,
and as the mountain hollows in which snow may gather are worn beneath the
snow line, the glaciers must disappear. Under present climatic conditions the
work of glaciers is therefore both local and of short duration.

Even the glacial epoch,
during which vast ice
sheets deposited drift over
northeastern North
America, must have been
brief as well as recent, for
many lofty mountains,
such as the Rockies and
Fig. 118. Longitudinal Section of a Tide Glacier the Alps, still bear the
occupying a Fjord and discharging Icebergs marks of great glaciers

Dotted Line, sea level which then filled their
valleys. Had the glacial
epoch been long, as the earth counts time, these mountains would have been
worn low by ice; had the epoch been remote, the marks of glaciation would
already have been largely destroyed by other agencies.

On the other hand, rivers are well-nigh universally at work over the land
surfaces of the globe, and ever since the dry land appeared they have been
constantly engaged in leveling the continents and in delivering to the seas the
waste which there is built into the stratified rocks.

Icebergs. Tide glaciers, such as those of Greenland and Alaska, are able to
excavate their beds to a considerable distance below sea level. From their fronts
the buoyancy of sea water raises and breaks away great masses of ice which float
out to sea as icebergs. Only about one seventh of a mass of glacier ice floats
above the surface, and a berg three hundred feet high may be estimated to have
been detached from a glacier not less than two thousand feet thick where it met
the sea.

Icebergs transport on their long journeys whatever drift they may have carried
when part of the glacier, and scatter it, as they melt, over the ocean floor. In this
way pebbles torn by the inland ice from the rocks of the interior of Greenland
and glaciated during their carriage in the ground moraine are dropped at last
among the oozes of the bottom of the North Atlantic.

CHAPTER VI

THE WORK OF THE WIND
We are now to study the geological work of the currents of the atmosphere,
and to learn how they erode, and transport and deposit waste as they sweep over
the land. Illustrations of the wind’s work are at hand in dry weather on any windy
day. Clouds of dust are raised from the street and driven along by the gale. Here
the roadway is swept bare; and there, in sheltered places, the dust settles in little
windrows. The erosive power of waste-laden currents of air is suggested as the

sharp grains of flying sand sting one’s face
or clatter against the window. In the country
one sometimes sees the dust whirled in
clouds from dry, plowed fields in spring and
left in the lee of fences in small drifts
resembling in form those of snow in winter.

The essential conditions for the wind’s
conspicuous work are illustrated in these
simple examples; they are aridity and the
absence of vegetation. In humid climates
these conditions are only rarely and locally
met; for the most part a thick growth of
vegetation protects the moist soil from the
Fig. 119. A sandy Region in a
wind with a cover of leaves and stems and a
Desert, the Sahara
mattress of interlacing roots. But in arid
regions either vegetation is wholly lacking, or scant growths are found huddled in
detached clumps, leaving interspaces of unprotected ground (Fig. 119). Here, too,
the mantle of waste, which is formed chiefly under the action of temperature
changes, remains dry and loose for long periods. Little or no moisture is present
to cause its particles to cohere, and they are therefore readily lifted and drifted by
the wind.

Transportation By The Wind
In the desert the finer waste is continually swept to and fro by the ever-
shifting wind. Even in quiet weather the air heated by contact with the hot sands
rises in whirls, and the dust is lifted in stately columns, sometimes as much as
one thousand feet in height, which march slowly across the plain. In storms the
sand is driven along the ground in a continuous sheet, while the air is tilled with
dust. Explorers tell of sand storms in the deserts of central Asia and Africa, in
which the air grows murky and suffocating. Even at midday it may become dark
as night, and nothing can be heard except the roar of the blast and the whir of
myriads of grains of sand as they fly past the ear.
Sand storms are by no means uncommon in the arid regions of the western United States. In a recent
year, six were reported from Yuma, Arizona. Trains on transcontinental railways are occasionally blockaded
by drifting sand, and the dust sifts into closed passenger coaches, covering the seats and floors. After such a
storm thirteen car loads of sand were removed from the platform of a station on a western railway.

Dust falls. Dust launched by upward-whirling winds on the swift currents of
the upper air is often blown for hundreds of miles beyond the arid region from
which it was taken. Dust falls from western storms are not unknown even as far
east as the Great Lakes. In 1896 a “black snow” fell in Chicago, and in another
dust storm in the same decade the amount of dust carried in the air over Rock
Island, Ill., was estimated at more than one thousand tons to the cubic mile.

Fig. 120. A Tract of Rocky Desert, Arabia
By what process have these rocks been broken up?
Why is finer waste here absent?

In March, 1901, a cyclonic storm carried vast quantities of dust from the Sahara northward across the
Mediterranean to fall over southern and central Europe. On March 8th dust storms raged in southern Algeria;
two days later the dust fell in Italy; and on the 11th it had reached central Germany and Denmark. It is
estimated that in these few days one million eight hundred thousand tons of waste were carried from
northern Africa and deposited on European soil.

We may see from these examples the importance of the wind as an agent of
transportation, and how vast in the aggregate are the loads which it carries. There
are striking differences between air and water as carriers of waste. Rivers flow in
fixed and narrow channels to definite goals. The channelless streams of the air
sweep across broad areas, and, shifting about continually, carry their loads back
and forth, now in one direction and now in another.

Wind Deposits

The mantle of waste of deserts is rapidly sorted by the wind. The coarser
rubbish, too heavy to be lifted into the air, is left to strew wide tracts with
residual gravels (Fig. 120). The sand derived from the disintegration of desert
rocks gathers in vast fields. About one eighth of the surface of the Sahara is said
to be thus covered with drifting sand. In desert mountains, as those of Sinai, it
lies like fields of snow in the high valleys below the sharp peaks. On more level
tracts it accumulates in seas of sand, sometimes, as in the deserts of Arabia, two
hundred and more feet deep.

Dunes. The sand thus accumulated by the
wind is heaped in wavelike hills called dunes.
In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is
laid in longitudinal dunes, i.e. in stripes
running parallel with the direction of the wind;
but commonly dunes lie, like ripple marks,
transverse to the wind current. On the
windward side they show a long, gentle slope,
up which grains of sand can readily be moved; Fig. 121. Longitudinal Dunes,
while to the lee their slope is frequently as Desert of Northwestern India
great as the angle of repose (Fig. 122). Dunes
whose sands are not fixed by vegetation travel Scale, 1 in = 3 miles
slowly with the wind; for their material is ever
shifted forward as the grains are driven up the windward slope and, falling over
the crest, are deposited in slanting layers in the quiet of the lee.

Like river deposits, wind-blown sands
are stratified, since they are laid by
currents of air varying in intensity, and
therefore in transporting power, which
carry now finer and now coarser materials
and lay them down where their velocity is
checked (Fig. 123). Since the wind varies
in direction, the strata dip in various
directions. They also dip at various angles,
Fig. 122. A Transverse Dune, according to the inclination of the surface
Seven Mile Beach, on which they were laid.
New Jersey
Account for the difference of slope in the two Dunes occur not only in arid regions,
ides of the dune. Is the dune marching? In
what direction? With what effect? Do the but also wherever loose sand lies

dges of the ripple marks upon the dune unpr
xtend along it or athwart it? Why?
otec
ted by vegetation from the wind. From the
beaches of sea and lake shores the wind
drives inland the surface sand left dry
between tides and after storms, piling it in
dunes which may invade forests and fields
and bury villages beneath their slowly
advancing waves. On flood plains during
summer droughts river deposits are often
worked over by the wind; the sand is Fig. 123. Stratified Wind-Blown
heaped in hummocks and much of the fine Sands,
silt is caught and held by the forests and Bermuda Islands
grassy fields of the bordering hills. These islands are made wholly of limestone,
the product of reef-building corals, and of lime
from the sea water. The limestone sand of the
The sand of shore beaches has been blown up into great dunes,
dunes differs little in some more than two hundred feet in height.
composition and the Much of the loose dune sand has been changed
to firm rock by percolating waters, which have
Fig. 124. Cross shape of its grains dissolved some of the limestone and deposited
Section of from that of the it again as a cement between the grains.
Transverse beach
Dune after from
Reversal of which it
Wind was
Redraw diagram, derived.
howing by dotted line But in
he original outline of deserts,
he dune
by the
long wear of grain on grain as
they are blown hither and
thither by the wind, all soft
minerals are ground to
powder and the sand comes to
consist almost wholly of
smooth round grams of hard Fig. 125. Dune Sands, Shore of Lake Michigan
quartz. Account for the dead forest, for its leaning tree trunks. Is the lake
shore to the right or left? What has been the history of the
landscape?
Some marine sandstones,
such as the St. Peter sandstone of the upper Mississippi valley, are composed so
entirely of polished spherules of quartz that it has been believed by some that

their grains were long blown about in ancient deserts before they were deposited
in the sea.

Dust deposits. As desert sands are composed almost wholly of quartz, we
may ask what has become of the softer minerals of which the rocks whose
disintegration has supplied the sand were in part, and often in large part,
composed. The softer minerals have been ground to powder, and little by little
the quartz sand also is worn by attrition to fine dust. Yet dust deposits are scant
and few in great deserts such as the Sahara. The finer waste is blown beyond its
limits and laid in adjacent oceans, where it adds to the muds and oozes of their
floors, and on bordering steppes and forest lands, where it is bound fast by
vegetation and slowly accumulates in deposits of unstratified loose yellow earth.
The fine waste of the Sahara has been identified in dredgings from the bottom of
the Atlantic Ocean, taken hundreds of miles from the coast of Africa.

Loess. In northern China an area as
large as France is deeply covered with a
yellow pulverulent earth called loess
(German, loose), which many consider a
dust deposit blown from the great
Mongolian desert lying to the west. Loess
mantles the recently uplifted mountains to
the height of eight thousand feet and
descends on the plains nearly to sea level.
Its texture and lack of stratification give it
a vertical cleavage; hence it stands in
Fig. 126. Crescentic Sand Dunes, steep cliffs on the sides of the deep and
Valley of the narrow trenches which have been cut in it
Columbia River by streams.
Did the wind which shaped them blow from
he left or from the right? On loess hillsides in China are thousands of villages
whose eavelike dwellings have been excavated in this
soft, yet firm, dry loam. While dust falls are common at the present time in this region, the loess is now
being rapidly denuded by streams, and its yellow silt gives name to the muddy Hwang-ho (Yellow River),
and to the Yellow Sea, whose waters it discolors for scores of miles from shore.

Wind deposits both of dust and of sand may be expected to contain the
remains of land shells, bits of wood, and bones of land animals, testifying to the
fact that they were accumulated in open air and not in the sea or in bodies of
fresh water.

Wind Erosion
Sand-laden currents of
air abrade and smooth and
polish exposed rock
surfaces, acting in much the
same way as does the jet of
steam fed with sharp sand,
which is used in the
manufacture of ground
glass. Indeed, in a single
storm at Cape Cod a plate
glass of a lighthouse was so
ground by flying sand that
its transparency was
destroyed and its removal
made necessary.
Fig. 127. Wind-Carved Rocks, Arizona
Telegraph poles and wires whetted
by wind-blown sands are destroyed within a few years. In rocks of unequal
resistance the harder parts are left in relief, while the softer are etched
away. Thus in the pass of San Bernardino, Cal., through which strong
winds stream from the west, crystals of garnet are left projecting on
delicate rock fingers from the softer rock in which they were imbedded.

Wind-carved pebbles are characteristically planed, the facets meeting
along a summit ridge or at a point like that of a pyramid. We may suppose
that these facets were ground by prevalent winds from certain directions, or
that from time to time the stone was undermined and rolled over as the
sand beneath it was blown away on the windward side, thus exposing fresh
surfaces to the driving sand. Such wind-carved pebbles are sometimes
found in ancient rocks and may be accepted as evidence that the sands of
which the rocks are composed were blown about by the wind.

Deflation. In the denudation of an arid region, wind
Fig. 128. A Wind- erosion is comparatively ineffective as compared with
Carved Pebble, Cape deflation (Latin, de, from; flare, to blow),—a term by
Cod which is meant the constant removal of waste by the
wind, leaving the rocks bare to the continuous attack of
the weather. In moist climates denudation is continually impeded by the mantle
of waste and its cover of vegetation, and the land surface can be lowered no
faster than the waste is removed by running water. Deep residual soils come to
protect all regions of moderate slope, concealing from view the rock structure,

and the various forms of the land are due more to the agencies of erosion and
transportation than to differences in the resistance of the underlying rocks.

But in arid regions the
mantle is rapidly removed,
even from well- nigh level
plains and plateaus, by the
sweep of the wind and the
wash of occasional rains. The
geological structure of these
regions of naked rock can be
read as far as the eye can see,
and it is to this structure that Fig. 129. Mesa Verde, Colorado
the forms of the land are there In the distance on the left are high volcanic mountains. On the
largely due. In a land mass of extreme right are seen outliers of strata which once covered the
region of the mesa
horizontal strata, for example,
any softer surface rocks wear down to some underlying, resistant stratum, and
this for a while forms the surface of a level plateau (Fig. 129). The edges of the
capping layer, together with those of any softer layers beneath it, wear back in
steep cliffs, dissected by the valleys of wet-weather streams and often swept bare
to the base by the wind. As they are little protected by talus, which commonly is
removed about as fast as formed, these escarpments and the walls of the valleys
retreat indefinitely, exposing some hard stratum beneath which forms the floor of
a widening terrace.

The high plateaus of northern Arizona and southern Utah (Fig. 130), north of
the Grand Canyon of the Colorado River, are composed of stratified rocks more
than ten thousand feet thick and of very gentle inclination northward. From the
broad plat form in which the canyon has been cut rises a series of gigantic stairs,
which are often more than one thousand feet high and a score or more of miles in
breadth. The retreating escarpments, the cliffs of the mesas and buttes which they
have left behind as outliers, and the walls of the ravines are carved into noble
architectural forms— into cathedrals, pyramids, amphitheaters, towers, arches,
and colonnades—by the processes of weathering aided by deflation. It is thus by
the help of the action of the wind that great plateaus in arid regions are dissected
and at last are smoothed away to waterless plains, either composed of naked
rock, or strewed with residual gravels, or covered with drifting residual sand.

Fig. 130. North-South Section, Eighty-Five Miles
Long, across the Plateau North of the Grand
Canyon of the Colorado River, Arizona, showing
Retreating Escarpments
O, outliers; V, canyon of the Colorado; A-H, rock systems from the
Archean to the Tertiary; P, platform of the plateau from which the
once overlying rocks have been stripped; dotted lines indicate
probable former extension of the strata. How thick is the mass of
strata which has been removed from over the platform? Has this
work been accomplished while the Colorado River has been
cutting its present canyon?

The specific gravity of air is 1⁄823 that of water. How does this fact affect the weight of the material
which each can carry at the same velocity?

If the rainfall should lessen in your own state to from five to ten inches a year, what changes would take
place in the vegetation of the country? in the soil? in the streams? in the erosion of valleys? in the agencies
chiefly at work in denuding the land?

In what way can a wind-carved pebble be distinguished from a river-worn pebble? from a glaciated
pebble?

CHAPTER VII

THE SEA AND ITS SHORES

Fig. 131. Sea Cliff and Rock Bench Cut in Chalk,
Dover, England

We have already seen that the ocean is the goal at which the waste of the land
arrives. The mantle of rock waste, creeping down slopes, is washed to the sea by
streams, together with the material which the streams have worn from their beds
and that dissolved by underground waters. In arid regions the winds sweep waste
either into bordering oceans or into more humid regions where rivers take it up
and carry it on to the sea. Glaciers deliver the load of their moraines either
directly to the sea or leave it for streams to transport to the same goal. All
deposits made on the land, such as the flood plains of rivers, the silts of lake
beds, dune sands, and sheets of glacial drift, mark but pauses in the process
which is to bring all the materials of the land now above sea level to rest upon the
ocean bed.

But the sea is also at work along all its shores as an agent of destruction, and
we must first take up its work in erosion before we consider how it transports and
deposits the waste of the land.

Sea Erosion
The sea cliff and the rock bench. On many coasts the land fronts the ocean
in a line of cliffs (Fig. 131). To the edge of the cliffs there lead down valleys and
ridges, carved by running water, which, if extended, would meet the water

surface some way out from shore. Evidently they are now abruptly cut short at
the present shore line because the land has been cut back.

Along the foot of the cliff lies a gently shelving
bench of rock, more or less thickly veneered with sand
and shingle. At low tide its inner margin is laid bare, but
at high tide it is covered wholly, and the sea washes the
base of the cliffs. A notch, of which the sea cliff and the
Fig. 132. Diagram of rock bench are the two sides, has been cut along the
Sea Cliff sc, and shore (Fig. 132).
Rock Bench rb
The broken line indicates Waves. The position of the rock bench, with its inner
the former extent of the margin slightly above low tide, shows that it has been
land. cut by some agent which acts like a horizontal saw set at
about sea level. This agent is clearly the surface agitation
of the water; it is the wind-raised wave.

As a wave comes up the shelving bench the crest topples forward and the
wave “breaks,” striking a blow whose force is measured by the momentum of all
its tons of falling water (Fig. 133). On the coast of Scotland the force of the
blows struck by the waves of the heaviest storms has sometimes exceeded three
tons to the square foot. But even a calm sea constantly chafes the shore. It heaves
in gentle undulations known as the ground swell, the result of storms perhaps a
thousand miles distant, and breaks on the shore in surf.

The blows of the waves are not struck
with clear water only, else they would have
little effect on cliffs of solid rock. Storm
waves arm themselves with the sand and
gravel, the cobbles, and even the large
bowlders which lie at the base of the cliff,
and beat against it with these hammers of
stone.
Where a precipice descends sheer into deep water, Fig. 133. Breaking Wave, Lake
waves swash up and down the face of the rocks but cannot Superior
break and strike effective blows. They therefore erode but
little until the talus fallen from the cliff is gradually built up beneath the sea to the level at which the waves
drag bottom upon it and break.

Compare the ways in which different agents abrade. The wind lightly brushes sand and dust over
exposed surfaces of rock. Running water sweeps fragments of various sizes along its channels, holding them
with a loose hand. Glacial ice grinds the stones of its ground moraine against the underlying rock with the

pressure of its enormous weight. The wave hurls fragments of rock against the sea cliff, bruising and
battering it by the blow. It also rasps the bench as it drags sand and gravel to and fro upon it.

Weathering of sea cliffs. The sea cliff furnishes the weapons for its own
destruction. They are broken from it not only by the wave but also by the
weather. Indeed the sea cliff weathers more rapidly, as a rule, than do rock ledges
inland. It is abundantly wet with spray. Along its base the ground water of the
neighboring land finds its natural outlet in springs which under mine it.
Moreover, it is unprotected by any shield of talus. Fragments of rock as they fall
from its face are battered to pieces by the waves and swept out to sea. The cliff is
thus left exposed to the attack of the weather, and its retreat would be
comparatively rapid for this reason alone.

Fig. 134. Sea Caves, La Jolla, California

Copyright, 1899, by the Detroit Photography
Company
Sea cliffs seldom overhang, but commonly, as in Figure 134, slope seaward, showing that the upper
portion has retreated at a more rapid rate than has the base. Which do you infer is on the whole the more
destructive agent, weathering or the wave?

Draw a section of a sea cliff cut in well jointed rocks whose joints dip toward the land. Draw a diagram
of a sea cliff where the joints dip toward the sea.

Sea caves. The wave does not merely batter the face of the cliff. Like a skillful quarryman it inserts
wedges in all natural fissures, such as joints, and uses explosive forces. As a wave flaps against a crevice it
compresses the air within with the sudden stroke; as it falls back the air as suddenly expands. On lighthouses
heavily barred doors have been burst outward by the explosive force of the air within, as it was released
from pressure when a partial vacuum was formed by the refluence of the wave. Where a crevice is filled
with water the entire force of the blow of the wave is transmitted by hydraulic pressure to the sides of the
fissure. Thus storm waves little by little pry and suck the rock loose, and in this way, and by the blows which
they strike with the stones of the beach, they quarry out about a joint, or wherever the rock may be weak, a

recess known as a sea cave, provided that the rock above is coherent enough to form a roof. Otherwise an
open chasm results.

Blowholes and sea arches. As a sea cave is drilled back into
the rock, it may encounter a joint or crevice opened to the
surface by percolating water. The shock of the waves soon
enlarges this to a blowhole, which one may find on the breezy
upland, perhaps a hundred yards and more back from the cliff’s
edge. In quiet weather the blowhole is a deep well; in storm it
plays a fountain as the waves drive through the long tunnel
below and spout their spray high in air in successive jets. As the
roof of the cave thus breaks down in the rear, there may remain
in front for a while a sea arch, similar to the natural bridges of
land caverns (Fig. 135).

Stacks and
wave-cut islands. Fig. 135. A Sea Arch,
As the sea drives its
tunnels and open California
drifts into the cliff,
it breaks through Copyright, 1899, by the
behind the
intervening portions Detroit Photography
and leaves them Company
isolated as stacks,
much as monuments are detached from inland escarpments by
the weather; and as the sea cliff retreats, these remnant masses
Fig. 136. Chasms worn by may be left behind as rocky islets. Thus the rock bench is often
set with stacks, islets in all stages of destruction, and sunken
Waves, Coast of Scotland reefs,—all wrecks of the land testifying to its retreat before the
incessant attack of the waves.

Fig. 137. A Stack, Scotland

Coves. Where zones of soft or closely
jointed rock outcrop along a shore, or where
minor water courses conic down to the sea and
aid in erosion, the shore is worn back in curved
reëntrants called coves; while the more resistant
rocks on either hand are left projecting as
headlands (Fig. 139). After coves are cut back a
short distance by the waves, the headlands come
to protect them, as with breakwaters, and
Fig. 138. Wave-Cut Islands,
prevent their indefinite retreat. The shore takes a
Scotland
curve of equilibrium, along which the hard rock
How far did the land once extend?
of the exposed headland and the weak rock of
the protected cove wear back at an equal rate.

Rate of recession. The rate at which a shore recedes depends on several
factors. In soft or incoherent rocks exposed to violent storms the retreat is so
rapid as to be easily measured. The coast of Yorkshire, England, whose cliffs are
cut in glacial drift, loses seven feet a year on the average, and since the Norman
conquest a strip a mile wide, with farmsteads and villages and historic seaports,

has been devoured by the sea. The sandy south
shore of Martha’s Vineyard wears back three feet a
year. But hard rocks retreat so slowly that their
recession has seldom been measured by the
records of history.

Fig. 139. Coves formed in
Softer Strata S, S; while
he Harder Strata H, H, are
left as Headlands

Fig. 140. A Pebble Beach, Cape Ann,
Massachusetts

Shore Drift
Bowlder and pebble beaches. About as fast as formed the waste of the sea
cliff is swept both along the shore and out to sea. The road of waste along shore
is the beach. We may also define the beach as the exposed edge of the sheet of
sediment formed by the carriage of land waste out to sea. At the foot of sea cliffs,
where the waves are pounding hardest, one commonly finds the rock bench
strewn on its inner margin with large stones, dislodged by the waves and by the
weather and some-what worn on their corners and edges. From this bowlder
beach the smaller fragments of waste from the cliff and the fragments into which
the bowlders are at last broken drift on to more sheltered places and there
accumulate in a pebble beach, made of pebbles well rounded by the wear which
they have suffered. Such beaches form a mill whose raw material is constantly
supplied by the cliff. The breakers of storms set it in motion to a depth of several

feet, grinding the pebbles together with a clatter to be heard above the roar of the
surf. In such a rock crusher the life of a pebble is short. Where ships have
stranded on our Atlantic coast with cargoes of hard-burned brick or of coal, a
year of time and a drift of five miles along the shore have proved enough to wear
brick and coal to powder. At no great distance from their source, therefore,
pebble beaches give place to beaches of sand, which occupy the more sheltered
reaches of the shore.

Sand beaches. The angular sand grains of various minerals into which
pebbles are broken by the waves are ground together under the beating surf and
rounded, and those of the softer minerals are crushed to powder. The process,
however, is a slow one, and if we study these sand grains under a lens we may be
surprised to see that, though their corners and edges have been blunted, they are
yet far from the spherical form of the pebbles from which they were derived. The
grains are small, and in water they have lost about half their weight in air; the
blows which they strike one another are therefore weak. Besides, each grain of
sand of the wet beach is protected by a cushion of water from the blows of its
neighbors.

The shape and size of these grains and the relative proportion of grains of the
softer minerals which still remain give a rough measure of the distance in space
and time which they have traveled from their source. The sand of many beaches,
derived from the rocks of adjacent cliffs or brought in by torrential streams from
neighboring highlands, is dark with grains of a number of minerals softer than
quartz. The white sand of other beaches, as those of the east coast of Florida, is
almost wholly composed of quartz grains; for in its long travel down the Atlantic
coast the weaker minerals have been worn to powder and the hardest alone
survive.
How does the absence of cleavage in quartz affect the durability of quartz sand?

How shore drift migrates. It is under the action of waves and currents that
shore drift migrates slowly along a coast. Where waves strike a coast obliquely
they drive the waste before them little by little along the shore. Thus on a north-
south coast, where the predominant storms are from the northeast, there will be a
migration of shore drift southwards.

All shores are swept also by currents produced by winds and tides. These are
usually far too gentle to transport of themselves the coarse materials of which
beaches are made. But while the wave stirs the grains of sand and gravel, and for
a moment lifts them from the bottom, the current carries them a step forward on

their way. The current cannot lift and the wave cannot carry, but together the two
transport the waste along the shore. The road of shore drift is therefore the zone
of the breaking waves.

The bay-head beach. As the waste
derived from the wear of waves and that
brought in by streams is trailed along a coast
it assumes, under varying conditions, a
number of distinct forms. When swept into
the head of a sheltered bay it constitutes the
bay-head beach. By the highest storm waves
the beach is often built higher than the
ground immediately behind it, and forms a
dam inclosing a shallow pond or marsh.
Fig. 141. A Bay Bar, Lake
Ontario The bay bar. As the stream of shore
drift reaches the mouth of a bay of some
size it often occurs that, instead of turning in, it sets directly across toward the
opposite headland. The waste is carried out from shore into the deeper waters of
the bay mouth; where it is no longer supported by the breaking waves, and sinks
to the bottom. The dump is gradually built to the surface as a stubby spur,
pointing across the bay, and as it reaches the zone of wave action current and
wave can now combine to carry shore drift along it, depositing their load
continually at the point of the spur. An embankment is thus constructed in much
the same manner as a railway fill, which, while it is building, serves as a roadway
along which the dirt from an adjacent cut is carted to be dumped at the end.
When the embankment is completed it bridges the bay with a highway along
which shore drift now moves without interruption, and becomes a bay bar.

Incomplete bay bars. Under certain
conditions the sea cannot carry out its
intention to bridge a bay. Rivers
discharging in bays demand open way to
the ocean. Strong tidal currents also are
able to keep open channels scoured by
their ebb and flow. In such cases the most
that land waste can do is to build spits and
shoals, narrowing and shoaling the channel Fig. 142. A Hook, Lake Michigan
as much as possible. Incomplete bay bars sometimes have their points recurved

by currents setting at right angles to the stream of shore drift and are then
classified as hooks (Fig. 142).

Sand reefs. On low coasts where shallow water
extends some distance out, the highway of shore drift
lies along a low, narrow ridge, termed the sand reef,
separated from the land by a narrow stretch of shallow
Fig. 143. Cross water called the lagoon (Fig. 143). At intervals the reef
Section of Sand Reef is held open by inlets,—gaps through which the tide
sr, and Lagoon; sl, flows and ebbs, and by which the water of streams
Sea Level finds way to the sea.
No finer example of this kind of shore line is to be found in the world than the
coast of Texas. From near the mouth of the Rio Grande a continuous sand reef
draws its even curve for a hundred miles to Corpus Christi Pass, and the reefs are
but seldom interrupted by inlets as far north as Galveston Harbor. On this coast the
tides are variable and exceptionally weak, being less than one foot in height, while
the amount of waste swept along the shore is large. The lagoon is extremely
shallow, and much of it is a mud flat too shoal for even small boats. On the coast of
New Jersey strong tides are able to keep open inlets at intervals of from two to
twenty miles in spite of a heavy alongshore drift.

Sand reefs are formed where the water is so shallow near
shore that storm waves cannot run in it and therefore break
some distance out from land. Where storm waves first drag
bottom they erode and deepen the sea floor, and sweep in
sediment as far as the line where they break. Here, where they
lose their force, they drop their load and beat up the ridge
which is known as the sand reef when it reaches the surface.

Shores of Elevation and Depression
Fig. 144. Sand
Reef and Our studies have already brought to our notice two distinct
Lagoon, Texas forms of strand lines,—one the high, rocky coast cut back to
cliffs by the attack of the waves, and the other the low, sandy
coast where the waves break usually upon the sand reef. To understand the origin
of these two types we must know that the meeting place of sea and land is
determined primarily by movements of the earth’s crust. Where a coast land
emerges the—shore line moves seaward; where it is being submerged the shore
line advances on the land.

Shores of elevation. The retreat of the sea, either because of a local uplift of
the land or for any other reason, such as the lowering of any portion of ocean
bottom, lays bare the inner margin of the sea floor. Where the sea floor has long
received the waste of the land it has been built up to a smooth, subaqueous plain,
gently shelving from the land. Since the new shore line is drawn across this even
surface it is simple and regular, and is bordered on the one side by shallow water
gradually deepening seaward, and on the other by low land composed of material
which has not yet thoroughly consolidated to firm rock. A sand reef is soon
beaten up by the waves, and for some time conditions will favor its growth. The
loss of sand driven into the lagoon beyond, and of that ground to powder by the
surf and carried out to sea, is more than made up by the stream of alongshore
drift, and especially by the drag of sediments to the reef by the waves as they
deepen the sea floor on its seaward side.

Meanwhile the lagoon gradually fills with waste from the reef and from the
land. It is invaded by various grasses and reeds which have learned to grow in
salt and brackish water; the marsh, laid bare only at low tide, is built above high
tide by wind drift and vegetable deposits, and becomes a meadow, soldering the
sand reef to the mainland.

While the lagoon has been filling, the waves have been so deepening the sea
floor off the sand reef that at last they are able to attack it vigorously. They now
wear it back, and, driving the shore line across the lagoon or meadow, cut a line
of low cliffs on the mainland. Such a shore is that of Gascony in southwestern
France,—a low, straight, sandy shore, bordered by dunes and unprotected by
reefs from the attack of the waves of the Bay of Biscay.

We may say, then, that on shores of elevation the presence of sand reefs and
lagoons indicates the stage of youth, while the absence of these features and the
vigorous and unimpeded attack by the sea upon the mainland indicate the stage
of maturity. Where much waste is brought in by rivers the maturity of such a
coast may be long delayed. The waste from the land keeps the sea shallow
offshore and constantly renews the sand reef. The energy of the waves is
consumed in handling shore drift, and no energy is left for an effective attack
upon the land. Indeed, with an excessive amount of waste brought down by
streams the land may be built out and encroach temporarily upon the sea; and not
until long denudation has lowered the land, and thus decreased the amount of
waste from it, may the waves be able to cut through the sand reef and thus the
coast reach maturity.

Shores of Depression
Where a coastal region is undergoing
submergence the shore line moves
landward. The horizontal plane of the sea
now intersects an old land surface
roughened by subaërial denudation. The
shore line is irregular and indented in
proportion to the relief of the land and the
amount of the submergence which the land
has suffered. It follows up partially
submerged valleys, forming bays, and
bends round the divides, leaving them to
project as promontories and peninsulas.
The outlines of shores of depression are as
varied as are the forms of the land partially
submerged. We give a few typical
illustrations.
The characteristics of
the coast of Maine are due
chiefly to the fact that a
Fig. 145. Map of New Jersey, with mountainous region of
that Portion of the State one hard rocks, once worn to a
Hundred Feet and more peneplain, and after a
subsequent elevation
above Sea Level shaded deeply dissected by north-
Describe the coast line which the state south valleys, has
would have if depressed one hundred subsided, the depression
amounting on its southern
feet. Compare it with the present margin to as much as six
coastline hundred feet below sea
level. Drowned valleys
penetrate the land in long, narrow bays, and rugged divides project in long,
narrow land arms prolonged seaward by islands representing the high portions
of their extremities. Of this exceedingly ragged shore there are said to be two
thousand miles from the New Brunswick boundary as far west as Portland,—a
straight-line distance of but two hundred miles. Since the time of its greatest
depression the land is known to have risen some three hundred feet; for the
bays have been shortened, and the waste with which their floors were strewn
is now in part laid bare as clay plains about the bay heads and in narrow
selvages about the peninsulas and islands. Fig. 146.
The coast of Dalmatia, on the Adriatic Sea, is characterized by long land Chesapeake Bay
arms and chains of long and narrow islands, all parallel to the trend of the Draw a sketch of this area
coast. A region of parallel mountain ranges has been depressed, and the
before its depression
longitudinal valleys which lie between them are occupied by arms of the sea.

Chesapeake Bay is a branching bay due to the depression of an ancient coastal plain which, after having
emerged from the sea, was channeled with broad, shallow valleys. The sea has invaded the valley of the
trunk stream and those of its tributaries, forming a shallow bay whose many branches are all directed toward
its axis (Fig. 146).

Hudson Bay, and the North, the Baltic, and the Yellow seas are examples where the sinking of the land
has brought the sea in over low plains of large extent, thus deeply indenting the continental out-line. The rise
of a few hundred feet would restore these submerged plains to the land.

The cycle of shores of depression. In its infantile stage the outline of a shore
of depression depends almost wholly on the previous relief of the land, and but
little on erosion by the sea. Sea cliffs and narrow benches appear where
headlands and outlying islands have been nipped by the waves. As yet, little
shore waste has been formed. The coast of Maine is an example of this stage.

In early youth all promontories have been strongly cliffed, and under a
vigorous attack of the sea the shore of open bays may be cut back also. Sea
stacks and rocky islets, caves and coves, make the shore minutely ragged. The
irregularity of the coast, due to depression, is for a while increased by differential
wave wear on harder and softer rocks. The rock bench is still narrow. Shore
waste, though being produced in large amounts, is for the most part swept into
deeper water and buried out of sight. Examples of this stage are the east coast of
Scotland and the California coast near San Francisco.

Later youth is characterized by a large accumulation of shore waste. The rock
bench has been cut back so that it now furnishes a good roadway for shore drift.
The stream of alongshore drift grows larger and larger, filling the heads of the
smaller bays with beaches, building spits and hooks, and tying islands with sand
bars to the mainland. It bridges the larger bays with bay bars, while their length is
being reduced as their inclosing promontories are cut back by the waves. Thus
there comes to be a straight, continuous, and easy road, no longer interrupted by
headlands and bays, for the transportation of waste alongshore. The Baltic coast
of Germany is in this stage.

All this while streams have been busy filling with delta deposits the bays into
which they empty. By these steps a coast gradually advances to maturity, the
stage when the irregularities due to depression have been effaced, when outlying
islands formed by subsidence have been planed away, and when the shore line
has been driven back behind the former bay heads. The sea now attacks the land
most effectively along a continuous and fairly straight line of cliffs. Although the
first effect of wave wear was to increase the irregularities of the shore, it sooner
or later rectifies it, making it simple and smooth. The northwest coast of France

is often cited as an example of a coast
which has reached this stage of
development (Fig. 147).

In the old age of coasts the rock bench
is cut back so far that the waves can no
longer exert their full effect upon the shore.
Their energy is dissipated in moving shore
drift hither and thither and in abrading the
bench when they drag bottom upon it. Little
by little the bench is deepened by tidal
currents and the drag of waves; but this Fig. 147. Portion of the Northwest
process is so slow that meanwhile the sea Coast of France
cliffs melt down under the weather, and the bench becomes a broad shoal where
waves and tides gradually work over the waste from the land to greater fineness
and sweep it out to sea.

Fig. 148. The South Shore of Martha’s Vineyard
The land is shaded. To what class of coasts does this belong? What
stage has it reached, and by what process? What changes will take
place in the future?

Plains of marine abrasion. While subaërial denudation reduces the land to
baselevel, the sea is sawing its edges to wave base, i.e. the lowest limit of the
wave’s effective wear. The widened rock bench forms when uplifted a plain of
marine abrasion, which like the peneplain bevels across strata regardless of their
various inclinations and various degrees of hardness.
How may a plain of marine abrasion be expected to differ from a peneplain in its mantle of waste?

Compared with subaërial denudation, marine abrasion is a comparatively
feeble agent. At the rate of five feet per century—a higher rate than obtains on

the youthful rocky, coast of Britain—it would require more than ten million years
to pare a strip one hundred miles wide from the margin of a continent, a time
sufficient, at the rate at which the Mississippi valley is now being worn away, for
subaërial denudation to lower the lands of the globe to the level of the sea.

Slow submergence favors the cutting of a wide rock bench. The water
continually deepens upon the bench; storm waves can therefore always ride in to
the base of the cliffs and attack them with full force; shore waste cannot impede
the onset of the waves, for it is continually washed out in deeper water below
wave base.

Basal conglomerates. As the sea marches across the land during a slow
submergence, the platform is covered with sheets of sea-laid sediments. Lowest
of these is a conglomerate,—the bowlder and pebble beach, widened indefinitely
by the retreat of the cliffs at whose base it was formed, and preserved by the finer
deposits laid upon it in the constantly deepening water as the land subsides. Such
basal conglomerates are not uncommon among the ancient rocks of the land, and
we may know them by their rounded pebbles and larger stones, composed of the
same kind of rock as that of the abraded and evened surface on which they lie.

CHAPTER VIII

OFFSHORE AND DEEP-SEA DEPOSITS
The alongshore deposits which we have now studied are the exposed edge of
a vast subaqueous sheet of waste which borders the continents and extends often
for as much as two or three hundred miles from land. Soundings show that
offshore deposits are laid in belts parallel to the coast, the coarsest materials lying
nearest to the land and the finest farthest out. The pebbles and gravel and the
clean, coarse sand of beaches give place to broad stretches of sand, which grows
finer and finer until it is succeeded by sheets of mud. Clearly there is an offshore
movement of waste by which it is sorted, the coarser being sooner dropped and
the finer being carried farther out.

Offshore Deposits
The débris torn by waves from rocky shores is far less in amount than the
waste of the land brought down to the sea by rivers, being only one thirty-third as
great, according to a conservative estimate. Both mingle alongshore in all the
forms of beach and bar that have been described, and both are together slowly
carried out to sea. On the shelving ocean floor waste is agitated by various
movements of the unquiet water,—by the undertow (an outward‑running bottom
current near the shore), by the ebb and flow of tides, by ocean currents where
they approach the land, and by waves and ground swells, whose effects are
sometimes felt to a depth of six hundred feet. By all these means the waste is
slowly washed to and fro, and as it is thus ground finer and finer and its soluble
parts are more and more dissolved, it drifts farther and farther out from land. It is
by no steady and rapid movement that waste is swept from the shore to its final
resting place. Day after day and century after century the grains of sand and
particles of mud are shifted to and fro, winnowed and spread in layers, which are
destroyed and rebuilt again and again before they are buried safe from further
disturbance.

These processes which are hidden from the eye are among the most important
of those with which our science has to do; for it is they which have given shape
to by far the largest part of the stratified rocks of which the land is made.

The continental delta. This fitting term has been recently suggested for the
sheet of waste slowly accumulating along the borders of the continents. Within a
narrow belt, which rarely exceeds two or three hundred miles, except near the
mouths of muddy rivers such as the Amazon and Congo, nearly all the waste of
the continent, whether worn from its surface by the weather, by streams, by
glaciers, or by the wind, or from its edge by the chafing of the waves, comes at
last to its final resting place. The agencies which spread the material of the
continental delta grow more and more feeble as they pass into deeper and more
quiet water away from shore. Coarse materials are therefore soon dropped along
narrow belts near land. Gravels and coarse sands lie in thick, wedge-shaped
masses which thin out seaward rapidly and give place to sheets of finer sand.

Sea muds. Outermost of the sediments derived from the waste of the
continents is a wide belt of mud; for fine clays settle so slowly, even in sea water,
—whose saltness causes them to sink much faster than they would in fresh water,
—that they are wafted far before they reach a bottom where they may remain

undisturbed. Muds are also found near shore, carpeting the floors of estuaries,
and among stretches of sandy deposits in hollows where the more quiet water has
permitted the finer silt to rest.

Sea muds are commonly bluish and consolidate to bluish shales; the red
coloring matter brought from land waste—iron oxide—is altered to other iron
compounds by decomposing organic matter in the presence of sea water. Yellow
and red muds occur where the amount of iron oxide in the silt brought down to
the sea by rivers is too great to be reduced, or decomposed, by the organic matter
present.

Green muds and green sand owe their color to certain chemical changes
which take place where waste from the land accumulates on the sea floor with
extreme slowness. A greenish mineral called glauconite—a silicate of iron and
alumina—is then formed. Such deposits, known as green sand, are now in
process of making in several patches off the Atlantic coast, and are found on the
coastal plain of New Jersey among the offshore deposits of earlier geological
ages.

Organic deposits. Living creatures swarm along the shore and on the
shallows out from land as nowhere else in the ocean. Seaweed often mantles the
rock of the sea cliff between the levels of high and low tide, protecting it to some
degree from the blows of waves. On the rock bench each little pool left by the
ebbing tide is an aquarium abounding in the lowly forms of marine life. Below
low-tide level occur beds of molluscous shells, such as the oyster, with countless
numbers of other humble organisms. Their harder parts—the shells of mollusks,
the white framework of corals, the carapaces of crabs and other crustaceans, the
shells of sea urchins, the bones and teeth of fishes—are gradually buried within
the accumulating sheets of sediment, either whole or, far more often, broken into
fragments by the waves.

By means of these organic remains each layer of beach deposits and those of
the continental delta may contain a record of the life of the time when it was laid.
Such a record has been made ever since living creatures with hard parts appeared
upon the globe. We shall find it sealed away in the stratified rocks of the
continents,— parts of ancient sea deposits now raised to form the dry land. Thus
we have in the traces of living creatures found in the rocks, i.e. in fossils, a
history of the progress of life upon the planet.

Fig. 149. Coquina, Florida

Molluscous shell deposits. The forms of marine life of importance in rock
making thrive best in clear water, where little sediment is being laid, and where at
the same time the depth is not so great as to deprive them of needed light, heat,
and of sufficient oxygen absorbed by sea water from the air. In such clear and
comparatively shallow water there often grow countless myriads of animals, such
as mollusks and corals, whose shells and skeletons of carbonate of lime gradually
accumulate in beds of limestone.
A shell limestone made of broken fragments cemented together is sometimes called coquina, a local term
applied to such beds recently uplifted from the sea along the coast of Florida (Fig. 149).

Oölitic limestone (ōon, an egg; lithos, a stone) is so named from the likeness of the tiny spherules which
compose it to the roe of fish. Corals and shells have been pounded by the waves to calcareous sand, and each
grain has been covered with successive concentric coatings of lime carbonate deposited about it from
solution.

The impalpable powder to which calcareous sand is ground by the waves
settles at some distance from shore in deeper and quieter water as a limy silt, and
hardens into a dense, fine-grained limestone in which perhaps no trace of fossil is
found to suggest the fact that it is of organic origin.
From Florida Keys there extends south to the trough of Florida Straits a limestone bank covered by from
five hundred and forty to eighteen hundred feet of water. The rocky bottom consists of limestone now slowly
building from the accumulation of the remains of mollusks, small corals, sea urchins, worms with calcareous
tubes, and lime-secreting seaweed, which live upon its surface.

Where sponges and other silica-secreting organisms abound on limestone
banks, silica forms part of the accumulated deposit, either in its original
condition, as, for example, the spicules of sponges, or gathered into concretions
and layers of flint.

Where considerable mud is being deposited along with carbonate of lime
there is in process of making a clayey limestone or a limy shale; where
considerable sand, a sandy limestone or a limy sandstone.

Consolidation of offshore deposits. We cannot doubt that all these loose
sediments of the sea floor are being slowly consolidated to solid rock. They are
soaked with water which carries in solution lime carbonate and other cementing
substances. These cements are deposited between the fragments of shells and
corals, the grains of sand and the particles of mud, binding them together into
firm rock. Where sediments have accumulated to great thickness the lower
portions tend also to consolidate under the weight of the overlying beds. Except
in the case of limestones, recent sea deposits uplifted to form land are seldom so
well cemented as are the older strata, which have long been acted upon by
underground waters deep below the surface within the zone of cementation, and
have been exposed to view by great erosion.

Fig. 150. Ripple Marks on Layers of Ancient Sandstone, Wisconsin

Ripple marks, sun cracks, etc. The pulse of waves and tidal currents
agitates the loose material of offshore deposits, throwing it into fine parallel
ridges called ripple marks. One may see this beautiful ribbing imprinted on beach
sands uncovered by the outgoing tide, and it is also produced where the water is
of considerable depth. While the tide is out the surface of shore deposits may be
marked by the footprints of birds and other animals, or by the raindrops of a
passing shower (Fig. 153). The mud of flats, thus exposed to the sun and dried,
cracks in a characteristic way (Figs. 151 and 152). Such markings may be
covered over with a thin layer of sediment at the next flood tide and sealed away
as a lasting record of the manner and place in which the strata were laid. In
Figure 150 we have an illustration of a very ancient ripple-marked sand
consolidated to hard stone, uplifted and set on edge by movements of the earth’s
crust, and exposed to open air after long erosion.

Stratification. For the most part the
sheet of sea-laid waste is hidden from our
sight. Where its edge is exposed along the
shore we may see the surface markings
which have just been noticed. Soundings
also, and the observations made in shallow
waters by divers, tell something of its
surface; but to learn more of its structures
we must study those ancient sediments
which have been lifted from the sea and
dissected by subaërial agencies. From
them we ascertain that sea deposits are
stratified. They lie in distinct layers which
often differ from one another in thickness,
Fig. 151. Sun Cracks
in size of particles, and perhaps in color.
They are parted by bedding planes, each of which represents either a change in
material or a pause during which deposition ceased and the material of one layer
had time to settle and become somewhat consolidated before the material of the
next was laid upon it. Stratification is thus due to intermittently acting forces,
such as the agitation of the water during storms, the flow and ebb of the tide, and
the shifting channels of tidal currents. Off the mouths of rivers, stratification is
also caused by the coarser and more abundant material brought down at time of
floods being laid on the finer silt which is discharged during ordinary stages.
How stratified deposits are built up is well illustrated in the flats which border estuaries, such as the Bay
of Fundy. Each advance of the tide spreads a film of mud, which dries and hardens in the air during low

water before another film is
laid upon it by the next
incoming tidal flood. In this
way the flats have been
covered by a clay which
splits into leaves as thin as
sheets of paper.

It is in fine
material, such as
clays and shales and
Fig. 153. Rain limestones, that the
Prints thinnest and most
uniform layers, as
well as those of widest extent, occur. On the
other hand, coarse materials are commonly
laid in thick beds, which soon thin out
seaward and give place to deposits of finer
stuff. In a general way strata are laid in well-
Fig. 152. The Under Side of a
nigh horizontal sheets, for the surface on
Layer deposited upon a Sun-
which they are laid is generally of very
Cracked Surface, showing Casts
gentle inclination. Each stratum, however, is
of the Cracks
lenticular, or lenslike, in form, having an area
where it is thickest, and thinning out thence to its edges, where it is overlapped
by strata similar in shape.
Cross bedding. There is an apparent exception
to this rule where strata whose upper and lower
surfaces may be about horizontal are made up of
layers inclined at angles which may be as high as
the angle of repose. In this case each stratum grew
by the addition along its edge of successive layers
of sediment, precisely as does a sand bar in a river,
the sand being pushed continuously over the edge
and coming to rest on a sloping surface. Shoals built
by strong and shifting tidal currents often show
successive strata in which the cross bedding is
inclined in different directions.

Thickness of sea deposits.
Fig. 154. Cross Bedding in Sandstone,
Remembering the vast amount of
England
material denuded from the land and
deposited offshore, we should expect that with the lapse of time sea deposits
would have grown to an enormous thickness. It is a suggestive fact that, as a rule,
the profile of the ocean bed is that of a soup plate,—a basin surrounded by a

flaring rim. On the continental shelf, as the rim is called, the water is seldom
more than six hundred feet in depth at the outer edge, and shallows gradually
towards shore. Along the eastern coast of the United States the continental shelf
is from fifty to one hundred and more miles in width; on the Pacific coast it is
much narrower. So far as it is due to upbuilding, a wide continental shelf, such as
that of the Atlantic coast, implies a massive continental delta thousands of feet in
thickness. The coastal plain of the Atlantic states may be regarded as the emerged
inner margin of this shelf, and borings made along the coast probe it to the depth
of as much as three thousand feet without finding the bottom of ancient offshore
deposits. Continental shelves may also be due in part to a submergence of the
outer margin of a continental plateau and to marine abrasion.

Deposition of sediments and subsidence. The stratified rocks of the land
show in many places ancient sediments which reach a thickness which is
measured in miles, and which are yet the product of well-nigh continuous
deposition. Such strata may prove by their fossils and by their composition and
structure that they were all laid offshore in shallow water. We must infer that,
during the vast length of time recorded by the enormous pile, the floor of the sea
along the coast was slowly sinking, and that the trough was constantly being
filled, foot by foot, as fast as it was depressed. Such gradual, quiet movements of
the earth’s crust not only modify the outline of coasts, as we have seen, but are of
far greater geological importance in that they permit the making of immense
deposits of stratified rock.

A slow subsidence continued during long time is recorded also in the
succession of the various kinds of rock that come to be deposited in the same
area. As the sea transgresses the land, i.e. encroaches upon it, any given part of
the sea bottom is brought farther and farther from the shore. The basal
conglomerate formed by bowlder and pebble beaches comes to be covered with
sheets of sand, and these with layers of mud as the sea becomes deeper and the
shore more remote; while deposits of limestone are made when at last no waste is
brought to the place from the now distant land, and the water is left clear for the
growth of mollusks and other lime-secreting organisms.

Fig. 155. Succession of Deposits recording
a Transgressing Sea
c, conglomerate; ss, sandstone; sh, shale; lm, limestone

Rate of deposition. As deposition in the sea corresponds to denudation on
the land, we are able to make a general estimate of the rate at which the former
process is going on. Leaving out of account the soluble matter removed, the
Mississippi is lowering its basin at the rate of one foot in five thousand years, and
we may assume this as the average rate at which the earth’s land surface of fifty-
seven million square miles is now being denuded by the removal of its
mechanical waste. But sediments from the land are spread within a zone but two
or three hundred miles in width along the margin of the continents, a line one
hundred thousand miles long. As the area of deposition—about twenty-five
million square miles—is about one half the area of denudation, the average rate
of deposition must be twice the average rate of denudation, i.e. about one foot in
twenty-five hundred years. If some deposits are made much more rapidly than
this, others are made much more slowly. If they were laid no faster than the
present average rate, the strata of ancient sea deposits exposed in a quarry fifty
feet deep represent a lapse of at least one hundred and twenty-five thousand
years, and those of a formation five hundred feet thick required for their
accumulation one million two hundred and fifty thousand years.

The sedimentary record and the
denudation cycle. We have seen that the
successive stages in a cycle of denudation, such as
that by which a land mass of lofty mountains is
worn to low plains, are marked each by its own
peculiar land forms, and that the forms of the
earlier stages are more or less completely effaced
as the cycle draws toward an end. Far more lasting Fig. 156. Thick Offshore
records of each stage are left in the sedimentary Deposits of Coarse
deposits of the continental delta. Thus, in the Waste recording the
youth of such a land mass as we have mentioned, Presence of a Young
torrential streams flowing down the steep Mountain Range near
mountain sides deliver to the adjacent sea their Shore
heavy loads of coarse waste, and thick offshore
deposits of sand and gravel (Fig. 156) record the high elevation of the bordering
land. As the land is worn to lower levels, the amount and coarseness of the waste
brought to the sea diminishes, until the sluggish streams carry only a fine silt
which settles on the ocean floor near to land in wide sheets of mud which harden

into shale. At last, in the old age of the region (Fig. 157), its low plains contribute
little to the sea except the soluble elements of the rocks, and in the clear waters
near the land lime-secreting organisms flourish and their remains accumulate in
beds of limestone. When long-weathered lands mantled with deep, well-oxidized
waste are uplifted by a gradual movement of the earth’s crust, and the mantle is
rapidly stripped off by the revived streams, the uprise is recorded in wide
deposits of red and yellow clays and sands upon the adjacent ocean floor.

Where the waste brought in is more than the waves can easily distribute, as
off the mouths of turbid rivers which drain highlands near the sea, deposits are
little winnowed, and are laid in rapidly alternating, shaly sandstones and sandy
shales.

Fig. 157. Offshore Deposits recording Old
Age of the Adjacent Land
ss, sandstone; sh, shale; lm, limestone

Where the highlands are of igneous rock, such as granite, and mechanical
disintegration is going on more rapidly than chemical decay, these conditions are
recorded in the nature of the deposits laid offshore. The waste swept in by
streams contains much feldspar and other minerals softer and more soluble than
quartz, and where the waves have little opportunity to wear and winnow it, it
comes to rest in beds of sandstone in which grains of feldspar and other soft
minerals are abundant. Such feldspathic sandstones are known as arkose.

On the other hand, where the waste supplied to the sea comes chiefly from
wide, sandy, coastal plains, there are deposited off- shore clean sandstones of
well-worn grains of quartz alone. In such coastal plains the waste of the land is
stored for ages. Again and again they are abandoned and invaded by the sea as
from time to time the land slowly emerges and is again submerged. Their
deposits are long exposed to the weather, and sorted over by the streams, and
winnowed and worked over again and again by the waves. In the course of long
ages such deposits thus become thoroughly sorted, and the grains of all minerals
softer than quartz are ground to mud.

Deep-Sea Oozes and Clays

Globigerina ooze. Beyond the reach of waste from the
land the bottom of the deep sea is carpeted for the most part
with either chalky ooze or a fine red clay. The surface
waters of the warm seas swarm with minute and lowly
animals belonging to the order of the Foraminifera, which
secrete shells of carbonate of lime. At death these tiny white
shells fall through the sea water like snowflakes in the air,
and, slowly dissolving, seem to melt quite away before they
can reach depths greater than about three miles. Near shore
they reach bottom, but are masked by the rapid deposit of Fig. 158.
waste derived from the land. At intermediate depths they Globigerina Ooze
mantle the ocean floor with a white, soft lime deposit under the
known as Globigerina ooze, from a genus of the Microscope
Foraminifera which contributes largely to its formation.

Red clay. Below depths of from fifteen to eighteen thousand feet the ocean
bottom is sheeted with red or chocolate colored clay. It is the insoluble residue of
seashells, of the débris of submarine volcanic eruptions, of volcanic dust wafted
by the winds, and of pieces of pumice drifted by ocean currents far from the
volcanoes from which they were hurled. The red clay builds up with such
inconceivable slowness that the teeth of sharks and the hard ear bones of whales
may be dredged in large numbers from the deep ocean bed, where they have lain
unburied for thousands of years; and an appreciable part of the clay is also
formed by the dust of meteorites consumed in the atmosphere,—a dust which
falls everywhere on sea and land, but which elsewhere is wholly masked by other
deposits.

The dark, cold abysses of the ocean are far less affected by change than any
other portion of the surface of the lithosphere. These vast, silent plains of ooze lie
far below the reach of storms. They know no succession of summer and winter,
or of night and day. A mantle of deep and quiet water protects them from the
agents of erosion which continually attack, furrow, and destroy the surface of the
land. While the land is the area of erosion, the sea is the area of deposition. The
sheets of sediment which are slowly spread there tend to efface any inequalities,
and to form a smooth and featureless subaqueous plain.

With few exceptions, the stratified rocks of the land are proved by their
fossils and composition to have been laid in the sea; but in the same way they are
proved to be offshore, shallow-water deposits, akin to those now making on
continental shelves. Deep- sea deposits are absent from the rocks of the land, and

we may therefore infer that the deep sea has never held sway where the
continents now are,—that the continents have ever been, as now, the elevated
portions of the lithosphere, and that the deep seas of the present have ever been
its most depressed portions.

The Reef-Building Corals
In warm seas the most conspicuous of rock-making organisms are the corals
known as the reef builders. Floating in a boat over a coral reef, as, for example,
off the south coast of Florida or among the Bahamas, one looks down through
clear water on thickets of branching coral shrubs perhaps as much as eight feet
high, and hemispherical masses three or four feet thick, all abloom with countless
minute flowerlike coral polyps, gorgeous in their colors of yellow, orange, green,
and red. In structure each tiny polyp is little more than a fleshy sac whose mouth
is surrounded with petal-like tentacles, or feelers. From the sea water the polyps
secrete calcium carbonate and build it up into the stony framework which
supports their colonies. Boring mollusks, worms, and sponges perforate and
honeycomb this framework even while its surface is covered with myriads of
living polyps. It is thus easily broken by the waves, and white fragments of coral
trees strew the ground beneath. Brilliantly colored fishes live in these coral
groves, and countless mollusks, sea urchins, and other forms of marine life make
here their home. With the débris from all these sources the reef is constantly built
up until it rises to low-tide level. Higher than this the corals cannot grow, since
they are killed by a few hours’ exposure to the air.

Fig. 159. Patch of Growing Corals exposed at an
Exceptionally Low Tide, Great Barrier Reef,
Australia

When the reef has risen to wave base, the waves abrade it on the windward
side and pile to leeward coral blocks torn from their foundation, filling the
interstices with finer fragments. Thus they heap up along the reef low, narrow
islands (Fig. 160).

Reef building is a comparatively rapid progress. It has been estimated that off
Florida a reef could be built up to the surface from a depth of fifty feet in about
fifteen hundred years.

Coral limestones. Limestones of various kinds are due to
the reef builders. The reef rock is made of corals in place and
broken fragments of all sizes, cemented together with calcium
carbonate from solution by infiltrating waters. On the island
Fig. 160.
beaches coral sand is forming oolitic limestone, and the white
Wave-Built
coral mud with which the sea is milky for miles about the reef
Island on
in times of storm settles and concretes into a compact
Coral Reef
limestone of finest grain. Corals have been among the most
, reef; sl, sea level
important limestone builders of the sea ever since they made
their appearance in the early geological ages.

The areas on which coral limestone is now forming are large. The Great
Barrier Reef of Australia, which lies off the north-eastern coast, is twelve
hundred and fifty miles long, and has a width of from ten to ninety miles. Most of
the islands of the tropics are either skirted with coral reefs or are themselves of
coral formation.

Conditions of coral growth. Reef-building corals cannot live except in clear
salt water less, as a rule, than one hundred and fifty feet in depth, with a winter
temperature not lower than 68° F. An important condition also is an abundant
food supply, and this is best secured in the path of the warm oceanic currents.

Coral reefs may be grouped in three classes,—fringing reefs, barrier reefs,
and atolls.
Fringing reefs. These take their name from the fact that they are attached as narrow fringes to the shore.
An example is the reef which forms a selvage about a mile wide along the northeastern coast of Cuba. The
outer margin, indicated by the line of white surf, where the corals are in vigorous growth, rises from about

forty feet of water. Between this and the shore lies a stretch of shoal across which one can wade at low
water, composed of coral sand with here and there a clump of growing coral.

Barrier reefs. Reefs separated from the shore by a ship channel of quiet
water, often several miles in width and sometimes as much as three hundred feet
in depth, are known as barrier reefs. The seaward face rises abruptly from water
too deep for coral growth. Low islands are cast up by the waves upon the reef,
and inlets give place for the ebb and flow of the tides. Along the west coast of the
island of New Caledonia a barrier reef extends for four hundred miles, and for a
length of many leagues seldom approaches within eight miles of the shore.

Atolls. These are ring-shaped or irregular coral islands, or island-studded
reefs, inclosing a central lagoon. The narrow zone of land, like the rim of a great
bowl sunken to the water’s edge, rises hardly more than twenty feet at most
above the sea, and is covered with a forest of trees such as the cocoanut, whose
seeds can be drifted to it uninjured from long distances. The white beach of coral
sand leads down to the growing reef, on whose outer margin the surf is constantly
breaking. The sea face of the reef falls off abruptly, often to depths of thousands
of feet, while the lagoon varies in depth from a few feet to one hundred and fifty
or two hundred, and exceptionally measures as much as three hundred and fifty
feet.

Theories of coral reefs. Fringing reefs require no explanation, since the
depth of water about them is not greater than that at which coral can grow; but
barrier reefs and atolls, which may rise from depths too great for coral growth
demand a theory of their origin.

Fig. 161. Diagram illustrating the Subsidence
Theory of Coral Reefs

Darwin’s theory holds that barrier reefs and atolls are formed from fringing
reefs by subsidence. The rate of sinking cannot be greater than that of the
upbuilding of the reef, since otherwise the corals would be carried below their

depth and drowned. The process is illustrated in Figure 161, where v represents a
volcanic island in mid ocean undergoing slow depression, and ss the sea level
before the sinking began, when the island was surrounded by a fringing reef. As
the island slowly sinks, the reef builds up with equal pace. It rears its seaward
face more steep than the island slope, and thus the intervening space between the
sinking, narrowing land and the outer margin of the reef constantly widens. In
this intervening space the corals are more or less smothered with silt from the
outer reef and from the land, and are also deprived in large measure of the
needful supply of food and oxygen by the vigorous growth of the corals on the
outer rim. The outer rim thus becomes a barrier reef and the inner belt of retarded
growth is deepened by subsidence to a ship channel, s´s´ representing sea level at
this time. The final stage, where the island has been carried completely beneath
the sea and overgrown by the contracting reef, whose outer ring now forms an
atoll, is represented by s´´s´´.

In very many instances, however, atolls and
barrier reefs may be explained without subsidence.
Thus a barrier reef may be formed by the seaward
growth of a fringing reef upon the talus of its sea
face. In Figure 162, f is a fringing reef whose outer
wall rises from about one hundred and fifty feet, the Fig. 162. Barrier Reef
lower limit of the reef-building species. At the foot of formed without
this submarine cliff a talus of fallen blocks t Subsidence
accumulates, and as it reaches the zone of coral a, zone of coral growth; f, former
growth becomes the foundation on which the reef is fringing reef; t, talus; b, barrier
steadily extended seaward. As the reef widens, the reef
polyps of the circumference flourish, while those of
the inner belt are retarded in their growth and at last perish. The coral rock of the
inner belt is now dissolved by sea water and scoured out by tidal currents until it
gives place to a gradually deepening ship channel, while the outer margin is left
as a barrier reef.

In much the same way atolls may be built on
any shoal which lies within the zone of coral
growth. Such shoals may be produced when
volcanic islands are leveled by waves and ocean
currents, and when submarine plateaus, ridges,
and peaks are built up by various organic
agencies, such as molluscous and foraminiferal
shell deposits (Fig. 163). The reef-building

Fig. 163. Section of Atoll on corals, whose eggs are drifted widely over the
a Shoal which has been tropic seas by ocean currents, colonize such
built up to near the submarine foundations wherever the conditions
Surface by Organic are favorable for their growth. As the reef
Deposits upon a approaches the surface the corals of the inner
Submarine Volcanic area are smothered by silt and starved, and their
Peak Submarine Volcanic Peak hard parts are
v, volcano; f, foraminiferal dissolved and scoured away; while those of the
deposits; m, molluscous shell circumference, with abundant food supply,
deposits; c, coral reef; sl, sea nourish and build the ring of the atoll. Atolls may
level be produced also by the backward drift of sand
from either end of a crescentic coral reef or
island, the spits uniting in the quiet water of the lee to inclose a lagoon. In the
Maldive Archipelago all gradations between crescent-shaped islets and complete
atoll rings have been observed.
In a number of instances where coral reefs have been raised by movements of the earth’s crust, the reef
formation is found to be a thin veneer built upon a foundation of other deposits. Thus Christmas Island, in
the Indian Ocean, is a volcanic pile rising eleven hundred feet above sea level and fifteen thousand five
hundred feet above the bottom of the sea. The summit is a plateau surrounded by a rim of hills of reef
formation, which represent the ring of islets of an ancient atoll. Beneath the reef are thick beds of limestone,
composed largely of the remains of foraminifers, which cover the lavas and fragmental materials of the old
submarine volcano.

Among the ancient sediments which now form the stratified rocks of the land there occur many thin reef
deposits, but none are known of the immense thickness which modern reefs are supposed to reach according
to the theory of subsidence.

Barrier and fringing reefs are commonly interrupted off the mouths of rivers. Why?

Summary. We have seen that the ocean bed is the goal to which the waste of
the rocks of the land at last arrives. Their soluble parts, dissolved by underground
waters and carried to the sea by rivers, are largely built up by living creatures into
vast sheets of limestone. The less soluble portions—the waste brought in by
streams and the waste of the shore—form the muds and sands of continental
deltas. All of these sea deposits consolidate and harden, and the coherent rocks of
the land are thus reconstructed on the ocean floor. But the destination is not a
final one. The stratified rocks of the land are for the most part ancient deposits of
the sea, which have been lifted above sea level; and we may believe that the
sediments now being laid offshore are the “dust of continents to be,” and will
some time emerge to form additions to the land. We are now to study the
movements of the earth’s crust which restore the sediments of the sea to the light
of day, and to whose beneficence we owe the habitable lands of the present.

PART II

INTERNAL GEOLOGICAL
AGENCIES

CHAPTER IX

MOVEMENTS OF THE EARTH’S CRUST
The geological agencies which we have so far studied—weathering, streams,
underground waters, glaciers, winds, and the ocean—all work upon the earth
from without, and all are set in motion by an energy external to the earth, namely,
the radiant energy of the sun. All, too, have a common tendency to reduce the
inequalities of the earth’s surface by leveling the lands and strewing their waste
beneath the sea.

But despite the unceasing efforts of these external agencies, they have not
destroyed the continents, which still rear their broad plains and great plateaus and
mountain ranges above the sea. Either, then, the earth is very young and the
agents of denudation have not yet had time to do their work, or they have been
opposed successfully by other forces.

We enter now upon a department of our science which treats of forces which
work upon the earth from within, and increase the inequalities of its surface. It is
they which uplift and recreate the lands which the agents of denudation are
continually destroying; it is they which deepen the ocean bed and thus withdraw
its waters from the shores. At times also these forces have aided in the
destruction of the lands by gradually lowering them and bringing in the sea.
Under the action of forces resident within the earth the crust slowly rises or sinks;
from time to time it has been folded and broken; while vast quantities of molten

rock have been pressed up into it from beneath and outpoured upon its surface.
We shall take up these phenomena in the following chapters, which treat of
upheavals and depressions of the crust, foldings and fractures of the crust,
earthquakes, volcanoes, the interior conditions of the earth, mineral veins, and
metamorphism.

Oscillations of the Crust
Of the various movements of the crust due to internal agencies we will
consider first those called oscillations, which lift or depress large areas so slowly
that a long time is needed to produce perceptible changes of level, and which
leave the strata in nearly their original horizontal attitude. These movements are
most conspicuous along coasts, where they can be referred to the datum plane of
sea level; we will therefore take our first illustrations from rising and sinking
shores.
New Jersey. Along the coasts of New Jersey one may find awash at high tide ancient shell heaps, the
remains of tribal feasts of aborigines. Meadows and old forest grounds, with the stumps still standing, are
now overflowed by the sea, and fragments of their turf and wood are brought to shore by waves. Assuming
that the sea level remains constant, it is clear that the New Jersey coast is now gradually sinking. The rate of
submergence has been estimated at about two feet per century.

On the other hand, the wide coastal plain of New Jersey is made of stratified sands and clays, which, as
their marine fossils show, were outspread beneath the sea. Their present position above sea level proves that
the land now subsiding emerged in the recent past.

The coast of New Jersey is an example of the slow and tranquil oscillations of
the earth’s unstable crust now in progress along many shores. Some are emerging
from the sea, some are sinking beneath it; and no part of the land seems to have
been exempt from these changes in the past.

Evidences of changes of level. Taking the surface of the sea as a level of
reference, we may accept as proofs of relative upheaval whatever is now found in
place above sea level and could have been formed only at or beneath it, and as
proofs of relative subsidence whatever is now found beneath the sea and could
only have been formed above it.

Thus old strand lines with sea cliffs, wave-cut rock benches, and beaches of
wave-worn pebbles or sand, are striking proofs of recent emergence to the
amount of their present height above tide. No less conclusive is the presence of
sea-laid rocks which we may find in the neighboring quarry or outcrop, although

it may have been long ages since they were lifted from the sea to form part of the
dry land.

Among common proofs of subsidence are roads and buildings and other
works of man, and vegetal growths and deposits, such as forest grounds and peat
beds, now submerged beneath the sea. In the deltas of many large rivers, such as
the Po, the Nile, the Ganges, and the Mississippi, buried soils prove subsidences
of hundreds of feet; and in several cases, as in the Mississippi delta, the
depression seems to be now in progress.

Other proofs of the same movement are drowned land forms which are
modeled only in open air. Since rivers cannot cut their valleys farther below the
baselevel of the sea than the depths of their channels, drowned valleys are among
the plainest proofs of depression. To this class belong Narragansett, Delaware,
Chesapeake, Mobile, and San Francisco bays, and many other similar drowned
valleys along the coasts of the United States. Less conspicuous are the submarine
channels which, as soundings show, extend from the mouths of a number of
rivers some distance out to sea. Such is the submerged channel which reaches
from New York Bay southeast to the edge of the continental shelf, and which is
supposed to have been cut by the Hudson River when this part of the shelf was a
coastal plain.

Warping. In a region undergoing changes of level the rate of movement
commonly varies in different parts. Portions of an area may be rising or sinking,
while adjacent portions are stationary or moving in the opposite direction. In this
way a land surface becomes warped. Thus, while Nova Scotia and New
Brunswick are now rising from the level of the sea, Prince Edward Island and
Cape Breton Island are sinking, and the sea now flows over the site of the famous
old town of Louisburg destroyed in 1758.
Since the close of the glacial epoch the coasts of Newfoundland and Labrador have risen hundreds of
feet, but the rate of emergence has not been uniform. The old strand line, which stands at five hundred and
seventy-five feet above tide at St. John’s, Newfoundland, declines to two hundred and fifty feet near the
northern point of Labrador (Fig. 164).

The Great Lakes is now under-going perceptible warping. Rivers
enter the lakes from the south and west with sluggish currents and deep
channels resembling the estuaries of drowned rivers; while those that
enter from opposite directions are swift and shallow. At the western end of
Lake Erie are found submerged caves containing stalactites, and old
meadows and forest grounds are now under water. It is thus seen that the
water of the lakes is rising along their southwestern shores, while from
their north-eastern shores it is being withdrawn. The region of the Great
Lakes is therefore warping; it is rising in the northeast as compared with
the southwest.
Fig. 164. Warped Strand
From old bench marks and records of lake levels it has been estimated
Line from St. John’s, that the rate of warping amounts to five inches a century for every one
Newfoundland, to Nachvak, hundred miles. It is calculated that the water of Lake Michigan is rising at
Labrador Chicago at the rate of nine or ten inches per century. The divide at this
point between the tributaries of the Mississippi and Lake Michigan is but
eight feet above the mean stage of the lake. If the canting of the region continues at its present rate, in a thousand
years the waters of the lake will here overflow the divide. In three thousand five hundred years all the lakes except
Ontario will discharge by this outlet, via the Illinois and Mississippi rivers, into the Gulf of Mexico. The present
outlet by the Niagara River will be left dry, and the divide between the St. Lawrence and the Mississippi systems will
have shifted from Chicago to the vicinity of Buffalo.

Physiographic effects of oscillations. We have already mentioned several of the
most important effects of movements of elevation and depression, such as their effects
on rivers, the mantle of waste (pp. 85, 86), and the forms of coasts (p. 166). Movements
of elevation—including uplifts by folding and fracture of the crust to be noticed later—
are the necessary conditions for erosion by whatever agent. They determine the various
agencies which are to be chiefly concerned m the wear of any land,—whether streams or
glaciers, weathering or the wind,—and the degree of their efficiency. The lands must be
uplifted before they can be eroded, and since they must be eroded before their waste can
be deposited, movements of elevation are a prerequisite condition for sedimentation
also. Subsidence is a necessary condition for deposits of great thickness, such as those
of the Great Valley of California and the Indo-Gangetic plain (p. 101), the Mississippi
delta (p. 109), and the still more important formations of the continental delta in
gradually sinking troughs (p. 183). It is not too much to say that the character and
thickness of each formation of the stratified rocks depend primarily on these crustal
movements.
Along the Baltic coast of Sweden, bench marks show that the sea is withdrawing from the land at a rate which at
the north amounts to between three and four feet per century; Towards the south the rate decreases. South of
Stockholm, until recent years, the sea has gained upon the land, and here in several seaboard towns streets by the
shore are still submerged. The rate of oscillation increases also from the coast inland. On the other hand, along the
German coast of the Baltic the only historic fluctuations of sea level are those which may be accounted for by
variations due to changes in rainfall. In 1730 Celsius explained the changes of level of the Swedish coast as due to a
lowering of the Baltic instead of to an elevation of the land. Are the facts just stated consistent with his theory?

Fig. 165. Old Strand Lines, Tadousac, Quebec
At the little town of Tadousac—where the Saguenay River empties into the St. Lawrence—there are terraces of
old sea beaches, some almost as fresh as recent railway fills, the highest standing two hundred and thirty feet above
the river (Fig. 165). Here the Saguenay is eight hundred and forty feet in depth, and the tide ebbs and flows far up its
stream. Was its channel cut to this depth by the river when the land was at its present height? What oscillations are
here recorded, and to what amount?

A few miles north of Naples, Italy, the ruins of an ancient Roman temple
lie by the edge of the sea, on a narrow plain which is overlooked in the rear
by an old sea cliff (Fig. 166). Three marble pillars are still standing. For
eleven feet above their bases these columns are uninjured, for to this height
they were protected by an accumulation of volcanic ashes; but from eleven
to nineteen feet they are closely pitted with the holes of boring marine
mollusks. From these facts trace the history of the oscillations of the region.
Fig. 166. Diagram showing
Ruins of Temple, North of
Naples
C, ancient sea cliff; m, marble pillars,
dotted where bored by mollusks; sl,
sea level

Fig. 167. Section in a Region of Folded Rocks

Foldings of the Crust
The oscillations which we have just described leave the strata not far from their
original horizontal attitude. Figure 167 represents a region in which movements of a
very different nature have taken place. Here, on either side of the valley v, we find
outcrops of layers tilted at high angles. Sections along the ridge r show that it is
composed of layers which slant inward from either side. In places the outcropping strata
stand nearly on edge, and on the right of the valley they are quite overturned; a shale sh
has come to overlie a limestone lm although the shale is the older rock, whose original
position was beneath the limestone.

It is not reasonable to suppose that these rocks were deposited in
the attitude in which we find them now; we must believe that, like
other stratified rocks, they were outspread in nearly level sheets
upon the ocean floor. Since that time they must have been deformed.
Layers of solid rock several miles in thickness have been crumpled
Fig. 168. Dip and folded like soft wax in the hand, and a vast denudation has worn
and Strike away the upper portions of the folds, in part represented in our
section by dotted lines.

Dip and strike. In districts where the strata have been disturbed it is desirable to
record their attitude. This is most easily done by taking the angle at which the strata are
inclined and the compass direction in which they slant. It is also convenient to record
the direction in which the outcrop of the strata trends across the country.

The inclination of a bed of rocks to the horizon is
its dip (Fig. 168). The amount of the dip is the angle
made with a horizontal plane. The dip of a horizontal
layer is zero, and that of a vertical layer is 90°. The
direction of the dip is taken with the compass. Thus a
geologist’s notebook in describing the attitude of
outcropping strata contains many such entries as
these: dip 32° north, or dip 8° south 20° west,—
meaning in the latter case that the amount of the dip is
8° and the direction of the dip bears 20° west of
south. Fig. 169. An Anticline,
Maryland
The line of intersection of a layer with the
horizontal plane is the strike. The strike always runs at right angles to the dip.
Dip and strike may be illustrated by a book set aslant on a shelf. The dip is the acute angle made with the shelf by
the side of the book, while the strike is represented by a line running along the book’s upper edge. If the dip is north
or south, the strike runs east and west.

Fig. 170. Folded Strata, Coast of England
A syncline in the center, with an anticline on either side

Folded structures. An upfold, in which the strata dip away from a line drawn along
the crest and called the axis of the fold, is known as an anticline (Fig. 169). A downfold,
where the strata dip from either side toward the axis of the trough, is called a syncline
(Fig. 170). There is sometimes seen a downward bend in horizontal or gently inclined
strata, by which they descend to a lower level. Such a single flexure is a monocline
(Fig. 171).

Degrees of folding. Folds vary in degree from broad,
low swells, which can hardly be detected, to the most
highly contorted and complicated structures. In
symmetric folds (Figs. 169 and 180) the dips of the rocks
on each side the axis of the fold are equal. In
unsymmetrical folds one limb is steeper than the other,
as in the anticline in Figure 167. In overturned folds
Fig. 171. A Monocline (Figs. 167 and 172) one limb is inclined beyond the
perpendicular. Fan folds have been so pinched that the
original anticlines are left broader at the top than at the bottom (Fig. 173).

Fig. 172. Overturned Fold, Vermont
In folds where the compression has been great the layers are often found thickened at the crest and thinned along
the limbs (174). Where strong rocks such as heavy limestones are folded together with weak rocks such as shales, the
strong rocks are often bent into great simple folds, while the weak rocks are minutely crumpled.

Systems of folds. As a rule, folds occur in
systems. Over the Appalachian mountain belt, for
example, extending from northeastern Pennsylvania
to northern Alabama and Georgia, the earth’s crust
has been thrown into a series of parallel folds whose
axes run from northeast to southwest (Fig. 175). In
Pennsylvania one may count a score or more of Fig. 173. Fan Folds, the Alps
these earth waves,— some but from ten to twenty
miles in length, and some extending as much as two hundred miles before they die
away. On the eastern part of this belt the folds are steeper and more numerous than on
the western side.

Cause and conditions of folding. The sections which we have
studied suggest that rocks are folded by lateral pressure. While a
single, simple fold might be produced by a heave, a series of folds,
including overturns, fan folds, and folds thickened on their crests at
the expense of their limbs, could only be made in one way,—by
Fig. 174. pressure from the side. Experiment has reproduced all forms of folds
Folds with by subjecting to lateral thrust layers of plastic material such as wax.
Layers
thickened at Vast as the force must have been which could fold the solid rocks
the Crest and of the crust as one may crumple the leaves of a magazine in the
thinned along fingers, it is only under certain conditions that it could have produced
the Limbs the results which we see. Rocks are brittle, and it is only when under a
heavy load and by great pressure slowly applied, that they can thus be
folded and bent instead of being crushed to pieces. Under these conditions, experiments
prove that not only metals such as steel, but also brittle rocks such as marble, can be
deformed and molded and made to flow like plastic clay.

Fig. 175. Relief Map of the Northern Appalachian Region
From Bingham’s Geographic Influences in American History

Zone of flow, zone of flow and fracture, and zone of fracture. We may believe
that at depths which must be reckoned in tens of thousands of feet the load of overlying
rocks is so great that rocks of all kinds yield by folding to lateral pressure, and flow
instead of breaking. Indeed, at such profound depths and under such inconceivable
weight no cavity can form, and any fractures would be healed at once by the welding of
grain to grain. At less depths there exists a zone where soft rocks fold and flow under
stress, and hard rocks are fractured; while at and near the surface hard and soft rocks
alike yield by fracture to strong pressure.

Structures developed in Compressed Rocks
Deformed rocks show the effects of the stresses to which they have yielded, not only
in the immense folds into which they have been thrown but in their smallest parts as
well. A hand specimen of slate, or even a particle under the microscope, may show
plications similar in form and origin to the foldings which have produced ranges of
mountains. A tiny flake of mica in the rocks of the Alps may be puckered by the same
resistless forces which have folded miles of solid rock to form that lofty range.

Slaty cleavage. Rocks which have yielded to pressure often split easily in a certain
direction across the bedding planes. This cleavage is known as slaty cleavage, since it is
most perfectly developed in fine-grained, homogeneous rocks, such as slates, which
cleave to the thin, smooth-surfaced plates with which we are familiar in the slates used

in roofing and for ciphering and blackboards. In coarse-grained rocks, pressure develops
more distant partings which separate the rocks into blocks.

Slaty cleavage cannot be due to lamination, since it commonly crosses bedding
planes at an angle, while these planes have been often well-nigh or quite obliterated.
Examining slate with a microscope, we find that its cleavage is due to the grain of the
rock. Its particles are flattened and lie with their broad faces in parallel planes, along
which the rock naturally splits more easily than in any other direction. The irregular
grains of the mud which has been altered to slate have been squeezed flat by a pressure
exerted at right angles to the plane of cleavage. Cleavage is found only in folded rocks,
and, as we may see in Figure 176, the strike of the cleavage runs parallel to the strike of
the strata and the axis of the folds. The dip of the cleavage is generally steep, hence the
pressure was nearly horizontal. The pressure which has acted at right angles to the
cleavage, and to which it is due, is the same lateral pressure which has thrown the strata
into folds.
We find additional proof that slates have undergone
compression at right angles to their cleavage in the fact that any
inclusions in them, such as nodules and fossils, have been squeezed
out of shape and have their long diameters lying in the planes of
cleavage.

That pressure is competent to cause cleavage is shown by
experiment. Homogeneous material of fine grain, such as beeswax,
when subjected to heavy pressure cleaves at right angles to the
direction of the compressing force.
Fig. 176. Slaty Cleavage
Rate of folding. All the facts known with
regard to rock deformation agree that it is a secular process, taking place so slowly that,
like the deepening of valleys by erosion, it escapes the notice of the inhabitants of the
region. It is only under stresses slowly applied that rocks bend without breaking. The
folds of some of the highest mountains have risen so gradually that strong, well-
intrenched rivers which had the right of way across the region were able to hold to their
courses, and as a circular saw cuts its way through the log which is steadily driven
against it, so these rivers sawed their gorges through the fold as fast as it rose beneath
them. Streams which thus maintain the course which they had antecedent to a
deformation of the region are known as antecedent streams. Examples of such are the
Sutlej and other rivers of India, whose valleys trench the outer ranges of the Himalayas
and whose earlier river deposits have been upturned by the rising ridges. On the other
hand, mountain crests are usually divides, parting the head waters of different drainage
systems. In these cases the original streams of the region have been broken or destroyed
by the uplift of the mountain mass across their paths.
On the whole, which have worked more rapidly, processes of deformation or of denudation?

Land Forms due to Folding

As folding goes on so slowly, it is never left to form
surface features unmodified by the action of other
agencies. An anticlinal fold is attacked by erosion as
soon as it begins to rise above the original level, and the
higher it is uplifted, and the stronger are its slopes, the
faster is it worn away. Even while rising, a young
upfold is often thus unroofed, and instead of appearing
as a long, Smooth, boat-shaped ridge, it commonly has
had opened along the rocks of the axis, when these are
weak, a valley which is overlooked by the infacing
escarpments of the hard layers of the sides of the fold Fig. 177. An Unroofed
Anticline
(Fig. 177). Under long-continued erosion, anticlines
may be degraded to valleys, while the synclines of the same system may be left in relief
as ridges (Fig. 167).

Folded mountains. The vastness of the forces which wrinkle the crust is best
realized in the presence of some lofty mountain range. All mountains, indeed, are not the
result of folding. Some, as we shall see, are due to upwarps or to fractures of the crust;
some are piles of volcanic material; some are swellings caused by the intrusion of
molten matter beneath the surface; some are the relicts left after the long denudation of
high plateaus.

Fig. 178. Mountain Peaks carved in Folded
Strata, Rocky Mountains, Montana

But most of the mountain ranges of the earth, and some of the greatest, such as the
Alps and the Himalayas, were originally mountains of folding. The earth’s crust has
wrinkled into a fold; or into a series of folds, forming a series of parallel ridges and
intervening valleys; or a number of folds have been mashed together into a vast
upswelling of the crust, in which the layers have been so crumpled and twisted,
overturned and crushed, that it is exceedingly difficult to make out the original structure.

The close and intricate folds seen in great mountain ranges were formed, as we have
seen, deep below the surface, within the zone of folding. Hence they may never have
found expression in any individual surface features. As the result of these deformations
deep under ground the surface was broadly lifted to mountain height, and the crumpled
and twisted mountain structures are now to be seen only because erosion has swept
away the heavy cover of surface rocks under whose load they were developed.

Fig. 179. Section of a Portion of the Alps
When the structure of mountains has been deciphered it is possible to estimate roughly the amount of horizontal
compression which the region has suffered. If the strata of the folds of the Alps were smoothed out, they would
occupy a belt seventy-four miles wider than that to which they have been compressed, or twice their present width. A
section across the Appalachian folds in Pennsylvania shows a compression to about two thirds the original width; the
belt has been shortened thirty-five miles in every hundred.

Considering the thickness of their strata, the compression which mountains have undergone accounts fully for
their height, with enough to spare for all that has been lost by denudation.

The Appalachian folds involve strata thirty thousand feet in thickness. Assuming that the folded strata rested on an
unyielding foundation, and that what was lost in width was gained in height, what elevation would the range have
reached had not denudation worn it as it rose?

The life history of mountains. While the disturbance and uplift of mountain masses
are due to deformation, their sculpture into ridges and peaks, valleys and deep ravines,
and all the forms which meet the eye in mountain scenery, excepting in the very
youngest ranges, is due solely to erosion. We may therefore classify mountains
according to the degree to which they have been dissected. The Juras are an example of
the stage of early youth, in which the anticlines still persist as ridges and the synclines
coincide with the valleys; this they owe as much to the slight height of their uplift as to
the recency of its date (Fig. 180).

Fig. 180. Section of a Portion of the Jura
Mountains

The Alps were upheaved at various times (p. 399), the last uplift being later than the
uplift of the Juras, but to so much greater height that erosion has already advanced them

well on towards maturity. The mountain mass has been cut to the core, revealing strange
contortions of strata which could never have found expression at the surface. Sharp
peaks, knife-edged crests, deep valleys with ungraded slopes subject to frequent
landslides, are all features of Alpine scenery typical of a mountain range at this stage in
its life history. They represent the survival of the hardest rocks and the strongest
structures, and the destruction of the weaker in their long struggle for existence against
the agents of erosion. Although miles of rock have been removed from such ranges as
the Alps, we need not suppose that they ever stood much, if any, higher than at present.
All this vast denudation may easily have been accomplished while their slow upheaval
was going on; in several mountain ranges we have evidence that elevation has not yet
ceased.

Fig. 181. Young Mountains, Rocky Mountains of Canada

Under long denudation mountains are subdued to the forms characteristic of old age.
The lofty peaks and jagged crests of their earlier life are smoothed down to low domes
and rounded crests. The southern Appalachians and portions of the Hartz Mountains in
Germany (Fig. 182) are examples of mountains which have reached this stage.

Fig. 182. Subdued Mountains, the Hartz
Mountains, Germany

There are numerous regions of upland and plains in which the rocks are found to
have the same structure that we have seen in folded mountains; they are tilted,
crumpled, and overturned, and have clearly suffered intense compression. We may infer
that their folds were once lifted to the height of mountains and have since been wasted
to low-lying lands. Such a section as that of Figure 67 illustrates how ancient mountains
may be leveled to their roots, and represents the final stage to which even the Alps and
the Himalayas must sometime arrive. Mountains, perhaps of Alpine height, once stood
about Lake Superior; a lofty range once extended from New England and New Jersey
southwestward to Georgia along the Piedmont belt. In our study of historic geology we
shall see more clearly how short is the life of mountains as the earth counts time, and
how great ranges have been lifted, worn away, and again upheaved into a new cycle of
erosion.

The sedimentary history of folded mountains. We may mention here some of the
conditions which have commonly been antecedent to great foldings of the crust.

1. Mountain ranges are made of belts of enormously and exceptionally thick
sediments. The strata of the Appalachians are thirty thousand feet thick, while the same
formations thin out to five thousand feet in the Mississippi valley. The folds of the
Wasatch Mountains involve strata thirty thousand feet thick, which thin to two thousand
feet in the region of the Plains.

2. The sedimentary strata of which mountains are made are for the most part the
shallow-water deposits of continental deltas. Mountain ranges have been upfolded along
the margins of continents.

3. Shallow-water deposits of the immense thickness found in mountain ranges can
be laid only in a gradually sinking area. A profound subsidence, often to be reckoned in
tens of thousands of feet, precedes the upfolding of a mountain range.

Thus the history of mountains of folding is as follows: For long ages the sea bottom
off the coast of a continent slowly subsides, and the great trough, as fast as it forms, is
filled with sediments, which at last come to be many thousands of feet thick. The
downward movement finally ceases. A slow but resistless pressure sets in, and gradually,
and with a long series of many intermittent movements, the vast mass of accumulated
sediments is crumpled and uplifted into a mountain range.

Fractures and Dislocations of the Crust
Considering the immense stresses to which the rocks of the crust are subjected, it is
not surprising to find that they often yield by fracture, like brittle bodies, instead of by
folding and flowing, like plastic solids. Whether rocks bend or break depends on the
character and condition of the rocks, the load of overlying rocks which they bear, and
the amount of the force and the slowness with which it is applied.

Joints. At the surface, where their load is least, we find rocks universally broken
into blocks of greater or less size by partings known as joints. Under this name are
included many division planes caused by cooling and drying; but it is now generally
believed that the larger and more regular joints, especially those which run parallel to
the dip and strike of the strata, are fractures due to up-and-down movements and
foldings and twistings of the rocks.

Fig. 183. Joints utilized by a River in widening
its Valley, Iowa

Joints are used to great advantage in quarrying, and we have seen how they are
utilized by the weather in breaking up rock masses, by rivers in widening their valleys,

by the sea in driving back its cliffs, by glaciers in plucking their beds, and how they are
enlarged in soluble rocks to form natural passageways for underground waters. The ends
of the parted strata match along both sides of joint planes; in. joints there has been little
or no displacement of the broken rocks.

Faults. In Figure 184 the rocks have been both broken and
dislocated along the plane ff´. One side must have been moved up
or down past the other. Such a dislocation is called a fault. The
amount of the displacement, as measured by the vertical distance
between the ends of a parted layer, is the throw (cd). The angle (ff
´v) which the fault plane makes with the vertical is the hade. In
Figure 184 the right side has gone down relatively to the left; the
right is the side of the downthrow, while the left is the side of the Fig. 184. A Normal
upthrow. Where the fault plane is not vertical the surfaces on the Fault
two sides may be distinguished as the hanging wall (that on the
right of Figure 184) and the foot wall (that on the left of the same figure). Faults differ in
throw from a fraction of an inch to many thousands of feet.
Slickensides. If we examine the walls of a fault, we may find further evidence of movement in the fact that the
surfaces are polished and grooved by the enormous friction which they have suffered as they have ground one upon
the other. These appearances, called slickensides, have sometimes been mistaken for the results of glacial action.

Normal faults. Faults are of two kinds,—normal faults and thrust faults. Normal
faults, of which Figure 184 is an example, hade to the downthrow; the hanging wall has
gone down. The total length of the strata has been increased by the displacement. It
seems that the strata have been stretched and broken, and that the blocks have readjusted
themselves under the action of gravity as they settled.

Thrust faults. Thrust faults hade to the upthrow; the hanging wall has gone up.
Clearly such faults, where the strata occupy less space than before, are due to lateral
thrust. Folds and thrust faults are closely associated. Under lateral pressure strata may
fold to a certain point and then tear apart and fault along the surface of least resistance.
Under immense pressure strata also break by shear without folding. Thus, in Figure 185,
the rigid earth block under lateral thrust has found it easier to break along the fault plane
than to fold. Where such faults are nearly horizontal they are distinguished as thrust
planes.

In all thrust faults one mass has been pushed over
another, so as to bring the underlying and older strata
upon younger beds; and when the fault planes are
nearly horizontal, and especially when the rocks have
been broken into many slices which have slidden far
Fig. 185. A Thrust Fault one upon another, the true succession of strata is
extremely hard to decipher.

In the Selkirk Mountains of Canada the basement rocks of the region have been driven east for seven miles on a
thrust plane, over rocks which originally lay thousands of feet above them.

Along the western Appalachians, from Virginia to Georgia, the mountain folds are broken by more than fifteen
parallel thrust planes, running from northeast to southwest, along which the older strata have been pushed westward
over the younger. The longest continuous fault has been traced three hundred and seventy-five miles, and the greatest
horizontal displacement has been estimated at not less than eleven miles.

Crush breccia. Rocks often do not fault with a clean and simple fracture, but along
a zone, sometimes several yards in width, in which they are broken to fragments. It may
occur also that strata which as a whole yield to lateral thrust by folding include beds of
brittle rocks, such as thin-layered limestones, which are crushed to pieces by the strain.
In either case the fragments when recemented by percolating waters form a rock known
as a crush breccia (pronounced bretcha)(Fig. 186).

Breccia is a term applied to any rock formed of
cemented angular fragments. This rock may be
made by the consolidation of volcanic cinders, of
angular waste at the foot of cliffs, or of fragments
of coral torn by the waves from coral reefs, as well
as of strata crushed by crustal movements.

Surface Features due to
Dislocations
Fault scarps. A fault of recent date may be
marked at surface by a scarp, because the face of
the upthrown block has not yet been worn to the
level of the downthrow side. Fig. 186. Breccia
After the upthrown block has been worn down to this level, differential erosion
produces fault scarps wherever weak rocks and resistant rocks are brought in contact
along the fault plane; and the harder rocks, whether on the upthrow or the downthrow
side, emerge in a line of cliffs. Where a fault is so old that no abrupt scarps appear, its
general course is sometimes marked by the line of division between highland and
lowland or hill and plain. Great faults have sometimes brought ancient crystalline rocks
in contact with weaker and younger sedimentary rocks, and long after erosion has
destroyed all fault scarps the harder crystallines rise in an upland of rugged or
mountainous country which meets the lowland along the line of faulting.

The vast majority of faults give rise to no surface features. The faulted region may
be old enough to have been baseleveled, or the rocks on both sides of the line of
dislocation may be alike in their resistance to erosion and therefore have been worn
down to a common slope. The fault may be entirely concealed by the mantle of waste,

and in such cases it can be inferred from abrupt changes in
the character or the strike and dip of the strata where they
may outcrop near it (Fig. 187).

Fig. 187. A Concealed
Fault Fig. 188. East-West Section across the Broken
This fault may be inferred from the
Plateau north of the Grand Canyon of the
changes in strata in passing along
Colorado River, Arizona
he strike, as from b to a´ and from
c to b´
The plateau trenched by the Grand Canyon of the Colorado River exhibits a series of magnificent fault scarps
whose general course is from north to south, marking the edges of the great crust blocks into which the country has
been broken. The highest part of the plateau is a crust block ninety miles long and thirty-five miles in maximum
width, which has been hoisted to nine thousand three hundred feet above, sea level. On the east it descends four
thousand feet by a monoclinal fold, which passes into a fault towards the north. On the west it breaks down by a
succession of terraces faced by fault scarps. The throw of these faults varies from seven hundred feet to more than a
mile. The escarpments, however, are due in a large degree to the erosion of weaker rock on the downthrow side.

The Highlands of Scotland (Fig. 189) meet the Lowlands on the south with a bold
front of rugged hills along a line of dislocation which runs across the country from sea
to sea. On the one side are hills of ancient crystalline rocks whose crumpled structures
prove that they are but the roots of once lofty mountains; on the other lies a lowland of
sandstone and other stratified rocks formed from the waste of those long-vanished
mountain ranges. Remnants of sandstone occur in places on the north of the great fault,
and are here seen to rest on the worn and fairly even surface of the crystallines. We may
infer that these ancient mountains were reduced along their margins to low plains, which Fig. 189. The Fault
were slowly lowered beneath the sea to receive a cover of sedimentary rocks. Still later separating the
came an uplift and dislocation. On the one side erosion has since stripped off the
sandstones for the most part, but the hard crystalline rocks yet stand in bold relief. On Highlands and the
the other side the weak sedimentary rocks have been worn down to lowlands. Lowlands,
Scotland
Rift valleys. In a broken region undergoing uplift or the
unequal settling which may follow, a slice inclosed between two fissures may sink
below the level of the crust blocks on either side, thus forming a linear depression
known as a rift valley, or valley of fracture.
One of the most striking examples of this rare type of valley is
the long trough which runs straight from the Lebanon Mountains
of Syria on the north to the Red Sea on the south, and whose
central portion is occupied by the Jordan valley and the Dead Sea.
The plateau which it gashes has been lifted more than three
thousand feet above sea level, and the bottom of the trough
reaches a depth of two thousand six hundred feet below that level
Fig. 190. Section from the in parts of the Dead Sea. South of the Dead Sea the floor of the
Mountains of Palestine to the trough rises somewhat above sea level, and in the Gulf of Akabah
Mountains of Moab across the again sinks below it. This uneven floor could be accounted for
either by the profound warping of a valley of erosion or by the
Dead Sea

a, ancient schists; b, Carboniferous strata; unequal depression of the floor of a rift valley. But that the trough
is a true valley of fracture is proved by the fact that on either side
c, d, and e, Cretaceous strata it is bounded by fault scarps and monoclinal folds. The keystone
of the arch has subsided. Many geologists believe that the Jordan-
Akabah trough, the long narrow basin of the Red Sea, and the chain of down-faulted valleys which in Africa extends
from the strait of Bab-el-Mandeb as far south as Lake Nyassa—valleys which contain more than thirty lakes—belong
to a single system of dislocation.

Should you expect the lateral valleys of a rift valley at the time of its formation to enter it as hanging valleys or at
a common level?

Block mountains. Dislocations take place on so grand a scale that by the upheaval
of blocks of the earth’s crust or the down- faulting of the blocks about one which is
relatively stationary, mountains known as block mountains are produced. A tilted crust
block may present a steep slope on the side upheaved and a more gentle descent on the
side depressed.
The Basin ranges. The plateaus of the United States bounded by
the Rocky Mountains on the east, and on the west by the ranges
which front the Pacific, have been profoundly fractured and faulted.
The system of great fissures by which they are broken extends north
and south, and the long, narrow, tilted crust blocks intercepted
between the fissures give rise to the numerous north-south ranges of
the region. Some of the tilted blocks, as those of southern Oregon,
Fig. 191. Block Mountains, are as yet but moderately carved by erosion, and shallow lakes lie on
Southern Oregon the waste that has been washed into the depressions between them
(Fig. 191). We may therefore conclude that their displacement is
somewhat recent. Others, as those of Nevada, are so old that they
have been deeply dissected; their original form has been destroyed by erosion, and the intermontane depressions are
occupied by wide plains of waste.

Dislocations and river valleys. Before geologists had proved that rivers can by their
own unaided efforts cut deep canyons, it was common to consider any narrow gorge as a
gaping fissure of the crust. This crude view has long since been set aside. A map of the
plateaus of northern Arizona shows how independent of the immense faults of the region
is the course of the Colorado River. In the Alps the tunnels on the Saint Gotthard railway
pass six times beneath the gorge of the Reuss, but at no point do the rocks show the
slightest trace of a fault.

Fig. 192. Fault crossing Valley in Japan

Rate of dislocation. So far as human experience goes, the earth movements which
we have just studied, some of which have produced deep-sunk valleys and lofty
mountain ranges, and faults whose throw is to be measured in thousands of feet, are
slow and gradual. They are not accomplished by a single paroxysmal effort, but by slow
creep and a series of slight slips continued for vast lengths of time.
In the Aspen mining district in Colorado faulting is now going on at a comparatively rapid rate. Although no
sudden slips take place, the creep of the rock along certain planes of faulting gradually bends out of shape the square-
set timbers in horizontal drifts and has closed some vertical shafts by shifting the upper portion across the lower.
Along one of the faults of this region it is estimated that there has been a movement of at least four hundred feet since
the Glacial epoch. More conspicuous are the instances of active faulting by means of sudden slips. In 1891 there
occurred along an old fault plane in Japan a slip which produced an earth rent traced for fifty miles (Fig. 192). The
country on one side was depressed in places twenty feet below that on the other, and also shifted as much as thirteen
feet horizontally in the direction of the fault line.

In 1872 a slip occurred for forty miles on the great line of dislocation which runs along the eastern base of the
Sierra Nevada Mountains. In the Owens valley, California, the throw amounted to twenty-five feet in places, with a
horizontal movement along the fault line of as much as eighteen feet. Both this slip and that in Japan just mentioned
caused severe earthquakes.

For the sake of clearness we have described oscillations, foldings, and fractures of
the crust as separate processes, each giving rise to its own peculiar surface features, but
in nature earth movements are by no means so simple,—they are often implicated with
one another: folds pass into faults; in a deformed region certain rocks have bent, while
others under the same strain, but under different conditions of plasticity and load, have
broken; folded mountains have been worn to their roots, and the peneplains to which
they have been denuded have been upwarped to mountain height and afterwards
dissected,—as in the case of the Allegheny ridges, the southern Carpathians, and other
ranges, —or, as in the case of the Sierra Nevada Mountains, have been broken and
uplifted as mountains of fracture.
Draw the following diagrams, being careful to show the direction in which the faulted blocks have moved, by the
position of the two parts of some well-defined layer of limestone, sandstone, or shale, which occurs on each side of

the fault plane, as in Figure 184.

1. A normal fault with a hade of 15°, the original fault scarp remaining.

2. A normal fault with a hade of 50°, the original fault scarp worn away, showing cliffs caused by
harder strata on the downthrow side.

3. A thrust fault with a hade of 30°, showing cliffs due to harder strata outcropping on the
downthrow. Fig.
4. A thrust fault with a hade of 80°, with surface baseleveled. 193
5. In a region of normal faults a coal mine is being worked along the seam of coal AB (Fig. 193). At B it is found
broken by a fault f which hades toward A. To find the seam again, should you advise tunneling up or down from B?

6. In a vertical shaft of a coal mine the same bed of coal is pierced twice at different levels because of a fault.
Draw a diagram to show whether the fault is normal or a thrust.

Fig. 194. Ridges to be
explained by Faulting
7. Copy the diagram in Figure 194, showing how the two ridges may be accounted for by a single resistant
stratum dislocated by a fault. Is the fault a strike fault, i.e. one running parallel with the strike of the strata, or a dip
fault, one running parallel with the direction of the dip?

Fig. 195. Earth
Block of Tilted
Strata, with
Included Seam
of Coal cc
8. Draw a diagram of the block in Figure 195 as it would appear if dislocated along the plane efg by a normal fault
whose throw equals one fourth the height of the block. Is the fault a strike or a dip fault? Draw a second diagram
showing the same block after denudation has worn it down below the center of the upthrown side. Note that the
outcrop of the coal seam is now deceptively repeated. This exercise may be done in blocks of wood instead of
drawings.

Fig. 196. A and B. Repeated Outcrops of
Same Strata
9. Draw diagrams showing by dotted lines the conditions both of A and B, Figure 196, after deformation had given
the strata their present attitude.

Fig. 197. A Block Mountain
10. What is the attitude of the strata of this earth block, Figure 197? What has taken place along the plane baf?
When did the dislocation occur compared with the folding of the strata? With the erosion of the valleys on the right-
hand side of the mountain? With the deposition of the sediments efg? Do you find any remnants of the original surface
baf produced by the dislocation? From the left-hand side of the mountain infer what was the relief of the region before
the dislocation. Give the complete history recorded in the diagram from the deposition of the strata to the present.

Fig. 199. Measurement
Fig. 198. A Faulted
of the Thickness of
Lava Flow aa´
Inclined Strata
11. Which is the older fault, in Figure 198, f or f´? When did the lava flow occur? How long a time elapsed
between the formation of the two faults as measured in the work done in the interval? How long a time since the
formation of the later fault?

12. Measure by the scale the thickness bc of the coal-bearing strata outcropping from a to b in Figure 199. On any
convenient scale draw a similar section of strata with a dip of 30° outcropping along a horizontal line normal to the
strike one thousand feet in length, and measure the thickness of the strata by the scale employed. The thickness may
also be calculated by trigonometry.

Unconformity
Strata deposited
one upon, another in
Fig. 201. Unconformity Fig. 200. Unconformity
an unbroken
between Non-parallel Strata between Parallel Strata
succession are said to
be conformable. But the continuous deposition of strata
is often interrupted by movements of the earth’s crust, Old sea floors are lifted to form
land and are again depressed beneath the sea to receive a cover of sediments only after
an interval during which they were carved by subaërial erosion. An erosion surface
which thus parts older from younger strata is known as an unconformity, and the strata
above it are said to be unconformable with the rocks below, or to rest unconformably
upon them. An unconformity thus records movements of the crust and a consequent
break in the deposition of the strata. It denotes a period of land erosion of greater or less
length, which may sometimes be roughly measured by the stage in the erosion cycle
which the land surface had attained before its burial. Unconformable strata may be
parallel, as in Figure 200, where the record includes the deposition of strata a, their
emergence, the erosion of the land surface ss, a submergence and the deposit of the
strata b, and lastly, emergence and the erosion of the present surface s´s´.

Often the earth movements to which the uplift or
depression was due involved tilting or folding of the
earlier strata, so that the strata are now nonparallel as
well as unconformable. In Figure 201, for example,
the record includes deposition, uplift, and tilting of a;
erosion, depression, the deposit of b; and finally the
uplift which has brought the rocks to open air and
permitted the dissection by which the unconformity is
revealed.
From this section we infer that during early Silurian times the area
was sea, and thick sea muds were laid upon it. These were later altered Fig. 202. Carboniferous
to hard slates by pressure and upfolded into mountains. During the later
Silurian and the Devonian the area was land and suffered vast Limestone resting
denudation. In the Carboniferous period it was lowered beneath the sea unconformably on Early
and received a cover of limestone.
Silurian Slates, Yorkshire,
England
The age of mountains.
It is largely by means of unconformities that we read the
history of mountain making and other deformations and

Fig. 203. Diagram movements of the crust. In Figure 203, for example, the
Illustrating how the deformation which upfolded the range of mountains took place
Age of Mountains is after the deposit of the series of strata a of which the
determined mountains are composed, and before the deposit of the
stratified rocks b, which rest unconformably on a
and have not shared their uplift.

Most great mountain ranges, like the Sierra
Nevada and the Alps, mark lines of weakness along
which the earth’s crust has yielded again and again
during the long ages of geological time. The strata
Fig. 204. Section of Mountain deposited at various times about their flanks have
Range showing repeated Uplifts been infolded by later crumplings with the original
, strata whose folding formed a mountain mountain mass, and have been repeatedly crushed,
ange; uu, baseleveled surface produced by
ong denudation of the mountains; b, tilted inverted, faulted, intruded with igneous rocks, and
trata resting unconformably on a; c, denuded. The structure of great mountain ranges
orizontal strata parted from b by the thus becomes exceedingly complex and difficult to
nconformity u´u´. The first uplift of the
ange preceded the period of time when b read. A comparatively simple case of repeated uplift
was deposited. The second uplift, to which is shown in Figure 204. In the section of a portion
he present mountains owe their height, was of the Alps shown in Figure 179 a far more
ater than this period but earlier than the
eriod when strata c were laid complicated history may be deciphered.

Fig. 205. Unconformity showing Buried Valleys
lm, limestone; sh, shale; r, r´, and r´´, river silts filling eroded valleys in the
limestone. The upper surface of the limestone is evidently a land surface developed
by erosion. The valleys which trench it are narrow and steep-sided; hence the land
surface had not reached maturity. The sands and muds, now hardened to firm rock,
which fill these valleys, r, r´, and r´´, contain no relics of the sea, but Instead the
remains of land animals and plants. They are river deposits, and we may infer that
owing to a subsidence the young rivers ceased to degrade their channels and
slowly filled their gorges with sands and silts. The overlying shale records a
further depression which brought the lanes below the level of the sea. A section
similar to this is to be seen in the coal mines of Bernissant, Belgium, where a
gorge twice as deep as that of Niagara was discovered within whose ancient river
deposits were found entombed the skeletons of more than a score of the huge
reptiles characteristic of the age when the gorge was cut and filled

Fig. 206. Unconformity showing Buried Mountains, Scotland

gn, ancient crystalline rocks; ss, marine sandstones. The surface bb of the ancient
crystalline rocks is mountainous, with peaks rising to a height of as much as three
thousand feet. It is one of the most ancient land surfaces on the planet and is
covered unconformably with pre-Cambrian sandstones thousands of feet in
thickness, in which the Torridonian Mountains of Scotland have been carved. What
has been the history of the region since the mountainous surface bb was produced
by erosion?

Unconformities in the Colorado Canyon, Arizona. How geological history may be read in unconformities is
further illustrated in Figures 207 and 208. The dark crystalline rocks a at the bottom of the canyon are among the
most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones b, whose total
thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip. Both a and b rise to
a common level nn´ and upon them rest the horizontal sea-laid strata c, in which the upper portion of the canyon has
been cut.

Note that the crystalline rocks a have been crumpled and crushed. Comparing their structure with that of folded
mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down,
mm´ or nm? Describe and account for the surface mm´. How does it differ from the surface of the crystalline rocks
seen in the Torridonian Mountains (Fig. 206), and why? This surface mm´ is one of the oldest land surfaces of which
any vestige remains. It is a bit of fossil geography buried from view since the earliest geological ages and recently
brought to light by the erosion of the canyon.

Fig. 208. View of the North Wall of
Fig. 207. Diagram of Wall of the
the Grand Canyon of the Colorado
Colorado
River, Arizona, showing the
Canyon, Arizona, showing
Unconformities illustrated in
Unconformities
Figure 207
How did the surface mm´ come to receive its cover of sandstones b? From the thickness and coarseness of these
sediments draw inferences as to the land mass from which they were derived. Was it rising or subsiding? high or low?
Were its streams slow or swift? Was the amount of erosion small or great?

Note the strong dip of these sandstones b. Was the surface mm´ tilted as now when the sandstones were deposited
upon it? When was it tilted? Draw a diagram showing the attitude of the rocks after this tilting occurred, and their
height relative to sea level.

The surface nn´ is remarkably even, although diversified by some low hills which rise into the bedded rocks of c,
and it may be traced for long distances up and down the canyon. Were the layers of b and the surface mm´ always thus
cut short by nn´ as now? What has made the surface nn´ so even? How does it come to cross the hard crystalline rocks
a and the weaker sandstones b at the same impartial level? How did the sediments of c come to be laid upon it? Give
now the entire history recorded in the section, and in addition that involved in the production of the platform P, shown

in Figure 130, and that of the cutting of the canyon. How does the time involved in the cutting of the canyon compare
with that required for the production of the surfaces mm´, nn´, and P?

Fig. 209. Unconformity between
the Cambrian and Pre-
Cambrian Rocks, Wisconsin
a, pre-Cambrian rocks, igneous and
metamorphic, greatly deformed; a´, zone of
decomposed pre-Cambrian rocks and residual
clays on which rest the Cambrian sandstones b.
What unconformity do you find here? What
two peneplains do you discover? Which is the
older? Which was the more complete? To what
stage has the present erosion cycle advanced?
Suggest a reason why the valleys in the
Cambrian are wider than those in the pre-
Cambrian. When did the decay of the pre-
Cambrian rocks of zone a´ take place, and
under what circumstances? Give the entire
history recorded in this section, stating the
successive cycles of erosion in their order and
the causes which brought each cycle to a close

CHAPTER X

EARTHQUAKES
Any sudden movement of the rocks of the crust, as when they tear apart when a
fissure is formed or extended, or slip from time to time along a growing fault, produces
a jar called an earthquake, which spreads in all directions from the place of disturbance.
The Charleston earthquake. On the evening of August 31, 1886, the city of Charleston, S.C., was shaken by one
of the greatest earthquakes which has occurred in the United States. A slight tremor which rattled the windows was
followed a few seconds later by a roar, as of subterranean thunder, as the main shock passed beneath the city. Houses
swayed to and fro, and their heaving floors overturned furniture and threw persons off their feet as, dizzy and
nauseated, they rushed to the doors for safety. In sixty seconds a number of houses were completely wrecked,
fourteen thousand chimneys were toppled over, and in all the city scarcely a building was left without serious injury.
In the vicinity of Charleston railways were twisted and trains derailed. Fissures opened in the loose superficial
deposits, and in places spouted water mingled with sand from shallow underlying aquifers.

The point of origin, or focus, of the earthquake was inferred from subsequent investigations to be a rent in the
rocks about twelve miles beneath the surface. From the center of greatest disturbance, which lay above the focus, a
few miles northwest of the city, the surface shock traveled outward in every direction, with decreasing effects, at the
rate of nearly two hundred miles per minute. It was felt from Boston to Cuba, and from eastern Iowa to the Bermudas,
over a circular area whose diameter was a thousand miles.

An earthquake is transmitted from the focus through the elastic rocks of the crust, as
a wave, or series of waves, of compression and rarefaction, much as a sound wave is
transmitted through the elastic medium of the air. Each earth particle vibrates with
exceeding swiftness, but over a very short path. The swing of a particle in firm rock
seldom exceeds one tenth of an inch in ordinary earthquakes, and when it reaches one
half an inch and an inch, the movement becomes dangerous and destructive.

Fig. 210. Block of the Earth’s Crust shaken by
an Earthquake
x, focus; a, b, c, d, successive spheroidal waves in the crust; a´, b
´, c´, d´, successive surface waves produced by the outcropping
of a, b, c, and d

The velocity of earthquake waves, like that of all elastic waves, varies with the
temperature and elasticity of the medium. In the deep, hot, elastic rocks they speed
faster than in the cold and broken rocks near the surface. The deeper the point of origin
and the more violent the initial shock, the faster and farther do the vibrations run.

Great earthquakes, caused by some sudden displacement or some violent rending of
the rocks, shake the entire planet. Their waves run through the body of the earth at the
rate of about three hundred and fifty miles a minute, and more slowly round its
circumference, registering their arrival at opposite sides of the globe on the exceedingly
delicate instruments of modern earthquake observatories.

Geological effects. Even great earthquakes seldom produce geological effects of
much importance. Landslides may be shaken down from the sides of mountains and
hills, and cracks may be opened in the surface deposits of plains; but the transient shiver,
which may overturn cities and destroy thousands of human lives, runs through the crust
and leaves it much the same as before.
The India earthquake of 1897. No earthquake of history has produced greater geological effects than that which
shook northeastern India in 1897. It laid in ruins a region thrice the size of the state of New York. In places not a
masonry building was left standing and hard-wood trees were snapped across. Foothills of the Himalayas were
stripped of soil and forests from base to summit by landslides. Streams which before were busily cutting down their
rocky beds were now overloaded with waste from the slides. They were compelled to cease eroding their beds while
they spread their valleys deep with sand, over which they now flow in broad and shallow channels. The incoherent
deposits of the alluvial plains were riddled with rents through which ground water was forced out in such quantities as
to flood considerable areas.

Certain other effects often attributed to the earthquake are rather the manifestations
of the dislocation to which the earthquake was due. Permanent changes of level were
effected. Some hills were found to have been lifted twenty feet, while others were
lowered, and resurveys proved that the entire region had been compressed horizontally
from north to south. Displacements occurred along several fault lines. One of these, with
a throw of twenty-five feet at the surface and a length of twelve miles, crossed a river
repeatedly, causing a series of waterfalls and lakes. All these disturbances are best
explained by the theory that the shock was due to a slip along a deep and hidden thrust
plane, accompanied by other movements of the strata along minor faults connected with
it, some of which reached the surface.

Earthquakes attending great displacements. Great earthquakes frequently attend
the displacement of large masses of the rocks of the crust. In 1822 the coast of Chile was
suddenly raised three or four feet, and the rise was five or six feet a mile inland. In 1835
the same region was again upheaved from two to ten feet. In each instance a destructive
earthquake was felt for one thousand miles along the coast.
Perhaps the most violent earthquake which ever visited the United States attended the depression, in 1812, of a
region seventy-five miles long and thirty miles wide, near New Madrid, Mo. Much of the area was converted into
swamps and some into shallow lakes, while a region twenty miles in diameter was bulged up athwart the channel of
the Mississippi. Slight quakes are still felt in this region from time to time, showing that the strains to which the
dislocation was due have not yet been fully relieved.

Earthquakes originating beneath the sea. Many earthquakes originate beneath the
sea, and in a number of examples they seem to have been accompanied, as soundings
indicate, by local subsidences of the ocean bottom. There have been instances where the
displacement has been sufficient to set the entire Pacific Ocean pulsating for many
hours. In mid ocean the wave thus produced has a height of only a few feet, while it may
be two hundred miles in width. On shores near the point of origin destructive waves two
or three score feet in height roll in, and on coasts thousands of miles distant the expiring
undulations may be still able to record themselves on tidal gauges.

Distribution of earthquakes. Every half hour some considerable area of the earth’s
surface is sensibly shaken by an earthquake, but earthquakes are by no means uniformly
distributed over the globe. As we might infer from what we know as to their causes,
earthquakes are most frequent in regions now undergoing deformation. Such are young
rising mountain ranges, fault lines where readjustments recur from time to time, and the
slopes of suboceanic depressions whose steepness suggests that subsidence may there be
in progress.
Earthquakes, often of extreme severity, frequently visit the lofty and young ranges of the Andes, while they are
little known in the subdued old mountains of Brazil. The Highlands of Scotland are crossed by a deep and singularly
straight depression called the Great Glen, which has been excavated along a very ancient line of dislocation. The
earthquakes which occur from time to time in this region, such as the Inverness earthquake in 1891, are referred to
slight slips along this fault plane.

In Japan, earthquakes are very frequent. More than a thousand are recorded every year, and twenty-nine world-
shaking earthquakes occurred in the three years ending with 1901. They originate, for the most part, well down on the
eastern flank of the earth fold whose summit is the mountainous crest of the islands, and which plunges steeply
beneath the sea to the abyss of the Tuscarora Deep.

Minor causes of earthquakes. Since any concussion within the crust sets up an
earth jar, there are several minor causes of earthquakes, such as volcanic explosions and
even the collapse of the roofs of caves. The earthquakes which attend the eruption of
volcanoes are local, even in the case of the most violent volcanic paroxysms known.
When the top of a volcano has been blown to fragments, the accompanying earth shock
has sometimes not been felt more than twenty-five miles away.

Depth of focus. The focus of the Charleston earthquake, estimated at about twelve
miles below the surface, was exceptionally deep. Volcanic earthquakes are particularly
shallow, and probably no earthquakes known have started at a greater depth than fifteen
or twenty miles. This distance is so slight compared with the earth’s radius that we may
say that earthquakes are but skin-deep.
Should you expect the velocity of an earthquake to be greater in a peneplain or in a river delta?

After an earthquake, piles on which buildings rested were found driven into the ground, and chimneys crushed at
base. From what direction did the shock come?

Chimneys standing on the south walls of houses toppled over on the roof. Should you infer that the shock in this
case came from the north or south?

How should you expect a shock from the east to affect pictures hanging on the east and the west walls of a room?
how the pictures hanging on the north and the south walls?

In parts of the country, as in southwestern Wisconsin, slender erosion pillars, or “monuments,” are common. What
inference could you draw as to the occurrence in such regions of severe earthquakes in the recent past?

CHAPTER XI

VOLCANOES
Connected with movements of the earth’s crust which take place so slowly that they
can be inferred only from their effects is one of the most rapid and impressive of all
geological processes,—the extrusion of molten rock from beneath the surface of the
earth, giving rise to all the various phenomena of volcanoes.

In a volcano, molten rock from a region deep below, which we may call its reservoir,
ascends through a pipe or fissure to the surface. The materials erupted may be spread

over vast areas, or, as is commonly the case, may accumulate about the opening,
forming a conical pile known as the volcanic cone. It is to this cone that popular usage
refers the word volcano; but the cone is simply a conspicuous part of the volcanic
mechanism whose still more important parts, the reservoir and the pipe, are hidden from
view.

Volcanic eruptions are of two types,—effusive eruptions, in which molten rock wells
up from below and flows forth in streams of lava (a comprehensive term applied to all
kinds of rock emitted from volcanoes in a molten state), and explosive eruptions, in
which the rock is blown out in fragments great and small by the expansive force of
steam.

Eruptions of the Effusive Type
The Hawaiian volcanoes. The Hawaiian Islands are all volcanic in origin, and have
a linear arrangement characteristic of many volcanic groups in all parts of the world.
They are strung along a northwest-southeast line, their volcanoes standing in two
parallel rows as if reared along two adjacent lines of fracture or folding. In the
northwestern islands the volcanoes have long been extinct and are worn low by erosion.
In the southeastern island, Hawaii, three volcanoes are still active and in process of
building. Of these Mauna Loa, the monarch of volcanoes, with a girth of two hundred
miles and a height of nearly fourteen thousand feet above sea level, is a lava dome the
slope of whose sides does not average more than five degrees. On the summit is an
elliptical basin ten miles in circumference and several hundred feet deep. Concentric
cracks surround the rim, and from time to time the basin is enlarged as great slices are
detached from the vertical walls and engulfed. Such a volcanic basin, formed by the
insinking of the top of the cone, is called a caldera.

Fig. 211. Mauna Loa Fig. 212. Caldera of Kilauea

On the flanks of Mauna Loa, four thousand feet above sea level, lies the caldera of Kilauea, an independent
volcano whose dome has been joined to the larger mountain by the gradual growth of the two. In each caldera the
floor, which to the eye is a plain of black lava, is the congealed surface of a column of molten rock. At times of an

eruption lakes of boiling lava appear which may be compared to air holes in a frozen river. Great waves surge up,
lifting tons of the fiery liquid a score of feet in air, to fall back with a mighty plunge and roar, and occasionally the
lava rises several hundred feet in fountains of dazzling brightness. The lava lakes may flood the floor of the basin, but
in historic times have never been known to fill it and overflow the rim. Instead, the heavy column of lava breaks way
through the sides of the mountain and discharges in streams which flow down the mountain slopes for a distance
sometimes of as much as thirty-five miles. With the drawing off of the lava the column in the duct of the volcano
lowers, and the floor of the caldera wholly or in part subsides. A black and steaming abyss marks the place of the lava
lakes (Fig. 213). After a time the lava rises in the duct, the floor is floated higher, and the boiling lakes reappear.

Fig. 213. A Portion of the Caldera of Kilauea after a
Collapse following an Eruption

The eruptions of the Hawaiian volcanoes are thus of the effusive type. The column
of lava rises, breaks through the side of the mountain, and discharges in lava streams.
There are no explosions, and usually no earthquakes, or very slight ones, accompany the
eruptions. The lava in the calderas boils because of escaping steam, but the vapor
emitted is comparatively little, and seldom hangs above the summits in heavy clouds.
We see here in its simplest form the most impressive and important fact in all volcanic
action, molten rock has been driven upward to the surface from some deep-lying source.

Fig. 214. Pahoehoe Lava, Hawaii

Lava flows. As lava issues from the side of a volcano or overflows from the summit,
it flows away in a glowing stream resembling molten iron drawn white-hot from an iron
furnace. The surface of the stream soon cools and blackens, and the hard crust of
nonconducting rock may grow thick and firm enough to form a tunnel, within which the
fluid lava may flow far before it loses its heat to any marked degree. Such tunnels may
at last be left as caves by the draining away of the lava, and are sometimes several miles
in length.
Pahoehoe and aa. When the crust of highly fluid lava remains unbroken after its first freezing, it presents a
smooth, hummocky, and ropy surface known by the Hawaiian term pahoehoe (Fig. 214). On the other hand, the crust
of a viscid flow may be broken and splintered as it is dragged along by the slowly moving mass beneath. The stream
then appears as a field of stones clanking and grinding on, with here and there from some chink a dull red glow or a
wisp of steam. It sets to a surface called aa, of broken, sharp-edged blocks, which is often both difficult and
dangerous to traverse (Fig. 215).

Fig. 215. Lava Flow of the Aa Type; Cinder
Cones in the Distance, Arizona

Fissure eruptions. Some of the largest and most important outflows of lava have
not been connected with volcanic cones, but have been discharged from fissures,
flooding the country far and wide with molten rock. Sheet after sheet of molten rock has
been successively outpoured, and there have been built up, layer upon layer, plateaus of
lava thousands of feet in thickness and many thousands of square miles in area.
Iceland. This island plateau has been rent from time to time by fissures from which floods of lava have
outpoured. In some instances the lava discharges along the whole length of the fissure, but more often only at certain
points upon it. The Laki fissure, twenty miles long, was in eruption in 1783 for seven months. The inundation of fluid
rock which poured from it is the largest of historic record, reaching a distance of forty-seven miles and covering two
hundred and twenty square miles to an average depth of a hundred feet. At the present time the fissure is traced by a
line of several hundred insignificant mounds of fragmental materials which mark where the lava issued (Fig. 216).

The distance to which the fissure eruptions of Iceland flow on slopes extremely gentle is noteworthy. One such
stream is ninety miles in length, and another seventy miles long has a slope of little more than one half a degree.

Where lava is emitted at one point and flows to a less
distance there is gradually built up a dome of the shape of an
inverted saucer with an immense base but comparatively low.
Many lava domes have been discovered in Iceland, although
from their exceedingly gentle slopes, often but two or three
degrees, they long escaped the notice of explorers.

The entire plateau of Iceland, a region as large as Ohio, is
composed of volcanic products,—for the most part of
successive sheets of lava whose total thickness falls little short
of two miles. The lava sheets exposed to view were outpoured
in open air and not beneath the sea; for peat bogs and old
forest grounds are interbedded with them, and the fossil plants
of these vegetable deposits prove that the plateau has long
been building and is very ancient. On the steep sea cliffs of the
island, where its structure is exhibited, the sheets of lava are
seen to be cut with many dikes,—fissures which have been
filled by molten rock,—and there is little doubt that it was
through these fissures that the lava outwelled in successive
flows which spread far and wide over the country and
gradually reared the enormous pile of the plateau. Fig. 216. Small Cinder Cones marking
an Eruptive Fissure, Iceland

Fig. 217. Diagram
llustrating the Structure of a
Lava Plateau such as Iceland
lf, lava flows; d, dikes

Eruptions of the Explosive Type
In the majority of volcanoes the lava which rises in the pipe is at least in part blown
into fragments with violent explosions and shot into the air together with vast quantities
of water vapor and various gases. The finer particles into—which the lava is exploded
are called volcanic dust or volcanic ashes, and are often carried long distances by the
wind before they settle to the earth. The coarser fragments fall about the vent and there
accumulate in a steep, conical, volcanic mountain. As successive explosions keep open
the throat of the pipe, there remains on the summit a cup-shaped depression called the
crater.
Stromboli. To study the nature of these explosions we may visit Stromboli, a low volcano built chiefly of
fragmental materials, which rises from the sea off the north coast of Sicily and is in constant though moderate action.

Over the summit hangs a cloud of vapor which strikingly resembles the column of smoke puffed from the
smokestack of a locomotive, in that it consists of globular masses, each the product of a distinct explosion. At night
the cloud of vapor is lighted with a red glow at intervals of a few minutes, like the glow on the trail of smoke behind
the locomotive when from time to time the fire box is opened. Because of this intermittent light flashing thousands of
feet above the sea, Stromboli has been given the name of the Lighthouse of the Mediterranean.

Looking down into the crater of the volcano, one sees a viscid lava slowly seething. The agitation gradually
increases. A great bubble forms. It bursts with an explosion which causes the walls of the crater to quiver with a
miniature earthquake, and an outrush of steam carries the fragments of the bubble aloft for a thousand feet to fall into
the crater or on the mountain side about it. With the explosion the cooled and darkened crust of the lava is removed,
and the light of the incandescent liquid beneath is reflected from the cloud of vapor which overhangs the cone.

At Stromboli we learn the lesson that the explosive force in volcanoes is that of
steam. The lava in the pipe is permeated with it much as is a thick boiling porridge. The
steam in boiling porridge is unable to escape freely and gathers into bubbles which in
breaking spurt out drops of the pasty substance; in the same way the explosion of great
bubbles of steam in the viscid lava shoots clots and fragments of it into the air.
Krakatoa. The most violent eruption of history, that of Krakatoa, a small volcanic island in the strait between
Sumatra and Java, occurred in the last week of August, 1883. Continuous explosions shot a column of steam and
ashes. seventeen miles in air. A black cloud, beneath which was midnight darkness and from which fell a rain of ashes
and stones, overspread the surrounding region to a distance of one hundred and fifty miles. Launched on the currents
of the upper air, the dust was swiftly carried westward to long distances. Three days after the eruption it fell on the
deck of a ship sixteen hundred miles away, and in thirteen days the finest impalpable powder from the volcano had
floated round the globe. For many months the dust hung over Europe and America as a faint lofty haze illuminated at
sunrise and sunset with brilliant crimson. In countries nearer the eruption, as in India and Africa, the haze for some
time was so thick that it colored sun and moon with blue, green, and copper-red tints and encircled them with coronas.

At a distance of even a thousand miles the detonations of the eruption sounded like the booming of heavy guns a
few miles away. In one direction they were audible for a distance as great as that from San Francisco to Cleveland.
The entire atmosphere was thrown into undulations under which all barometers rose and fell as the air waves thrice
encircled the earth. The shock of the explosions raised sea waves which swept round the adjacent shores at a height of
more than fifty feet, and which were perceptible halfway around the globe.

At the close of the eruption it was found that half the mountain had been blown away, and that where the central
part of the island had been the sea was a thousand feet deep.

Fig. 218. Ruins of St. Pierre, Martinique; Mt.
Pelée in the Distance

Martinique and St. Vincent. In 1902 two dormant volcanoes of the West Indies, Mt. Pelee in Martinique and
Soufriére in St. Vincent, broke into eruption simultaneously. No lava was emitted, but there were blown into the air
great quantities of ashes, which mantled the adjacent parts of the islands with a pall as of gray snow. In early stages of
the eruption lakes which occupied old craters were discharged and swept down the ash-covered mountain valleys in
torrents of boiling mud.

On several occasions there was shot from the crater of each volcano a thick and heavy cloud of incandescent ashes
and steam, which rushed down the mountain side like an avalanche, red with glowing stones and scintillating with
lightning flashes. Forests and buildings in its path were leveled as by a tornado, wood was charred and set on fire by
the incandescent fragments, all vegetation was destroyed, and to breathe the steam and hot, suffocating dust of the
cloud was death to every living creature. On the morning of the 8th of May, 1902, the first of these peculiar
avalanches from Mt. Pelee fell on the city of St. Pierre and instantly destroyed the lives of its thirty thousand
inhabitants.

Fig. 219. An Eruption of Vesuvius, 1872
The huge column of dust and steam rises to a height of about four
miles above the sea. Drifting down the wind, the vapor condenses
into copious rains. Such often produce destructive torrents of mud
as they sweep down the ash-covered mountain side, and during
the historic eruption of Vesuvius in A.D. 69 the city of
Herculaneum was thus buried. Lava flows are marked by the
overhanging clouds of aqueous vapor condensed from the steam
which the molten rock gives off.

The eruptions of many volcanoes partake of both the effusive and the explosive
types: the molten rock in the pipe is in part blown into the air with explosions of steam,
and in part is discharged in streams of lava over the lip of the crater and from fissures in
the sides of the cone. Such are the eruptions of Vesuvius, one of which is illustrated in
Figure 219.

Submarine eruptions. The many volcanic islands of the ocean and the coral islands
resting on submerged volcanic peaks prove that eruptions have often taken place upon
the ocean floor and have there built up enormous piles of volcanic fragments and lava.
The Hawaiian volcanoes rise from a depth of eighteen thousand feet of water and lift
their heads to about thirty thousand feet above the ocean bed. Christmas Island (see
p. 194), built wholly beneath the ocean, is a coral-capped volcanic peak, whose total
height, as measured from the bottom of the sea, is more than fifteen thousand feet.
Deep-sea soundings have revealed the presence of numerous peaks which fail to reach
sea level and which no doubt are submarine volcanoes. A number of volcanoes on the
land were submarine in their early stages, as, for example, the vast pile of Etna, the
celebrated Sicilian volcano, which rests on stratified volcanic fragments containing
marine shells now uplifted from the sea.

Submarine outflows of lava and deposits of volcanic fragments become covered
with sediments during the long intervals between eruptions. Such volcanic deposits are
said to be contemporaneous, because they are formed during the same period as the
strata among which they are imbedded. Contemporaneous lava sheets may be expected
to bake the surface of the stratum on which they rest, while the sediments deposited
upon them are unaltered by their heat. They are among the most permanent records of
volcanic action, far outlasting the greatest volcanic mountains built in open air.

From upraised submarine volcanoes, such as Christmas Island, it is learned that lava
flows which are poured out upon the bottom of the sea do not differ materially either in
composition or texture from those of the land.

Volcanic Products
Vast amounts of steam are, as we have seen, emitted from volcanoes, and
comparatively small quantities of other vapors, such as various acid and sulphurous
gases. The rocks erupted from volcanoes differ widely in chemical composition and in
texture.

Fig. 220. Cellular Lava

Acidic and basic lavas. Two classes of volcanic rocks may be distinguished,—those
containing a large proportion of silica (silicic acid, SiO2) and therefore called acidic,
and those containing less silica and a larger proportion of the bases (lime, magnesia,
soda, etc.) and therefore called basic. The acidic lavas, of which rhyolite and thrachyte
are examples, are comparatively light in color and weight, and are difficult to melt. The
basic lavas, of which basalt is a type, are dark and heavy and melt at a lower
temperature.

Scoria and pumice. The texture of volcanic rocks depends in part on the degree to
which they were distended by the steam which permeated them when in a molten state.
They harden into compact rock where the steam cannot expand. Where the steam is
released from pressure, as on the surface of a lava stream, it forms bubbles (steam blebs)
of various sizes, which give the hardened rock a cellular structure (Fig. 220), In this way
are formed the rough slags and clinkers called scoria, which are found on the surface of
flows and which are also thrown out as clots of lava in explosive eruptions.

On the surface of the seething lava in the throat of the volcano there gathers a rock
foam, which, when hurled into the air, is cooled and falls as pumice,—a spongy gray
rock so light that it floats on water.

Fig. 221. Amygdules in Lava

Amygdules. The steam blebs of lava flows are often drawn out from a spherical to
an elliptical form resembling that of an almond, and after the rock has cooled these
cavities are gradually filled with minerals deposited from solution by underground
water. From their shape such casts are called amygdules (Greek, amygdalon, an
almond). Amygdules are commonly composed of silica. Lavas contain both silica and
the alkalies, potash and soda, and after dissolving the alkalies, percolating water is able
to take silica also into solution. Most agates are banded amygdules in which the silica
has been laid in varicolored, concentric layers (Fig. 222).

Glassy and stony lavas. Volcanic rocks differ
in texture according also to the rate at which they
have solidified. When rapidly cooled, as on the
surface of a lava flow, molten rock chills to a
glass, because the minerals of which it is
composed have not had time to separate
themselves from the fused mixture and form
crystals. Under slow cooling, as in the interior of
the flow, it becomes a stony mass composed of
crystals set in a glassy paste. In thin slices of
volcanic glass one may see under the microscope Fig. 222. Polished Section of an
the beginnings of crystal growth in filaments and Agate
needles and feathery forms, which are the
rudiments of the crystals of various minerals.
Spherulites, which also mark the first changes of glassy lavas toward a stony condition, are little balls within the
rock, varying from microscopic size to several inches in diameter, and made up of radiating fibers.

Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to
contraction.

Fig. 223.
Microsection
Fig. 224. Perlitic
showing the
Structure and
Beginnings of
Spherulites, a, a
Crystal Growth
in Glassy Lava

Flow lines are exhibited by volcanic rocks both to the naked eye and under the
microscope. Steam blebs, together with crystals and their embryonic forms, are left
arranged in lines and streaks by the currents of the flowing lava as it stiffened into rock.

Fig. 225. Flow Lines in Lava

Porphyritic structure. Rocks whose ground mass has scattered through it large
conspicuous crystals(Fig. 226) are said to be porphyritic, and it is especially among
volcanic rocks that this structure occurs. The ground mass of porphyries either may be
glassy or may consist in part of a felt of minute crystals; in either case it represents the
consolidation of the rock after its outpouring upon the surface. On the other hand, the
large crystals of porphyry have slowly formed deep below the ground at an earlier date.

Fig. 226. Porphyritic Structure

Columnar structure. Just as wet starch contracts on drying to prismatic forms, so
lava often contracts on cooling to a mass of close-set, prismatic, and commonly six-
sided columns, which stand at right angles to the cooling surface. The upper portion of a
flow, on rapid cooling from the surface exposed to the air, may contract to a confused
mass of small and irregular prisms; while the remainder forms large and beautifully
regular columns, which have grown upward by slow cooling from beneath (Fig. 227).

Fragmental Materials
Rocks weighing many tons are often thrown from a volcano at the beginning of an
outburst by the breaking up of the solidified floor of the crater; and during the progress
of an eruption large blocks may be torn from the throat of the volcano by the outrush of
steam. But the most important fragmental materials are those derived from the lava
itself. As lava rises in the pipe, the steam which permeates it is released from pressure
and explodes, hurling the lava into the air in fragments of all sizes,—large pieces of
scoria, lapilli (fragments the size of a pea or walnut), volcanic “sand” and volcanic
“ashes.” The latter resemble in appearance the ashes of wood or coal, but they are not in
any sense, like them, a residue after combustion.

Fig. 227. Columnar Structure in Basaltic Lava, Scotland

Volcanic ashes are produced in several ways: lava rising in the volcanic duct is
exploded into fine dust by the steam which permeates it; glassy lava, hurled into the air
and cooled suddenly, is brought into a state of high strain and tension, and, like Prince
Rupert’s drops, flies to pieces at the least provocation. The clash of rising and falling
projectiles also produces some dust, a fair sample of which may be made by grating
together two pieces of pumice.
Beds of volcanic ash occur widely among recent deposits in the western United States. In Nebraska ash beds are
found in twenty counties, and are often as white as powdered pumice. The beds grow thicker and coarser toward the
southwestern part of the state, where their thickness sometimes reaches fifty feet. In what direction would you look
for the now extinct volcano whose explosive eruptions are thus recorded?

Tuff. This is a convenient term designating any rock composed of volcanic
fragments. Coarse tuffs of angular fragments are called volcanic breccia, and when the
fragments have been rounded and sorted by water the rock is termed a volcanic
conglomerate. Even when deposited in the open air, as on the slopes of a volcano, tuffs
may be rudely bedded and their fragments more or less rounded, and unless marine
shells or the remains of land plants and animals are found as fossils in them, there is
often considerable difficulty in telling whether they were laid in water or in air. In either
case they soon become consolidated. Chemical deposits from percolating waters fill the
interstices, and the bed of loose fragments is cemented to hard rock.

The materials of which tuffs are composed are easily recognized as volcanic in their
origin. The fragments are more or less cellular, according to the degree to which they
were distended with steam when in a molten state, and even in the finest dust one may
see the glass or the crystals of lava from which it was derived. Tuffs often contain
volcanic bombs,—balls of lava which took shape while whirling in the air, and solidified
before falling to the ground.

Ancient volcanic rocks. It is in these materials
and structures which we have described that
volcanoes leave some of their most enduring records.
Even the volcanic rocks of the earliest geological
ages, uplifted after long burial beneath the sea and
exposed to view by deep erosion, are recognized and
their history read despite the many changes which
they may have undergone. A sheet of ancient lava
may be distinguished by its composition from the
sediments among which it is imbedded. The direction
Fig. 228. Volcanic Bombs, of its flow lines may be noted. The cellular and
Cinder Cone, California slaggy surface where the pasty lava was distended by
escaping steam is recognized by the amygdules
which now fill the ancient steam blebs. In a pile of successive sheets of lava each flow
may be distinguished and its thickness measured; for the surface of each sheet is glassy
and scoriaceous, while beneath its upper portions the lava of each flow is more dense
and stony. The length of time which elapsed before a sheet was buried beneath the
materials of succeeding eruptions may be told by the amount of weathering which it had
undergone, the depth of ancient soil—now baked to solid rock—upon it, and the erosion
which it had suffered in the interval.

If the flow occurred from some submarine volcano, we may recognize the fact by
the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface
and containing pieces of lava washed from it in their basal layers.

Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing
action of underground waters; but their flow lines and perlitic and spherulitic structures
remain to tell of their original state.

Ancient tuffs are known by the fragmental
character of their volcanic material, even though they
have been altered to firm rock. Some remains of land
animals and plants may be found imbedded to tell that
the beds were laid in open air; while the remains of
marine organisms would prove as surely that the tuffs
were deposited in the sea.

In these ways ancient volcanoes have been Fig. 229. A Volcanic Cone,
recognized near Boston, in southeastern Pennsylvania, Arizona
about Lake Superior, and in other regions of the
United States.

The Life History of a Volcano
The invasion of a region by volcanic forces is attended by movements of the crust
heralded by earthquakes. A fissure or a pipe is opened and the building of the cone or
the spreading of wide lava sheets is begun.

Volcanic cones. The shape of a volcanic cone depends chiefly on the materials
erupted. Cones made of fragments may have sides as steep as the angle of repose, which
in the case of coarse scoria is sometimes as high as thirty or forty degrees. About the
base of the mountain the finer materials erupted are spread in more gentle slopes, and
are also washed forward by rains and streams. The normal profile is thus a symmetric
cone with a flaring base.

Fig. 230. Sarcoui, a Trachyte Dome, France

Cones built of lava vary in form according to the liquidity of the lava. Domes of
gentle slope, as those of Hawaii, for example, are formed of basalt, which flows to long
distances before it congeals. When superheated and emitted from many vents, this easily
melted lava builds great plateaus, such as that of Iceland. On the other hand, lavas less
fusible, or poured out at a lower temperature, stiffen when they have flowed but a short
distance, and accumulate in a steep cone. Trachyte has been extruded in a state so viscid
that it has formed steepsided domes like that of Sarcoui (Fig. 230).

Most volcanoes are built, like Vesuvius, both of lava flows and of tuffs, and sections
show that the structure of the cone consists of outward-dipping, alternating layers of
lava, scoria, and ashes.

Fig. 231. Section of Vesuvius
V, Vesuvius; S, Somma, a mountainous rampart half encircling
Vesuvius, and like it built of outward-dipping sheets of tuff and
lava; a, crystalline rocks; b, marine strata; c, tuffs containing
seashells. Which is the older mountain, Vesuvius or Somma? Of
what is Somma a remnant? Draw a diagram showing its original
outline. Suggest what processes may have brought it to its
present form. What record do you find of the earliest volcanic
activity? What do you infer as to the beginnings of the volcano?

From time to time the cone is rent by the violence of explosions and by the weight of
the column of lava in the pipe. The fissures are filled with lava and some discharge on
the sides of the mountain, building parasitic cones, while all form dikes, which
strengthen the pile with ribs of hard rock and make it more difficult to rend.

Great catastrophes are recorded in the shape of
some volcanoes which consist of a circular rim
perhaps miles in diameter, inclosing a vast crater or a
caldera within which small cones may rise. We may
infer that at some time the top of the mountain has
Fig. 232. Crater Lake, Oregon
been blown off, or has collapsed and been engulfed
How wide and deep is the basin which
because some reservoir beneath had been emptied by holds the lake? The mountain walls which
long-continued eruptions (Fig. 232). enclose it are made of outward-dipping
sheets of lava. Draw a diagram restoring
the volcano of which they are the remnant.
The cone-building stage may be said to continue No volcanic fragments of the same nature
until eruptions of lava and fragmental materials as the materials of which the volcano is
cease altogether. Sooner or later the volcanic forces built are found about the region. What
theory of the destruction of the cone does
shift or die away, and no further eruptions add to the this fact favor? W´, Wizard Island, is a
pile or replace its losses by erosion during periods of cinder cone. When was it built?
repose. Gases however are still emitted, and, as
sulphur vapors are conspicuous among them, such vents are called solfataras. Mount
Hood, in Oregon, is an example of a volcano sunk to this stage. From a steaming rift on
its side there rise sulphurous fumes which, half a mile down the wind, will tarnish a
silver coin.

Geysers and hot springs. The hot springs of volcanic regions are among the last
vestiges of volcanic heat. Periodically eruptive boiling springs are termed geysers. In

each of the geyser regions of the earth—the
Yellowstone National Park, Iceland, and New Zealand
—the ground water of the locality is supposed to be
heated by ancient lavas that, because of the poor
conductivity of the rock, still remain hot beneath the
surface.
Old Faithful, one of the many geysers of the Yellowstone National
Park, plays a fountain of boiling water a hundred feet in air; while clouds
of vapor from the escaping steam ascend to several times that height. The
eruptions take place at intervals of from seventy to ninety minutes. In
repose the geyser is a quiet pool, occupying a craterlike depression in a
conical mound some twelve feet high. The conduit of the spring is too
irregular to be sounded. The mound is composed of porous silica
deposited by the waters of the geyser.

Geysers erupt at intervals instead of continuously
boiling, because their long, narrow, and often tortuous
Fig. 233. Old Faithful Geyser conduits do not permit a free circulation of the water.
in Eruption, Yellowstone After an eruption the tube is refilled and the water
National Park again gradually becomes heated. Deep in the tube
where it is in contact with hot lavas the water sooner or
later reaches the boiling point, and bursting into steam shoots the water above it high in
air.

Fig. 234. Terrace and Cones of Siliceous
Sinter deposited by Geysers, Yellowstone
National Park

Carbonated springs. After all the other signs of life have gone, the ancient volcano
may emit carbon dioxide as its dying breath. The springs of the region may long be
charged with carbon dioxide, or carbonated, and where they rise through limestone may
be expected to deposit large quantities of travertine. We should remember, however, that
many carbonated springs, and many hot springs, are wholly independent of volcanoes.

Fig. 235. Mount Shasta, California

Fig. 236. Mount Hood, Oregon

The destruction of the cone. As soon as the volcanic cone ceases to grow by
eruptions the agents of erosion begin to wear it down, and the length of time that has
elapsed since the period of active growth may be roughly measured by the degree to
which the cone has been dissected. We infer that Mount Shasta, whose conical shape is
still preserved despite the gullies one thousand feet deep which trench its sides
(Fig. 235), is younger than Mount Hood, which erosive agencies have carved to a
pyramidal form (Fig. 236). The pile of materials accumulated about a volcanic vent, no
matter how vast in bulk, is at last swept entirely away. The cone of the volcano, active or
extinct, is not old as the earth counts time; volcanoes are short- lived geological
phenomena.

Fig. 237. Crandall Volcano
Crandall volcano. This name is given to a dissected ancient volcano in the Yellowstone National Park, which
once, it is estimated, reared its head thousands of feet above the surrounding country and greatly exceeded in bulk
either Mount Shasta or Mount Etna. Not a line of the original mountain remains; all has been swept away by erosion
except some four thousand feet of the base of the pile. This basal wreck now appears as a rugged region about thirty
miles in diameter, trenched by deep valleys and cut into sharp peaks and precipitous ridges. In the center of the area is
found the nucleus (N, Fig. 237),—a mass of coarsely crystalline rock that congealed deep in the old volcanic pipe.
From it there radiate in all directions, like the spokes of a wheel, long dikes whose rock grows rapidly finer of grain as

it leaves the vicinity of the once heated core. The remainder of the base of
the ancient mountain is made of rudely bedded tuffs and volcanic breccia,
with occasional flows of lava, some of the fragments of the breccia
measuring as much as twenty feet in diameter. On the sides of canyons the
breccia is carved by rain erosion to fantastic pinnacles. At different levels
in the midst of these beds of tuff and lava are many old forest grounds. The
stumps and trunks of the trees, now turned to stone, still in many cases
stand upright where once they grew on the slopes of the mountain as it was
building (Fig. 238). The great size and age of some of these trees indicate,
the lapse of time between the eruption whose lavas or tuffs weathered to
the soil on which they grew and the subsequent eruption which buried
them beneath showers of stones and ashes.

Near the edge of the area lies Death Gulch, in which carbon dioxide is
given off in such quantities that in quiet weather it accumulates in a heavy
layer along the ground and suffocates the animals which may enter it.

CHAPTER XII Fig. 238. Fossil Tree Trunks,
Yellowstone National Park

UNDERGROUND STRUCTURES OF IGNEOUS
ORIGIN
It is because long-continued erosion lays bare the innermost anatomy of an extinct
volcano, and even sweeps away the entire pile with much of the underlying strata, thus
leaving the very roots of the volcano open to view, that we are able to study
underground volcanic structures. With these we include, for convenience, intrusions of
molten rock which have been driven upward into the crust, but which may not have
succeeded in breaking way to the surface and establishing a volcano. All these structures
are built of rock forced when in a fluid or pasty state into some cavity which it has
found or made, and we may classify them therefore, according to the shape of the molds
in which the molten rock has congealed, as (1) dikes, (2) volcanic necks, (3) intrusive
sheets, and (4) intrusive masses.

Dikes. The sheet of once molten rock with which a fissure has been filled is known
as a dike. Dikes are formed when volcanic cones are rent by explosions or by the weight
of the lava column in the duct, and on the dissection of the pile they appear as radiating
vertical ribs cutting across the layers of lava and tuff of which the cone is built. In
regions undergoing deformation rocks lying deep below the ground are often broken and
the fissures are filled with molten rock from beneath, which finds no outlet to the
surface. Such dikes are common in areas of the most ancient rocks, which have been
brought to light by long erosion.

In exceptional cases dikes may reach the length of fifty or one hundred miles. They
vary in width from a fraction of a foot to even as much as three hundred feet.

Fig. 239. Dikes, Spanish Peaks, Colorado
Dikes are commonly more fine of grain on the sides than in the center, and may have a glassy and crackled
surface where they meet the inclosing rock. Can you account for this on any principle which you have learned?

Fig. 240. A Dissected Volcanic Cone
N, volcanic neck; l, l, lava-topped table mountains; t, t, beds of tuff;
d, d, dikes; dotted lines indicate the initial profile

Volcanic necks. The pipe of a volcano rises from far below the base of the cone,—
from the deep reservoir from which its eruptions are supplied. When the volcano has
become extinct this great tube remains filled with hardened lava. It forms a cylindrical
core of solid rock, except for some distance below the ancient crater, where it may
contain a mass of fragments which had fallen back into the chimney after being hurled
into the air.

As the mountain is worn down, this central
column known as the volcanic neck is left standing
as a conical hill (Fig. 240). Even when every other
trace of the volcano has been swept away, erosion
will not have passed below this great stalk on
which the volcano was borne as a fiery flower
whose site it remains to mark. In volcanic regions
of deep denudation volcanic necks rise solitary and Fig. 241. Mount Johnson, a
abrupt from the surrounding country as dome- Volcanic Neck near Montreal
shaped hills. They are marked features in the
landscape in parts of Scotland and in the St. Lawrence valley about Montreal
(Fig. 241).

Fig. 242. The Palisades of the Hudson, New
Jersey

Intrusive sheets. Sheets of igneous rocks are sometimes found interleaved
with sedimentary strata, especially in regions where the rocks have been deformed
and have suffered from volcanic action. In some instances such a sheet is seen to
be contemporaneous (p. 248). In other instances the sheet must be intrusive. The
overlying stratum, as well as that beneath, has been affected by the heat of the once
molten rock. We infer that the igneous rock when in a molten state was forced
between the strata, much as a card may be pushed between the leaves of a closed
book. The liquid wedged its way between the layers, lifting those above to make
room for itself. The source of the intrusive sheet may often be traced to some dike
(known therefore as the feeding dike), or to some mass of igneous rock.

Intrusive sheets may extend a score and more of miles, and, like the longest
surface flows, the most extensive sheets consist of the more fusible and fluid lavas,
—those of the basic class of which basalt is an example. Intrusive sheets are
usually harder than the strata in which they lie and are therefore often left in relief
after long denudation of the region (Fig. 315).
On the west bank of the Hudson there extends from New
York Bay north for thirty miles a bold cliff several hundred feet
high,— the Palisades of the Hudson. It is the outcropping edge of
a sheet of ancient igneous rock, which rests on stratified
sandstones and is overlain by strata of the same series.
Sandstones and lava sheet together dip gently to the west and the
latter disappears from view two miles back from the river. Fig. 243. Diagram of the
It is an interesting question whether the Palisades sheet is
Palisades of the Hudson
contemporaneous or intrusive. Was it outpoured on the i, intrusive sheet; s, sandstone; d, feeding
sandstones beneath it when they formed the floor of the sea, and dike;
covered forthwith by the sediments of the strata above, or was it
HR, Hudson River
intruded among these beds at a later date?

The latter is the case: for the overlying stratum is
intensely baked along the zone of contact. At the west
edge of the sheet is found the dike in which the lava
rose to force its way far and wide between the strata.

Electric Peak, one of the prominent mountains of
the Yellowstone National Park, is carved out of a mass
of strata into which many sheets of molten rock have
Fig. 244. Section of Electric Peak. E. been intruded. The western summit consists of such a
sheet several hundred feet thick. Studying the section
and Gray Peak, G, of Figure 244, what inference do you draw as to the
Yellowstone National Park source of these intrusive sheets?
Intrusive sheets and masses of igneous rock are
drawn in black
Intrusive Masses
Bosses. This name is generally applied to
huge irregular masses of coarsely crystalline
igneous rock lying in the midst of other
formations. Bosses vary greatly in size and may
reach scores of miles in extent. Seldom are there
any evidences found that bosses ever had
connection with the surface. On the other hand, it
is often proved that they have been driven, or
have melted their way, upward into the formations
Fig. 245. Stone Mountain, in which they lie; for they give off dikes and
Georgia, a Granite Boss intrusive sheets, and have profoundly altered the
rocks about them by their heat.

The texture of the rock of bosses proves that
consolidation proceeded slowly and at great
depths, and it is only because of vast denudation
that they are now exposed to view. Bosses are
commonly harder than the rocks about them, and
stand up, therefore, as rounded hills and
mountainous ridges long after the surrounding
country has worn to a low plain (Fig. 245).

Figure 246 exhibits a few small bosses of
granite near Baltimore as examples of numerous
areas of igneous rock within the Piedmont Belt
which represent bodies of molten rock which
solidified deep below the surface.
The Spanish Peaks of southeastern Colorado were formed by
the upthrust of immense masses of igneous rock, bulging and Fig. 246. Map of Granite
breaking the overlying strata. On one side of the mountains the
throw of the fault is nearly a mile, and fragments of deep-lying Bosses near Baltimore (areas
beds were dragged upward by the rising masses. The adjacent horizontally Lined)
rocks were altered by heat to a distance of several thousand feet.
No evidence appears that the molten rock ever reached the surface, and if volcanic eruptions ever took place
either in lava flows or fragmental materials, all traces of them have been effaced. The rock of the intrusive
masses is coarsely crystalline, and no doubt solidified slowly under the pressure of vast thicknesses of
overlying rock, now mostly removed by erosion.

A magnificent system of dikes radiates from the Peaks to a distance of fifteen miles, some now being left
by long erosion as walls a hundred feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate the
surrounding strata, and their edges are cut by canyons as much as twenty-five miles from the mountain. In
these strata are valuable beds of lignite, an imperfect coal, which the heat of dikes and sheets has changed to
coke.

Laccoliths. The laccolith (Greek laccos,
cistern; lithos, stone) is a variety of intrusive
masses in which molten rock has spread between
the strata, and, lifting the strata above it to a
dome- shaped form, has collected beneath them
in a lens-shaped body with a flat base.
The Henry Mountains, a small group of detached peaks in
southern Utah, rise from a plateau of horizontal rocks. Some of Fig. 247. Section of a Laccolith
the peaks are carved wholly in separate domelike uplifts of the
strata of the plateau. In others, as Mount Hillers, the largest of the group, there is exposed on the summit a core
of igneous rock from which the sedimentary rocks of the flanks dip steeply outward in all directions. In still
others erosion has stripped off the covering strata and has laid bare the core to its base; and its shape is here
seen to be that of a plano-convex lens or a baker’s bun, its flat base resting on the undisturbed bedded rocks
beneath. The structure of Mount Hillers is shown in Figure 248. The nucleus of igneous rock is four miles in
diameter and more than a mile in depth.

Regional intrusions. These vast bodies
of igneous rock, which may reach hundreds
of miles in diameter, differ little from
bosses except in their immense bulk. Like
bosses, regional intrusions give off dikes
and sheets and greatly change the rocks
about them by their heat. They are now
Fig. 248. Section of Mount Hillers exposed to view only because of the
profound denudation which has removed
the upheaved dome of rocks beneath which they slowly cooled. Such intrusions are
accompanied —whether as cause or as effect is still hardly known—by
deformations, and their masses of igneous rock are thus found as the core of many
great mountain ranges. The granitic masses of which the Bitter Root Mountains
and the Sierra Nevadas have been largely carved are each more than three hundred
miles in length. Immense regional intrusions, the cores of once lofty mountain
ranges, are found upon the Laurentian peneplain.

Physiographic effects of intrusive masses. We have already seen examples of
the topographic effects of intrusive masses in Mount Hillers, the Spanish Peaks,
and in the great mountain ranges mentioned in the paragraph on regional
intrusions, although in the latter instances these effects are entangled with the
effects of other processes. Masses of igneous rock cannot be intruded within the
crust without an accompanying deformation on a scale corresponding to the bulk
of the intruded mass. The overlying strata are arched into hills or mountains, or, if
the molten material is of great extent, the strata may conceivably be floated
upward to the height of a plateau. We may suppose that the transference of molten
matter from one region to another may be among the causes of slow subsidences
and elevations. Intrusions give rise to fissures, dikes, and intrusive sheets, and
these dislocations cannot fail to produce earthquakes. Where intrusive masses open
communication with the surface, volcanoes are established or fissure eruptions
occur such as those of Iceland.

The Intrusive Rocks
The igneous rocks are divided into two general classes,—the volcanic or
eruptive rocks, which have been outpoured in open air or on the floor of the sea,
and the intrusive rocks, which have been intruded within the rocks of the crust and
have solidified below the surface. The two classes are alike in chemical
composition and may be divided into acidic and basic groups. In texture the
intrusive rocks differ from the volcanic rocks because of the different conditions

under which they have solidified. They cooled far more slowly beneath the cover
of the rocks into which they were pressed than is permitted to lava flows in open
air. Their constituent minerals had ample opportunity to sort themselves and
crystallize from the fluid mixture, and none of that mixture was left to congeal as a
glassy paste.

They consolidated also under pressure. They are never scoriaceous, for the
steam with which they were charged was not allowed to expand and distend them
with steam blebs. In the rocks of the larger intrusive masses one may see with a
powerful microscope exceedingly minute cavities, to be counted by many millions
to the cubic inch, in which the gaseous water which the mass contained was held
imprisoned under the immense pressure of the overlying rocks.

Naturally these characteristics are best developed in the intrusives which
cooled most slowly, i.e. in the deepest-seated and largest masses; while in those
which cooled more rapidly, as in dikes and sheets, we find gradations approaching
the texture of surface flows.

Varieties of the intrusive rocks. We will now describe a few of the varieties
of rocks of deep-seated intrusions. All are even grained, consisting of a mass of
crystalline grains formed during one continuous stage of solidification, and no
porphyritic crystals appear as in lavas.

Granite, as we have learned already, is composed of three minerals,—quartz,
feldspar, and mica. According to the color of the feldspar the rock may be red, or
pink, or gray. Hornblende—a black or dark green mineral, an iron-magnesian
silicate, about as hard as feldspar—is sometimes found as a fourth constituent, and
the rock is then known as hornblendic granite. Granite is an acidic rock
corresponding to rhyolite in chemical composition. We may believe that the same
molten mass which supplies this acidic lava in surface flows solidifies as granite
deep below ground in the volcanic reservoir.

Syenite, composed of feldspar and mica, has consolidated from a less siliceous
mixture than has granite.

Diorite, still less siliceous, is composed of hornblende and feldspar,—the latter
mineral being of different variety from the feldspar of granite and syenite.

Gabbro, a typical basic rock, corresponds to basalt in chemical composition. It
is a dark, heavy, coarsely crystalline aggregate of feldspar and augite (a dark

mineral allied to hornblende). It often contains magnetite (the magnetic black
oxide of iron) and olivine (a greenish magnesian silicate).

In the northern states all these types, and many others also of the vast number
of varieties of intrusive rocks, can be found among the rocks of the drift brought
from the areas of igneous rock in Canada and the states of our northern border.

Summary. The records
of geology prove that since
the earliest of their annals
tremendous forces have
been active in the earth. In
all the past, under
pressures inconceivably
great, molten rock has
been driven upward into Fig. 250. A and B.
the rocks of the crust. It Mountains of coarsely
has squeezed into fissures Crystalline Igneous i,
Fig. 249. Ground Plan of forming dikes; it has surrounded by
Dikes in Granite. (Scale burrowed among the strata Sedimentary Strata s
80 feet to the inch) as intrusive sheets; it has and s´
What is the relative age of the melted the rocks away or Copy each diagram and
dikes lifted the overlying strata, complete it, so as to show
aa, bb, and cc? filling the chambers which whether the mass of igneous
rock is a volcanic neck, a boss,
it has made with intrusive or a laccolith
masses. During all geological ages molten rock has
found way to the surface, and volcanoes have darkened the sky with clouds of
ashes and poured streams of glowing lava down their sides. The older strata,—the
strata which have been most deeply buried,—and especially those which have
suffered most from folding and from fracture, show the largest amount of igneous
intrusions. The molten rock which has been driven from the earth’s interior to
within the crust or to the surface during geologic time must be reckoned in
millions of cubic miles.

Fig. 251. Fig. 252.
1, limestone; 2, tuff; 3, 5, 7, shale with
a, sedimentary strata with intrusive sheets; b,
marine shells; 4, 6, lava, dotted portions
sedimentary strata; c, lava flow; d, dike. Give the
scoriaceous. Give the history recorded in
succession of events recorded in this section
this section

Fig. 253.
Which of the lava sheets of this section are
contemporaneous anti which intrusive,—A, Fig. 254. Mato Tepee, Wyoming
whose upper surface is overlain with a This magnificent tower of igneous rock three
conglomerate of rolled lava pebbles; B, the hundred feet in height has been called by some a
cracks and seams of whose upper surface volcanic neck. Is the direction of the columns that
are filled with the material of the overlying which would obtain in the cylindrical pipe of a
sandstone; C, which breaks across the strata volcano? The tower is probably the remnant of a
in which it is imbedded; D, which includes small laccolith, an outlying member of a group of
fragments of both the underlying and laccoliths situated not far distant
overlying strata and penetrates their
crevices and seams?

The Interior Condition of the Earth and Causes of
Vulcanism and Deformation
The problems of volcanoes and of deformation are so closely connected with
that of the earth’s interior that we may consider them together. Few of these
problems are solved, and we may only state some known facts and the probable
conclusions which may be drawn as inferences from them.

The interior of the earth is hot. Volcanoes prove that in many parts of the
earth there exist within reach of the surface regions of such intense heat that the

rock is in a molten condition. Deep wells and mines show everywhere an increase
in temperature below the surface shell affected by the heat of summer and the cold
of winter,—a shell in temperate latitudes sixty or seventy feet thick. Thus in a
boring more than a mile deep at Schladebach, Germany, the earth grows warmer at
the rate of 1° F. for every sixty-seven feet as we descend. Taking the average rate
of increase at one degree for every sixty feet of descent, and assuming that this
rate, observed at the moderate distances open to observation, continues to at least
thirty-five miles, the temperature at that depth must be more than three thousand
degrees,—a temperature at which all ordinary rocks would melt at the earth’s
surface. The rate of increase in temperature probably lessens as we go downward,
and it may not be appreciable below a few hundred miles. But there is no reason to
doubt that the interior of the earth is intensely hot. Below a depth of one or two
score miles we may imagine the rocks everywhere glowing with heat.

Although the heat of the interior is great enough to melt all rocks at
atmospheric pressure, it does not follow that the interior is fluid. Pressure raises
the fusing point of rocks, and the weight of the crust may keep the interior in what
may be called a solid state, although so hot as to be a liquid or a gas were the
pressure to be removed.

The interior of the earth is dense and heavy. The earth behaves as a globe
more rigid than glass under the strains to which it is subjected by the attractions of
the sun and moon and other heavenly bodies. The jar of world-shaking earthquakes
passes through the earth’s interior with nearly twice the velocity with which it
would traverse solid steel, and since the speed of elastic waves depends on the
density and elasticity of the medium, it follows that the globe is as a whole more
dense and rigid than steel. The interior of the earth is extremely dense and rigid.

The common rocks of the crust are about two and a half times heavier than
water, while the earth as a whole weighs five and six-tenths times as much as a
globe of water of the same size. The interior is therefore much more heavy than the
crust. This may be caused in part by compression of the interior under the
enormous weight of the crust, and in part also by an assortment of material, the
heavier substances, such as the heavy metals, having gravitated towards the center.

Between the crust, which is solid because it is cool, and the interior, which is
hot enough to melt were it not for the pressure which keeps it dense and rigid,
there may be an intermediate zone in which heat and pressure are so evenly
balanced that here rock liquefies whenever and wherever the pressure upon it may
be relieved by movements of the crust. It is perhaps from such a subcrustal layer
that the lava of volcanoes is supplied.

The causes of volcanic action. It is now generally believed that the heat of
volcanoes is that of the earth’s interior. Other causes, such as friction and crushing
in the making of mountains and the chemical reactions between oxidizing agents
of the crust and the unoxidized interior, have been suggested, but to most
geologists they seem inadequate.

There is much difference of opinion as to the force which causes molten rock
to rise to the surface in the ducts of volcanoes. Steam is so evidently concerned in
explosive eruptions that many believe that lava is driven upward by the expansive
force of the steam with which it is charged, much as a viscid liquid rises and boils
over in a test tube or kettle.

But in quiet eruptions, and still more in the irruption of intrusive sheets and
masses, there is little if any evidence that steam is the driving force. It is therefore
believed by many geologists that it is pressure due to crustal movements and
internal stresses which squeezes molten rock from below into fissures and ducts in
the crust. It is held by some that where considerable water is supplied to the rising
column of lava, as from the ground water of the surrounding region, and where the
lava is viscid so that steam does not readily escape, the eruption is of the explosive
type; when these conditions do not obtain, the lava outwells quietly, as in the
Hawaiian volcanoes. It is held by others not only that volcanoes are due to the
outflow of the earth’s deep-seated heat, but also that the steam and other emitted
gases are for the most part native to the earth’s interior and never have had place in
the circulation of atmospheric and ground waters.

Volcanic action and deformation. Volcanoes do not occur on wide plains or
among ancient mountains. On the other hand, where movements of the earth’s
crust are in progress in the uplift of high plateaus, and still more in mountain
making, molten rock may reach the surface, or may be driven upward toward it
forming great intrusive masses. Thus extensive lava flows accompanied the
upheaval of the block mountains of western North America and the uplift of the
Colorado plateau. A line of recent volcanoes may be traced along the system of rift
valleys which extends from the Jordan and Dead Sea through eastern Africa to
Lake Nyassa. The volcanoes of the Andes show how conspicuous volcanic action
may be in young rising ranges. Folded mountains often show a core of igneous
rock, which by long erosion has come to form the axis and the highest peaks of the
range, as if the molten rock had been squeezed up under the rising upfolds. As we
decipher the records of the rocks in historical geology we shall see more fully how,
in all the past, volcanic action has characterized the periods of great crustal
movements, and how it has been absent when and where the earth’s crust has
remained comparatively at rest.

The causes of deformation. As the earth’s interior, or nucleus, is highly heated
it must be constantly though slowly losing its heat by conduction through the crust
and into space; and since the nucleus is cooling it must also be contracting. The
nucleus has contracted also because of the extrusion of molten matter, the loss of
constituent gases given off in volcanic eruptions, and (still more important) the
compression and consolidation of its material under gravity. As the nucleus
contracts, it tends to draw away from the cooled and solid crust, and the latter
settles, adapting itself to the shrinking nucleus much as the skin of a withering
apple wrinkles down upon the shrunken fruit. The unsupported weight of the
spherical crust develops enormous tangential pressures, similar to the stresses of an
arch or dome, and when these lateral thrusts accumulate beyond the power of
resistance the solid rock is warped and folded and broken.

Since the planet attained its present mass it has thus been lessening in volume.
Notwithstanding local and relative upheavals the earth’s surface on the whole has
drawn nearer and nearer to the center. The portions of the lithosphere which have
been carried down the farthest have received the waters of the oceans, while those
portions which have been carried down the least have emerged as continents.

Although it serves our convenience to refer the movements of the crust to the
sea level as datum plane, it is understood that this level is by no means fixed.
Changes in the ocean basins increase or reduce their capacity and thus lower or
raise the level of the sea. But since these basins are connected, the effect of any
change upon the water level is so distributed that it is far less noticeable than a
corresponding change would be upon the land.

CHAPTER XIII

METAMORPHISM AND MINERAL VEINS
Under the action of internal agencies rocks of all kinds may be rendered
harder, more firmly cemented, and more crystalline. These processes are known as
metamorphism, and the rocks affected, whether originally sedimentary or igneous,
are called metamorphic rocks. We may contrast with metamorphism the action of
external agencies in weathering, which render rocks less coherent by dissolving
their soluble parts and breaking down their crystalline grains.

Contact metamorphism. Rocks beneath a lava flow or in contact with igneous
intrusions are found to be metamorphosed to various degrees by the heat of the
cooling mass. The adjacent strata may be changed only in color, hardness, and
texture. Thus, next to a dike, bituminous coal may be baked to coke or anthracite,
and chalk and limestone to crystalline marble. Sandstone may be converted into
quartzite, and shale into argillite, a compact, massive clay rock. New minerals may
also be developed. In sedimentary rocks there may be produced crystals of mica
and of garnet (a mineral as hard as quartz, commonly occurring in red, twelve-
sided crystals). Where the changes are most profound, rocks may be wholly made
over in structure and mineral composition.

In contact metamorphism, thin sheets of molten rock produce less effect than
thicker ones. The strongest heat effects are naturally caused by bosses and regional
intrusions, and the zone of change about them may be several miles in width. In
these changes heated waters and vapors from the masses of igneous rocks
undoubtedly play a very important part.
Which will be more strongly altered, the rocks about a closed dike in which lava began to cool as soon as it
filled the fissure, or the rocks about a dike which opened on the surface and through which the molten rock
flowed for some time?

Taking into consideration the part played by heated waters, which will produce the most far-reaching
metamorphism, dikes which cut across the bedding planes or intrusive sheets which are thrust between the
strata?

Regional metamorphism. Metamorphic rocks occur wide-spread in many
regions, often hundreds of square miles in area, where such extensive changes
cannot be accounted for by igneous intrusions. Such are the dissected cores of lofty
mountains, as the Alps, and the worn-down bases of ancient ranges, as in New
England, large areas in the Piedmont Belt, and the Laurentian peneplain.

In these regions the rocks have yielded to immense pressure. They have been
folded, crumpled, and mashed, and even their minute grains, as one may see with a
microscope, have often been puckered, broken, and crushed to powder. It is to
these mechanical movements and strains which the rocks have suffered in every
part that we may attribute their metamorphism, and the degree to which they have
been changed is in direct proportion to the degree to which they have been
deformed and mashed.

Other factors, however, have played important parts. Rock crushing develops
heat, and allows a freer circulation of heated waters and vapors. Thus chemical
reactions are greatly quickened; minerals are dissolved and redeposited in new
positions, or their chemical constituents may recombine in new minerals, entirely

changing the nature of the rock, as when, for example, feldspar recrystallizes as
quartz and mica.
Early stages of metamorphism are seen in slate. Pressure has hardened the marine muds, the arkose
(p. 186), or the volcanic ash from which slates are derived, and has caused them to cleave by the
rearrangement of their particles.
Under somewhat greater pressure, slate becomes phyllite, a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes. The
same pressure which has caused the rock to cleave has set free some of its mineral constituents along the cleavage planes to crystallize there as
mica.

Foliation. Under still stronger
pressure the whole structure of the rock is
altered. The minerals of which it is
composed, and the new minerals which
develop by heat and pressure, arrange
themselves along planes of cleavage or of
shear in rudely parallel leaves, or folia. Of
this structure, called foliation, we may
distinguish two types,—a coarser
feldspathic type, and a fine type in which
other minerals than feldspar predominate.

Gneiss is the general name under
which are comprised coarsely foliated
rocks banded with irregular layers of
feldspar and other minerals. The gneisses
appear to be due in many cases to the
crushing and shearing of deep-seated Fig. 255. A Foliated Rock
igneous rocks, such as granite and gabbro.

The crystalline schists, representing the finer types of foliation, consist of thin,
parallel, crystalline leaves, which are often remarkably crumpled. These folia can
be distinguished from the laminae of sedimentary rocks by their lenticular form
and lack of continuity, and especially by the fact that they consist of platy,
crystalline grains, and not of particles rounded by wear.
Mica schist, the most common of schists, and in fact of all metamorphic rocks, is composed of mica and
quartz in alternating wavy folia. All gradations between it and phyllite may be traced, and in many cases we
may prove it due to the metamorphism of slates and shales. It is widespread in New England and along the
eastern side of the Appalachians. Talc schist consists of quartz and talc, a light-colored magnesian mineral of
greasy feel, and so soft that it can be scratched with the thumb nail.

Hornblende schist, resulting in many cases from the foliation of basic igneous rocks, is made of folia of
hornblende alternating with bands of quartz and feldspar. Hornblende schist is common over large areas in the
Lake Superior region.

Quartz schist is produced from quartzite by the development of fine folia of mica along planes of shear. All
gradations may be found between it and unfoliated quartzite on the one hand and mica schist on the other.

Under the resistless pressure of crustal movements almost any rocks, sandstones, shales, lavas of all kinds,
granites, diorites, and gabbros may be metamorphosed into schists by crushing and shearing. Limestones,
however, are metamorphosed by pressure into marble, the grains of carbonate of lime recrystallizing freely to
interlocking crystals of calcite.

These few examples must suffice of the great class of metamorphic rocks. As
we have seen, they owe their origin to the alteration of both of the other classes of
rocks—the sedimentary and the igneous—by heat and pressure, assisted usually by
the presence of water. The fact of change is seen in their hardness arid
cementation, their more or less complete recrystallization, and their foliation; but
the change is often so complete that no trace of their original structure and mineral
composition remains to tell whether the rocks from which they were derived were
sedimentary or igneous, or to what variety of either of these classes they belonged.

Fig. 256. Contorted Gneiss, the Ottawa River, Canada

In many cases, however, the early history of a metamorphic rock can be
deciphered. Fossils not wholly obliterated may prove it originally water-laid.
Schists may contain rolled-out pebbles, showing their derivation from a
conglomerate. Dikes of igneous rocks may be followed into a region where they

have been foliated by pressure. The most
thoroughly metamorphosed rocks may
sometimes be traced out into unaltered
sedimentary or igneous rocks, or among
them may be found patches of little
change where their history maybe read.

Metamorphism is most common
among rocks of the earlier geological
ages, and most rare among rocks of
recent formation. No doubt it is now in
progress where deep-buried sediments
Fig. 257. Quartz Veins in Slate are invaded by heat either from intrusive
igneous masses or from the earth’s
interior, or are suffering slow deformation under the thrust of mountain-making
forces.
Suggest how rocks now in process of metamorphism may sometimes be exposed to view. Why do
metamorphic rocks appear on the surface to-day?

Mineral Veins
In regions of folded and broken rocks fissures are frequently found to be filled
with sheets of crystalline minerals deposited from solution by underground water,
and fissures thus filled are known as mineral veins. Much of the importance of
mineral veins is due to the fact that they are often metalliferous, carrying valuable
native metals and metallic ores disseminated in fine particles, in strings, and
sometimes in large masses in the midst of the valueless nonmetallic minerals
which make up what is known as the vein stone.
The most common vein stones are quartz and calcite. fluorite (calcium fluoride), a mineral harder than
calcite and crystallizing in cubes of various colors, and barite (barium sulphate), a heavy white mineral, are
abundant in many veins.

The gold-bearing quartz veins of California traverse the
metamorphic slates of the Sierra Nevada Mountains. Below
the zone of solution (p. 45) these veins consist of a vein
stone of quartz mingled with pyrite (p. 13), the latter
containing threads and grains of native gold. But to the
depth of about fifty feet from the surface the pyrite of the
vein has been dissolved, leaving a rusty, cellular quartz with
grains of the insoluble gold scattered through it. Fig. 258. Placer Deposits in
California
The placer deposits of California and other regions are
gold- bearing deposits of gravel and sand in river beds. The g, gold-bearing gravels in present river beds; g

heavy gold is apt to be found mostly near or upon the solid ´, ancient gold-bearing river gravels; a, a, lava
rock, and its grains, like those of the sand, are always
rounded. How the gold came in the placers we may leave flows capping table mountains; s, slate. Draw
the pupil to suggest. a diagram showing by dotted lines conditions
before the lava flows occurred. What changes
Copper is found in a number of ores, and have since taken place?
also in the native metal. Below the zone of
surface changes the ore of a copper vein is often a double sulphide of iron and
copper called chalcopyrite, a mineral softer than pyrite—it can easily be scratched
with a knife—and deeper yellow in color. For several score of feet below the
ground the vein may consist of rusty quartz from which the metallic ores have
been dissolved; but at the base of the zone of solution we may find exceedingly
rich deposits of copper ores,— copper sulphides, red and black copper oxides, and
green and blue copper carbonates, which have clearly been brought down in
solution from the leached upper portion of the vein.

Origin of mineral veins. Both vein stones and ores have been deposited
slowly from solution in water, much as crystals of salt are deposited on the sides of
a jar of saturated brine. In our study of underground water we learned that it is
everywhere circulating through the permeable rocks of the crust, descending to
profound depths under the action of gravity and again driven to the surface by
hydrostatic pressure. Now fissures, wherever they occur, form the trunk channels
of the underground circulation. Water descends from the surface along these rifts;
it moves laterally from either side to the fissure plane, just as ground water seeps
through the surrounding rocks from every direction to a well; and it ascends
through these natural water ways as in an artesian well, whenever they intersect an
aquifer in which water is under hydrostatic pressure.

The waters which deposit vein stones and ores are commonly hot, and in many
cases they have derived their heat from intrusions of igneous rock still uncooled
within the crust. The solvent power of the water is thus greatly increased, and it
takes up into solution various substances from the igneous and sedimentary rocks
which it traverses. For various reasons these substances stances are deposited in
the vein as ores and vein stones. On rising through the fissure the water cools and
loses pressure, and its capacity to hold minerals in solution is therefore lessened.
Besides, as different currents meet in the fissure, some ascending, some
descending, and some coming in from the sides, the chemical reaction of these
various weak solutions upon one another and upon the walls of the vein
precipitates the minerals of vein stuffs and ores.
As an illustration of the method of vein deposits we may cite the case of a wooden box pipe used in the
Comstock mines, Nevada, to carry the hot water of the mine from one level to another, which in ten years was
lined with calcium carbonate more than half an inch thick.

The Steamboat Springs, Nevada, furnish examples of mineral veins in process of formation. The steaming
water rises through fissures in volcanic rocks and is now depositing in the rifts a vein stone of quartz, with
metallic ores of iron, mercury, lead, and other metals.

Fig. 259. Reconcentration of Ores in Mineral Veins
A, original vein; B, same after reconcentration; v, mineral vein; s,
surface of ground (dotted line, former surfaces of the ground); sp,
spring; o, vein leached of ores by descending waters in zone of
solution; m, rich ore deposits reconcentrated from above; n,
unchanged portion of vein

Reconcentration. Near the base of the zone of solution veins are often stored
with exceptionally large and valuable ore deposits. This local enrichment of the
vein is due to the reconcentration of its metalliferous ores. As the surface of the
land is slowly lowered by weathering and running water, the zone of solution is
lowered at an equal rate and encroaches constantly on the zone of cementation.
The minerals of veins are therefore constantly being dissolved along their upper
portions and carried down the fissures by ground water to lower levels, where they
are redeposited.

Many of the richest ore deposits are thus due to successive concentrations: the
ores were leached originally from the rocks to a large extent by laterally seeping
waters; they were concentrated in the ore deposits of the vein chiefly by ascending
currents; they have been reconcentrated by descending waters in the way just
mentioned.

The original source of the metals. It is to the igneous rocks that we may look
for the original source of the metals of veins. Lavas contain minute percentages of
various metallic compounds, and no doubt this was the case also with the igneous
rocks which formed the original earth crust. By the erosion of the igneous rocks

the metals have been distributed among sedimentary strata, and even the sea has
taken into solution an appreciable amount of gold and other metals, but in this
widely diffused condition they are wholly useless to man. The concentration which
has made them available is due to the interaction of many agencies. Earth
movements fracturing deeply the rocks of the crust, the intrusion of heated masses,
the circulation of underground waters, have all cooperated in the concentration of
the metals of mineral veins.
While fissure veins are the most important of mineral veins, the latter term is applied also to any water way
which has been filled by similar deposits from solution. Thus in soluble rocks, such as limestones, joints
enlarged by percolating water are sometimes filled with metalliferous deposits, as, for example, the lead and
zinc deposits of the upper Mississippi valley. Even a porous aquifer may be made the seat of mineral deposits,
as in the case of some copper-bearing and silver-bearing sandstones of New Mexico.

Fig. 260. Geological Map of the United states and Part of Canada
Click on map to view larger version

PART III

HISTORICAL GEOLOGY

CHAPTER XIV

THE GEOLOGICAL RECORD
What a formation records. We have already learned that each individual body
of stratified rock, or formation, constitutes a record of the time when it was laid.
The structure and the character of the sediments of each formation tell whether the
area was land or sea at the time when they were spread; and if the former, whether
the land was river plain, or lake bed, or was covered with wind-blown sands, or by
the deposits of an ice sheet. If the sediments are marine, we may know also
whether they were laid in shoal water near the shore or in deeper water out at sea,
and whether during a period of emergence, or during a period of subsidence when
the sea transgressed the land. By the same means each formation records the stage
in the cycle of erosion of the land mass from which its sediments were derived
(p. 185). An unconformity between two marine formations records the fact that
between the periods when they were deposited in the sea the area emerged as land
and suffered erosion (p. 227). The attitude and structure of the strata tell also of the
foldings and fractures, the deformation and the metamorphism, which they have
suffered; and the igneous rocks associated with them as lava flows and igneous
intrusions add other details to the story. Each formation is thus a separate local
chapter in the geological history of the earth, and its strata are its leaves. It
contains an authentic record of the physical conditions—the geography—of the
time and place when and where its sediments were laid.

Past cycles of erosion. These chapters in the history of the planet are very
numerous, although much of the record has been destroyed in various ways. A
succession of different formations is usually seen in any considerable section of
the crust, such as a deep canyon or where the edges of upturned strata are exposed
to view on the flanks of mountain ranges; and in any extensive area, such as a state
of the Union or a province of Canada, the number of formations outcropping on
the surface is large.

It is thus learned that our present continent is made up for the most part of old
continental deltas. Some, recently emerged as the strata of young coastal plains,
are the records of recent cycles of erosion; while others were deposited in the early
history of the earth, and in many instances have been crumpled into mountains,
which afterwards were leveled to their bases and lowered beneath the sea to
receive a cover of later sediments before they were again uplifted to form land.

The cycle of erosion now in progress and recorded in the layers of stratified
rock being spread beneath the sea in continental deltas has therefore been preceded
by many similar cycles. Again and again movements of the crust have brought to
an end one cycle— sometimes when only well under way, and sometimes when
drawing toward its close—and have begun another. Again and again they have
added to the land areas which before were sea, with all their deposition records of
earlier cycles, or have lowered areas of land beneath the sea to receive new
sediments.

The age of the earth. The thickness of the stratified rocks now exposed upon
the eroded surface of the continents is very great. In the Appalachian region the
strata are seven or eight miles thick, and still greater thicknesses have been
measured in several other mountain ranges. The aggregate thickness of all the
formations of the stratified rocks of the earth’s crust, giving to each formation its
maximum thickness wherever found, amounts to not less than forty miles.
Knowing how slowly sediments accumulate upon the sea floor (p.184), we must
believe that the successive cycles which the earth has seen stretch back into a past
almost inconceivably remote, and measure tens of millions and perhaps even
hundreds of millions of years.

How the formations are correlated and the geological record made up.
Arranged in the order of their succession, the formations of the earth’s crust would
constitute a connected record in which the geological history of the planet may be
read, and therefore known as the geological record. But to arrange the formations
in their natural order is not an easy task. A complete set of the volumes of the
record is to be found in no single region. Their leaves and chapters are scattered
over the land surface of the globe. In one area certain chapters may be found,
though perhaps with many missing leaves, and with intervening chapters wanting,
and these absent parts perhaps can be supplied only after long search through
many other regions.

Adjacent strata in any region are arranged according to the law of
superposition, i.e. any stratum is younger than that on which it was deposited, just
as in a pile of paper, any sheet was laid later than that on which it rests. Where

rocks have been disturbed, their original attitude must be determined before the
law can be applied. Nor can the law of superposition be used in identifying and
comparing the strata of different regions where the formations cannot be traced
continuously from one region to the other.

The formations of different regions are arranged in their true order by the law
of included organisms; i.e. formations, however widely separated, which contain a
similar assemblage of fossils are equivalent and belong to the same division of
geological time.
The correlation of formations by means of fossils may be explained by the formations now being deposited
about the north Atlantic. Lithologically they are extremely various. On the continental shelf of North America
limestones of different kinds are forming off Florida, and sandstones and shales from Georgia northward.
Separated from them by the deep Atlantic oozes are other sedimentary deposits now accumulating along the
west coast of Europe. If now all these offshore formations were raised to open air, how could they be
correlated? Surely not by lithological likeness, for in this respect they would be quite diverse. All would be
similar, however, in the fossils which they contain. Some fossil species would be identical in all these
formations and others would be closely allied. Making all due allowance for differences in species due to local
differences in climate and other physical causes, it would still be plain that plants and animals so similar lived
at the same period of time, and that the formations in which their remains were imbedded were
contemporaneous in a broad way. The presence of the bones of whales and other marine mammals would
prove that the strata were laid after the appearance of mammals upon earth, and imbedded relics of man would
give a still closer approximation to their age. In the same way we correlate the earlier geological formations.

For example, in 1902 there were collected the first fossils ever found on the antarctic continent. Among the
dozen specimens obtained were some fossil ammonites (a family of chambered shells) of genera which are
found on other continents in certain formations classified as the Cretaceous system, and which occur neither
above these formations nor below them. On the basis of these few fossils we may be confident that the strata
in which they were found in the antarctic region were laid in the same period of geologic time as were the
Cretaceous rocks of the United States and Canada.

The record as a time scale. By means of the law of included organisms and
the law of superposition the formations of different countries and continents are
correlated and arranged in their natural order. When the geological record is thus
obtained it may be used as a universal time scale for geological history. Geological
time is separated into divisions corresponding to the times during which the
successive formations were laid. The largest assemblages of formations are known
as groups, while the corresponding divisions of time are known as eras. Groups are
subdivided into systems, and systems into series. Series are divided into stages and
substages,—subdivisions which do not concern us in this brief treatise. The
corresponding divisions of time are given in the following table.

Strata Time
Group Era
System Period
Series Epoch

The geologist is now prepared to read the physical history—the geographical
development—of any country or of any continent by means of its formations,
when he has given each formation its true place in the geological record as a time
scale.

The following chart exhibits the main divisions of the record, the name given
to each being given also to the corresponding time division. Thus we speak of the
Cambrian system, meaning a certain succession of formations which are classified
together because of broad resemblances in their included organisms; and of the
Cambrian period, meaning the time during which these rocks were deposited.

Group and Era System and Period Series and Epoch
Recent
Quaternary
Pleistocene
Cenozoic Pliocene
Tertiary Miocene
Eocene
Cretaceous
Mesozoic Jurassic
Triassic
Permian
Carboniferous Pennsylvanian
Mississippian
Paleozoic Devonian
Silurian
Ordovician
Cambrian
Algonkian
Archean

Fossils and what they teach
The geological formations contain a record still more important than that of the
geographical development of the continents; the fossils imbedded in the rocks of
each formation tell of the kinds of animals and plants which inhabited the earth at
that time, and from these fossils we are therefore able to construct the history of
life upon the earth.
Fossils. These remains of organisms are found in the strata in all degrees of perfection, from trails and
tracks and fragmentary impressions, to perfectly preserved shells, wood, bones, and complete skeletons. As a

rule, it is only the hard parts of animals and plants which have left any traces in the rocks. Sometimes the
original hard substance is preserved, but more often it has been replaced by some less soluble material.
Petrifaction, as this process of slow replacement is called, is often carried on in the most exquisite detail.
When wood, for example, is undergoing petrifaction, the woody tissue may be replaced, particle by particle,
by silica in solution through the action of underground waters, even the microscopic structures of the wood
being perfectly reproduced. In shells originally made of aragonite, a crystalline form of carbonate of lime, that
mineral is usually replaced by calcite, a more stable form of the same substance. The most common petrifying
materials are calcite, silica, and pyrite (p. 13).

Often the organic substance has neither been preserved nor replaced, but the form has been retained by
means of molds and casts. Permanent impressions, or molds, may be made in sediments not only by the hard
parts of organisms, but also by such soft and perishable parts as the leaves of plants, and, in the rarest
instances, by the skin of animals and the feathers of birds. In fine-grained limestones even the imprints of
jellyfish have been retained.

The different kinds of molds and casts may be illustrated by means of a clam
shell and some moist clay, the latter representing the sediments in which the
remains of animals and plants are entombed. Imbedding the shell in the clay and
allowing the clay to harden, we have a mold of the exterior of the shell, as is seen
on cutting the clay matrix in two and removing the shell from it. Filling this mold
with clay of different color, we obtain a cast of the exterior, which represents
accurately the original form and surface markings of the shell. In nature, shells and
other relics of animals or plants are often removed by being dissolved by
Fig. 261. Section percolating waters, and the molds are either filled with sediments or with minerals
deposited from solution.
of Cast and
Mold of a Shell Where the fossil is hollow, a cast of the interior is made in the same way.
a, shell; b, mold of Interior casts of shells reproduce any markings on the inside of the valves, and
casts of the interior of the skulls of ancient vertebrates show the form and size of
exterior; c, cast of their brains.
interior
Imperfection of the life record. At the present time only
the smallest fraction of the life on earth ever gets entombed in rocks now forming.
In the forest great fallen tree trunks, as well as dead leaves, decay, and only add a
little to the layer of dark vegetable mold from which they grew. The bones of land
animals are, for the most part, left unburied on the surface and are soon destroyed
by chemical agencies. Even where, as in the swamps of river, flood plains and in
other bogs, there are preserved the remains of plants, and sometimes insects,
together with the bones of some animal drowned or mired, in most cases these
swamp and bog deposits are sooner or later destroyed by the shifting channels of
the stream or by the general erosion of the land.

In the sea the conditions for preservation are more favorable than on land; yet
even here the proportion of animals and plants whose hard parts are fossilized is
very small compared with those which either totally decay before they are buried
in slowly accumulating sediments or are ground to powder by waves and currents.

We may infer that during each period of the past, as at the present, only a very
insignificant fraction of the innumerable organisms of sea and land escaped

destruction and left in continental and oceanic deposits permanent records of their
existence. Scanty as these original life records must have been, they have been
largely destroyed by metamorphism of the rocks in which they were imbedded, by
solution in underground waters, and by the vast denudation under which the
sediments of earlier periods have been eroded to furnish materials for the
sedimentary records of later times. Moreover, very much of what has escaped
destruction still remains undiscovered. The immense bulk of the stratified rocks is
buried and inaccessible, and the records of the past which it contains can never be
known. Comparatively few outcrops have been thoroughly searched for fossils.
Although new species are constantly being discovered, each discovery may be
considered as the outcome of a series of happy accidents,—that the remains of
individuals of this particular species happened to be imbedded and fossilized, that
they happened to escape destruction during long ages, and that they happened to be
exposed and found.

Some inferences from the records of the history of life upon the planet.
Meager as are these records, they set forth plainly some important truths which we
will now briefly mention.

1. Each series of the stratified rocks, except the very deepest, contains vestiges
of life. Hence the earth was tenanted by living creatures for an uncalculated length
of time before human history began.

2. Life on the earth has been everchanging. The youngest strata hold the
remains of existing species of animals and plants and those of species and varieties
closely allied to them. Strata somewhat older contain fewer existing species, and in
strata of a still earlier, but by no means an ancient epoch, no existing species are to
be found; the species of that epoch and of previous epochs have vanished from the
living world. During all geological time since life began on earth old species have
constantly become extinct and with them the genera and families to which they
belong, and other species, genera, and families have replaced them. The fossils of
each formation differ on the whole from those of every other. The assemblage of
animals and plants (the fauna-flora) of each epoch differs from that of every other
epoch.

In many cases the extinction of a type has been gradual; in other instances
apparently abrupt. There is no evidence that any organism once become extinct has
ever reappeared. The duration of a species in time, or its “vertical range” through
the strata, varies greatly. Some species are limited to a stratum a few feet in
thickness; some may range through an entire formation and be found but little
modified in still higher beds. A formation may thus often be divided into zones,