Elementary physical geography and outline of physiography

Front matter

EOGRAPH\

R. E pBABLJJ

"i^te'vi jUmg

Preface

The science of Geography sets forth the relations of life and its environment to the earth, and it is the function of both the writer and the teacher of geography to explain these relations. In the Elementary Natural Geography the pupil studies the various peoples of the earth and the countries in which they live ; in the Advanced Geography there is presented in addition a discussion of the industries of life and their geographic distribution. In the present volume, which the author has prepared as a logical sequel, it is designed to show that the distribution of life is governed very largely by the conditions of geographic environment, and that human history and industries are always closely connected with geographic laws — in many instances the direct- resultants of them.

The science of Geography as now understood includes something more than a mere description of topographic forms — it comprehends the gradual and progressive development of these forms and their results as regards life, as well. It includes also the effects of temperature and moisture, for life and its activities depend also on them. That is, it naturally involves the principles of Descriptive Geography, Physiography, and Economics; and the present volume is designed to show their interrelation.

In scope this book contains all the principles recommended by the Committee; of Fifteen, and such other features as have suggested themselves to the author. It is designed to be used in the junior grades of the High vi PEEFACE

School, and in Normal Schools. With judgment in the selection of the topics, it may be begun in the last half of the eighth year of the Grammar School. The arrangement of the subjects is logical, but the teacher may readily organize a course of study in the subject without reference to the present arrangement. To make this more easily accomplished, the principles of the subject are set forth in the larger type ; relevant matter that is illustrative but non-essential is confined to the notes. In general, the teacher should not hesitate to omit a topic the discussion of which is too difficult for the class.

The Questions and Exercises are designed to stimulate observation and independent thought. If, occasionally, they leave the pupil in doubt, the design of the author will be fulfilled. The pupil must learn by experience that knowledge does not come in cut-and-dried packages ; it comes only after long and painstaking investigation.

In the preparation of the work the author takes pleasure in acknowledging the material assistance of Miss Frances Bronson, and of his daughter, Miss Elizabeth Ebert Redway. But to more than anyone else, however, thanks are due to Miss Stella Wilson, Instructor in Physical Geography in the Central High School, Columbus, Ohio. To Miss Wilson's keen judgment, excellent criticism, and experience are largely due the usefulness as a text-book which this volume may have.

The books designated for reference and collateral reading are intentionally few in number, and those most commonly cited should be in the school library. The teacher will also find it very advisable to get in close touch with the United States Geological Survey and the Weather Bureau. The Bureau of Geography recently established at Winona, Minn., will also be helpful.

J. W. R.

Contents

PAGE

Introductory, . . . 1

CHAPTER

Chapter II

In the long period of time that has elapsed since the earth was glowing with intense heat, the substances composing it seem to have adjusted themselves in accordance with the laws of gravitation 1 — that is, the heaviest kinds of

IDEAL SECTION THROUGH THE EARTH The thickness of the "various envelopes is greatly distorted.

matter are nearest the centre. Structurally the earth consists of a dense and practically solid globe, the lithosphere, nearly covered with a comparatively thin layer of water, the hydrosphere or water envelope, the whole surrounded by an envelope of gaseous matter, the atmosphere.

The shape of the lithosphere and the condition of the substances composing it, all go to show that in times past it was intensely heated, and that much of the rock composing it has been iu a molten condition. The globular form is the only one that would naturally result from the action of gravitation on a plastic or fluid body ; and the flattening at the poles is most reasonably explained by the supposition of a rotation on its axis while it was still plastic.

The density of the lithosphere, together with the waters, is about that of iron ore a — that is, bulk for bulk, it is about five and one-half times as heavy as water. At the surface, however, the density of the rocks is not much more than half as great ; it is certain, therefore, that the substances forming the interior are much heavier than those occurring at the surface.

The Rock Envelope. — The outer part of the lithosphere is a shell of more or less friable material called the rock envelope, or, popularly, the " crust of the earth." It surrounds an intensely heated interior. 3 The rock envelope itself has lost so much of the heat it once had that it is comparatively cold ; the amount of heat it radiates is about equal to that which it receives from the sun.

That the interior of the lithosphere is very hot, however, cannot be doubted ; for in every place where the rock envelope has been penetrated by deep borings, a constant increase of temperature is observed — the greater the depth the higher the temperature. 4 The thickness of the rock envelope is not known, but at a depth of less than forty miles it is thought that the temperature is high enough to fuse the most refractory substances. The broken folds of the outer surface have revealed something of its character to the depth of several miles. Borings have been made to a depth of a little more than a mile (Table II., Appendix), but beyond the slight knowledge obtained from these, nothing positive is known about its interior.

The Water Envelope. — About four-fifths of the surface of the rock envelope is covered by a comparatively thin layer of water, the hydrosphere. The water not only exists in a free state, at the surface, but in chemical combination it is a constituent of various kinds of rock 5 that occur at or near the surface.

The waters of the earth form a most important constituent so far as life is concerned. Water is an essential element to the existence of life ; for not only does it form the greater part of every plant or animal, but it is also the chief vehicle by which nutrition is distributed throughout the various parts of the body of the animal or the plant. Within a range of a very few degrees of temperature, water exists in one or another of three forms — a solid, ice ; a liquid, water ; and an invisible vapor, often called "steam." Water in one or the other of its forms is the agent by which, more than any other, the surface of the rock envelope has been sculptured ; therefore it has a very important part in the science of physiography.

The Atmosphere. — The atmosphere consists of a mixture of gaseous substances — namely: nitrogen, oxygen, water vapor, and carbon dioxide. Of these oxygen is the substance required in the breathing of animals ; carbon dioxide, the gas formed when coal or carbon " burns," is essential in the breathing of plants ; nitrogen forms a part of the body structure in both animals and plants ; and water vapor is the form in which the fresh water is carried from the sea to the land. The atmosphere, therefore, is just as essential to life as the water envelope.

The thickness of the atmospheric envelope is not known. Various estimates place it between one hundred and two hundred miles. At the latter estimate, on a globe one yard in diameter, the depth of the atmosphere in proportion would be about one-half an inch. 6 Illustrate by diagram.

Keeping Nature's Balance. — The three envelopes are constantly acting and reacting upon each other, and at the same time each has certain movements of its own. The movements of the rock envelope have changed the level of its surface so that the waters are divided from the land, and the surface of the land has been wrinkled, crumpled, and folded so as to form the plateaus, ranges, and valleys. The heat of the sun causes a part of the ocean waters to take the form of vapor, and the latter, mingled with the air, flows over the land. Being chilled, the vapor again takes the form of rain, or of snow, and falling on the land wears away its surface. The water gathers into channels and, carrying the mingled particles of rock waste in its flood, flows back to the sea and there deposits them.

And so the c} T cle of change ever goes on. At the plane where the atmosphere rests upon the land and the sea the physiographic processes that modif}" the earth's surface are ever in action.

Vertical Movements of the Rock Envelope. — The changes in the surface of the rock envelope that are most noticeable are the wearing away of the land and the transportation of the rock waste to lower levels. That is, water falling as rain loosens particles of rock, while streams carry it seaward. If the land were everywhere level, the run-off of water could wear away but little of it ; but vertical movements of the surface that are apparent only after long intervals of time are taking place, and these, making new slopes, have given the run-off waters increased wearing power.

Movements of the rock envelope in times past have diversified its surface with highlands and lowlands, inountains and valleys, and similar movements are going on at the present time. Probably no part of the earth is free from them, but they are most clearly observed along sea-shores. Thus, the coast of New Jersey is sinking ; 7 and

AN UPLIFTED COAST— SAN PEDRO, CAL.

From a survey made by Merick Reynolds, Jr. The successively formed beaches are shown

by the strata of shells and sand.

so also is much of the coast around the Gulf of Mexico, the Zuyder Zee, and the delta of the Ganges-Brahmaputra. The coast of the New England Plateau has subsided until the sea has flooded the coast plain and the lower valleys, and has buried most of the old river mouths. The multitude of bays and fjords that characterize this coast are examples of "drowned" valleys. On the other hand, parts of the Mediterranean basin, of the California coast, 8 the Scandinavian peninsula, and the basin of Great Salt Lake are rising.

In nearly every instance in areas to which extensive sediments are being carried there is evidence of sinking ; while, as a rule, areas that are being denuded are rising. It is evident, therefore, that vertical movements of the rock envelope — that is, uplift and subsidence — are definitely connected with the wasting of the land and the transfer of sediment.

The cause or causes of these earth movements are not known, but it is believed that the gradual contraction of the rock envelope to fit itself around a more rapidly shrinking interior is the chief cause. There is evidence, too, that gravitation is a factor. The removal of great amounts of rock waste — often many cubic miles in volume — from one locality to another, relieves weight at one place and increases it at the other. 9 Therefore it is inferred that a sinking, because of the increased load, occurs at the latter place, and an uplift at the former, on account of the lessened weight.

The effects of these earth movements are very far-reaching. The great highland regions of the earth, with their ridges and folds, are probably direct results, and it is not improbable that the uplift of the continents themselves was also due to them.

IGNEOUS ROCK: A FLOW OF LAVA

Rock and Its Formation. — To almost every mineral substance that forms a part of the earth, the term /'"•/,• is applied. Thus, clay, sand, gravel, limestone, quartz, granite, lava, and even the fine, wind-blown rock waste, are each called rock ; and so also is a combination or any mixture of them. In many instances there is no doubt at all how the rock has been formed, or whether it has been altered or not, because the whole process of its formation has been carried on in plain sight. Thus, when a volcano or a fissure pours out a flood of molten lava there is no question about how the rock got into place, or whence it came. The lava, when it has hardened, may be glassy, or metallic in appearance, or it may be like cinder or furnace slag ; but there are always qualities about it that determine its origin.

Beyond a depth of a feAV thousand feet from the surface, nothing positive is known about the substances of which the rock envelope is composed. It is certain, however, that most of the rock now at the surface consists of sediments carried into place by running water and deposited in the form of layers or strata that afterward hardened into compact rock. But these sediments must have come from somewhere, and there is but one place from which they could be derived — namely, from the rock envelope itself.

Now, no one knows what the 'primitive or first rock that formed the crust of the earth may have been, but certain kinds of rock have been found underlying the water-formed sediments from which the latter seem to be derived. Ordinarjr granite is an example of this kind of rock, and granitic rocks are very abundant. There are various kinds of granite, but the most common varieties contain minerals of which nearly all the elementary rocks themselves are composed.

One of these minerals is silica, of which quartz and sea sand are the best examples. Another is felspar, a mineral which, decomposed, yields clay, potash, lime, and soda. Another mineral is hornblende, which decomposes mainly into iron, lime, and silica. Still another constituent usually present is mica, popularly called " isinglass ; " like felspar it also decomposes into clay, silica, lime, and a number of other substances.

EXERCISE.— Procure one or more specimens of granite, 10 and with the aid of a magnifying-glass observe the following directions. Look for small clusters of foliated or " leafy " mineral ; it may be whitish or, perhaps, green or brown ; this mineral is mica. If no mica is found, look for jet black crystals or masses ; this is hornblende ; it is usually opaque, but sometimes translucent. Find the white, translucent mineral with glassy lustre ; it is quartz, or silica, and it is apt to form the chief bulk of the rock. Look also for an opaque mineral varying from yellowish-white to pink in color ; possibly it will break into fragments having flat sides, or cleavage planes ; this mineral is felspar ; it has different crystalline forms accordingly as it contains lime, potash, or soda.

Igneous Rocks. — There are certain surface rocks that have cooled from a molten condition, and of these the lavas of volcanoes, though not the most abundant, are perhaps the best known. The Hawaiian Islands are mainly great piles or domes of lava, and this kind of rock is common in most mountainous regions. In many instances the molten rock has been ejected from long fissures and has cooled slowly ; in this form it is usually known as basalt, or, if it breaks into regular blocks, trap. The Palisades of the Hudson, Fingal's Cave, and the Giant's Causeway are examples.

All the foregoing are commonly called vulcanic or igneous rocks ; consult a good dictionary and learn why these names are applied. Igneous rocks are usually found in mountainous regions, or in localities from which the sedimentary rock has been removed. Granite rocks prevail in the New England Plateau ; igneous rocks are abundant in the Western Highlands.

Sedimentary Rocks. — Although the sedimentary rocks that prevail in such a great extent of the land are derived from the granitic and other vulcanic rocks, there is nothing about them to indicate their close relation to the latter. The making of firm rock out of loose sediments is a somewhat complex process. Let us follow the formation of sandstone. In the first place the grains of quartz are rounded, and in the second place they are uniform in size. The rock from which they came, probably granite, has crumbled, and water has sorted the various minerals from one another. The waves, beating the fragments of quartz and rubbing them against one another, have not only rounded the grains, but they have also sorted them according to size, and piled them in a nearly flat layer. True sand, therefore, is nearly always a formation of beaches or of water in motion.

In time the beach is lifted up above sea-level and covered deep with vegetable remains mixed with loam. Water, in one form or another, flows over or stands upon the surface ; and if the water contains lime in solution it will leach through the layer of sand and cement the grains, forming sandstone.

In most instances, clay banks are derived from granitic and similar rocks. Felspar decomposes into clay, and the latter, being very light and fine, is carried off by the water, settling by itself, while the heavier materials remain. In many instances the clay is spread over large areas. Possibly it remains in the stiff, pasty form by which it is commonly known ; more likely pressure, heat, and moisture, acting together, convert it into slate.

It is not difficult to understand how rivers and other running waters are active workers in making rock, because one can almost always find clay-banks, gravel-beds, and other sediments that have been brought down stream and distributed by the water. 11 It is not so easy to understand how rocks are found at the bottom of the sea ; as a matter

SEDIMENTARY ROCK, NEAR OLEAN, N. Y.

The face of the cliff is one side of a channel of the river.

of fact, however, probably more sedimentary rock has been formed in ocean and lake beds than in any other places. In very many instances these rocks are largely composed of the remains of animals so small that several thousand of them together would not be so large as the head of a pin.

The sea, especially in regions of warm water, contains many thousand species of such animals ; moreover they multiply with great rapidity. But the animals are shortlived, and as soon as tbey die their bodies sink to the bottom. The mineral remains of these organisms consist mainly of lime or silica, and in time the thick layer that accumulates finally becomes cemented into rock. The growth of rock in this way is slow, it is true, but time alone is required to make such layers of very great thickness. The chalk cliffs of England and France were formed in this manner, and they aggregate nearly half a mile in thickness. The limestones of the Mississippi Valley also accumulated on sea-bottoms and have about the same thickness.

Metamorphic Rocks. — There are many instances in which the character of sedimentary rocks has been subsequently changed. Thus, by pressure and heat in the presence of moisture, beds of clay have been transformed into layers of gritty slate; chalk and limestone have become crystalline marble ; and bituminous coal has become anthracite. Certain kinds of granitic rock, especially gneiss or " stratified granite," are metamorphic. Older granitic rock has crumbled, and the rock waste has been cemented into firm rock again with but little alteration.

One might infer, therefore, that the older and deeper stratified rocks would be thus changed. 12 This is usually the case. The weight of the overlying rock produces immense pressure, and the changes resulting from the moisture within them greatly alter their appearance. Many of the older rocks, indeed, are much like igneous rock in appearance. Bocks that form a part of mountain folds are apt to be metamorphic on account of the pressure that results from the folding and crumpling.

EXERCISE.— Procure specimens of clay and slate, chalk (not crayon) and marble, bituminous (soft) coal and anthracite. Examine each pair with reference to hardness, foliation, crystalline appearance, and density (weight of pieces of equal size). Make a list of the rocks occurring in the neighborhood in which you live, and classify them as igneous, sedimentary, or metamorphic.

Order of the Strata. — Most of the sedimentary rocks were deposited in horizontal layers, but, on account of the vertical movements of the rock envelope, they are often found in oblique positions. Sometimes they occur in gentle folds ; but in mountainous regions they are much crumpled and broken. In some of the old sea-beds now raised above the surface the strata are undisturbed.

SEDIMENTARY ROCK: SECTION THROUGH THE CANON OF THE COLO- RADO RIVER

The level of the strata has not been disturbed.

It is by studying the upturned edges of broken and tilted strata that the story of the earth has been read. Each stratum is a chapter by itself, and to read the history properly it is best to begin with the lowest. It is not always easy to tell the relative position of strata at some distance from one another, but as each stratum has fossils, or animal remains peculiar to itself, the position is usually determined by the kind and character of these.

SEDIMENTARY ROCK: TILTED STRATA

The total thickness of the stratified rocks is estimated at upward of twenty miles. There is no locality known, and none exists, in which all the various strata are found — no locality is known in which even any considerable number occur. Not infrequently very old rocks are overlaid by those of the most recent formations ; all the intermediate strata are missing. 13

To the lowest strata, that do not differ much from the granitic rocks and possibly include some of them, the name

Archcean u is given, the continents and

They seem to be the foundation of

the floor of the oceans. The decay

and wearing away of these has

formed the material of which

- nearly all the sedimentary rock

|§ is composed. " The waste of the

| old land is the material of the

11 neiv."

The remaining strata are named in accordance with the character of the life forms that existed when the rocks that compose them were undergoing formation. Upon the Archaean, rest the rocks of the Palceozoic era — the age of the earliest life forms. Then follow the rocks of the Mesozoic, or middle-life era; the Cenozoic, or era of recent life ; and, last of all, the era of man.

Archaean Era. — In Archaean times North America consisted mainly of a narrow, V-shaped strip of land south of Hudson Bay. The crests of the Appalachian Mountains were just above the sea level ; the Black Hills and one or two peaks of the Rocky Mountains had also just emerged. The general form of the American continent was outlined in Archaean times. With the possible exception of a few species resembling the sponge, no forms of life are found in Archaean rocks. (See illustration, p. 34.)

Palaeozoic Era. — The Palaeozoic era was of very long duration. The sediments composing it are 25,000 feet thick in places. The greater part of Europe and North America were above sea-level during this period, but the

UNCONFORMABLE STRATA : CANON OF THE COLORADO RIVER

The tilted strata, originally horizontal, were deposited on the surface of the igneous rock. Subsequently the upper layers were deposited on the broken surface of the tilted layers.

THE STKUCTUEE OF THE EARTH land was many times upheaved and submerged. In North America the greater part of the Mississippi Valley was a shallow inland sea, that later became an immense marsh.

In the variety and extent of life forms the Palaeozoic era is the most noteworthy of all the geological periods. It began with the lowest form of sponges and closed with the advent of mammoth reptiles.

During this period animals with backbones appeared for the first time. Insects Avere numerous, and toward the close reptiles existed. Fishes and mollusks seem to have been the prevailing forms.

The climate was warm and moist. The vast accumulations of vegetable matter that are now the coal fields

$1 as"

Quaternary ] Champiaii (■ Glacial

Tertiary

Cretaceous

Jurassic

Devonian

Silurian

Cambrian

ORDER OI- STRATA were found in swamps of this age. 15 In North America these swamps covered much of the area that is now the central United States.

Mesozoic Era. — During the Mesozoic era both North America and Europe had grown to about their present shape. In the former division the Gulf of Mexico reached as far north as the mouth of the Ohio, and a northwestern branch of it extended nearly to the 50th parallel. In Europe all the principal mountain ranges 16 and the higher elevations of land most probably had been raised permanently above sea-level.

It was an age of gigantic reptiles. The animals of some species were from sixty to eighty feet in length. Eor the first time birds appeared. They were very much like reptiles, however, and in some species they had, instead of horny beaks, heavy jaws with socket teeth.

Cenozoic Era. — This era was largely one of uplift and mountain-making, although both in North America and Europe the various ranges and systems had received definite forms. The former was a continent of vast fresh-water lakes ; the latter of inland seas.

Most of the life forms that flourished in preceding ages were common, but one great step in advance maybe noted — the appearance of mammals. Their genera included

NORTH AMERICA IN ARCHAEAN TIMES

The shaded area shows the part of the continent above sea-ienjel.

the elephant, camel, 17 rhinoceros, wolf, deer, and horse. 13 There was a considerable advance in plant-life, and the forest trees both of North America and Europe included most of the sjDecies found to-day.

Quaternary Age. — The abrupt close of the Cenozoic era was probably due to an elevation of a large part of North America and Europe from 1,000 to 2,000 feet, and a decided lowering of temperature. The ice and snow of the north polar regions crept southward until it enveloped nearly all of Europe and the greater part of the United

States. This accession of ice is commonly known as the glacial epoch. It is marked on a stupendous scale by a movement of drift similar to that which marks the glaciers of the present time.

The changes of the Quaternary age were disastrous to life. In the area covered by glacial ice most of the species of larger mammals perished. The cave bear, horse, wolf, and reindeer survived. Many species of plants were destroyed, but many escaped.

That man existed before the close of the glacial epoch seems certain. In the caverns of Belgium, Germany, and Italy the bones of man have been found in caves along with the skeletons of animals and various implements of the chase. From the few scraps of unwritten history it

NORTH AMERICA IN CENOZOIC TIMES

The shaded area shows the part of the continent above sea-level.

seems that primitive man was a savage of the lowest type. He lived in caves and obtained his food by hunting and fishing. He did not cultivate the soil nor did he have any

THE UNITED STATES AT THE BEGINNING OF THE QUATERNARY AGE The shaded area shows the part added in recent times.

domestic animals. He had learned the use of fire, however, and from that moment his intellectual development was a question of time only.

QUESTIONS AND EXERCISES.— A mixture of iron filings, sand, and meal is gently shaken in a glass : what position will the components take when they come to rest ? Explain why.

It is sometimes assumed that the rock envelope is about forty miles, and the atmosphere about two hundred miles, in thickness. Construct a diagram on the blackboard or on paper, showing the relative thickness of each on scale in the ratio of 4000 : 40 : 200.

Obtain specimens of iron ore, marble, and dry clay, and compare the weight of pieces of the same size. If possible find the specific gravity of each. Determine, or judge by " hefting," the relative weight of the various kinds of rock in the neighborhood in which you live.

Note and describe any instances within your personal knowledge of the action of water on the rock envelope ; explain the nature of the changes and how they have been brought about.

Study the various rock formations in the neighborhood in which you live and classify them according to their origin— that is, as sedimentary or igneous.

Make a collection of them for future use.

A stream flows over a bed of limestone rock that is slightly soluble, into a lake without an outlet ; what changes in the formation of rock are likely to occur ? Will the rock formed be stratified or unstratified ? In what way may it become fossiliferous ?

From the official State reports find the order and distribution of rock strata in the State in which you live, and from the information given construct a geological map.

COLLATEEAL READING

Powell.— Physiography of the United States, pp. 22-29. Le Conte. — Elements of Geology, pp. 127-132. Mill.— Realm of Nature, pp. 211-230, 249-261. Shaler. — First Book of Geology, pp. 107-124.