Chapter 18

Fig. 342.Map showing the Lava Sheet (shaded area) of Western India

Erosion of Tertiary mountains and plateaus.The mountains and plateaus built at various times during the Tertiary and at its commencement have been profoundly carved by erosive agents. The Sierra Nevada Mountains have been dissected onthe western slope by such canyons as those of King’s River and the Yosemite. Six miles of strata have been denuded from parts of the Wasatch Mountains since their rise at the beginning of the era. From the Colorado plateaus, whose uplift dates from the same time, there have been stripped off ten thousand feet of strata over thousands of square miles, and the colossal canyon of the Colorado has been cut after this great denudation had been mostly accomplished (Fig. 130).

On the eastern side of the continent, as we have seen, a broad peneplain had been developed by the close of the Cretaceous. The remnants of this old erosion surface are now found upwarped to various heights in different portions of its area. In southern New England it now stands fifteen hundred feet above the sea in western Massachusetts, declining thence southward and eastward to sea level at the coast. In southwestern Virginia it has been lifted to four thousand feet above the sea. Manifestly this upwarp occurred since the peneplain was formed; it is later than the Mesozoic, and the vast dissection which the peneplain has suffered since its uplift must belong to the successive cycles of Cenozoic time.

Revived by the uplift, the streams of the area trenched it as deeply as its elevation permitted, and reaching grade, opened up wide valleys and new peneplains in the softer rocks. The Connecticut valley is Tertiary in age, and in the weak Triassic sandstones (p. 370) has been widened in places to fifteen miles. Dating from the same time are the valleys of the Hudson, the Susquehanna, the Delaware, the Potomac, and the Shenandoah.

In Pennsylvania and the states lying to the south the Mesozoic peneplain lies along the summits of the mountain ridges. On the surface of this ancient plain, Tertiary erosion etched out the beautifully regular pattern of the Allegheny mountain ridges and their intervening valleys. The weaker strata of the long, regular folds were eroded into longitudinal valleys, while the hard Paleozoic sandstones, such as the Medina (p. 335)and the Pocono (p. 350), were left in relief as bold mountain walls whose even crests rise to the common level of the ancient plain. From Virginia far into Alabama the great Appalachian valley was opened to a width in places of fifty miles and more, along a belt of intensely folded and faulted strata where once was the heart of the Appalachian Mountains. InFigure 70, the summit of the Cumberland plateau (ab) marks the level of the Mesozoic peneplain, while the lower erosion levels are Tertiary and Quaternary in age.

Fig. 343.Diagram of the Allegheny Mountains, PennsylvaniaFrom Davis’Elementary Physical Geography

Life of the Tertiary Period

Vegetation and climate.The highest plants in structure, thedicotyls(such as our deciduous forest trees) and themonocotyls(represented by the palms), were introduced during the Cretaceous. The vegetable kingdom reached its culmination before the animal kingdom, and if the dividing line between the Mesozoic and the Cenozoic were drawn according to the progress ofplant life, the Cretaceous instead of the Tertiary would be made the opening period of the modern era.

The plants of the Tertiary belonged, for the most part, to genera now living; but their distribution was very different from that of the flora of to-day. In the earlier Tertiary, palms flourished over northern Europe, and in the northwestern United States grew the magnolia and laurel, along with the walnut, oak, and elm. Even in northern Greenland and in Spitzbergen there were lakes covered with water lilies and surrounded by forests of maples, poplars, limes, the cypress of our southern states, and noble sequoias similar to the “big trees” and redwoods of California. A warm climate like that of the Mesozoic, therefore, prevailed over North America and Europe, extending far toward the pole. In the later Tertiary the climate gradually became cooler. Palms disappeared from Europe, and everywhere the aspect of forests and open lands became more like that of to-day. Grasses became abundant, furnishing a new food for herbivorous animals.

Animal life of the tertiary.Little needs to be said of the Tertiary invertebrates, so nearly were they like the invertebrates of the present. Even in the Eocene, about five per cent of marine shells were of species still living, and in the Pliocene the proportion had risen to more than one half.

Fishes were of modern types. Teleosts were now abundant. The ocean teemed with sharks, some of them being voracious monsters seventy- five feet and even more in length, with a gape of jaw of six feet, as estimated by the size of their enormous sharp-edged teeth.

Snakes are found for the first time in the early Tertiary. These limbless reptiles, evolved by degeneration from lizardlike ancestors, appeared in nonpoisonous types scarcely to be distinguished from those of the present day.

Mammals of the early tertiary.The fossils of continental deposits of the earliest Eocene show that a marked advance hadnow been made in the evolution of the Mammalia. The higher mammals had appeared, and henceforth the lower mammals—the monotremes and the marsupials—are reduced to a subordinate place.

Fig. 344.Phenacodus

These first true mammals were archaic and generalized in structure. Their feet were of the primitive type, with five toes of about equal length. They were alsoplantigrades,—that is, they touched the ground with the sole of the entire foot from toe to heel. No foot had yet become adapted to swift running by a decrease in the number of digits and by lifting the heel and sole so that only the toes touch the ground,—a tread calleddigitigrade. Nor was there yet any foot like that of the cats, with sharp retractile claws adapted to seizing and tearing the prey. The forearm and the lower leg each had still two separate bones (ulna and radius, fibula and tibia), neither pair having been replaced with a single strong bone, as in the leg of the horse. The teeth also were primitive in type and of full number. The complex heavy grinders of the horse and elephant, the sharp cutting teeth of the carnivores, and the cropping teeth of the grass eaters were all still to come.

Phenacodus is a characteristic genus of the early Eocene, whose species varied in size from that of a bulldog to that of an animal a littlelarger than a sheep. Its feet were primitive, and their five toes bore nails intermediate in form between a claw and a hoof. The archaic type of teeth indicates that the animal was omnivorous in diet. A cast of the brain cavity shows that, like its associates of the time, its brain was extremely small and nearly smooth, having little more than traces of convolutions.

The long ages of the Eocene and the following epochs of the Tertiary were times of comparatively rapid evolution among the Mammalia. The earliest forms evolved along diverging lines toward the various specialized types of hoofed mammals, rodents, carnivores, proboscidians, the primates, and the other mammalian orders as we know them now. We must describe the Tertiary mammals very briefly, tracing the lines of descent of only a few of the more familiar mammals of the present.

The horse.The pedigree of the horse runs back into the early Eocene through many genera and species to a five-toed,[3]short-legged ancestor little bigger than a cat. Its descendants gradually increased in stature and became better and better adapted to swift running to escape their foes. The leg became longer, and only the tip of the toes struck the ground. The middle toe (digit number three), originally the longest of the five, steadily enlarged, while the remaining digits dwindled and disappeared. The inner digit, corresponding to the great toe and thumb, was the first to go. Next number five, the little finger, was also dropped. By the end of the Eocene a three-toed genus of the horse family had appeared, as large as a sheep. The hoof of digit number three now supported most of the weight, but the slender hoofs of digits two and four were still serviceable. In the Miocene the stature of the ancestors of the horse increased to that of a pony. The feet were still three-toed, but the side hoofs were now mere dewclaws and scarcely touched the ground. The evolution of the family was completed in the Pliocene. The middle toe was enlarged still more, the side toes were dropped, and the palm and foot bones which supported them were reduced to splints.

[3]Or, more accurately, with four perfect toes and a rudimentary fifth corresponding to the thumb.

Fig. 345.Development of Forefoot (A), the Forearm (B), the Molar (C), of the Horse Family

While these changes were in progress the radius and ulna of the fore limb became consolidated to a single bone; and in the hind limb the fibula dwindled to a splint, while the tibia was correspondingly enlarged. The molars, also gradually lengthened, and became more and more complex on their grinding surface; the neck became longer; the brain steadily increased in size and its convolutions became more abundant. The evolution of the horse has made for greater fleetness and intelligence.

The rhinoceros and tapir.These animals, which are grouped with the horse among theodd-toed(perissodactyl) mammals, are now verging toward extinction. In the rhinoceros, evolution seems to have taken the opposite course from that of the horse. As the animal increased in size it became more clumsy, its limbs became shorter and more massive, and, perhaps because of its great weight, the number of digitswere not reduced below the number three. Like other large herbivores, the rhinoceros, too slow to escape its enemies by flight, learned to withstand them. It developed as its means of defense a nasal horn.

Peculiar offshoots of the line appeared at various times in the Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks, resembling in aspect the hippopotamus, lived along the water courses of the plains east of the Rockies, and its bones are now found by the thousands in the Miocene of Kansas. Another developed along a line parallel to that of the horse, and herds of these light-limbed and swift-footed running rhinoceroses ranged the Great Plains from the Dakotas southward.

Fig. 346.A Tertiary Mastodon

The tapirs are an ancient family which has changed but little since it separated from the other perissodactyl stocks in the early Tertiary. At present, tapirs are found only in South America and southern Asia,—a remarkable distribution which we could not explain were it not that the geological record shows that during Tertiary times tapirs ranged throughout the northern hemisphere, making their way to South America late in that period. During the Pleistocene they became extinct over all the intervening lands between the widely separated regions where now they live. The geographic distribution of animals, as well as their relationships and origins, can be understood only through a study of their geological history.

Fig. 347.Head of Dinothere

The proboscidians.This unique order of hoofed mammals, of which the elephant is the sole survivor, has been traced back to the close of the Eocene. In the middle and later Tertiary it was represented by huge creatures so nearly akin to the mastodons of the Pleistocene that they are often included in that genus. The TertiaryMastodonwas furnished with a long, flexible proboscis, and armed with two pairs of long, straight ivory tusks, the pair of the lower jaw being smaller.

TheDinotherewas a curious offshoot of the line, which developed in the Miocene in Europe. In this immense proboscidian, whose skull was three feet long, the upper pair of tusks had disappeared, and those of the lower jaw were bent down with a backward curve in walrus fashion.

Fig. 348.Crown of Mastodon Tooth

Fig. 349.Tooth of an Extinct Elephant, the Mammoth

In the trueelephants, which do not appear until near the close of the Tertiary, the lower jaw loses its tusks and the grinding teeth become exceedingly complex in structure. The grinding teeth of the mastodon had long roots and low crowns crossed by four or five peaked enameled ridges. In the teeth of the true elephants the crown has become deep, and the ridges of enamel have changed to numerous upright, platelike folds, their interspaces filled with cement. The two genera—Mastodon and Elephant—are connected by species whose teeth are intermediate in pattern. The proboscidians culminated in the Pliocene, when some of the giant elephants reached a height of fourteen feet.

Fig. 350. Evolution of the Artiodactyl Foot, Illustrated by Existing FamiliesA, pig;B, roebuck;C, sheep;D, camel

The artiodactylscomprise the hoofed Mammalia which have an even number of toes, such as cattle, sheep, and swine. Like the perissodactyls, they are descended from the primitive five-toed plantigrade mammals of the lowest Eocene. In their evolution, digit number one was first dropped, and the middle pair became larger and more massive, while the side digits, numbers two and five, became shorter, weaker, and less serviceable. Thefour-toed artiodactylsculminated in the Tertiary; at present they are represented only by the hippopotamus and the hog.Along the main line of the evolution of the artiodactyls the side toes, digits two and five, disappeared, leaving as proof that they once existed the corresponding bones of palm and sole as splints. Thetwo-toed artiodactyls, such as the camels, deer, cattle, and sheep, are now the leading types of the herbivores.

Swine and peccariesare two branches of a common stock, the first developing in the Old World and the second in the New. In the Miocene a noticeable offshoot of the line was a gigantic piglike brute, a root eater, with a skull a yard in length, whose remains are now found in Colorado and South Dakota.

Camels and llamas.The line of camels and llamas developed in North America, where the successive changes from an early Eocene ancestor, no larger than a rabbit, are traced step by step to the present forms, as clearly as is the evolution of the horse. In the late Miocene some of the ancestral forms migrated to the Old World by way of a land connection where Bering Strait now is, and there gave rise to the camels and dromedaries. Others migrated into South America, which had now been connected with our own continent, and these developed into the llamas and guanacos, while those of the race which remained in North America became extinct during the Pleistocene.Some peculiar branches of the camel stem appeared in North America. In the Pliocene arose a llama with the long neck and limbs of a giraffe, whose food was cropped from the leaves and branches of trees. Far more generalized in structure was theOreodon, an animal related to the camels, but with distinct affinities also with other lines, such as those of the hog and deer. These curious creatures were much like the peccary in appearance, except for their long tails. In the middle Eocene they roamed in vast herds from Oregon to Kansas and Nebraska.

Some peculiar branches of the camel stem appeared in North America. In the Pliocene arose a llama with the long neck and limbs of a giraffe, whose food was cropped from the leaves and branches of trees. Far more generalized in structure was theOreodon, an animal related to the camels, but with distinct affinities also with other lines, such as those of the hog and deer. These curious creatures were much like the peccary in appearance, except for their long tails. In the middle Eocene they roamed in vast herds from Oregon to Kansas and Nebraska.

The ruminants.This division of the artiodactyls includes antelopes, deer, oxen, bison, sheep, and goats,—all of which belong to a common stock which took its rise in Europe in the upper Eocene from ancestral forms akin to those of the camels. In the Miocene the evolution of the two-toed artiodactyl foot was well-nigh completed. Bonelike growths appeared on the head, and the two groups of the ruminants became specialized,—thedeer with bony antlers, shed and renewed each year, and the ruminants with hollow horns, whose two bony knobs upon the skull are covered with permanent, pointed, horny sheaths.

The ruminants evolved in the Old World, and it was not until the later Miocene that the ancestors of the antelope and of some deer found their way to North America. Mountain sheep and goats, the bison and most of the deer, did not arrive until after the close of the Tertiary, and sheep and oxen were introduced by man.The hoofed mammals of the Tertiary included many offshoots from the main lines which we have traced. Among them were a number of genera of clumsy, ponderous brutes, some almost elephantine in their bulk.

The hoofed mammals of the Tertiary included many offshoots from the main lines which we have traced. Among them were a number of genera of clumsy, ponderous brutes, some almost elephantine in their bulk.

The carnivores.The ancestral lines of the families of the flesh eaters—such as the cats (lions, tigers, etc.), the bears, the hyenas, and the dogs (including wolves and foxes)—converge in thecreodontsof the early Eocene,—an order so generalized that it had affinities not only with the carnivores but also with the insect eaters, the marsupials, and the hoofed mammals as well. From these primitive flesh eaters, with small and simple brains, numerous small teeth, and plantigrade tread, the different families of the carnivores of the present have slowly evolved.

Dogs and bears.The dog family diverged from the creodonts late in the Eocene, and divided into two branches, one of which evolved the wolves and the other the foxes. An offshoot gave rise to the family of the bears, and so closely do these two families, now wide apart, approach as we trace them back in Tertiary times that theAmphicyon, a genus doglike in its teeth and bearlike in other structures, is referred by some to the dog and by others to the bear family. The well-known plantigrade tread of bears is a primitive characteristic which has survived from their creodont ancestry.

Cats.The family of the cats, the most highly specialized of all the carnivores, divided in the Tertiary into two main branches. One, thesaber-tooth tigers(Fig. 351), which takes its name fromtheir long, saberlike, sharp-edged upper canine teeth, evolved a succession of genera and species, among them some of the most destructive beasts of prey which ever scourged the earth. They were masters of the entire northern hemisphere during the middle Tertiary, but in Europe during the Pliocene they declined, from unknown causes, and gave place to the other branch of cats,—which includes the lions, tigers, and leopards. In the Americas the saber-tooth tigers long survived the epoch.

Fig. 351.Saber-Tooth Tiger

Marine mammals.The carnivorous mammals of the sea—whales, seals, walruses, etc.—seem to have been derived from some of the creodonts of the early Tertiary by adaptation to aquatic life. Whales evolved from some land ancestry at a very early date in the Tertiary; in the marine deposits of the Eocene are found the bones of theZeuglodon, a whalelike creature seventy feet in length.

Primates.This order, which includes lemurs, monkeys, apes, and man, seems to have sprung from a creodont or insectivorous ancestry in the lower Eocene. Lemur-like types, with small, smooth brains, were abundant in the United States in the early Tertiary, but no primates have been found here in the middle Tertiary and later strata. In Europe true monkeys wereintroduced in the Miocene, and were abundant until the close of the Tertiary, when they were driven from the continent by the increasing cold.

Advance of the mammalia during the tertiary.During the several millions of years comprised in Tertiary time the mammals evolved from the lowly, simple types which tenanted the earth at the beginning of the period, into the many kinds of highly specialized mammals of the Pleistocene and the present, each with the various structures of the body adapted to its own peculiar mode of life. The swift feet of the horse, the horns of cattle and the antlers of the deer, the lion’s claws and teeth, the long incisors of the beaver, the proboscis of the elephant, were all developed in Tertiary times. In especial the brain of the Tertiary mammals constantly grew larger relatively to the size of body, and the higher portion of the brain—the cerebral lobes—increased in size in comparison with the cerebellum. Some of the hoofed mammals now have a brain eight or ten times the size of that of their early Tertiary predecessors of equal bulk. Nor can we doubt that along with the increasing size of brain went a corresponding increase in the keenness of the senses, in activity and vigor, and in intelligence.

CHAPTER XXII

THE QUATERNARY

The last period of geological history, the Quaternary, may be said to have begun when all, or nearly all, living species of mollusks and most of the existing mammals had appeared.

It is divided into two great epochs. The first, thePleistoceneorGlacial epoch, is marked off from the Tertiary by the occupation of the northern parts of North America and Europe by vast ice sheets; the second, theRecent epoch, began with the disappearance of the ice sheets from these continents, and merges into the present time.

The Pleistocene Epoch

We now come to an episode of unusual interest, so different was it from most of the preceding epochs and from the present, and so largely has it influenced the conditions of man’s life.

The records of the Glacial epoch are so plain and full that we are compelled to believe what otherwise would seem almost incredible,—that following the mild climate of the Tertiary came a succession of ages when ice fields, like that of Greenland, shrouded the northern parts of North America and Europe and extended far into temperate latitudes.

The drift.Our studies of glaciers have prepared us to decipher and interpret the history of the Glacial epoch, as it is recorded in the surface deposits known as the drift. Over most of Canada and the northern states this familiar formation is exposed to view in nearly all cuttings which pass below the surface soil. The drift includes two distinct classes of deposits,—the unstratified drift laid down by glacier ice, and the stratified drift spread by glacier waters.

The materials of the drift are in any given place in part unlike the rock on which it rests. They cannot be derived from the underlying rock by weathering, but have been brought from elsewhere. Thus where a region is underlain by sedimentary rocks, as is the drift-covered area from the Hudson River to the Missouri, the drift contains not only fragments of limestone, sandstone, and shale of local derivation, but also pebbles of many igneous and metamorphic rocks, such as granites, gneisses, schists, dike rocks, quartzites, and the quartz of mineral veins, whose nearest source is the Archean area of Canada and the states of our northern border. The drift received its name when it was supposed that the formation had been drifted by floods and icebergs from outside sources,—a theory long since abandoned.

Fig. 352.Stratified Drift overlaying Unstratified Drift, Massachusetts

The distribution also of the drift points clearly to its peculiar origin. Within the limits of the glaciated area it covers the country without regard to the relief, mantling with its débris not only lowlands and valleys but also highlands and mountain slopes.The boundary of the drift is equally independent of the relief of the land, crossing hills and plains impartially, unlike water-laid deposits, whose margins, unless subsequently deformed, are horizontal. The boundary of the drift is strikingly lobate also, bending outward in broad, convex curves, where there are no natural barriers in the topography of the country to set it such a limit. Under these conditions such a lobate margin cannot belong to deposits of rivers, lakes, or ocean, but is precisely that which would mark the edge of a continental glacier which deployed in broad tongues of ice.

The boundary of the drift is equally independent of the relief of the land, crossing hills and plains impartially, unlike water-laid deposits, whose margins, unless subsequently deformed, are horizontal. The boundary of the drift is strikingly lobate also, bending outward in broad, convex curves, where there are no natural barriers in the topography of the country to set it such a limit. Under these conditions such a lobate margin cannot belong to deposits of rivers, lakes, or ocean, but is precisely that which would mark the edge of a continental glacier which deployed in broad tongues of ice.

The rock surface underlying the drift.Over much of its area the drift rests on firm, fresh rock, showing that both the preglacial mantle of residual waste and the partially decomposed and broken rock beneath it have been swept away. The underlying rock, especially if massive, hard, and of a fine grain, has often been ground down to a smooth surface and rubbed to a polish as perfect as that seen on the rock beside an Alpine glacier where the ice has recently melted back. Frequently it has been worn to the smooth, rounded hummocks known as roches moutonnées, and even rocky hills have been thus smoothed to flowing outlines like roches moutonnées on a gigantic scale. The rock pavement beneath the drift is also marked by long, straight, parallel scorings, varying in size from deep grooves to fine striae as delicate as the hair lines cut by an engraver’s needle. Where the rock is soft or closely jointed it is often shattered to a depth of several feet beneath the drift, while stony clay has been thrust in among the fragments into which the rock is broken.

In the presence of these glaciated surfaces we cannot doubt that the area of the drift has been overridden by vast sheets of ice which, in their steady flow, rasped and scored the rock bed beneath by means of the stones with which their basal layers were inset, and in places plucked and shattered it.

Till.The unstratified portion of the drift consists chiefly of sheets of dense, stony clay called till, which clearly are theground moraines of ancient continental glaciers. Till is an unsorted mixture of materials of all sizes, from fine clay and sand, gravel, pebbles, and cobblestones, to large bowlders. The stones of the till are of many kinds, some having been plucked from the bed rock of the locality where they are found, and others having been brought from outside and often distant places. Land ice is the only agent known which can spread unstratified material in such extensive sheets.

Thefine materialof the till comes from two different sources. In part it is derived from old residual clays, which in the making had been leached of the lime and other soluble ingredients of the rock from which they weathered. In part it consists of sound rock ground fine; a drop of acid on fresh, clayey till often proves by brisk effervescence that the till contains much undecayed limestone flour. The ice sheet, therefore, both scraped up the mantle of long-weathered waste which covered the country before its coming, and also ground heavily upon the sound rock underneath, and crushed and wore to rock flour the fragments which it carried.

The color of unweathered till depends on that of the materials of which it is composed. Where red sandstones have contributed largely to its making, as over the Triassic sandstones of the eastern states and the Algonkian sandstones about Lake Superior, the drift is reddish. When derived in part from coaly shales, as over many outcrops of the Pennsylvanian, it may when moist be almost black. Fresh till is normally a dull gray or bluish, so largely is it made up of the grindings of unoxidized rocks of these common colors.

Except where composed chiefly of sand or coarser stuff, unweathered till is often exceedingly dense. Can you suggest by what means it has been thus compacted? Did the ice fields of the Glacial epoch bear heavy surface moraines like the medial and lateral moraines of valley glaciers? Where was the greater part of the load of these ice fields carried, judging from what you know of the glaciers of Greenland?

Bowlders of the drift.The pebbles and bowlders of the drift are in part stream gravels, bowlders of weathering, and other coarse rock waste picked up from the surface of the country by the advancing ice, and in part are fragments plucked from ledges of sound rock after the mantle of waste had been removed. Many of the stones of the till are dressed as only glacier ice can do; their sharp edges have been blunted and their sides faceted and scored.

Fig. 353.A Drumlin, Wisconsin

We may easily find all stages of this process represented among the pebbles of the till. Some are little worn, even on their edges; some are planed and scored on one side only; while some in their long journey have been ground down to many facets and have lost much of their original bulk. Evidently the ice played fast and loose with a stone carried in its basal layers, now holding it fast and rubbing it against the rock beneath, now loosening its grasp and allowing the stone to turn.Bowlders of the drift are sometimes found on higher ground than their parent ledges. Thus bowlders have been left on the sides of Mount Katahdin, Maine, which were plucked from limestone ledges twelve miles distant and three thousand feet lower than their resting place. In other cases stones have been carried over mountain ranges, as in Vermont, where pebbles of Burlington red sandstone were dragged over the Green Mountains, three thousand feet in height, and left in the Connecticut valley sixty miles away. No other geological agent than glacier ice could do this work.The bowlders of the drift are often large. Bowlders ten and twenty feet in diameter are not uncommon, and some are known whose diameter exceeds fifty feet. As a rule the average size of bowlders decreases with increasing distance from their sources. Why?

Bowlders of the drift are sometimes found on higher ground than their parent ledges. Thus bowlders have been left on the sides of Mount Katahdin, Maine, which were plucked from limestone ledges twelve miles distant and three thousand feet lower than their resting place. In other cases stones have been carried over mountain ranges, as in Vermont, where pebbles of Burlington red sandstone were dragged over the Green Mountains, three thousand feet in height, and left in the Connecticut valley sixty miles away. No other geological agent than glacier ice could do this work.

The bowlders of the drift are often large. Bowlders ten and twenty feet in diameter are not uncommon, and some are known whose diameter exceeds fifty feet. As a rule the average size of bowlders decreases with increasing distance from their sources. Why?

Till plains.The surface of the drift, where left in its initial state, also displays clear proof of its glacial origin. Over largeareas it is spread in level plains of till, perhaps bowlder- dotted, similar to the plains of stony clay left in Spitzbergen by the recent retreat of some of the glaciers of that island. In places the unstratified drift is heaped in hills of various kinds, which we will now describe.

Fig. 354.Map of a portion of a Drumlin Area near Oswego, New York

Drumlins.Drumlins are smooth, rounded hills composed of till, elliptical in base, and having their longer axes parallel to the movement of the ice as shown by glacial scorings. Theycrowd certain districts in central New York and in southern Wisconsin, where they may be counted by the thousands. Among the numerous drumlins about Boston is historic Bunker Hill.

Drumlins are made of ground moraine. They were accumulated and given shape beneath the overriding ice, much as are sand bars in a river, or in some instances were carved, like roches moutonnées, by an ice sheet out of the till left by an earlier ice invasion.

Terminal moraines.The glaciated area is crossed by belts of thickened drift, often a mile or two, and sometimes even ten miles and more, in breadth, which lie transverse to the movement of the ice and clearly are the terminal moraines of ancient ice sheets, marking either the limit of their farthest advance or pauses in their general retreat.

Fig. 355.Terminal Moraine, Staten Island

The surface of these moraines is a jumble of elevations and depressions, which vary from low, gentle swells and shallow sags to sharp hills, a hundred feet or so in height, and deep, steep-sided hollows. Such tumultuous hills and hummocks, setwith depressions of all shapes, which usually are without outlet and are often occupied by marshes, ponds, and lakes, surely cannot be the work of running water. The hills are heaps of drift, lodged beneath the ice edge or piled along its front. The basins were left among the tangle of morainic knolls and ridges (Fig. 105) as the margin of the ice moved back and forth. Some bowl-shaped basins were made by the melting of a mass of ice left behind by the retreating glacier and buried in its débris.

Fig. 356.Esker, New York

The stratified drift.Like modern glaciers the ice sheets of the Pleistocene were ever being converted into water about their margins. Their limits on the land were the lines where their onward flow was just balanced by melting and evaporation. On the surface of the ice along the marginal zone, rivulets no doubt flowed in summer, and found their way through crevasses to the interior of the glacier or to the ground. Subglacial streams, like those of the Malaspina glacier, issued from tunnels in the ice, and water ran along the melting ice front as it is seen to do about the glacier tongues of Greenland. All these glacier waters flowed away down the chief drainage channels in swollen rivers loaded with glacial waste.

It is not unexpected therefore that there are found, over all the country where the melting ice retreated, deposits made of the same materials as the till, but sorted and stratified by running water. Some of these were deposited behind the ice front in ice-walled channels, some at the edge of the glaciers by issuing streams, and others were spread to long distances in front of the ice edge by glacial waters as they flowed away.

Eskersare narrow, winding ridges of stratified sand and gravel whose general course lies parallel with the movement of the glacier. These ridges, though evidently laid by running water, do not follow lines of continuous descent, but may be found to cross river valleys and ascend their sides. Hence the streams by which eskers were laid did not flow unconfined upon the surface of the ground. We may infer that eskers were deposited in the tunnels and ice-walled gorges of glacial streams before they issued from the ice front.

Fig. 357. Kames, New York

Kamesare sand and gravel knolls, associated for the most part with terminal moraines, and heaped by glacial waters along the margin of the ice.

Fig. 358.Diagram Illustrating the Formation of Kame Terracesi, glacier ice;t,t, terraces

Kame terracesare hummocky embankments of stratified drift sometimes found in rugged regions along the sides of valleys. In these valleys long tongues of glacier ice lay slowly melting. Glacial waters took their way between the edges of the glaciers and the hillside, and here deposited sand and gravel in rude terraces.

Outwash plainsare plains of sand and gravel which frequently border terminal moraines on their outward face, and were spread evidently by outwash from the melting ice. Outwash plains are sometimes pitted by bowl-shaped basins where ice blocks were left buried in the sand by the retreating glacier.

Valley trainsare deposits of stratified drift with which river valleys have been aggraded. Valleys leading outward from the ice front were flooded by glacial waters and were filled often to great depths with trains of stream-swept drift. Since the disappearance of the ice these glacial flood plains have been dissected by the shrunken rivers of recent times and left on either side the valley in high terraces. Valley trains head in morainic plains, and their material grows finer down valley and coarser toward their sources. Their gradient is commonly greater than that of the present rivers.

The extent of the drift.The extent of the drift of North America and its southern limits are best seen inFigure 359. Its area is reckoned at about four million square miles. The ice fields which once covered so much of our continent were all together ten times as large as the inland ice of Greenland, and about equal to the enormous ice cap which now covers the antartic regions.

The ice field of Europe was much smaller, measuring about seven hundred and seventy thousand square miles.

Centers of dispersion.The direction of the movement of the ice is recorded plainly in the scorings of the rock surface, in the shapes of glaciated hills, in the axes of drumlins and eskers, and in trains of bowlders, when the ledges from which they were plucked can be discovered. In these ways it has been proved that in North America there were three centers where ice gathered to the greatest depth, and from which it flowed in all directions outward. There were thus three vast ice fields,—one theCordilleran, which lay upon the Cordilleras of British America; one theKeewatin, which flowed out from theprovince of Keewatin, west of Hudson Bay; and one theLabradorice field, whose center of dispersion was on the highlands of the peninsula of Labrador. As shown inFigure 359, the western ice field extended but a short way beyond the eastern foothills of the Rocky Mountains, where perhaps it met the far-traveled ice from the great central field. The Keewatin and the Labrador ice fields flowed farthest toward the south, and in the Mississippi valley the one reached the mouth of the Missouri and the other nearly to the mouth of the Ohio. In Minnesota and Wisconsin and northward they merged in one vast field.

Fig. 359. Hypothetical Map of the Pleistocene Ice Sheets of North AmericaFrom Salisbury’sGlacial Geology of New Jersey

The thickness of the ice was so great that it buried the highest mountains of eastern North America, as is proved by the transported bowlders which have been found upon their summits. If the land then stood at its present height above sea level, and if the average slope of the ice were no more than ten feet to the mile,—a slope so gentle that the eye could not detect it and less than half the slope of the interior of the inland ice of Greenland,—the ice plateaus about Hudson Bay must have reached a thickness of at least ten thousand feet.

Fig. 360.Hypothetical Map of the Pleistocene Ice Sheet of Europe

In Europe the Scandinavian plateau was the chief center of dispersion. At the time of greatest glaciation a continuous field of ice extended from the Ural Mountains to the Atlantic, where, off the coasts of Norway and the British Isles, it met the sea in an unbroken ice wall. On the south it reached to southern England, Belgium, and central Germany, and deployed on the eastern plains in wide lobes over Poland and central Russia (Fig. 360).At the same time the Alps supported giant glaciers many times the size of the surviving glaciers of to-day, and a piedmont glacier covered the plains of northern Switzerland.

At the same time the Alps supported giant glaciers many times the size of the surviving glaciers of to-day, and a piedmont glacier covered the plains of northern Switzerland.

The thickness of the drift.The drift is far from uniform in thickness. It is comparatively thin and scanty over the Laurentian highlands and the rugged regions of New England, while from southern New York and Ontario westward over the Mississippi valley, and on the great western plains of Canada, it exceeds an average of one hundred feet over wide areas, and in places has five and six times that thickness. It was to thismarginal belt that the ice sheets brought their loads, while northwards, nearer the centers of dispersion, erosion was excessive and deposition slight.

Successive ice invasions and their drift sheets.Recent studies of the drift prove that it does not consist of one indivisible formation, but includes a number of distinct drift sheets, each with its own peculiar features. The Pleistocene epoch consisted, therefore, of several glacial stages,—during each of which the ice advanced far southward,—together with the intervening interglacial stages when, under a milder climate, the ice melted back toward its sources or wholly disappeared.

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