The Elements of Geology - William Harmon Norton (best beach reads TXT) 📗

During the long ages of the Mesozoic, mammals continued small and few, and were completely dominated by the reptiles. Their remains are exceedingly rare, and consist of minute scattered teeth,—with an occasional detached jaw,—which prove them to have been flesh or insect eaters. In the same way their affinities are seen to be with the lowest of mammals,—the MONOTREMES and MARSUPIALS. The monotremes,—such as the duckbill mole and the spiny ant-eater of Australia, reproduce by means of eggs resembling those of reptiles; the marsupials, such as the opossum and the kangaroo, bring forth their young alive, but in a very immature condition, and carry them for some time after birth in the marsupium, a pouch on the ventral side of the body.

THE CENOZOIC ERA. The last stages of the Cretaceous are marked by a decadence of the reptiles. By the end of that period the reptilian forms characteristic of the time had become extinct one after another, leaving to represent the class only the types of reptiles which continue to modern times. The day of the ammonite and the belemnite also now drew to a close, and only a few of these cephalopods were left to survive the period. It is therefore at the close of the Cretaceous that the line is drawn which marks the end of the Middle Age of geology and the beginning of the Cenozoic era, the era of modern life,—the Age of Mammals.

In place of the giant reptiles, mammals now become masters of the land, appearing first in generalized types which, during the long ages of the era, gradually evolve to higher forms, more specialized and ever more closely resembling the mammals of the present. In the atmosphere the flying dragons of the Mesozoic give place to birds and bats. In the sea, whales, sharks, and teleost fishes of modern types rule in the stead of huge swimming reptiles. The lower vertebrates, the invertebrates of land and sea, and the plants of field and forest take on a modern aspect, and differ little more from those of to-day than the plants and animals of different countries now differ from one another. From the beginning of the Cenozoic era until now there is a steadily increasing number of species of animals and plants which have continued to exist to the present time.

The Cenozoic era comprises two divisions,—the TERTIARY period and the QUATERNARY period.

In the early days of geology the formations of the entire geological record, so far as it was then known, were divided into three groups,—the PRIMARY, the SECONDARY (now known as the Mesozoic), and the TERTIARY, When the third group was subdivided into two systems, the term Tertiary was retained for the first system of the two, while the term QUATERNARY was used to designate the second.

DIVISIONS OF THE TERTIARY. The formations of the Tertiary are grouped in three divisions,—the PLIOCENE (more recent), the MIOCENE (less recent), and the EOCENE (the dawn of the recent). Each of these epochs is long and complex. Their various sub- divisions are distinguished each by its own peculiar organisms, and the changes of physical geography recorded in their strata. In the rapid view which we are compelled to take we can note only a few of the most conspicuous events of the period.

PHYSICAL GEOGRAPHY OF THE TERTIARY IN EASTERN NORTH AMERICA. The Tertiary rocks of eastern North America are marine deposits and occupy the coastal lowlands of the Atlantic and Gulf states (Fig. 260). In New England, Tertiary beds occur on the island of Martha's Vineyard, but not on the mainland; hence the shore line here stood somewhat farther out than now. From New Jersey southward the earliest Tertiary sands and clays, still unconsolidated, leave only a narrow strip of the edge of the Cretaceous between them and the Triassic and crystalline rocks of the Piedmont oldland; hence the Atlantic shore here stood farther in than now, and at the beginning of the period the present coastal plain was continental delta. A broad belt of Tertiary sea- laid limestones, sandstones, and shales surrounds the Gulf of Mexico and extends northward up the Mississippi embayment to the mouth of the Ohio River; hence the Gulf was then larger than at present, and its waters reached in a broad bay far up the Mississippi valley.

Along the Atlantic coast the Mesozoic peneplain may be traced shoreward to where it disappears from view beneath an unconformable cover of early Tertiary marine strata. The beginning of the Tertiary was therefore marked by a subsidence. The wide erosion surface which at the close of the Mesozoic lay near sea level where the Appalachian Mountains and their neighboring plateaus and uplands now stand was lowered gently along its seaward edge beneath the Tertiary Atlantic to receive a cover of its sediments.

As the period progressed slight oscillations occurred from time to time. Strips of coastal plain were added to the land, and as early as the close of the Miocene the shore lines of the Atlantic and Gulf states had reached well-nigh their present place. Louisiana and Florida were the last areas to emerge wholly from the sea,— Florida being formed by a broad transverse upwarp of the continental delta at the opening of the Miocene, forming first an island, which afterwards was joined to the mainland.

THE PACIFIC COAST. Tertiary deposits with marine fossils occur along the western foothills of the Sierra Nevadas, and are crumpled among the mountain masses of the Coast Ranges; it is hence inferred that the Great Valley of California was then a border sea, separated from the ocean by a chain of mountainous islands which were upridged into the Coast Ranges at a still later time. Tertiary marine strata are spread over the lower Columbia valley and that of Puget Sound, showing that the Pacific came in broadly there.

THE INTERIOR OF THE WESTERN UNITED STATES. The closing stages of the Mesozoic were marked, as we have seen, by the upheaval of the Rocky Mountains and other western ranges. The bases of the mountains are now skirted by widespread Tertiary deposits, which form the highest strata of the lofty plateaus from the level of whose summits the mountains rise. Like the recent alluvium of the Great Valley of California, these deposits imply low-lying lands when they were laid, and therefore at that time the mountains rose from near sea level. But the height at which the Tertiary strata now stand—five thousand feet above the sea at Denver, and twice that height in the plateaus of southern Utah—proves that the plateaus of which the Tertiary strata form a part have been uplifted during the Cenozoic. During their uplift, warping formed extensive basins both east and west of the Rockies, and in these basins stream-swept and lake-laid waste gathered to depths of hundreds and thousands of feet, as it is accumulating at present in the Great Valley of California and on the river plains of Turkestan. The Tertiary river deposits of the High Plains have already been described. How widespread are these ancient river plains and beds of fresh-water lakes may be seen in the map of Figure 260.

THE BAD LANDS. In several of the western states large areas of Tertiary fresh-water deposits have been dissected to a maze of hills whose steep sides are cut with innumerable ravines. The deposits of these ancient river plains and lake beds are little cemented and because of the dryness of the climate are unprotected by vegetation; hence they are easily carved by the wet-weather rills of scanty and infrequent rains. These waterless, rugged surfaces were named by the early French explorers the BAD LANDS because they were found so difficult to traverse. The strata of the Bad Lands contain vast numbers of the remains of the animals of Tertiary times, and the large amount of barren surface exposed to view makes search for fossils easy and fruitful. These desolate tracts are therefore frequently visited by scientific collecting expeditions.

MOUNTAIN MAKING IN THE TERTIARY. The Tertiary period included epochs when the earth's crust was singularly unquiet. From time to time on all the continents subterranean forces gathered head, and the crust was bent and broken and upridged in lofty mountains.

The Sierra Nevada range was formed, as we have seen, by strata crumpling at the end of the Jurassic. But since that remote time the upfolded mountains had been worn to plains and hilly uplands, the remnants of whose uplifted erosion surfaces may now be traced along the western mountain slopes. Beginning late in the Tertiary, the region was again affected by mountain-making movements. A series of displacements along a profound fault on the eastern side tilted the enormous earth block of the Sierras to the west, lifting its eastern edge to form the lofty crest and giving to the range a steep eastern front and a gentle descent toward the Pacific.

The Coast Ranges also have had a complex history with many vicissitudes. The earliest foldings of their strata belong to the close of the Jurassic, but it was not until the end of the Miocene that the line of mountainous islands and the heavy sediments which had been deposited on their submerged flanks were crushed into a continuous mountain chain. Thick Pliocene beds upon their sides prove that they were depressed to near sea level during the later Tertiary. At the close of the Pliocene the Coast Ranges rose along with the upheaval of the Sierra, and their gradual uplift has continued to the present time.

The numerous north-south ranges of the Great Basin and the Mount
Saint Elias range of Alaska were also uptilted during the
Tertiary.

During the Tertiary period many of the loftiest mountains of the earth—the Alps, the Apennines, the Pyrenees, the Atlas, the Caucasus, and the Himalayas—received the uplift to which they owe most of their colossal bulk and height, as portions of the Tertiary sea beds now found high upon their flanks attest. In the Himalayas, Tertiary marine limestones occur sixteen thousand five hundred feet above sea level.

VOLCANIC ACTIVITY IN THE TERTIARY. The vast deformations of the Tertiary were accompanied on a corresponding scale by outpourings of lava, the outburst of volcanoes, and the intrusion of molten masses within the crust. In the Sierra Nevadas the Miocene river gravels of the valleys of the western slope, with their placer deposits of gold, were buried beneath streams of lava and beds of tuff. Volcanoes broke forth along the Rocky Mountains and on the plateaus of Utah, New Mexico, and Arizona.

Mount Shasta and the immense volcanic piles of the Cascades date from this period. The mountain basin of the Yellowstone Park was filled to a depth of several thousand feet with tuffs and lavas, the oldest dating as far back as the beginning of the Tertiary. Crandall volcano was reared in the Miocene and the latest eruptions of the Park are far more recent.

THE COLUMBIA AND SNAKE RIVER LAVAS. Still more important is the plateau of lava, more than two hundred thousand square miles in area, extending from the Yellowstone Park to the Cascade Mountains, which has been built from Miocene times to the present.

Over this plateau, which occupies large portions of Idaho, Washington, and Oregon, and extends into northern California and Nevada, the country rock is basaltic lava. For thousands of square miles the surface is a lava plain which meets the boundary mountains as a lake or sea meets a rugged and deeply indented coast. The floods of molten rock spread up the mountain valleys for a score of miles and more, the intervening spurs rising above the lava like long peninsulas, while here and there an isolated peak was left to tower above the inundation like an island off a submerged shore.

The rivers which drain the plateau—the Snake, the Columbia, and their tributaries—have deeply trenched it, yet their canyons, which reach the depth of several thousand feet, have not been worn to the base of the lava except near the margin and where they cut the summits of mountains drowned beneath the flood. Here and there the plateau has been deformed. It has been upbent into great folds, and broken into immense blocks of bedded lava, forming mountain ranges, which run parallel with the Pacific coast line. On the edges of these tilted blocks the thickness of the lava is seen to be fully five thousand feet. The plateau has been built, like that of Iceland, of innumerable overlapping sheets of lava. On the canyon walls they weather back in horizontal terraces and long talus slopes. One may distinguish each successive flow by its dense central portion, often jointed with large vertical columns, and the upper portion with its mass of confused irregular columns and scoriaceous surface. The average thickness of the flows seems to be about seventy-five feet.

The plateau was long in building. Between the layers are found in places old soil beds and forest grounds and the sediments of lakes. Hence