Throughout the glacial period, other activities were in progress, but the action of the ice overshadows all other phenomena in the northern latitudes. No other terrestrial activity leaves such characteristic evidence on the topography. The distinctive boulders and surface cleavages, such as potholes, lakes, displaced stone, and rock flour, are to be found in connection with no other phenomena in nature. The ice is also responsible for those gentle swells, or surface undulations, known as drumlins. And a glacier, as it advances, displaces rivers and changes the whole face of the earth. Glaciers alone leave behind them those telltale drifts – the ground, lateral, and terminal moraines. These drifts, particularly the ground moraines, extend from the eastern seaboard north and westward in North America and are found in Europe and Siberia.
750,000 years ago, the fourth ice sheet, a union of the North American central and eastern ice fields, was well on its way south; at its height, it reached to southern Illinois, displacing the Mississippi River fifty miles to the west, and in the east, it extended as far south as the Ohio River and central Pennsylvania.
In Asia, the Siberian ice sheet made its southernmost invasion, while in Europe the advancing ice stopped just short of the mountain barrier of the Alps.
500,000 years ago, during the fifth advance of the ice, a new development accelerated the course of human evolution. Suddenly and in one generation the six colored races of our planet mutated from the aboriginal human stock.
In North America, the advancing fifth glacier consisted of a combined invasion by all three ice centers. The eastern lobe, however, extended only a short distance below the St. Lawrence valley, and the western ice sheet made little southern advance. But the central lobe reached south to cover most of the State of Iowa. In Europe, this invasion of the ice was not so extensive as the preceding one.
250,000 years ago, the sixth and last glaciation began. And despite the fact that the northern highlands had begun to sink slightly, this was the period of greatest snow deposition on the northern ice fields.
In this invasion, the three great ice sheets coalesced into one vast ice mass, and all of the western mountains participated in this glacial activity. This was the largest of all ice invasions in North America; the ice moved south over fifteen hundred miles from its pressure centers, and North America experienced its lowest temperatures.
150,000 years ago, the sixth and last glacier reached its farthest points of southern extension, the western ice sheet crossing just over the Canadian border; the central coming down into Kansas, Missouri, and Illinois; the eastern sheet advancing south and covering the greater portion of Pennsylvania and Ohio.
This is the glacier that sent forth the many tongues, or ice lobes, which carved out the present-day lakes, great and small. During its retreat, the North American system of Great Lakes was produced. And our geologists have very accurately deduced the various stages of this development and have correctly surmised that these bodies of water did, at different times, empty first into the Mississippi valley, then eastward into the Hudson valley, and finally by a northern route into the St. Lawrence. It is thirty-seven thousand years since the connected Great Lakes system began to empty out over the present Niagara route.
100,000 years ago, during the retreat of the last glacier, the vast polar ice sheets began to form, and the center of ice accumulation moved consider- ably northward. And as long as the polar regions continue to be covered with ice, it is hardly possible for another glacial age to occur, regardless of future land elevations or modification of ocean currents.
This last glacier was one hundred thousand years advancing, and it required a like span of time to complete its northern retreat. The temperate regions have been free from the ice for a little over fifty thousand years.
The rigorous glacial period destroyed many species and radically changed numerous others. Many were sorely sifted by the to-and-fro migration which was made necessary by the advancing and retreating ice. Those animals which followed the glaciers back and forth over the land were the bear, bison, reindeer, musk ox, mammoth, and mastodon.
The mammoth sought the open prairies, but the mastodon preferred the sheltered fringes of the forest regions. The mammoth, until a late date, ranged from Mexico to Canada; the Siberian variety became wool covered. The mastodon persisted in North America until exterminated by the red man much as the white man later killed off the bison.
In North America, during the last glaciation, the horse, tapir, llama, and saber-toothed tiger became extinct. In their places sloths, armadillos, and water hogs came up from South America.
The enforced migration of life before the advancing ice led to an extraordinary commingling of plants and of animals, and with the retreat of the final ice invasion, many arctic species of both plants and animals were left stranded high upon certain mountain peaks, whither they had journeyed to escape destruction by the glacier. And so, today, these dislocated plants and animals may be found high up on the Alps of Europe and even on the Appalachian Mountains of North America.
The ice age is the last completed geologic period, the so-called Pleistocene, over two million years in length.
35,000 years ago, marks the termination of the great ice age excepting in the polar regions of this planet. This state is also significant in that it approximates the (from the year A.D. 2003, 37,917 years ago) arrival of Adam and Eve [detailed in the Urantia Book], the biologic uplifters of Urantia, and the beginning of the Adamic dispensation, roughly corresponding to the beginning of the Holocene or post-glacial period.
This narrative, extending from the rise of mammalian life to the retreat of the ice and on down to historic times, covers a span of almost fifty million years. This is the last – the current – geologic period and is known to our researchers as the Cenozoic or recent-times era.
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