By the close of the preceding period the lands of the northeastern part of North America and of northern Europe were highly elevated on an extensive scale, in North America vast areas rising up to 30,000 feet and more. Mild climates had formerly prevailed over these northern regions, and the arctic waters were all open to evaporation, and they continued to be ice-free until almost the close of the glacial period. Simultaneously with these land elevations the ocean currents shifted, and the seasonal wind changed their direction. These conditions eventually produced an almost constant precipitation of moisture from the movement of the heavily saturated atmosphere over the northern highlands. Snow began to fall on these elevated and therefore cool regions, and it continued to fall until it had attained a depth of 20,000 feet. The areas of the greatest depth of snow, together with altitude, determined the central points of subsequent glacial pressure flows. And the ice age persisted just as long as this excessive precipitation continued to cover these northern highlands with this enormous mantle of snow, which soon metamorphosed into solid but creeping ice.
The great ice sheets of this period were all located on elevated high- lands, not in mountainous regions where they are found today. One half of the glacial ice was in North America, one fourth in Eurasia, and one fourth elsewhere, chiefly in Antarctica. Africa was little affected by the ice, but Australia was almost covered with the Antarctic ice blanket.
The northern regions of this world have experienced six separate and distinct ice invasions, although there were scores of advances and recessions associated with the activity of each individual ice sheet. The ice in North America collected in two and, later, three centers. Greenland was covered, and Iceland was completely buried beneath the ice flow. In Europe, the ice at various times covered the British Isles excepting the coast of southern England, and it overspread western Europe down to France.
2,000,000 years ago, the first North American glacier started its southern advance. The ice age was now in the making, and this glacier consumed nearly one million years in its advance from, and retreat back toward the northern pressure centers. The central ice sheet extended south as far as Kansas; the eastern and western ice centers were not then so extensive.
1,500,000 years ago, the first great glacier was retreating northward. In the meantime, enormous quantities of snow had been falling on Greenland and on the northeastern part of North America, and erelong this eastern ice mass began to flow southward. This was the second invasion of the ice.
These first two ice invasions were not extensive in Eurasia. During these early epochs of the ice age, North America was overrun with mastodons, woolly mammoths, horses, camels, deer, musk oxen, bison, ground sloths, giant beavers, saber-toothed tigers, sloths as large as elephants, and many groups of the cat and dog families. But from this time forward they were rapidly reduced in numbers by the increasing cold of the glacial period. Toward the close of the ice age the majority of these animal species were extinct in North America.
Away from the ice, the land and water life of the world was little changed. Between the ice invasions the climate was about as mild as at present, perhaps a little warmer. The glaciers were, after all, local phenomena, though they spread out to cover enormous areas. The coastwise climate varied greatly between the times of glacial inaction and those times when enormous icebergs were sliding off the coast of Maine into the Atlantic, slipping out through Puget Sound into the Pacific, and thundering down Norwegian fiords into the North Sea.
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