The New Student's Reference Work/Ice-Age, The
Ice-Age, The. The ice age or glacial period is the time when the northern part of North America as well as much of Europe and Asia, was covered with a sheet of ice, above which rose only the highest mountain peaks. In North America the ice probably was thickest in Labrador, near the western shore of Hudson Bay and in the mountains of British Columbia. From these points the ice seems to have flowed in all directions, but especially southward. For the movement of ice, which is very slow, depends, like the movement of water, upon the slope, not of the bottom, but of the surface, of the ice. The White Mountains were covered, the ice at this point being 6,000 feet thick at least. In southern Connecticut it was 1,000 feet thick. In Norway it was not less than 6,000 feet in depth. Central Europe and the plateau of central Asia were covered, and the ice flowed down the southern slopes of the Himalayas toward the plains of India, reaching within 2,000 feet of sealevel. Some think that the ice age closed as late as 40,000 years ago or even later; but most suppose that it was more remote. It seems that within the ice age there were many glacial periods, between which the area affected became very much warmer. Then the glaciers retreated northward, often blocking up the rivers with broken ice and drift, while the waters from the melted ice covered the land with great lakes. Lake Agassiz, which once covered much of Minnesota and Manitoba, was thus formed. It was 700 miles long from north to south. The last invasion of the glaciers left the thickest deposits of rocks, soil and clay, which the ice had scraped or broken off from the land it passed over or from the sides of the valleys it passed through and carried down to the end of its course. This line of thickest drift, called the great terminal moraine (see Glacier), passes through Perth Amboy, N. J., near Elmira, N. Y., near Cincinnati, then in an “irregular, sinuous line” through Indiana, Illinois and Iowa and northwesterly through the Dakotas and Montana to Canada.
This ice age is not only wonderful to contemplate; its effects are most important. New England it has covered with broken rocks of all kinds, some of them of enormous size, brought hundreds of miles from where the ice must have broken them off from the bedrock. New York and New England owe to this age the many beautiful lakes which are but river-valleys flooded by the deposits of drift that block their ancient channels. The many falls in the east and west are in most cases due to the same cause; for the old rivers were turned from their old beds which they had worn smooth, and they still have rough paths to follow in the new beds that they then found. In Kansas, Iowa, Minnesota and Dakota are many peatbeds, some of which are due to the floods which prevailed during the warm interglacial periods; while much of the rich soil in those states is due to the great lakes of fresh water that existed then. Less important but interesting signs of the ice age are the kettle holes that abound in the northern states, depressions without drainage, due to the great masses of ice cut off from the glacier, as it retreated, and buried in heaps of deposit. The ice melted and left the hollow that still endures. Then in many places there are surfaces polished by the gritty surface of the ice or grooved by the rocks which the ice carried on in its vice-like grip.
The waves of ice gradually encroaching upon the previously warm regions of the earth must have submitted all living things to many new and disturbing conditions. In consequence many great mammals became extinct at this time; and it is possible that at the same time man assumed those features that separate him so markedly from all other animals.
The cause of this wonderful ice age is still unknown. Some say that it was due to the elevation at that time of those parts of the earth where the ice gathered; when the land sank again, the glaciers disappeared. Others attribute it to the fact that sometimes the earth is farthest from the sun in winter time; while thousands of years afterwards it is farthest from the sun in summer. In the former case more ice is supposed to accumulate in the winter than the summer can remove. Thus there are a constant increase of ice and an ice age. In the latter case the ice is removed each summer, and the ice age or glacial period ends.