Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 48.—Sketch map of the region between Lake George and Schenectady, New York, showing how certain of the main drainage courses have been revolutionized by the great retreating ice sheet and the deposits it left. Preglacial courses shown by dotted lines only where essentially different from the present streams. (By the author, as published by New York State Museum.)

Only a few of the numerous stream changes directly due to the Ice Age will be briefly referred to. Certain of the principles involved are exceptionally well illustrated in the general vicinity of Saratoga Springs and Lake George, New York. During the retreat of the great glacier a lobe of ice occupied the Lake George Valley and forced the Hudson River west over a divide at Stony Creek. Then, because of heavy glacial deposits near Corinth, the Hudson could not continue south through what had been the preglacial valley of Luzerne River, but it was forced eastward over a divide in a low mountain ridge to Glens Falls. The remarkable shift of the Sacandaga River from its preglacial channel was caused by the building up of a great morainic ridge across the valley in the vicinity of Broadalbin.

The drainage of the basin of the upper Ohio River has also been revolutionized as a result of the glaciation. All the drainage of western Pennsylvania passed northward into Lake Erie just before the Ice Age instead of southwestward through the Ohio River as at present.

Rivers as large as the Mississippi and the Missouri were also more or less locally deflected from their preglacial courses. Thus the Missouri, which in preglacial time followed the James River Valley of eastern South Dakota, was forced, by a great lobe of retreating ice, to find its present course many miles farther west.

How long ago did the Ice Age end? In seeking an answer to this question we should bear in mind not only the fact that the Ice Age ended at different times, according to latitude, the more southern districts having been first freed from ice, but also the fact that approximately 4,000,000 square miles of the polar regions are now ice-covered, so that in a real sense those portions of the earth are still in an Ice Age. Some of the best estimates of the length of postglacial time for a given place are based upon the rate of recession of Niagara Falls, the average of the estimates being about 25,000 years. The evidence for this conclusion is briefly set forth in [Chapter III]. A careful study of the rate of recession of St. Anthony Falls, Minnesota, has led to the conclusion that the last retreat of the ice occurred there from 10,000 to 16,000 years ago. Certain clays deposited under tidewater since the last withdrawal of ice in Sweden show a remarkable succession of alternating layers thought to represent seasonal changes. By counting the layers it has been estimated that Stockholm was freed from ice only 9,000 years ago.

Although the actual duration of the Ice Age is by no means accurately known, we can be quite sure that the time represented is far longer than that of postglacial time. That it must have lasted fully 500,000 years seems certain when due consideration is given to amount of time necessary to bring about the repeated changes of climate between the glacial and interglacial stages; the amount of plant accumulation during the interglacial stages; the amount of weathering and erosion of the various layers of glacial deposits. Some estimates run as high as 1,500,000 years for the duration of the Ice Age, and an average is about 1,000,000 years, which probably indicates, at least roughly, the order of magnitude of the time involved.

When it is considered not only that the fact of the great Ice Age was not even thought of until 1837, but also that many factors enter into the general problem of the climate of geologic time, it is not surprising that the cause (or causes) of the glacial climate is still not definitely known. A few of the various hypotheses which have been advocated to account for the glacial climate will now be very briefly referred to. One is that the increased cold (not more than 10 to 15 degrees for the yearly average) was brought about by the notably increased altitudes of late Tertiary and early Quaternary times in northern North America and Europe. In this connection it is interesting to note that the four times of real glaciation during geologic time (mid-Proterozoic, early Paleozoic, late Paleozoic, and early Cenozoic) did occur directly after great crustal disturbances and notable uplifts of land. According to this hypothesis the interglacial stages would have to be explained by a rather unreasonable assumption of repeated rising and sinking of the glaciated lands.

Another hypothesis, long held in favor, is based upon certain astronomical considerations. Thus we now have winter in the northern hemisphere when the earth is nearest the sun, but in about 10,500 years, due to wobbling of the earth on its axis, our winter will occur when the earth is farthest from the sun, thus making the winters longer and colder, and the summers shorter and hotter. After a much longer period of time the earth will be millions of miles farther from the sun in winter than in summer and this would still further accentuate the length and coldness of the winters. The interglacial stages represent the 10,500 year periods when the earth in winter (northern hemisphere) is nearest the sun. A difficulty in the way of accepting this hypothesis is that it is inconceivable that each glacial and interglacial stage lasted only 10,500 years. Another objection to the hypothesis as an explanation of Ice Ages is that it is directly opposed by the fact of widespread glaciation at low latitudes either side of the equator during the late Paleozoic Ice Age.