Records of glaciation, such as glacial scratches, bowlders, lakes, etc., occur high up in the White and Green Mountains, Adirondacks, Catskills, and the Berkshire Hills, thus proving that the ice must have been at least some thousands of feet thick over New England and New York. We have good reason to believe that even the highest summits, except possibly in the Catskills, from 4,000 to over 6,000 feet above sea level, were completely submerged under the ice. On top of a mountain of Archeozoic granite nearly 4,000 feet in altitude, facing the St. Lawrence Valley in northern New York, the writer has found many fragments of sandstone which were picked off by the ice in the low valley, moved southward a good many miles, and uphill several thousand feet to the top of the mountain. The reader may wonder how a great glacier at least a mile thick in northern New York could have thinned out to disappearance within the short distance to the southern border of the State, but observations on existing large glaciers show that it is quite the habit for them to thin out very rapidly near their margins, thus producing steep ice fronts.
The fact that glacial ice flows as though it were a viscous substance is well known from studies of valley glaciers in the Alps and Alaska, and the great ice sheet of Greenland. A common assumption, either that the land at one of the great centers of ice accumulation during the Ice Age must have been many thousands of feet higher, or that the ice must there have been immensely thick, in order to permit ice flowage so far out from the center, is not necessary. Viscous tar slowly poured upon a level surface will gradually flow out in all directions, and at no time need the tar at the center of accumulation be very much thicker than elsewhere. The movement of glacial ice from the great centers of dispersal during the Ice Age was much the same in principle, only in the case of the glaciers the accumulations of snow and ice were by no means confined to the immediate centers.
The fronts of the vast ice sheets, like those of ordinary valley glaciers, must have undergone many advances and retreats of greater or less consequence. In the northern Mississippi Valley, and also in Europe, there is positive proof for five or six important advances and retreats of the ice which gave rise to the true interglacial stages. The strongest evidence is the presence of successive layers of glacial (morainic) débris piled one upon another, a given layer often having been oxidized, eroded, and even covered with plant life before the next or overlying layer was deposited. Such is the condition of things throughout much of Iowa, where wells sunk into the glacial deposits commonly pass through layers of partly decomposed vegetable matter at depths of from 100 to 300 feet. Near Toronto, Canada, the finding of warm climate plants between two glacial deposits proves that the climate there during an interglacial stage was much like that of the southern States to-day. During the great interglacial stages the vast glaciers were notably restricted in size, and in some or possibly all, cases they may have wholly disappeared from the continent.
In former years there was a tendency to ascribe mighty erosive power to the vast slow-moving ice sheets, but to-day scarcely any geologist would hold that the ice really produced large valleys solely by ice erosion, or that mountains were notably cut down. Throughout the glaciated region, especially toward the north, the deep preglacial residual soils and rotten rocks were nearly all scoured off by the passage of the ice. That the ice, where properly shod with rock fragments, actually eroded to at least little depths into hard and fresh rocks is well known, but the evidence is clear and conclusive that the preglacial hills and mountains, and most of the valleys (including all the large ones), were rarely more than a little modified in shape and size.
One of the principal effects of the Ice Age is the widespread distribution of glacial deposits, and other deposits which were formed under water in direct association with the ice. Such materials have been described in the chapter on “Glaciers and Their Work.”
As a direct result of the Ice Age, many thousands of lakes came into existence throughout the glaciated region where few, if any, previously existed. Many of these lasted only while the ice was present because their waters were held up by walls of ice acting as dams. Thousands of others still persist, most of these having their water levels maintained by dams of glacial débris left by the ice across valleys. Good examples of lakes of both types, including a summary of the remarkable history of the Great Lakes, are considered in the chapter on “A Study of Lakes.”
Many drainage changes, gorges, and waterfalls have also directly resulted from the great Ice Age. In fact it is not too much to say that practically all true gorges and waterfalls of the glaciated region have originated as a direct result of the Ice Age. The most remarkable combination of waterfall and gorge thus produced is that of the world-famous Niagara, described in the chapter on “Stream Work.” Not only are Niagara Falls and gorge of postglacial origin but there was no Niagara River as such before the Ice Age. In New York the well-known Ausable Chasm, Trenton Falls Gorge, and Watkins Glen are all excellent examples of gorges cut since the Ice Age by streams which, because their old valleys were filled with glacial débris, have been forced to take new courses. A gorge of very special interest is that at Little Falls in central New York. This gorge, two miles long, with its precipitous walls hundreds of feet high, is the most important gateway for traffic between the Atlantic border and the Great Lakes region. The bottom of this defile contains six tracks of the New York Central and West Shore Railroads, the Barge Canal, an important highway, and the Mohawk River. Before the Ice Age there was a stream divide instead of a gorge, several hundred feet above the present river level. During a late stage of the Ice Age, when the Great Lakes drained through the Mohawk Valley, a tremendous volume of water passed over the divide and cut it down to form nearly all of the gorge except the inner or bottom trench which has since been eroded by the Mohawk River.