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.