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.
[Illustration: 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.