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Page:Popular Science Monthly Volume 49.djvu/23

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culating currents was the form of the basin started, and afterward deepened in the soft shales. The rocks at the end of the basin are always obscured by the landslides of the overlying drift materials,

Niagara Falls crossing the Ancient and Buried Tonawanda River.—Reference has been made to the ancient buried valley westward of the Niagara. In olden days the rains, rills, and rivulets were everywhere acting upon the surface of the land and producing broad, flattened features which are characteristic of old topography. Through such a valley flowed the ancient Tonawanda River (partially recognized by Dr. J, Pohlman), draining the Niagara district (as indicated in Fig. 5). This valley in the vicinity of the falls was about a mile and a half wide and ninety feet lower than the rocky rim which bordered the northern side (see Fig. 8), which barrier is now exposed between the railway and the carriage bridges over the river. In wells this ancient valley has been found to extend in the direction of the St. Davids Valley (Fig. 5), which is comparable in size to it, in place of turning off at right angles, as does the modern river at the location of the falls. This ancient Tonawanda River never drained the Erie basin, and when it afterward became filled with drift it did not determine the character of the modern river, except to give rise to the magnificent rapids above the falls (as shown in Fig. 17).

Effects of the Depressions of the Ancient Surface and the Geological Structure upon the Recession of the Falls.—The partial scooping out of the superficial limestones in the vicinity of the falls and at the whirlpool is the only important feature which has noticeably affected the excavation of the modern river channel, and this only to a very small extent, for the ancient depressions were filled with the rubbish of the drift period, which loose material was protected from being carried away by the flowing currents; and even after the last barrier of rock had been removed by the retreat of the falls, the river had nearly as much work as ever to do, for the recession of the falls is by the undermining of the capping limestones, and not on account of their being worn away by the river to an appreciable extent. Furthermore, the regularity of the recession has been largely maintained by the remarkably uniform character of the beds of rocks, which for a considerable portion of the length of the cañon are almost horizontal, and only at the lower end do they dip as much as fifteen or twenty feet in a mile. Now all these explanations mean that the character of the country and the geological formations would not cause any great variation in the rate of the recession of the falls, but those changes were due to the other and farther reaching causes.