NIAGARA AS A TIMEPIECE. n 



culating currents was the form of the basin started, and after- 

 ward 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 Tona- 

 WANDA 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), drain- 

 ing 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 rail- 

 way and the carriage bridges over the river. In wells this an- 

 cient 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 ofi^ 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, ex- 

 cept 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 op the 

 Falls. — The partial scooping out of the superficial limestones in 

 the vicinity of the falls and at the whirlpool is the only impor- 

 tant 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 ac- 

 count 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 caiion 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. 



