Niagara Falls 



1901 individually, the depth of the excavations must have been small. 

 Hitchcock The profound depths of the upper great gorge are wholly want- 

 ing below the island. The width of the lower gorge is nearly as 

 great as that above the railroad bridges, but the water is not 

 very deep. Even if the v/hole volume of the present river was 

 concerned in the erosion, the depth would be small. 



Of the 330 feet exposed in the cliff at the mouth of the gorge, 

 about 200 belong to the Medina terrane, and must have been 

 the material over which the lower fall poured. This dips south- 

 erly, so that only 40 feet of it rise above the river at the whirl- 

 pool. While this fall receded slowly at first, its pace would be 

 greatly quickened in passing southerly. The second fall must 

 have receded uniformly, because the thickness of the Clinton is 

 the same throughout. The upper limestone is twice as thick 

 at the whirlpool as at Lewiston. So the upper fall could not 

 recede so fast as at first, and the lower cataract gained constantly 

 upon it. 



Looking at the amount of water in the early Ontario, the 

 Niagara must have fallen into it at a level 135 higher than 

 it is now. This would have brought the level of the water to 

 the top of the lower grey sandstone, or that just at the river's 

 surface at the whirlpool. The lower fall, then, did not start so 

 early as the others. But in a later epoch the lake receded so that 

 its surface was over 80 feet lower than now. With the greater 

 depth of fall the erosion of the lower rock would have proceeded 

 more energetically. Perhaps the average recession has been the 

 same as if there had been no variation in its rate. We do not con- 

 sider in this sketch the excavation of the softer parts of the Medina 

 terrane below Lewiston. Its presence neither accelerated nor 

 retarded the excavation of the greater gorge. 



A mile below the whirlpool there seems to have been an island 

 in the midst of the channel analogous to Goat Island between 

 the American and Canadian falls. Foster's flat shows a surface 

 that was swept by the river before the excavation of the gorge. 

 About fifteen feet below the level of the electric road at Winter- 



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