444 J. W. SPENCER RELATIVE WORK OF TWO FALLS OF NIAGARA 



below, passes the top of the resistant Clinton limestone (20 to 22 feet 

 thick) at 12 feet above the surface of the basin below the falls, and 

 reaches the blocks of the underlying talus at 72 feet below the surface, as 

 found by my soundings; but the soundings farther away and other fea- 

 tures indicate the depth of the river near the falls (where not covered by 

 blocks) to be about 100 feet, although, farther down, the river is deeper, 

 as explained in "Evolution of the Falls" (page 56). Toward the middle 

 of the falls there is no shelf of Clinton limestone, although there is a 

 projecting ledge supporting a talus and extending outward for nearly 

 1,000 feet from the original edge of the falls on the Canadian side. Here 

 have been two stages in the recession of the rock wall, or an undercutting 

 beneath each of the two series of overhanging limestones. 



The conditions at the American Falls are different. They are 167.5 

 feet high, and the hard Clinton limestones rise to 40 feet above the river 

 at the northern end of the cataract, with the underlying shales also ap- 

 pearing above the river beneath the talus heap of fallen limestones, which 

 also in places rise to more than 40 feet above the lower river; but the 

 fallen mass must be much less than that beneath the water at the foot of 

 the main falls. Thus it would seem that a precise measurement of the rela- 

 tive efficiency in erosion by the two falls is impossible. Add to the diffi- 

 culty the fact that the falls have changed their height by a reduction 

 from 220 feet to 167 feet, owing to the recently falling rocks at the 

 Whirlpool Eapids forming a dam and raising the surface of the pool 

 below the falls, within the last 300 years, though more than 160 years 

 ago.^ 



Before the time of the rising of the waters in the basin below the falls 

 there was a wall of shales and perishable sandstone layers (70 feet or more 

 in height) beneath the hard Clinton limestones under the American 

 Falls, so that the conditions of recession were much more favorable then 

 than now ; but after the rising of the river these lower beds were protected, 

 and with the subsequent collapse of the overhanging walls the accumu- 

 lated talus became further protective. The talus is now being slowly 

 removed, owing to the direct impact of the falling water, aided by frost 

 action. The observations on the deserted floor of the river at Fosters 

 Flats left the conclusion that the fallen blocks of limestone have been 

 largely carried away by solution.^ The talus heap under the American 

 Falls (due in part to the undermining by the former higher cataract) is 

 now being worn away with the maximum efficiency, checked only by occa- 

 sional gales of wind. 



* Peter Kalm, writing in 1750, said : "This fall, by all the accounts that have been 

 given of it, has grown less and less, and those Avho have measured it with mathematical 

 instruments find the perpendicular fall of water to be exactly 137 feet" (French). . 



» Op. cit, pp. 174 and 177. 



