12 



is about 47 feet (14-3 m.) above Lake Ontario, so that the 

 sandstone declines about 80 feet (24 m.) from the mouth 

 of the gorge. On the west side opposite the American 

 fall, a bench disclosed by Spencer's soundings at 90 to 100 

 feet (27 to 30 m.) is presumably of this sandstone, 

 indicating a descent of about 50 to 60 feet (15 to 18 m.) 

 from the whirlpool to the falls, for the water surface at the 

 base of the falls is about 100 feet (30 m.) above Lake 

 Ontario. 



THE PROCESS OF GORGE MAKING. 



The gorge is being elongated by boring at the base 

 of the falls, where the heavy mass of falling water strikes 

 at the end of its vertical descent. The softer strata are 

 slowly worn away by the impact of the water itself, but 

 the hard capping layer is removed chiefly by undermining, 

 until it falls away in huge blocks. The blocks which drop 

 into the caldron at the foot from above become highly 

 efficient tools for grinding away the shale, for no doubt 

 many of them are spun around in the violent currents 

 like pestle stones in the making of potholes. The limestone 

 is much harder than the shale, and while the spinning 

 undoubtedly wears away the blocks, it wears away the 

 shale in the walls and bottom of the caldron much faster. 



When the falls had large volume and full height, 

 the thinner, hard layers were bored through and removed, 

 but when the height of the falls was reduced, as it was 

 in one part of the gorge, or where the water sheet passing 

 over the crest became thin, as it did in several places, 

 parts of these beds were not removed and now form 

 benches or terraces of more or less extent as described 

 above. When the volume of the river was relatively 

 small, as it was in two sections of the gorge, these layeis 

 were not both bored through by one vertical plunge, 

 but in all probability formed separate cataracts, one 

 for each hard layer. As Spencer has pointed out, this 

 was probably the condition for a time in the older part 

 of the gorge, north of Niagara University. In the gorge 

 of the Whirlpool rapids the Clinton limestone probably 

 formed a separate fall, but the Whirlpool sandstone was not 

 bored through, except towards the north end, and probably 

 now forms the block-covered floor of the channel. At 



