146 THE ICE AGE IN CANADA. 



whirlpool, and have cleaned out an old channel above 

 this, and cut back the present face of the fall some 

 distance since the close of the glacial period, and the 

 careful observations of Dr. Spencer* have shown that 

 the existing relations of the Niagara escarpment and the 

 lakes were established antecedent to the time when the 

 present fall was established. Claypolef has also shown 

 that the terraces with fresh- water shells on the Niagara 

 river prove that the retaining ridge between lakes Erie 

 and Ontario was then as high as now. As I have else- 

 where argued also, the thickness of the harder bed, the 

 Niagara limestone, which the river has to cut, has, owing 

 to the southerly dip of the rocks, been increasing as the 

 falls were cut back, and there is reason to believe that a 

 part of the gorge above the whirlpool was formed in pre- 

 glacial times, and has merely been cleaned out by the 

 modern river. There is also some reason to believe that 

 the amount of water in the fall may have been greater in 

 the early modern period than now. 



What, then, is the rate of recession of this great 

 cataract, and how long has it been cutting its gorge ? 

 The rate of cutting has been variousiy estimated at from 

 one foot to three feet annually ; but the actual measured 

 rate for the last forty-two years, as given on the authority 

 of Mr. R S. Woodward, of the U. S. Geological Survey, 

 is 24 feet, or nearly two-and-a-half feet per year. This 

 will give, say 12,000 to 15,000 years for the time required, 

 and, making allowance for the deductions above stated, 

 we may confidently affirm that the great cataract began 

 its labour somewhere between seven and twelve thousand 



* Iroquois Beach. Trans. R. S. Can., 1889. 



t American Naturalist, Oct., 1886. Trans. Geol. Socy., 1888. 



