NIAGARA FALLS AND VICINITY 83 



The American falls 



Feet a year 



From 1842 to 1875 .74 



1875 " 1886 .II 



1886 '' 1890 ' 1.65 



averaging 



From 1842 to 1890 .64 



The Horseshoe falls 



From 1842 to 1875 2.01 



1875 " 1886 1.86 



1886 '' 1890 5.01 

 averaging 



From 1842 to 1890 2.18 



This shows a most rapid increase in the rate of recession during 

 the four years between the last two surveys. From this we may 

 assume that the mean recession of a cataract combining the volumes 

 of both American and Horseshoe falls, such as existed throughout 

 the greater period of gorge excavation, is at least three feet a year 

 and may be as high as four or even five feet a year. 



The first to make use of this known rate of recession in estimat- 

 ing the age of the gorge was Dr Julius Pohlman. He considered 

 that the gorge of the whirlpool rapids and other portions of the 

 present gorge were of preglacial origin, and so reduced the length 

 of post-glacial time to 3500 years. Since that time numerous esti- 

 mates of the age of the gorge have been made, the results often 

 varying widely, owing to different interpretations given to the nar- 

 row portions of the gorge. It is perfectly evident that, if Niagara 

 was deprived of seven eighths of its water supply, for the period of 

 time during which the gorge of the whirlpool rapids was excavated 

 a very slow rate of recession must have obtained, and hence the age 

 of the gorge is greatly increased. Upham, who does not believe 

 in the withdrawal of the waters, makes the age of the gorge be- 

 tween 5000 and 10,000 years. Spencer and Taylor are ardent ad- 

 vocates of the reduction of the volume of water during a prolonged 

 period, when the supply from the upper Great lakes was cut off. 



