THE QUATERNARY PERIOD 359 



by the statement that " little value is to be placed on estimates of 

 this kind, except as a means for developing a conception of the 

 order of magnitude of the time involved." l 



Length of Time since the Glacial Epoch 



Estimates of the length of time since the close of the Ice age 

 are perhaps more satisfactory, though it must be remembered that 

 the close of the Ice age was not the same for all places. The ice 

 retreated northward very slowly and when, for example, southern 

 New York was free from the ice, northern New York was still 

 occupied by the glacier. The best estimates of the length of 

 time since the close of the Ice age are based upon the rate of 

 recession of Niagara Falls. We have learned that Niagara River 

 began its work about the time the glacial waters in the Erie-On- 

 tario basins dropped to the Iroquois level, and that the falls were 

 first formed by the plunging of the river over the limestone es- 

 carpment at Lewiston. Studies based upon actual surveys, draw- 

 ings, daguerreotypes, photographs, etc., made between the years 

 1842 and 1905, have shown that the Horseshoe Fall had receded 

 about 5 feet a year, while the American Fall, between 1827 and 

 1905, had receded about 3 inches a year. Thus the gorge cutting 

 is clearly taking place on the Canadian side. The length of the 

 gorge is 7 miles, and if we consider the rate of recession to have 

 been always 5 feet a year, the length of time necessary to cut the 

 gorge would be something over 7000 years. But the problem is 

 not so simple, since we know that at the time of, or shortly after, 

 the beginning of the river, the upper lakes drained out through 

 the Trent River, and then still later through the Ottawa River. 

 So it is evident that, for a good part of the time since the ice 

 retreated from the Niagara region, the volume of water passing 

 over the falls was notably diminished, and hence the length of 

 time for the gorge cutting increased. The best estimates for the 

 length of time since the ice retreated from the Niagara region 

 vary from 7000 to 50,000 years, an average being about 25,000 

 years. In a similar way, the time based upon the recession of St. 

 Anthony's Falls, Minnesota, ranges from about 10,000 to 16,000 



1 Obviously, the determination of the number of years equivalent to one 

 time unit involves the determination of the time since the disappearance of 

 the last ice sheet, and this is discussed under the next heading. 



