THE GEOLOGICAL HISTORY OF NEW YORK STATE IT5 
advances and retreats of the ice sheets etc. Although a closer 
calculation is well nigh impossible because of the variability of the 
factors involved from time to time, it is nevertheless certain that, 
from the geological standpoint, the Ice age was of short duration, 
while, from the standpoint of known human history, it was 
immensely long. 
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 at the same time 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 glaciated. The best estimates for the length of time since the 
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Fic. 39 Sketch map showing the relation of the crest of 
Niagara Falls in 1842 to that of 1905. Based upon actual 
surveys. The retreat of the inner portion of the Horseshoe 
Fall was more than 300 feet. 
Modified after Gilbert, U. S. G. S. Bul. 306, p. 20 
close of the Ice age in New York State are based upon the rate , 
of recession of Niagara falls. We have learned that the Niagara 
river began its work about the time the glacial waters in the Erie- 
Ontario regions had dropped to the Iroquois level, and that the 
falls were first formed by the plunging of the river over the Niagara 
limestone escarpment at Queenston and Lewiston. Studies based 
upon actual surveys, drawings, daguerreotypes, photographs etc. 
made between the years 1842 and 1905, have shown that the Horse- 
shoe 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 
