THE PLEISTOCENE OR GLACIAL PERIOD 893 



If the length of the Niagara gorge be divided by the average an- 

 nual retreat since the successive positions of the falls were located 

 by accurate surveys, the quotient is about 7,000. This result is, 

 however, subject to several qualifications, chief of which is the fact 

 that at the time of the beginning of the cutting of the gorge, the 

 waters of the upper lakes flowed by a more northerly route to the 

 sea, leaving only the waters of the Erie basin to pass over the falls. 

 If the history is correctly read, it was only at a comparatively late 

 date that the waters of the Upper Great Lakes went out through 

 the Niagara River. These and other considerations have led Gil- 

 bert, Taylor, Spencer, and others to the view that the cutting of 

 the narrower portion of the gorge was probably the work' of the 

 lesser volume of water from the Erie basin, and that the recession 

 at this stage proceeded at a relatively slow rate, but that the rate 

 of recession was accelerated when the upper lakes began to discharge 

 their waters to Lake Erie. It is this accelerated rate that is used 

 as the divisor in the simple computation that gives 7,000 years. In 

 view of these considerations, it is thought that 7,000 should be mul- 

 tiplied several times to give the true time-estimate. Spencer places 

 the period at about 39,000 years, and Taylor at 50,000 years as an 

 approximate maximum. There are, however, those who do not 

 accept the conclusions, and who appeal to other phenomena that 

 cannot be discussed here. 



From a comparison of the earlier and later surveys of St. Anthony 

 Falls, the time of recession from the mouth of the gorge has been 

 estimated at about 8,000 years. But considerations not taken into 

 account in this estimate, make it clear that this estimate should be 

 increased to 12,000 or 16,000 at least. 



It will be seen therefore that even in these cases of best data, 

 there are possible errors, and that these errors may affect the results 

 to the extent of several hundred per cent. If the range of the esti- 

 mates for Niagara Falls be placed at 10,000 to 50,000 years, and if 

 this be added to the range of estimates for the time of retreat of the 

 ice before the falls came into existence, 10,000 to 30,000 years more, 

 the result is 20,000 to 80,000 years for the time since the beginning 

 of the Late Wisconsin ice retreat. These may be taken for a rough, 

 wide-ranging estimate of the time since the climax of the Late Wis- 



