the quotient is about 7,000, but it is not safe to assume that this 

 number of years is the time since the last glacial epoch. At the 

 he-winning of the cutting of the gorge, the waters of the upper lakes 

 (lowed by a more northerly route to the sea (Figs. 530 and 531), 

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

 the history is correctly read, it was at a comparatively late date that 

 the waters of the Upper Great Lakes went out through the Niagara 

 River. The early cutting was therefore much slower than the later. 

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

 multiplied several times to give the true time-estimate. Spencer 

 places the period at about 39,000 years, and Taylor at about 25,000 

 years. 



It is to be noted that cutting of the Niagara Gorge could not have begun until 

 the Mohawk outlet of the lakes (p. 639) was abandoned, and that the time measured 

 by the Niagara cutting is only that which has elapsed since the ice melted back 

 from the Adirondacks far enough to permit the waters of the ancestral Lake 

 Ontario to find an outlet lower than the Niagara escarpment, and no very effective 

 cutting could take place until the waters were withdrawn to something near their 

 present level. 



If the border of the ice-sheet at this stage (Fig. 531) is compared with the 

 border of the ice at the maximum Wisconsin stage, it will be seen that it had re- 

 treated some 600 miles. The rate of recession of the ice is unknown, but 200 feet 

 per year is an improbably high rate; but at this rate, the ice must have been reced- 

 ing some 15.000 years, before the falls came into existence. If this be added to the 

 time occupied in the development of the gorge, say 25,000 to 40,000 years (esti- 

 mated), the result is 40,000 to 55,000 years since the beginning of the retreat of the 

 kst great ice-sheet. 



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

 Falls, the time of recession of the falls 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 15,000 at least. If to these figures 

 20,000 years be added for the time of retreat (700 or 800 miles) before 

 the falls began to develop, we have a total of more than 30,000 

 years since the climax of glaciation in the late Wisconsin epoch. 



Little value is to be placed on estimates of this kind, except as 

 means for developing a conception of the order of magnitude of the 

 time involved. 



Foreign 



In Europe, the succession of ice epochs and formations is not less 

 complex than in North America, though there is not complete agree- 



