398 THE ICE AGE IN NORTH AMERICA. 



should be if it really had occurred. The subsidence of the 

 lake region to such an extent would have left countless 

 other marks over a wide extent of country ; but such marks 

 are not to be found. Especially is there an absence of 

 evidences of marine life. The cause was evidently more 

 local than that of a general subsidence. The theory of the 

 elevation of a rocky barrier would also seem to be ruled out 

 of the held by the fact that no other direct evidence can be 

 found of such recent local disturbances. Such facts as we 

 have point to a subsidence at the east rather than to an eleva- 

 tion. 



But a glance at the course of the terminal moraine, and 

 at the relation of the outlets of these lakes to the great ice- 

 movements of the Glacial period, brings to view a most 

 likely cause for this former enlargement and increase in 

 height of the surface of the lower lakes. It will be no- 

 ticed that the glacial front near New York city was about 

 one hundred miles farther south than it was in the vicinity 

 of Buffalo. Hence the natural outlet to the Great Lakes 

 through the Mohawk Valley would not have been opened 

 until the ice-front over New England and eastern New York 

 had retreated to the north well-nigh one hundred and fifty 

 miles. A similar amount of retreat of the ice-front from 

 its farthest extension in Cattaraugus county, in New York, 

 would have carried it back thirty miles to the north of Lake 

 Ontario, while a similar amount of retreat from eastern Ohio 

 would have left nearly all the present bed of Lake Erie free 

 from glacial ice. With little doubt, therefore, we have, in 

 the lake ridges of Upper Canada, New York, and Ohio, evi- 

 dence of the existence of an ice-barrier which continued to 

 fill the valley of the Mohawk, and choke up the outlet through 

 the St. Lawrence, long after the glacial front farther to the 

 west had withdrawn itself to Canada soil. A study of these 

 ridges may yet shed important light upon the length of time 

 during which this ice-barrier continued across the valley of 

 the Mohawk. 



