STAGES OF RECESSION OF NORTH AMERICAN ICE-SHEET 



207 



Age of Lake Ikoquois 



Beginning with the age of the upper shelf worn in the deposits at 

 Toronto, approximately about 200 feet above the level of Lake Ontario, 

 it is to be noted as bearing on the subject : 



1. That it is younger by several thousand years than the beaches of 

 glacial origin about the south end of Lake Michigan and than those 

 formed on the shores of Lake Warren on the south side of Lake Erie. 'No 

 one will question the statement that the final retreat of glacial ice from 

 the United States and Canada proceeded from the southwest, uncovering 

 successively the western areas of the Great Lakes, until finally the ice 

 barriers to the east over central New York and Quebec gave away and 



FIGURE 2. — Map showing Stages of Recession of the North American Ice-sheet (Upliam) 



permitted the drainage of the Great Lakes to pursue its present outlet 

 (see map on opposite page). But for a considerable period there was a 

 drainage leading to the Hudson Valley at a higher level along the ice- 

 front through the valley of the Mohawk Eiver in central New York. 

 Whatever age, therefore, we assign to the erosion of the 200-foot shelf at 

 Toronto, the shorelines at the south of Lake Michigan and those of Lake 

 Warren in Ohio must be much older. 



2. But we are compelled to set moderate limits to the time which has 

 elapsed since the ice withdrew from the southern end of Lake Michigan. 



a. The dunes at the south end of Lake Michigan represent accumula- 

 tions which have been going on ever since the ice withdrew from that 

 region. They are composed of sand, which is borne along by the waves 

 and currents on the west shore of the lake and is finally caught up by 



