638 W. A. JOHNSTON 



behind the removal of the great mass of the Wisconsin ice sheets 

 and was only completed after the final retreat following the latest 

 advance of the ice. 



SUMMARY 



Evidence bearing on the life-history of Lake Agassiz, found in 

 the Rainy River-Lake of the Woods district, Ontario, confirms 

 Tyrrell's conclusion that Lake Agassiz had at first a rising stage. 

 The evidence is based largely on the fact that an unconformity 

 exists at the base of the Lake Agassiz lacustrine sediments. The 

 lake was associated with the latest advance of ice sheets, chiefly 

 of the Labradorean glacier, during the Wisconsin stage of glacia- 

 tion. An earlier glacial marginal lake, which is herein referred 

 to as Early Lake Agassiz, was associated with a lobe of the Kee- 

 watin glacier; but this lake was largely if not wholly drained before 

 Lake Agassiz came into existence. The latest advance of the ice 

 into the Lake Agassiz basin did not extend farther south than the 

 southern portion of Lake Winnipeg, so that the ice border of Lake 

 Agassiz was at least 250 miles north of the southern end of the lake 

 during the entire existence of the lake. 



The acceptance of this interpretation of the genesis of Lake 

 Agassiz has an important bearing on the question of the character 

 and cause of the differential uplift which is shown to have affected 

 the region by the deformation of the shore lines. The evidence 

 suggests, but does not prove, that if the uplift was due to isostatic 

 readjustment following the removal of the burden of the ice sheets, 

 there was no close sympathetic relation, but that, as Le Conte and 

 Wright have supposed, uplift lagged considerably behind and was 

 only completed after the final retreat following the latest advance 

 of the ice sheets of the Wisconsin stage of glaciation. 



