636 W. A. JOHNSTON 



would seem to show that they are lacustrine in origin and not fluvial. 

 It seems probable that they are Lake Agassiz deposits and that they 

 are unconformable upon the underlying sediments. 



It is concluded, therefore, with Tyrrell, that after the retreat 

 of the Keewatin glacier well toward the north there was compara- 

 tively free drainage to the north and that a later advance of the ice 

 from the northeast was met by a slight readvance of the Keewatin 

 glacier, which resulted in the ponding of the northward drainage 

 and the inception of Lake Agassiz. It is not certain just how far 

 the latest advance of the ice extended. It did not reach Rainy 

 River district, for the calcareous till derived from Manitoba is not 

 overlain by till derived from the northeast, and the southeastward- 

 and eastward-bearing striae are not crossed by later striae. At 

 Stony Mountain, near Winnipeg, southeastward-bearing striae 

 cross striae trending nearly south, but are not themselves crossed 

 by later striae. Tyrrell found that along the east side of Lake 

 Winnipeg southwestward-bearing striae cross earlier striae bearing 

 nearly southward. 1 Tyrrell'' also held that the "Winnipeg Moraine" 

 represented by islands in Lake Winnipeg and developed in places 

 along the western shore of the lake marked the termination of the 

 Labradorean glacier. It seems evident that during the life of the 

 last great glacial marginal lake of the region, viz., Lake Agassiz, 

 the ice margin in Manitoba was at no time farther south than the 

 southern portion of Lake Winnipeg, and that the whole southern 

 portion of the lake was practically free from ice. Lake Agassiz 

 was associated with a readvance of the ice sheet, chiefly of the 

 Labradorean glacier at a very late time during the Wisconsin stage 

 oi glaciation, and its disappearance followed the final withdrawal of 

 ice sheets from the region. 



BEARING OF THE LIFE-HISTORY OF LAKE AGASSIZ ON THE QUESTION 

 OF THE CHARACTER AND CAUSE OF THE DIFFERENTLA.L 



UPLIFT 



If it is true, as seems probable, that during the existence of 

 Lake Agassiz the ice border was far north of the southern end of 

 the lake, this fact lias an important bearing on the character and 



'J. B. Tyrrell, Amer. Geo!., VIII. 21. 



* Bull. G inter., XXIII (1911), 733. 



