26 A. G. LEONARD 



Lake Agassiz: The early history of Lake Agassiz, according to 

 Upham/ was intimately connected with the recession of the ice 

 front, since when the glacier had retreated across the divide between 

 the Minnesota and Red rivers the lake was formed by the ponding 

 of the water at the south end of the Red River Valley. According 

 to this view, Lake Agassiz began as a small body of water and 

 expanded northward as the ice melted until its maximum was 

 attained, its area at that time being about 110,000 square miles. 



Recently W. A. Johnson, of the Canadian Geological Survey, 

 has attributed a different life-history to Lake Agassiz.^ He 

 believes with Tyrrell that after the retreat of the Keewatin glacier 

 northward into Manitoba there was comparatively free drainage 

 in that direction, so that an earlier glacial marginal lake associated 

 with a lobe of the Keewatin glacier was largely or wholly drained. 

 Lake Agassiz proper did not come into existence until a later 

 advance of the ice from the northeast was met by a slight advance 

 of the Keewatin glacier, which resulted in the ponding of the north- 

 ward drainage and the initial stage of the lake. The waters gradu- 

 ally rose and extended southward, filling the Red River Valley and 

 overflowing to the south. 



It will be noted that these two views differ radically, one holding 

 that the lake began at the upper or south end of the valley and 

 expanded northward with the retreat of the ice margin; the other, 

 that the lake originated well to the north in Manitoba after much of 

 the Red River Valley was already free of ice, and had first a rising 

 stage as it increased in size and extended southward over the valley 

 floor. But in either case Lake Agassiz owed its existence to the 

 presence of the ice barrier to the north and northeast, higher land 

 holding in its waters on the other sides of the basin. 



This Pleistocene lake left its mark on the region in the form of 

 beaches, deltas, and lacustrine silts. The gradual retreat of the 

 ice barrier which held the lake in place afforded outlets at different 

 levels, and at many of these stages the water remained long enough 

 to form more or less distinct beach lines. A series of beach ridges 



' Warren Upham, "Glacial Lake Agassiz," U.S. Geol. Survey, Mono. No. 25. 

 ^W. A. Johnson, "The Genesis of Lake Agassiz," Jour. Geol., XXIV (1916), 

 625-38. 



