208 THE GLACIAL LAKE AGASglZ. 



their deposition in each of the numerous basins in which they are found. 

 But I believe that ultimately they will be shown to be everywhere attribu- 

 table either to fluvial deposition attendant on the recession of the ice-sheet 

 or to deposition as deltas in glacial lakes which owed their existence to 

 ice dams or to depressions where the land had sunk beneath the ice weight 

 and has since been reelevated. For example, the Kootanie basin may 

 well have been filled by a glacial lake obstructed in the present course of 

 drainage by the retreating ice-sheet and outflowing by the way of Pack 

 River and Lake Pend d'Oreille, which Professor Chamberlin finds to have 

 been covered by the maximum advance of the ice, while gravel-bearing 

 floods from the glacial melting poured thence to the south and west.^ 

 Again, the silts on the Peace River east of the Rocky Mountains seem 

 referable, as will be stated more fully on a later page, to a glacial lake 

 held by the baliTier of the departing ice-sheet on the north and northeast, 

 with outflow southeastward into Lake Agassiz. 



EXTENSION OF LAKE AGASSIZ WITH THE DEPARTURE OF THE 



ICE-SHEET. 



On the west side of Lake Agassiz the Dakota lobe of the ice-sheet, 

 from its junction with the Minnesota lobe near the Head of the Coteau 

 des Prairies, 25 miles west of Lake Traverse and Browns Valley, at the 

 beginning of the moraine-forming or Wisconsin division of the Glacial 

 period, reached about 200 miles south along the valley of the James or 

 Dakota River to Yankton and the Missouri ; but it was gradually dimin- 

 ished in its extent until, at the time of formation of the Kiester, Elysian, 

 Waconia, and Dovre moraines, it no longer retained its lobate outline. 

 While these moraines were being formed in Minnesota the southwestern 

 boundary of the ice-sheet in South and North Dakota passed from the 

 vicinity of Big Stone Lake and Lake Traverse northwesterly along 

 morainic belts which have been traced through Sargent, Ransom, Barnes, 

 and Griggs counties, N. Dak., and by the sources of the James and 

 Sheyenne rivers. During the later stages, represented by the Fergus Falls 

 and Leaf Hills moraines, the Dakota ice front appears to have become 



'U. S. Geol. Survey, Bulletin No. 40, p. 8. 



