34 



J. E. TODD — HYDROGRAPHIC HISTORY OF SOUTH DAKOTA 



If our conclusion is right, we may believe that the ice of the Kansan 

 epoch, coming from the northeast into the Minnesota valley, was pre- 

 vented, by the abruptness of its western side, the height and unbroken 

 character of the plateau west of it, from entering the James River valley, 

 although it succeeded in overriding the eastern edge and entering the 

 valley of the Big Sioux. Glacial striae in that valley are found in such 

 relations as to indicate that it was occupied by a narrow lobe pushing 

 down the Big Sioux, west of a small area, which is comparatively drift- 

 less, north of Garretson. South Dakota. 



When the ice-sheet crowded against this plateau of the Coteau des 

 Prairies the whole drainage from its western border farther north must 



Figure 2. — Hydrographic Conditions in South Dakota in pre-Wisconsin Pleistocene. 



have found its way over the divide between the James and Red rivers, 

 we may suppose, at some lower and narrower portion, such as we may 

 readily suppose, from the present conditions, may have been a little 

 north of our state, as is represented on the map. There may have been 

 first formed a temporary lake east of the ridge, and from it there must 

 have been a rapid descent to the level of the James River valley. As the 

 materials of the ridge were mostly clay and sand, such a barrier as that 

 could not long resist the eroding influence of a stream which was fed by 

 the draining from the whole western slope of the ice-sheet farther north. 

 We can scarcely doubt that in a comparatively short time, it would cut 

 down a canyon through this ridge to the level of the James river, which 



