130 IOWA ACADEMY OF SCIENCES. 



accounted for, not only by the ridge, as we have before stated, 

 but by the depth of the Red river valley together with the 

 delaying influence of a north slope. For we conceive it 

 reasonable to suppose that the ice would be more plastic in 

 the region of greater warmth and that there would be more 

 rapid accumulation along the southern side of the zone of 

 accumulation. Both relations would favor such a conclusion.* 

 If such a state of affairs is conceivable, we may not only 

 account for the Kansan till, so far as it is sub-glacial, but we 

 may have found a partial explanation of the more difficult 

 phenomena of the course of the ice during the lowan stage. 

 One of the strange things connected with that stage is the 

 persistent course of the ice toward the southeast. Now, if the 

 summit of the ice lobe, during the Kansan stage, rose to the 

 altitude of the zone of accumulation in western Minnesota, we 

 may conceive that it might for a time act as a secondary 

 center of glacial motion. The persistent easterly tendency of 

 the ice might be partially accounted for in this way, but we 

 may find another factor in the possible subsistence of the 

 driftless area. The very existence of that area has suggested 

 its former greater elevation, and we have learned to expect 

 subsidence as one of the effects of ice occupation. The 

 Kansan load, acting for a time on the west, and subsequently, 

 if not in part contemporaneously, the Illinoian on the east and 

 south may have at last brought it down to a considerable 

 lower level. The movement of the lowan ice lobes, both in 

 Iowa and Illinois, would harmonize with such a view. See 

 Leverett's map, " Interglacial Deposits in Iowa, " page 8. 



* Moreover, Mr. Upham's study of Lake Agasslz would lead us to think there was 

 then greater northward elevation. 



