130 
IOWA ACADEMY OP 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 Iowan 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 And 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 Iowan 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 Agassiz would lead us to think there was 
then greater northward elevation. 
