Dfift i?i South Dakota. — Todd. 105 
The failure of the ice to press equally northward may be 
accounted for, not only by the ridge, as we have before stated, 
but by the depth of the Red river vaUey 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 cen- 
ter 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 subsidence of the driftless 
area. The very existence of that area has suggested its for- 
mer greater elevation, and we have learned to expect subsi- 
dence 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, 
than it occupied at the beginning of the Pleistocene. The 
movement of the lowan ice lobes, both in Iowa and Illinois, 
would harmonize with such a view.f 
Moreover, Mr. Upham's study of lake Agassiz would lead 
us to think there was then greater northward elevation over 
that area than in the Wisconsin epoch. 
♦Glacial Lake Agassiz, Mon. XXV, U. S. G. S., p. 501. 
"fSee Leverett's map, "Interglacial Deposits in Iowa," p. 8. 
