TODD: PLEISTOCENE CRUSTAL MOVEMENTS. 317 
curacy when we know the impossibility of a slender ice lobe 
maintaining itself for 500 miles from Dakota to Kansas along 
the edge of a dry and probably warm region like the great 
plains, doubtless traversed then as now by southwest winds. 
Second. Another significant fact is that the ice overrode 
points now over 1500 feet A. T. This was true in northern Pot- 
tawatomie county, about Blaine and Wheaton, Kan., where the 
preglacial surface rises to that height. The surface of drift 
deposits lies at that altitude at Summerfield in Marshall county, 
Kansas, and along the divide northward past Virginia and 
other points in northern Gage county, Nebraska. 
This fact should also be contrasted with the fact that the 
highest points of the limit of the ice on the east side of the 
same lobe in northeastern Iowa is only about 1200 feet A. T. 
in Winneshiek, Allamakee and Dubuque counties. It should 
also be compared with the fact that southeastern Iowa now lies 
only 750 feet A. T., or only half the height in Kansas. 
Taking the altitudes as we now find them, we can not see 
why, if the ice sheet reached 1500 feet in Pottawatomie 
county, Kansas, it should not have pushed over the 1300-foot 
levels in northeastern Iowa and scores of miles farther into the 
Wisconsin driftless area, and well across Illinois southeast, 
in which direction there was an open field and lower levels. 
We may reason it thus: Taking a point in northern Kossuth 
county, lowa, as a common point of passage for all points south 
from Kansas to Illinois, then taking Blaine, Kan., 1500 feet 
A. T., and assuming an average slope for the surface of ice of 
25 feet per mile, we should find the top of the ice sheet over 
northern Kossuth county to be over 9000 feet A. T. If a similar 
slope prevailed also southeast to West Union, Iowa, the ice 
would have reached 5275, or 4000 feet above the height of the 
present surface there, and in a southeast direction the slope 
would have carried it beyond Bloomington, Ill. This is also 
far beyond the observed limit in that direction. 
Mr. J. E. Carman, in the Illinois Geological Survey Bulletin 
13, represents the limit of the Kansan till reaching nearly to 
Savannah, Ill., and a little beyond Fulton, Ill.; and Leverett, 
in the U. S. G. S. Monograph XXXVIII, places the margin 
through Hancock and Adams counties, and crossing the Mis- 
sissippi river near Hannibal, Mo. With the slope assumed and 
