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IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
distance be increased to a thousand miles, the length of time con- 
sumed in this retreat would be brief geologically, probably not 
permitting of any perceptible amount of weathering. The state- 
ment, therefore, seems safe that the loess was deposited imme- 
diately after the closing stages of the Iowan glacial epoch in 
which case the loess should be regarded as early Peorian in age 
rather than Iowan. 
If the loess is early Peorian in age, the relations of the loess 
to the Wisconsin drift should show it. In an excellent ex- 
posure made by the Chicago, Milwaukee & St. Paul Railway 
Company during the recent reconstruction of their line in Mar- 
shall county, at the Wisconsin margin, tangible evidence was 
found supporting this view. Here the loess, which is typically de- 
veloped to the south of the Iowan border, passes beneath twenty- 
five feet of Wisconsin drift. The loess itself also is twenty- 
five feet thick. The larger part of the Wisconsin drift has 
been scarcely changed by weathering, whereas the loess is al- 
most wholly oxidized, there being some gray left at the base, 
and the top of the loess in one place is leached four to five feet. 
Therefore, the conclusion seems warranted that the loess which 
is associated with the Iowan drift is chiefly early Peorian in 
age. 
4. Bearing of the Loess on the Problem of the Iowan Drift. 
— Inasmuch as the deposition of the loess dates back to the time 
when the Iowan drift was yet unleached, the loess may be used 
for correlation purposes in determining the time relations of the 
Iowan drift to the Kansan and Illinoian drifts. The great un- 
conformities between the loess and the Kansan drift, and between 
the loess and the Illinoian drift, which have already been de- 
scribed, show that the Iowan ice-sheet invaded Iowa a long 
time after the Kansan and Illinoian glaciations, the Kansan, of 
course, having much preceded the Illinoian. 
Department of Geology, 
University of Washington. 
