Age of the Kaiisaii Drift Sheet — Hershey. 23 
orig-inal drift plain is preserved, and that, too, on the imme- 
diate borders of the ^Mississippi where conditions for erosion 
are more favorable than in the area to the west which is occu- 
pied by the eroded Kansan sheet. The great contrast in 
amount of erosion supports strongly the view that a longer in- 
terval elapsed between the Kansan and Illinoian glaciations 
than between the Illinoian and the present time." To one 
who has given any attention to eroeion studies in the district 
referred to the above conclusion appears quite conservative. 
In situations such as these old drift plains, the Wisconsin 
drift sheet has suffered practically no erosion, and the lowan 
also very little. These two sheets are, therefore, very, very 
recent as compared with the older. In northwestern Illinois 
where the country was hilly after the deposition of the drift, 
there is abundant evidence that the Illinoian or earlier sheet 
is much older than the lowan, but if it were so old as the 
Kansan it must have been almost completeb' destroyed by 
erosion. The eskers show great erosion, yet if of Kansan age 
they should be reduced to low rounded mounds instead of the 
sharp peaks which they present. I think as a result of these 
comparative studies that there can ,be little doubt of the Illi- 
noian age of the drift in northwestern Illinois formerly known 
as Kaihsan. 
There is another phase of the subject to which I wish to 
call attention as it is a matter which I believe has not yet 
been discussed by anyone: As the evidence shows that the 
Kansan drift sheet is very much older than the Illinoian and 
later sheets, it may be suspected that it is even of greater age 
than the Quaternary era as the term is usually understood. 
The erosion effected on the Kansan drift plain is as great as 
many have been willing to credit as the capability of the entire 
Quaternary era. 
In passing off of the Kansan drift on to the unglaciated area 
of southern Missouri, I was surprised to find that the type of 
the erosion topography did not materially change. Where the 
tilting of the original plain surface was slight and the streams 
did not cut down deeper than in northern jNIissouri, the vallr*/s 
had the same shape and about the same amount of the plain re- 
mained. In the vicinity of Tipton, along the main divide be- 
tween the ]Missouri and Osage rivers, there is a flat plain of 
