PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY 621 
with that of 365 feet at Fort Madison. Attention is directed 
particularly to this evidence that the lowest valley excavation is . 
preglacial, since Trowbridge, of the University of Iowa, has for 
some years been advocating that a large part of the valley deepen- 
ing in the upper Mississippi region was accomplished between the 
pre-Kansan and Kansan stages of glaciation. 
The Kansan drift extends beyond the present Mississippi Valley 
into the western edge of Illinois, but does not appear to reach the 
Illinois Valley. Its southern limits on the border of the Mis- 
sissippi Valley are at the city of St. Louis. The amount of filling 
with glacial deposits is difficult to determine because of the great 
amount of erosion since they were laid down. It was sufficient in 
southeastern Iowa to prevent the stream from occupying the old 
valley again. But below the mouth of the Des Moines it was 
reoccupied by drainage when the ice of the Kansan stage melted 
away. ‘The course of post-Kansan drainage from the part of the 
valley between Muscatine and the Des Moines rapids is more 
difficult to determine, for it was later occupied by ice in the Ilhi- 
nolan stage of glaciation. Possibly it drained eastward from 
Muscatine, and together with the upper Mississippi discharged 
through the lower Illinois Valley. It is probable that nearly all 
the erosion of the gorge at the Des Moines rapids has taken place 
since the Illinoian stage. A comparison of the valley to the east 
from Muscatine with that across the Des Moines rapids indicates 
that it was opened earlier. Its rock bed has been cut below the 
level of the present Mississippi, and it is a broader valley with 
gentler slopes than that across the rapids. The difference may be 
seen by comparing charts 148 and 149 of the Mississippi River 
Commission, embracing the part east of Muscatine, with charts 
136 and 137, which embrace the rapids. The valley east from 
Muscatine is also embraced in the Edgington and Milan quadrangles 
of the U.S. Geological Survey. The opening of this valley eastward 
from Muscatine may date from the Nebraskan stage and have been 
occupied by east-flowing drainage down to the Illinoian stage, 
when it was changed to a west-flowing stream. 
t Proc. Iowa Academy of Science, Vol. XXI (1915), pp. 208-9; University of 
Towa Studies, Vol. IX (1921), pp. 123-27. 
