PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY 623 
It is probable that the aggradation at the Illinoian stage was 
sufficient to enable the river to make use of a low pass across the 
rock ridge below Thebes, Illinois, and thus join the Ohio more 
directly than by the old course. The contours of this new course 
seem to be about the same as in the Des Moines rapids and those 
_ above Rock Island, and thus to favor a similar age. 
The Iowan glaciation appears to have been too light and transi- 
tory, or to have encroached on the Mississippi Valley too little, 
to produce any permanent deflections of the stream. It may, how- 
ever, have reached to the valley both from the east and west in 
the vicinity of Clinton, as indicated by maps and descriptions in 
Monograph XXXVIII.*. There are deposits of sandy gravel along 
some of the tributaries of the Mississippi in southeastern Minnesota 
and in western Wisconsin that are outside the glacial drainage 
lines which were connected with the Wisconsin ice sheet, and which 
are tentatively referred to the Iowan glacial drainage. The ma- 
terial is nearly as fresh as gravel of Wisconsin age, and its preserva- 
tion from erosion is but little different from that of Wisconsin 
deposits. It seems, therefore, to be too young to refer to the 
Illinoian glacial drainage. 
The Wisconsin glaciation covered only the headwaters of the 
Mississippi down to a point a little below St. Paul. But it covered 
the lower Illinois, and built great moraines to the north and west 
of Hennepin that have effectually barred the upper Mississippi 
from returning to its old course into the lower Illinois Valley. 
The glacial drainage into the Mississippi from the Wisconsin 
drift at first had its head on the outer slope of a moraine that 
crosses the river in the southeast part of the St. Paul quadrangle. 
The drainage was extended up the valley step by step with the 
recession of the ice border. At the border of the Wisconsin drift 
there is an extensive outwash plain, 940 to 960 feet above sea level, 
or 260 to 280 feet above the Mississippi River. From this outwash 
plain there was drainage down the course of the Vermillion Valley 
as well as down the present Mississippi, with a descent of fully 
100 feet in about 20 miles. ‘The filling is a fine, sandy gravel, and 
t Op. cit., pp. 131-153, Pls. VI and XII. 
