OUTLINE OF PLEISTOCENE HISTORY OF MISSISSIPPI 
VALLEY! 
FRANK LEVERETT 
Ann Arbor, Michigan 
RELATION OF PRESENT STREAM TO PREGLACIAL VALLEYS 
The headwater portion of the Mississippi River, above St. Paul, 
Minnesota, is in a region so thickly covered by glacial deposits 
that the present streams are entirely independent of the preglacial 
valleys, and their history begins with the recession of the ice in the 
last, or Wisconsin, stage of glaciation. ‘The courses of preglacial 
drainage lines in this region have been only partially traced by 
means of deep borings. This paper deals, therefore, mainly with 
the part below St. Paul. 
For 15 miles below St. Paul the Mississippi follows the valley 
of a small tributary of the preglacial river, the course of the main 
valley being a few miles to the west, passing beneath Lake Min- 
netonka, and across the lower end of Minnesota Valley, and 
continuing through Dakota County to the present stream at Pine 
Bend, 6 miles above Hastings. From Hastings to Clinton, Iowa, 
the river practically follows the course of the preglacial valley, 
though it cuts off projecting points from the west bluff at ‘Trem- 
pealeau, Wisconsin, and in the north part of Clinton, Iowa (Fig. 1). 
‘At the mouth of the Wapsipinicon River, below Clinton, the 
Mississippi leaves the preglacial valley, passes into a rock gorge, 
and flows across rapids to Rock Island. Two courses for the 
preglacial valley have been suggested, one to the southwest from 
the lower part of Wapsipinicon Valley to Muscatine, on the present 
Mississippi, the other to the southeast.to Hennepin, on the Illinois 
Valley. Along both lines the glacial deposits are very thick, and 
the rock surface much lower than the present streams. The 
southeastward course seems on the whole more likely to have been 
t Published by permission of Director, U.S. Geological Survey. 
615 
