LOWER RAPIDS OF THE MISSISSIPPI g 



exposures of the blue-gray Kansan till are found in the vicinity 

 of Ft. Madison. They there constitute for several miles the 

 upper ioo feet of the Mississippi bluff, except a thin coating 

 of loess. 



The filling produced by the blue-gray till was sufficient to 

 prevent the return of the stream to its preglacial course, the 

 altitude of the surface along the part of the preglacial channel 

 west of the lower rapids being as great as in border districts. In 

 this case, therefore, it is only necessary to decide whether the 

 stream assumed its present course across the lower rapids at the 

 time the Kewatin ice-field made its final withdrawal from that 

 region, or whether it drained eastward to the Illinois until it was 

 forced from that course by the advance of the Labrador ice- 

 field, at the Illinoian stage of glaciation. Concerning this 

 question it is thought that evidence of some value has been 

 collected, as appears below. 



Erosion preceding the Illinoia?i stage of glaciation. — The 

 Mississippi valley for about fifty miles below the lower rapids 

 was greatly filled by the drift from the Kewatin ice-field. 

 Immediately below the rapids the filling on the borders of the 

 valley reached a level about 150 feet above the present stream. 

 It seems not improbable that there was a filling to nearly this 

 height in the middle of the valley, for the abandoned section 

 just above was filled in its middle part to as great a height as on 

 its borders. Upon passing down the valley the height of filling 

 gradually decreases to the limits of the Kewatin drift near 

 Hannibal. From the filling of tributaries near Hannibal it is 

 estimated that the Mississippi valley could not have been filled 

 to a height greater than seventy-five feet above the present 

 stream. Below Hannibal the filling was produced by stream 

 action rather than by glacial deposition and appears to have 

 reached but little, if any, above the sand terraces of the valley, 

 say fifty feet above the river. Now if this filling suffered but 

 little erosion before the Illinoian stage of glaciation, it can 

 reasonably be inferred that the drainage of the upper Mississippi 

 did not pass across the lower rapids and through this part of 



