IOWA ACADEMY OP SCIENCES. 81 



Illinois beneath the Illinoian till sheet an undetermined 

 distance. The tendency to break into rectangular blocks often 

 serves to distinguish it from the overlying Illinoian till, as 

 well as from the underlying blue-black till, though the 

 Illinoian, in places, takes on this phase of fracture. Probably 

 the most extensive of the exposures of the blue-gray Kansan 

 till are found in the vicinity of Ft. Madison. It there consti- 

 tutes, for several miles, the upper 100 feet of the Mississippi 

 blulf, 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, for 

 the altitude of the surface, along the part of the preglacial 

 channel west of the lower rapids, is 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 east- 

 ward 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 evi- 

 dence of some value has been collected, as appears below: 



EROSION PRECEDING THE ILLINOIAN 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 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 



