IOWA ACADEMY OF 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 
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, 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 OP 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. Prom 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 
6 
