82 
IOWA ACADEMY OP SCIENCES. 
and through this part of the valley until forced westward by 
the advance of the Labrador ice field. But if a great erosion 
took place in this part of the valley prior to the Illinoian stage 
of glaciation, there would seem good grounds for supposing 
that the stream assumed its present course sooh after the 
Kewatin ice field made its final withdrawal. 
Examining into this question, it is found that after this 
drift was deposited by the Kewatin ice field an erosion so 
great took place that it was removed, throughout the greater 
part of the width of the valley, down to a level scarcely fifty 
feet above the present stream at the mouth of the Des Moines, 
and to an equally low level at Hannibal. The depth of cutting 
appears, therefore, to have been about 100 feet at the mouth 
of the Des Moines and perhaps twenty-five feet at Hannibal. 
It seems safe to assume an average depth of fifty feet for the 
entire section and a width of five or six miles, making an 
erosion of nearly three cubic miles of drift in the fifty miles 
below the mouth of the Des Moines river. It is scarcely 
necessary to raise the question whether this erosion could 
have been accomplished by the Des Moines and other 
tributaries of the Mississippi below the rapids, for it is 
evidently out of proportion to the work which these small 
streams would be able to accomplish since the Kansan stage 
of glaciation. It seems certain that the Mississippi river is 
responsible for the principal part of the erosion. This makes 
necessary the opening of the new channel across the rapids, 
since the old channel west of the rapids was not utilized by 
the river after the Kansan stage of glaciation, and no other 
line of drainage could have been adopted by the river that 
would pass through the portion of the valley below the rapids. 
Evidence is found within the new channel, of an erosion 
such as the interpretation just given demands. In the south 
part of Keokuk, between the foot of Main street and the 
mouth of Soap creek, the rock bluff rises but fifty to sixty feet 
above low water and is capped by a bed of bowlders about 
twenty feet in depth. Attention was called to this bed some 
thirty years ago by Mr. S. J. Wallace of Keokuk,^ and the 
view expressed that it is “old river shingle.” Mr. Wallace 
stated that Dr. George Kellogg, of Keokuk, regarded it as an 
indication of an ancient fall at this place, but that he did not 
so regard it. 
* Proc. A. A. A. s., Vol. XVII, 1869, p. 344. 
