82 IOWA ACADEMY OF 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 soon 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 seeois 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 siace 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. 



