LOWER RAPIDS OF THE MISSISSIPPI I I 



S. J. Wallace, of Keokuk, 1 and the view expressed that it is "old 

 river shingle." Mr. Wallace stated that Dr. George Kellogg, of 

 Keokuk, regarded it as the indication of an old fall at this place, 

 but that he did not so regard it. This bed has been discussed 

 at some length by Dr. C. H. Gordon in the Geology of Iowa, 2 

 and three interpretations for its origin are presented. (i) That 

 it was formed by river action alone, i. e., as an alluvial bar; (2) 

 that it is due to the cutting down of a till sheet, the coarse 

 material being left as a residue ; (3) that it is a bowldery 

 moraine dropped at the edge of the ice sheet at the Illinoian 

 staafe of orlaciation. 



Of the three interpretations the second seems to Mr. Gordon, 

 as well as to the present writer, the most applicable. Dr. Kel- 

 logg's suggestion of a fall as the cause seems at least to be 

 poorly sustained. A similar bowlder bed occurs near Warsaw, 

 111. It there forms the capping of an eroded till surface and 

 bears clear evidence of removal of the fine material by a stream 

 and the retention of the bowlders as a residue. A bowlder bed 

 is also found along the face of the west bluff of the rapids near 

 Sandusky, about six miles above Keokuk, at a level forty to 

 sixty feet above the stream, that probably was derived from the 

 erosion of a sheet of till. It seems referable to the period of 

 erosion that produced the beds at Keokuk and Warsaw. The 

 exposure, however, is not sufficiently extensive to show clearly 

 its relation to the till sheet. 



The amount of erosion effected is so great that the beginning 

 of this new channel seems to date from near the close of the 

 Kansan stage of glaciation. This becomes more evident as we 

 study into the later stages of the history of the river. Even if 

 the river had been forced into a channel farther east than the 

 lower rapids, it seems scarcely probable that it remained long in 

 that course. It apparently began its work of opening the course 

 across the rapids long before the Labrador ice-field had reached 

 the region. 



1 Proc. A. A. A. S., Vol. XVII, 1869, p. 344. 



2 Geology of Iowa, Vol. Ill, 1893, PP- 252-255. 



