LOWER RAPIDS OF THE MISSISSIPPI I 7 



Kansan glaciation. This excavation in the section embraced 

 within the rapids was mainly rock, for the loess and alluvium had 

 built up the channel scarcely thirty feet above the rock floor of 

 the post-Kansan erosion. But for some distance, both above 

 and below these rapids, the excavation was largely in till. The 

 channel across the rapids was opened to a width but little greater 

 than the stream, or about one mile. Elsewhere the channel is 

 three to six times the width of the stream. 



This erosion seems to have continued until the early part of 

 the Wisconsin glacial stage, when, as indicated below, another 

 filling occurred. The extent and depth of the erosion which 

 took place prior to the Wisconsin filling is well shown in the 

 broad portion of the valley above the rapids. Numerous wells 

 indicate that the till had been removed nearly to present river 

 level over the greater part of the width of the valley before 

 that filling set in. 



The amount of erosion in the Mississippi valley seems to 

 have been nearly as great in this interval as in the post-Kansan 

 interval of erosion. It is doubtful, however, if the time involved 

 was so great as in that interval, for the gradient appears to have 

 been higher. To properly estimate the time involved, it is 

 necessary, also, to know the volume of water discharged through 

 the valley at each interval, a matter concerning which very little 

 is yet known. 



Filling at the Wisconsin stage of glaciation. — At the Wiscon- 

 sin stage of glaciation the Mississippi and several of its tribu- 

 taries, which flowed away from the ice-sheet, became so burdened 

 by glacial detritus that they were unable to completely transport 

 their load, much less to continue the erosion of their valleys. 

 The Mississippi headed in the ice-sheet near St. Paul, Minnesota, 

 while the Chippewa and Wisconsin rivers brought material from 

 the Chippewa and Green Bay lobes of Wisconsin. Rock River, 

 also, brought material from the Green Bay lobe, and through its 

 tributaries, Kishwaukee and Green rivers, from the Lake Michi- 

 gan lobe. Just above St. Louis the Illinois River contributed a 

 large amount of material derived from the Lake Michigan lobe. 



