88 
IOWA ACADEMY OF SCIENCES. 
Whether the deposition took place by water or by wind, 
there seems to have been a suspension of erosion on the lower 
rapids, and the length of this suspension must certainly be 
sufficient to affect materially their duration. An estimate of 
the time involved seems at present impossible, there being 
fewer data for an estimate than in the filling which occurred 
at the Illinoian stage. 
EROSION FOLLOWING THE LOESS FILLING. 
After the deposition of the loess, the valleys throughout 
much of the Mississippi basin experienced a marked deepen- 
ing, which brought their bottoms to a lower level than before 
the loess filling. In the portion of the Mississippi valley 
which lies within and near the rapids the deepening seems to 
have proceeded continuously to a level nearly as low as the 
present stream, or fifty to seventy -five feet below the excava- 
tion which occurred in the interval following the 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. Else- 
where 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 Ailing, 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 Ailing 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 dis- 
charged through the valley at each interval, a matter concern- 
ing which very little is yet known. 
