LOWER RAPIDS OF THE MISSISSIPPI 3 



tially abandoned preglacial valley, is shown in cross-section in 

 Fig. 1, furnished by the Iowa Geological Survey. The depth 



Fig. 1. Cross-section from Sonora, 111., to Argyle, la., showing old and new chan- 

 nels of the Mississippi River (Iowa Geol. Survey). 



of the new channel is but little more than half, and the width 

 scarcely one-fifth, that of the preglacial channel. In size it is, 

 therefore, scarcely one-tenth as large as the preglacial valley. 



The small size of the Mississippi Valley at the lower rapids, 

 compared with its size above and below, was noted by Worthen 

 more than forty years ago, and interpreted to be an evidence 

 that the greater valley is preglacial, while the portion of the 

 valley across the rapids is postglacial. In the report of Hall, 

 made in 1856, the following statement is found in the discus- 

 sion of Lee county : J " The valley thus scooped out of the solid 

 rocks extends from Montrose to the mouth of Skunk River, and 

 is from six to eight miles in width. The eastern portion of this 

 ancient basin, except the bluffs on the river above Ft. Madison, 

 is now covered by the alluvial deposits before mentioned, 

 while the western part is occupied by deposits of drift material 

 from 100 to 185 feet in thickness. That this valley was formed 

 by ancient currents previous to the drift period is proved by the 

 fact that a considerable portion of it is now occupied by deposits 

 of that age, and which must have been formed after those cur- 

 rents ceased to act." Again, in his first volume of the "Geology 

 of Illinois," published in 1866, Worthen remarks (p. 9) that the 

 present river has shown, by the work done in the upper and 

 lower rapids, how inadequate its erosive power would be to 



'Geol. of Iowa, Vol. I, 1858, p. 188. 



