76 IOWA ACADEMY OF SCIENCES 



which reach its rock floor. The comparative size of the valley 

 of the Mississippi, in its new channel across the lower rapids, 

 and the partially abandoned preglacial valley, is shown in 

 cross section in figure 6, of Vol. Ill of the Iowa Geological Sur- 

 vey. The depth 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." 



Again, in his first volume of the Geology of Illinois, pub- 

 lished in 1866, Worthen remarks (page 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 excavate 

 in postglacial time the entire valley which it now but partially 

 occupies. 



A few years later Gen. G. K Warren discovered the aban- 

 doned section of the preglacial valley which crosses Lee 

 county, Iowa, a few miles west of the lower rapids, and con- 

 nects the portion occupied by the stream above the rapids with 

 that below. In his report in 1878 he presented a discussion 

 illustrated by a map setting forth the position of the old 

 channel. + 



General Warren based his interpretations upon the absence 

 of rock outcrops in the valleys which traverse the old course 



*In his report to Hall, made in 1856, the following' htatement is found in the discus- 

 sion of Lee county. (Geol. of Iowa, Vol. 1, 1858, p. 188): 



"The valley thus scooped out of the solid rocks e.\tends from Montrose to the 

 mouth of Skunk river, and is from six to eight miles in width. The eastern portion of 

 this ancieut basio, 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 consider- 

 able portion of it is now occupied by deposits of that age, and wliich must have been 

 formed after those currents ceased to act." 



+ Report of the U. S. Army Engineers for 1878-9, Vol IV, part 2, pp. 916-&17, Diagram 

 E, also Jjiiigram 1, sheet 4. 



