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IOWA ACADEMY OP 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 III 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, f 
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 statement 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 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 depcjsits 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 which 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. 916-917, Diagram 
E, also Diagram 1, sheet 4. 
