340 
They have, however, penetrated 40 to 50 feet without striking rock. The 
bed rock gradually descends from each side toward the middle of the low- 
land, and some of the creeks coming into the lowland occupy large and 
deep channels which have been only partially filled with drift. This rather 
throws the balance of evidence in favor of the view that the preglacial 
stream flowed southeastward into the Illinois. 
“Tt should be observed that in case the southwestward route proves to 
have been the course of the Mississippi, the present line of the stream de- 
parts from it only a few miles and enters the same old valley below Mus- 
eatine, which it occupies above Clinton. But in case the southeastward 
route proves to have been the preglacial course from Clinton, the pre- 
glacial valley above Clinton finds its continuation down the Illinois in- 
stead of down the Mississippi, and the present Mississippi passes from one 
drainage system to another in its course between Clinton and Muscatine.” 
Carmon gives many more interesting details, but he concludes with 
Leverett and Udden: “It is quite possible that in one or the other of these 
courses the preglacial Mississippi flowed. Both appear to have rock floors 
to carry the waters of the streams which excavated the Mississippi valley 
above Clinton, but the data are not complete enough to allow us to decide 
which of these two courses was the real one” (16). Carmen gives an 
interesting discussion of the changes produced by each ice invasion upon 
the Mississippi and a reading of this will help detract from the complexity 
of the situation in this region. 
From Muscatine southward the Mississippi is flowing in a broad pre- 
glacial channel except for a few miles above Keokuk, Iowa, where it is 
flowing in a post-glacial gorge known as the Lower Rapids (63). The old 
drift-filled valley which has been studied by C. H. Gordon (41), is about 
twice as wide and 100 feet deeper than the present valley, and lies to the 
westward in Lee county, Iowa’ (Fig. 1). Below Keokuk the Mississippi 
follows the preglacial channel. 
Not much space can be devoted to a discussion of the tributaries be- 
‘ause the map shows the ones that can be mapped with any certainty, and 
the reference in regard to each one are full. 
Regarding the preglacial history of the Minnesota valley, Upham sayer 
(1381): “There is evidence . . . . in the terraces of modified drift 
2 Leverett also gives a map and cross sections of this channel, See bib. 62, 
63, 64; also J. E. Todd, 114. 
