340 



They have, however, peiietratecl 40 to 50 feet without striking roelc. 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. 



"It 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- 

 catine, 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 interesthig 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 tliis 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-fllled 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, lowa^ (Fig. 1). Below Keokuk the Mississippi 

 follows the preglacial channel. 



Not much space can be devoted to a discussion of the tributaries be- 

 cause the map shows the ones that can be mapi^ed with any certainty, and 

 the reference in regard to each one are full. 



Regarding the preglacial history of the Minnesota valley, Upham says 

 (131) : "There is evidence .... in the terraces of modified drift 



' Leverett also gives a map and cross sections of this cliannel. See bib. 62, 

 63, 64; also J. E. Todd, 114, 



