THE MISSISSIPPI VALLEY. 4(ft 



^ With the exception of drainage systems which are new throughout, the 

 drainage lines often embrace two or more of the phases just outlined. This 

 can not, therefore, well be made a basis for classification. A grouping 

 about the main drainage lines will perhaps best serve our purpose. The 

 discussion begins at the northwest with the Mississippi Valley and proceeds 

 south and east. 



THE MISSISSIPPI VALIiBY. 



The present Mississippi River has evidently a system of drainage widely 

 different from the system or systems which were operative in preglacial 

 times within the region now drained by it. Besides opening a new channel 

 at each of the rapids, the stream apparently is occupying sections of two or 

 more independent preglacial valleys. It may not be possible at present to 

 determine what relation the several sections sustained to one another in pre- 

 glacial time, much less to show the relation to the great systems by which 

 the interior of North America was drained. 



ACCESSION FROM THE NORTH ( ?). 



Hershey has recently written a paper in which he maintains that the 

 present Mississippi at Dubuque is out of proportion to the valley that it 

 occupies. 1 Upon comparing it with neighboring streams whose sources are 

 within the Driftless Area, and whose valleys are cut in formations similar to 

 those of the Mississippi at Dubuque, he concludes that the preglacial stream 

 flowing past Dubuque could not have been larger than the present Rock 

 River, and was possibly as small as the Pecatonica. The Mississippi pre- 

 sents bluffs which are somewhat steeper than those of tributary valleys. 

 In many places they are perpendicular precipices, and they are rarely suffi- 

 ciently sloping to support a growth of trees. The tributary valleys exca- 

 vated in the same rock formations have wooded slopes, rather steep, but 

 rarely bare precipices. Hershey remarks concerning this portion of the 

 Mississippi: "I have never yet come upon its canyon valley without beino- 

 impressed with its general appearance of greater youth than others of 

 apparently the same system." 



'The physiographic development of the Upper Mississippi Valley, hy Oscar H. Hershey Am 

 Geologist, Vol. XX, 1897, pp. 246-268. 



