CHAPTER XTI, 



INFLUENCE OF THE DRIFT UPON DRAINAGE SYSTEMS AND 

 DRAINAGE CONDITIONS. 



Throughout almost the whole extent of the Wisconsin drift in Illinois 

 and Indiana the preglacial valleys are filled so effectually that the present 

 drainage is independent of them. The only notable exceptions are found 

 in the lower part of the Illinois and Wabash valleys. Within the Wiscon- 

 sin drift the morainic ridges often constitute water partings, while the plains 

 between them carry drainage lines having courses nearly parallel with the 

 ridges. In the Iowan drift area and in the portion of the Illinoian drift in 

 western Illinois and southeastern Iowa, it is frequently possible to deter- 

 mine the courses of preglacial drainage lines and of the divides between 

 them, though the streams conform to preglacial features only in a rude 

 way. In southern Illinois and southwestern Indiana, for about 75 miles 

 north from the extreme limits of glaciation, or to about the latitude of 

 St. Louis, Missouri, and Terre Haute, Indiana, the drift is so thin that it has 

 not greatly changed the main arteries of drainage from the lines followed 

 by preglacial streams. The smaller streams, however, are often found to 

 follow courses independent of preglacial lines, and the great majority are 

 only partly coincident with them. The influence of glaciation, as already 

 noted, has also extended into outlying districts and caused the deflections 

 of streams whose lower courses or outlets were so situated as to be 

 obstructed by the ice sheet or by its deposits. The influence of the drift 

 is, therefore, very marked in all parts of the region under discussion and 

 becomes a question of importance in the interpretation of the drainag'e 

 development. 



Inasmuch as the influence of the drift varies greatly, the following 

 designations are made, to indicate, in their order of importance, the relative 

 amounts of change produced by it : 



(a) New drainage systems. 



(b) Major stream deflections. 



(c) Minor stream deflections. 



(d) Reestablished or nondeflected drainage. 



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