CHAPTER XII I. 

 AVERAGE THICKNESS OF THE DRIFT IN ILLINOIS. 



The region under discussion is one which may perhaps furnish a more 

 complete series of data concerning the thickness of the drift than any region 

 of equal size yet studied. It is especially favorable, not only because of 

 the large number of sections of borings obtained, but also because of the 

 comparative smoothness of the region, an estimate of average thickness 

 being more readily obtained in a comparatively plane region than in a very 

 hilly one. 



The inequalities of the rock surface, as indicated above, are sufficient to 

 give the drift considerable variation in thickness, for in much of the region 

 the amount of drift is sufficient to fill the valleys or basins nearly to a level 

 with the uplands, only the most prominent parts of the uplands rising above 

 the general level of the drift surface. 



- The thickness of the drift also varies because of aggregation in 

 morainic ridges, there being several prominent morainic belts in which the 

 thickness is as much greater than that of the bordering plains as the measure 

 of the relief of the moraines. The inequalities in thickness resulting from 

 morainic accumulations are, however, much less than those resulting 

 from variations in the altitude of the underlying rock, being seldom greater 

 than 100 feet and often but 40 to 50 feet, while variations due to inequalities 

 of the rock surface often reach 200 feet and may in places exceed 300 feet. 



In a general sense it is true that in the portions of this region where 

 but a single ice invasion has occurred the drift is thinner than where there 

 has been a succession of invasions. Thus the southeastern, southern, 

 and western portions show thinner deposits of drift than the central and 

 northeastern. Yet extensive areas of comparatively thin drift occur in 

 the northeastern portion on the borders of the Kankakee River and in the 

 vicinity of the Chicago Outlet, i. e., in places where the number of invasions 

 has been greatest. 



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