MODE OF FORMATION OF TILL AS ILLUSTRATED 



BY THE KANSAN DRIFT OF NORTHERN 



ILLINOIS. 



Perhaps at no time during the Quaternary era were the 

 climatal conditions of Illinois of such a nature as to originate a 

 glacier independently of the ice introduced into the territory by 

 outflow from the vast ?ievc to the northeast. Consequently, 

 when near the culmination of the Kansan epoch, a very early 

 representative of the Lake Michigan glacier overspread nearly 

 the whole of the territory of the present state of Illinois, it 

 advanced across a region in which the indurated rock was but 

 thinly covered by a nearly continuous mantle of residuary 

 material, mostly clay, sand and angular gravel ; and the ice 

 thereupon proceeded to manufacture this into till. In a part of 

 northwestern Illinois, especially in Stephenson county, the glaci- 

 ation was of short duration and never repeated, and this 

 district therefore, presents one of the best fields for the study of 

 the contact phenomena between the base of the glacier and the 

 preglacial land surface. There are scattered over the area 

 incomplete deposits representing every stage in the process from 

 solid rock to typical till. By a careful study of these imperfectly 

 formed masses of till we may infer the process by which the ice 

 manufactured the residuary material and underlying rock into 

 the various types of deposits due directly to glacial action. The 

 necessarily limited nature of this paper will compel me to state 

 the hypothesis which seems to best explain the phenomena 

 known to me, and simply refer to the localities where each stage 

 is illustrated, mentioning a few of their most significant fea- 

 tures. 



The ice, in advancing across Stephenson county, moved 

 westwardly, and within fifteen miles of the Driftless Area, in a 

 decidedly northwesterly direction. During this time of general 



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