RIDGED DRIFT OF KASKASKIA BASIN. 73 



ridges, often rise abruptly from very fiat tracts, and seldom shade into the 

 bordering plain, a feature which distinguishes them from the moraines of 

 the Wisconsin series, which usually grade into plane tracts on their inner 

 border. 



The ridge leading southeastward from the Kaskaskia across north- 

 eastern Randolph and western Jackson counties is not so sharp as the 

 majority of ridges west of the Kaskaskia, and has a gently undulating sur- 

 face, similar to that of the ridges formed on the drift border in western 

 Illinois and southeastern Iowa. It seems, however, to be definitely con- 

 nected with the system of ridges following the west side of the Kaskaskia, 

 and can scarcely be included with the ridges which characterize the drift 

 border. 



The entire system of ridges is composed largely of typical till, blue 

 till being present in the lower portions and brown till near the surface. In 

 a few cases gravel and sand have been found, but such material is so rare 

 that railways have not found it expedient to obtain ballast from these ridges. 



The origin or mode of formation of these ridges is problematical. 

 Their trend is nearly in line with the ice movement, as shown by stria? 

 in the neighborhood of Alton, in Madison County, and is about at right 

 angles with the course of the boundary of the Illinoian drift sheet. Several 

 working hypotheses were employed during the field investigations, among 

 which may be mentioned the following: (1) That the ridges are similar in 

 origin to drumlins. (2) That the ridges constitute an interlobate morainic 

 system formed in a reentrant between a supposable lobe which covered 

 western Illinois and one which covered southern Illinois and southwestern 

 Indiana. (3) That the ridges mark the western border of a lobe which 

 persisted in southern Illinois after the ice had retreated from western Illinois. 

 (4) That the ridges are dependent in some way upon obstructing rock hills 

 which constitute their nuclei. (5) That the ridges are a remnant of a sheet 

 of drift which once filled that region to the height of their crests. 



In the study of these ridges it was found that in a few instances they 

 bear resemblance to drumlins in their form, but the great majority bear 

 little or no likeness to that class of ridges. The hypothesis of an origin 

 similar to drumlins is therefore held somewhat lightly, but is not rejected. 



The hypothesis that these ridges were formed as an interlobate belt 



