CHAPTER IX. 



THE ILLINOIAN ICE INVASION AND THE SANGAMON INTER- 

 GLACIAL INTERVAL 



I. The Illinoian Ice Invasion 



"The typical formation of this stage was a sheet of till occupying the sur- 

 face in the southern and western portions of Illinois, and running back under 

 the later formations to the northeast toward the Labradorean center of radia- 

 tion. Its surface exposure is traceable northerly into Wisconsin and easterly 

 into Indiana and Ohio, but it is not identified with any confidence farther 

 east, where the margin seems to have fallen back, and to have been overridden 

 by the ice of the Wisconsin epoch. The identification of the Illinoian drift 

 in the Keewatin area is yet an open question. Like the Kansan drift, the 

 Illinoian is made up of clayey till, without marked association with assorted 

 drift in most regions. There is appreciably more assortment of the material, 

 however, than in the Kansan drift. There are tracts of kames in some sec- 

 tions, notably a belt running southwest from Tower Hill, Illinois, to the mar- 

 gin of the drift. The original surface was generally plane, and only a limited 

 tendency to ridging in the fasion of terminal moraines has been found. The 

 west edge of the Illinoian ice-lobe crossed the present course of the Mississippi 

 between Rock Island and Fort Madison, and pushed out into Iowa a score of 

 miles, forcing the river in front of it. Previously the Kansan lobe had forced 

 the Mississippi east of its present course, if indeed it did not already have a 

 course east of its present one before the Kansan ice appeared. Efforts to 

 trace out the early courses of the Mississippi under the thick mantle of drift 

 in Illinois have not been entirely successful " J 



II. The Sangamon Interglacial Interval 



A. SANGAMON SOIL AND WEATHERED ZONE; TYPICAL EXPOSURES 



"Between the disappearance of the Illinoian ice sheet and the deposition 

 of the Iowan till and loess there occurred an interval of deglaciation about as 

 marked as that between the Kansan and Illinoian stages of glaciation, a period 

 marked by leaching and oxidation of the Illinoian drift, of peat and soil accumu- 

 lation, and of erosion. This interval was long since brought to notice by Prof. 

 A. H. Worthen in his report on Sangamon County, Illinois. For this reason, 



J Chamberlin and Salisbury, Geology, HI, p. 391. 



