INTRODUCTION. 27 



drift sheet in the State of Illinois. In its peripheral portion it shows complexity that is 

 not as yet fully understood. Loess is interbedded to some extent with till, and silt thought 

 to be of aqueous deposition fills many of the valleys. The loess may prove to be derived 

 from pre-Illinoian loess gathered by the wind from districts outside and redeposited in such 

 manner as to become interbedded with Illiiioian till. 



The presence or extent of the Illinoian drift in the Keewatin field remains to be determined. 

 The only drift yet reported in that region to occur between the Kansan and Wisconsin drift 

 is termed Iowan. That drift, however, has been placed by the Iowa Geological Survey either 

 as a correlative or a close forerunner of the main or post-Sangamon loess. 



THIRD OR SANGAMON INTERGLACIAL STAGE. 



A well-defined interval of soil forming and leaching on the surface of the Illinoian drift 

 came in prior to the deposition of the overlying loess. The soil formed is named from the 

 Sangamon River basin in Illinois, where its occurrence was brought to notice by Worthen 

 many years ago in reports of the State Survey. The soil and the weathering are well developed 

 in Indiana and Ohio as well as in Illinois. 



POST-SANGAMON OR MAIN LOESS (IOWANP). 



The chief loess deposit east of Mississippi River overlies the Sangamon soil. West of that 

 stream it extends beyond the limits of the Ilhnoian drift and rests either on a soil developed 

 on the surface of the Kansan drift or on an earlier loess (pre- Ilhnoian) that has a patchy preser- 

 vation in that region. As noted above, it has been considered a somewhat close correlative 

 or possibly close successor of the Iowan drift. So far as the region under discussion is concerned 

 no Iowan drift has been recognized. 



PEORIAN OR POSTLOESSIAL SOIL AND WEATHERED ZONE. 



The post-Sangamon loess and its associated silts suffered some weathering before the 

 Wisconsin ice invasion. Exposures of the loess beneath the Wisconsin drift near Peoria, 111., 

 in which the upper part of the loess is more weathered than the lower, have led to the use of 

 the name Peorian to represent the pre- Wisconsin weathering, erosion, and general alteration 

 of the loess. 



WISCONSIN DRIFT. 



LOBATION OF THE BORDER. 



At the culmination of the last or Wisconsin ice invasion the outline of the border differed 

 considerably from that at the culmination of the Illinoian invasion, falling short in places by 

 more than 100 miles and extending beyond hi other places. The portion in Illinois fell short 

 the most. From Illinois eastward to central Ohio the borders gradually approach each other, 

 and beyond central Ohio the Wisconsin border overlaps the Illinoian and in places extends 

 beyond the Jerseyan ? drift. In the Green Bay lobe of Wisconsin the Wisconsin drift overlaps 

 the border of the Ilhnoian and encroaches on what had been driftless area in the Illinoian stage 

 of glaciation. 



The drift border of the Wisconsin invasion is more lobed than that of the Ilhnoian, especially 

 in Indiana and Ohio, where it includes the deposits of the East White, Miami, Scioto, and Grand 

 River lobes. In Illinois the Wisconsin had but one pronounced lobe, the Lake Michigan, whose 

 outline was rudely concentric with that of the Illinoian drift border. This lobe was an exten- 

 sion from the Lake Michigan basin. It formed, in the course of its withdrawal from Illinois, 

 a succession of bulky but rather smooth morainic ridges, which extend into western Indiana 

 for a few miles to the head of the reentrant between it and the portion of the Labrador ice field 

 that passed through the Huron and Erie basins. This reentrant is a few miles farther west 

 than the great reentrant between the Illinois and the Huron-Erie lobes of the Illinoian stage 

 (see PL V, p. 62), the latter being in south-central rather than southwestern Indiana. 



