876 GEOLOGY 



IV. The Yarmouth interglacial stage. 1 Where the Illinois till 

 overlaps the Kansan (eastern Iowa), an old soil with deep subsoil 

 weathering was formed on the surface of the latter before its burial. 



V. The Illinois glacial stage. The exposed part of the Illinois 

 drift appears at the surface in the southern and western portions 

 of Illinois. It runs Lack under later formations to the northeast, 

 toward the Labradorean center of radiation. It is not identified 

 with any confidence east of Ohio, where its margin seems to have 

 been overridden by the ice of the Wisconsin epoch. The identifi- 

 cation of the Illinois drift in the Keewatin area is yet an open 

 question. The Illinois till is clayey, with little assorted drift asso- 

 ciated. The west edge of the Illinois ice-lobe pushed out into Iowa 

 a score of miles, forcing the Mississippi in front of it. 2 The Kansan 

 lobe had earlier invaded Illinois from the west, and probably forced 

 the Mississippi east of its present course, if such an easterly course 

 had not been taken before the Kansan epoch. Efforts to trace out 

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

 in Illinois and Iowa have not been entirely successful. 



VI. The Sangamon interglacial stage. 3 Like the preceding 

 interglacial stages, this is marked by peat, muck, old soil and sub- 

 soil, weathering, surface erosion, etc., on the surface of the Illinois 

 drift. Judged by these criteria, the interval was not as long as the 

 Yarmouth. 



VII. The lowan glacial stage. 4 The lowan ice-sheet left a thin 

 sheet of till (Fig. 582), marked by a profusion of large granitoid 

 bowlders many of which lie on the surface. The known lowan 

 drift was formed by a lobe of the Keewatin ice-sheet, which ex- 

 tended down into the north-central part of Iowa, but fell short of 

 the Kansan invasion of the same region. 



VIII. The Peorian interglacial stage. 5 This is characterized in 

 the same way as the preceding interglacial intervals, but less strongly. 



1 Leverett, Mono. XXXVIII, U. S. Geol. Surv. 



2 Idem. 



3 Idem. 



4 See Calvin, Bain, and others, Reports Iowa Geol. Surv. The ro.ility of 

 this sheet of drift as a separate formation has recently been called into qur-s- 

 tion by Leverett. 



5 Leverett, op. cit. 



