THE PLEISTOCENE OR GLACIAL PERIOD. 391 



V. The Illinoian glacial stage. — The typical formation of this stage 

 was a sheet of till occupying the surface in the southern and western 

 portions of Illinois (Fig. 514), and running back under the later forma- 

 tions to the northeast toward the Labradorean center of radiation. 

 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 identifica- 

 tion 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 sections, notably a, 

 belt running southwest from Tower Hill, Illinois, to the margin of the 

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

 tendency to ridging in the fashion 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. 1 Previously, the 

 Kansan lobe had invaded the border of Illinois, and probably 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. 



VI. The Sangamon interglacial stage. 2 — Like the preceding inter- 

 glacial stages, this is characterized by peat, muck, old soil and sub- 

 soil, weathering, surface erosion, etc. Judged by these, the interval 

 was not as long as the Yarmouth. 



VII. The Iowan glacial stage. 3 — The Iowan ice invasion is recorded 

 in a thin sheet of till (Fig. 512), marked by an exceptional profusion 

 of large granitoid bowlders which lie chiefly on the surface and are 

 somewhat aggregated into a bowlder belt on the eastern border of 

 the tract. The typical Iowan drift was formed by a lobe of the Kee- 

 watin ice-sheet, occupying the north-central part of Iowa (see map, 

 Fig. 514). It fell much short of the Kansan invasion of the same 



1 Leverett, Mono.#XXVIII, U. S. Geol. Survey. 



2 Idem. 



3 See Calvin, Bain, and others, Reports Iowa Geol. Surv. 



