SUCCESSION OF ICE-SHEETS 635 



assorted drift associated. The west edge of the Illinoian ice-lobe 

 pushed out into Iowa a score of miles, forcing the Mississippi in 

 front of it. Ice of the Kansan epoch 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 be- 

 fore the K.ansan epoch. 



V. B. The lowan drift (Keewatin field). In northeastern Iowa 

 the ice of this stage left a thin sheet of till marked by a profusion of 

 lari^e granitoid bowlders most of which lie on the surface. To the 

 northward in Minnesota these bowlders are less abundant, and the 

 formation passes beneath later drift. To the northeast it appears 

 to be connected with the third drift of Wisconsin. 



VI. Sangamon interglacial stage. In central Illinois a sheet 

 of sandy material marked by remnants of old soil, muck, peat, 

 weathering and erosion overlies the Illinoian glacial drift. Above 

 this lies a mantle of loess and the Peorian peaty beds. According 

 to the older view, the lowan was placed between the Sangamon 

 and the Peorian, now regarded, tentatively, as equivalents. 



VII. Wisconsin or Wisconian stage. The ice radiated from the 

 Labradorean, Keewatin, Cordilleran, and from many mountain 

 centers. It had probably done this at each of the preceding glacial 

 stages, but the record is much obscured by erosion and concealment. 

 The margin of the Wisconian ice was pronouncedly lobate, and the 

 drift which it left is characterized by stout terminal moraines, 

 numerous kames, eskers, drumlins, outwash aprons, valley trains, 

 and other features distinctive of glacial action and glacio-fluvial 

 cooperation. This drift-sheet, more than any of the others, bears 

 the stamp of the great agency of the period. The distinctive topog- 

 raphy of the various phases of this formation is in contrast with the 

 relatively expressionless surfaces of the older sheets of drift. Part 

 of this difference is due to the fact that the Wisconsin formation has 

 been eroded less than the older drifts; but the larger part, apparently, 

 is assignable to a stronger original expression. 



Unlike the earlier sheets of drift, the Later Wisconsin drift was 

 not overriden by later sheets of ice, and its original development 

 is therefore better shown at the surface. It has nearly a score of 

 concentric terminal moraines in some places. Some of them repre- 

 sent re-advances of the ice in the course of its general retreat, while 

 others mark halts in the retreat sufficient to permit an exceptional 

 accumulation of drift at the border of the ice. 



