THE PLEISTOCENE OR GLACIAL PERIOD 877 



IX. The Wisconsin glacial stage (or stages). Following this 

 epoch of readjustment, the ice radiated from the Labradorean, Kee- 

 watin, and Cordilleran centers (Fig. 560), and from many isolated 

 heights. Nearly all the well-known mountain glaciation of the west 

 is referred to this epoch. The margin of the ice-sheets assumed a 

 pronounced lob ate form, and the drift which they left is character- 

 ized by stout terminal moraines, numerous kames, eskers, drumlins, 

 outwash aprons, valley trains, and other features distinctive of 



Fig. 582. Map showing the relations of the several drift-sheets in Iowa 

 and northern Illinois. 1, Kansan; 2, Illinoian; 3, lowan; 4, Early 

 Wisconsin; 5, Late Wisconsin; 6, Driftless area; 7, Course of the Mis- 

 sissippi during the Illinoian glacial epoch. (Leverett, U. S. Geol. Surv., 

 and Calvin, la., Geol. Surv.) 



glacial action and glacio-fluvial co-operation. This drift-sheet, far 

 beyond all the others, bears the stamp of the great agency of the 

 period. The pronounced topographic expression of this formation 

 is in contrast with the relatively expressionless surfaces of the older 

 sheets of drift. A part of this difference is due to the greater fresh- 

 ness of the Wisconsin formation; but the larger part, apparently, is 

 assignable to a stronger original expression. 



The Wisconsin glacial epoch has been divided into two, an 

 Earlier and a Later. They are, however, less distinct than the 

 other epochs mentioned above,, and so are grouped together here. 



