DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 695 
reep SAN \S 
along the post-Il]inoian cour: ock River. 
Fic. 1.—Glacial map of portions of southern Wisconsin and northern Illinois. 
(Boundaries for that part of Illinois not examined by the writer from Frank 
Leverett, “Illinois Glacial Lobe,” Mon. U.S. Geol. Surv., XXXVIII, Pl. XII.) 
