VOLUME XXXI NUMBER 4 



THE 



JOURNAL OF GEOLOGY 



May-yune ig2J 



THE DIFFERENTIATION OF THE DRIFT SHEETS 

 OF NORTHWESTERN ILLINOIS 



MORRIS M, LEIGHTON 

 Illinois Geological Survey, Urbana, Illinois 



THE PROBLEM 



Ever since the present classification of the Pleistocene has been 

 evolved with its five glacial stages — the Nebraskan, Kansan, Illi- 

 noian, lowan, and Wisconsin — the drift of northwestern Illinois, 

 west of the mapped Wisconsin, has had an uncertain status. After 

 the important discovery of the Illinoian stage of glaciation, about 

 three decades ago, Mr. Leverett referred the outermost drift in 

 northwestern Illinois tentatively to the Illinoian stage of glacia- 

 tion, and the next inner drift to the lowan."" Subsequently the 

 existence of an lowan drift in the Labrador field was ignored by Mr. 

 Leverett in at least two publications, and its existence in the Kee- 

 watin field questioned.^ In 1908, Dr. William C. Alden carried on 

 studies along Rock River and to the west, and reached the conclu- 



^ Frank Leverett, "The Illinois Glacial Lobe," U.S. Geological Survey Monograph 

 XXXVIII (1899), Plates VI and XII. 



^ "Weathering and Erosion as Time Measures," American Journal of Science, 

 Vol. XXVII (1909), p. 351, and " Comparison of North American and European Glacial 

 Drift Sheets, Zeit.jiir Gletsch., Book 4 (1910), p. 248. 



265 



