266 MORRIS M. LEIGHTON 



sion that no post-Illinoian ice sheet had invaded the territory/ It 

 should be noted, however, that the area which he studied Hes mainly- 

 west of that which the present author has called the Belvidere lobe. 

 In 1913, Leverett suggested that the drift of northwestern Illinois 

 and certain other drift in western Wisconsin and southeastern 

 Minnesota may be a late stage of the Illinoian and a correlative of 

 the lowan drift in Iowa. Accordingly the name "Late Illinoian or 

 lowan" was proposed for such drift.^ More recently the question 

 was raised as to whether some of the drift south of Belvidere and 

 east of Dixon may be the product of a greater advance of the Early 

 Wisconsin ice than had formerly been suspected.^ Because of this 

 uncertain status, the drift of northwestern Illinois, particularly 

 with reference to its age, has been the first to be restudied by the 

 Illinois Geological Survey in its program for the re-examination of 

 the Pleistocene of the state. 



Throughout the investigation, the questions which were kept 

 in mind were: (i) Is there but one drift sheet or more than one in 

 northern Illinois, west of the mapped Wisconsin moraine? (2) 

 What is the age of the drift or drifts ? 



MODE OF ATTACK 



Information for answering these questions was sought in a care- 

 ful study of the topographic expression of the drift, the composition 

 of the drift, the stratigraphic sequence of the Pleistocene materials, 

 the state of weathering of these materials, and the drainage changes 

 which have occurred. 



Heretofore in the gathering of Pleistocene data in this area, 

 reliance has been placed chiefly on road cuts, railroad cuts, and 

 stream exposures, but they are comparatively few and the data 

 derived from these must be used with great discretion. The major- 



' William C. Alden, "Concerning Certain Criteria for the Discrimination of the 

 Age of Glacial Drift Sheets as Modified by Topographic Situation and Drainage 

 Relations," Journal of Geology, Vol. XVII (1909), p. 695. Somewhat further data was 

 included by Dr. Alden in "The Quaternary Geology of Southeastern Wisconsin," 

 U.S. Geological Survey, Prof. Paper 106 (19 18), pp. 137-60. Most of the succeeding 

 authors have followed Alden's mapping, p. 153. 



^ "lowan drift" (abstract), Bulletin of the Geological Society of America, Vol. XXIV, 

 (1923), p. 698. 



3 U.S. Geological Survey, Prof. Paper 106 (1918), p. 153, referring to the question 

 raised by Leverett. 



