DRIFT SHEETS OF NORTHWESTERN ILLINOIS 



279 



in 119 different borings in the Green River drift area was 5.7 feet 

 as compared with an average of 8.1 feet in the Illinoian drift area, 

 based on 150 borings. (See Fig. 4 for a graphical presentation of 



Fig. 4. — Graphs showing the number of borings recording a certain depth of 

 leaching in the Belvidere, Green River, and Illinoian drift areas. 



these data.) The average thickness of leached till is more than 

 three times greater for the Illinoian drift than for the Green River 

 drift, while the thickness of the overlying non-calcareous loess or 

 loess-Uke silts is about the same.^ It is also of interest to note that 



' The thickness of the calcareous loess is not given, but it is much greater on the 

 Illinoian than on the Green River drift; in many cases so thick that the auger failed to 

 reach the till below, and hence such borings could not be used. 



