AVERAGE THICKNESS OF THE DRIFT IN ILLINOIS. 549 



the State may safely be placed at not more than 130 feet and not less than 

 100 feet; probably it lies not far from midway between these amounts, or 

 115 feet. 1 



1 In the above estimates everything which overlies the rock has been included, not only glacial 

 drift, but residuary clay, alluvium, and loess. In order to ascertain the proportion of till, a careful 

 computation has been made from the records of 1,687 borings obtained within the the area covered by 

 the Illinois lobe. These give the following results: 



Per cent. 



Till, including all glacial clays 69. 38 



Sand, gravel, and allnvinm 25.25 



Loess and associated silts 4. 25 



Buried soil, residuary clay, etc 1.12 



Total 100.00 



In this computation it is probable that the amount of residuary material is underestimated, 

 since it is only occasionally recognized in borings. 



