AVERAGE THICKNESS OP THE DRIFT IN ILLINOIS. 547 



The following tabular statement sets forth the proportion of reported 

 borings between each 100 feet within the entire district covered by the Illi- 

 nois lobe, and includes both those which reach rock and those which do not. 



Number. 



With 400 feet or more 4 



With 300 to 400 feet 18 



With 200 to 300 feet 138 



With 100 to 200 feet 698 



With less than 100 feet 2,251 



Total 3,109 



After the estimates just recorded were completed, the thickness of drift 

 in Illinois was computed on the basis of the requirements of the topography, 

 the data obtained by Chamberlin and Salisbury in the Driftless Area serving 

 as a guide. The investigations in the Driftless Area led to the opinion that 

 150 to 200 feet of drift is necessary to fill the valleys up to the level of the 

 divides, all the summits of the ridges being still left bare, while 300 feet 

 would be necessary to bury the region as deeply as in the heavy drift regions 

 of the four adjoining States. 1 



Upon turning to Illinois, it is found that the drift in places has filled the 

 valleys completely and brought the surface up to a level perhaps 100 feet 

 or more above the summits of the ridges. In much more extensive districts 

 it has barely filled the valleys, while in fully half the State it has fallen 

 short of filling the valleys, the amount in the different localities being two- 

 thirds, one-half, one-third, or one-fourth as great as is necessary to com- 

 pletely fill the valleys. 



It is estimated that there may be about 4,160 square miles along the 

 bulky moraines of the Wisconsin drift in which the average thickness 

 reaches 300 feet; but in the greater part of the Wisconsin drift area it can 

 scarcely exceed 200 feet, for the filling extends but little above the level 

 of the rock divides. There is estimated to be 10,975 square miles in which 

 the average thickness may reach 200 feet, this district lies mainly within 

 the Wisconsin drift but extends beyond this drift down the Illinois Valley 

 through Mason and the adjoining portion of Logan County into Cass 

 County. The area of Iowan drift in eastern Winnebago, Boone, and neigh- 

 boring portions of McHenry and Kane counties may possibly have 150 



1 Sixth Ann. Rept. U. S. Geol. Survey, pp. 257-258. 



