10 DRIFT DEPOSITS OF ILLINOIS. 



The drift deposits, as may be seen by the sections to be found 

 on the following pages, attain their maximum thickness in the 

 central portions of the State and thin out to the southward, 

 all the beds, except the loess, disappearing before reaching the 

 Ohio river. 



Throughout the southern counties of the State, the loess, in- 

 cluding the sandy marl and the brown clay which lies above 

 it. with a few feet of local drift beneath it, the whole aggregat- 

 ing a thickness of ten to thirty feet, are the only remaining 

 representatives of the drift deposits of Central Illinois. The 

 local drift appears to be entirely derived from the country rock 

 on which it rests, for where the underlying rock is limestone, the 

 embedded fragments of the superincumbent bed will be com- 

 posed of the cherty material derived therefrom. Where the un- 

 derlying rock is sandstone or sandy shale, sand and nodules of 

 sandstone will be found to constitute the main portion of the 

 overlying deposit. 



The sections that are to follow, extend over two-thirds of the 

 entire area of the State, including the central and northern 

 portions, where the greatest accumulation of foreign drift occurs. 



In Carroll county, on the northwestern border of the State, 

 the following section of a well in Mt. Carroll, was obtained by 

 the Hon. James Shaw, and is given in his report on the geology 

 of that county, as typical r of the drift deposits of that region: 



Feet. 



Black prairie soil 2 



Yellow flue grained clay 13 



Common blue clay 2 



Reddish clay and gravel 15 



Tough blue clay 2 



Coarse stratified gravel 3 



Pure yellow sand 11 



Black mucky clay , 5 



Total S3 



If there is any representative of the boulder clay in the above 

 section, it must be the fifteen feet of clay and gravel near the 

 middle of the section. 



In the counties to the eastward of Carroll, Mr. Shaw esti- 

 mates the average thickness of the drift deposits in Stephenson 

 and Winnebago counties at about twenty-five feet, while in Ogle 

 county he places its maximum thickness at "more than a hun- 



