62 THE ILLINOIS GLACIAL LOBE. 



there considered a "post-Tertiary soil" older than the drift proper, and 

 formed under very different conditions. The following is the section there 

 published: 



Section in a boring for coal, a mile east of Coatsburg, Adams County, Illinois. 



Feet. 



Soil and yellow clay 6 



Bluish-colored clay and gravel - 45 



Clay with large bowlders 40 



Black soil 2+ 



Clay (stratified) 6 



Very tough blue clay 20 



Rock entered at 119 



It seems not improbable that the silt under the till of central Adams 

 County is attributable to ponded waters held in front of the Keewatin ice 

 sheet in the Kansan stage, for that ice sheet apparently crossed the Missis- 

 sippi into Illinois near Hannibal, Missouri, and covered the lower courses of 

 its eastern tributaries. 



In Pike County typical till has been seen at but few points. The 

 several drift ridges which traverse the county are composed largely of clay 

 and sand in which only a few small pebbles occur. There is, however, on 

 the borders of Hadley Creek, in the northern part of the county and the 

 adjacent portion of Adams County, considerable waterworn chert in the 

 base of the drift. This chert is apparently a residuary product from 

 the decay of cherty limestone in that region, but it was worked over to 

 some extent by the ice sheet and its associated waters, and this has resulted 

 in the introduction into the chert of occasional bowlders and smaller stones 

 of distant derivation, as well as the wearing and rounding of the chert frag- 

 ments. These chert beds are in places 10 or 15 feet thick. They usually 

 present the appearance of gravel beds, there being very little clay present. 

 Exposures were found, however, east and southeast of Baylis, on the west 

 side of Bay Creek, in which a large amount of clay is mingled with the 

 chert and other stony material. In the southwest part of the count}', which 

 appears to have been but slightly glaciated, if at all, the chert beds remain 

 intact at the surface of the limestone. Quarries along the east bluff of the 

 Mississippi afford good exposures. 



A few well sections and hillside sections obtained in the vicinity of the 

 drift border are here given to illustrate and make clearer the above state- 

 ments. 



