150 GEOLOGY OF ILLINOIS. 



The Loess, the most recent of the geological formations after the Alluvium, 

 occurs in this county only along the Illinois river bluffs, in which it attains a 

 thickness of from sixty to eighty feet. Back from the bluffs, it rapidly thins 

 out, and is seldom seen extending more than a mile or two up the side ravines, 

 and, indeed, it frequently disappears entirely within a much less distance. 

 The material is generally an ash or buff colored, marly sand, containing fossil 

 fresh water shells of existing species, here as elsewhere, forming high, conical 

 bluffs, which constitute a peculiar feature in the landscape. So resistant is this 

 material to atmospheric influences, that many of the bluffs are crowned by steep, 

 mural escarpments of compacted sand, which preserve their shape from year to 

 year, in spite of the wearing action of the frosts and showers. 



The deposits of the Drift extend over nearly the whole surface of the county, 

 their thickness ranging all the way from twenty to eighty, or one hundred feet, 

 and at Jacksonville, its thickness amounts to even one hundred and forty-seven 

 feet. The material of this formation is generally a blue or yellow clay, with 

 occasional seams or strata, of quicksand or gravel. Good sections of this for- 

 mation are, however, rarely met with, both on account of the infrequency of 

 shafts or wells of sufficient depth, and of the frequent lack of reliable informa- 

 tion in regard to those wells which have been sunk. In general, however, the 

 brown clays are uppermost, and are underlaid by bluish clays and hard-pan. 

 A little distance north of Prentice Station, on the St. Louis, Jacksonville and 

 Chicago railroad, in the extreme northeastern part of the county, a shaft passed 

 through eighty-five feet of the beds of the Drift, aad the following section was 

 reported : 



FKKT. 



1. Surface soil, and brown and yellow clays 25 



2. Bluish hard-pan 50 



3. Sandy clay, containing a log eighteen or twenty inches in diameter 10 



Logs and drift-wood are reported to have been frequently found in the clays' 

 etc., of the Drift in this county, but seldom as deep as in this instance, at the 

 very base of the formation. 



Boulders are abundant in all parts of the county, but in this region are sel- 

 dom of such very large size as farther north. Many of the transported boul- 

 ders show polished and striated surfaces on two or more sides, but no such sur- 

 faces were observed in any of the exposures of rock in situ. 



The older geological formations which appear in the surface exposures of 

 this county, are the Coal Measures, and the St. Louis limestone. Of the for- 

 mer, there is between the uppermost and lowest exposures a considerable 

 aggregate thickness, it is difficult to state exactly how much, but probably sev- 

 eral hundred feet, including the horizon of at least three or four workable coal 

 seams. Of the St. Louis limestone, only a limited thickness of the upper beds 

 is exposed. 



