178 GEOLOGY OF ILLINOIS. 



which form so characteristic a feature in the landscape along the river below. 

 In the northern part of Tazewell county, although this buff marly sand appears 

 to some extent in the bluffs along the Illinois river, it is not by any means as 

 well exposed, or prominent, as in the counties farther to the south. 



The Drift formation, which covers the older rocks in almost every part of 

 this district, is here composed of beds of blue and brown clay, sand, and gravel, 

 and varies in thickness, in different portions, from fifty feet in the western part 

 of Tazewell county, to two hundred and fifty feet in the Bloomington shafts. 

 It has been penetrated, however, at comparatively but few points, and over the 

 greater portion of this region, its depth can only be approximately estimated. 

 It seems probable, indeed, that it may be of this thickness over considerable 

 portions of McLean county, as a boring at Chatsworth, in the adjoing portion 

 of Livingston county, was reported to have penetrated to a depth of two 

 hundred and fifty feet before striking rock. 



The material of the Drift in this region, appears to be generally roughly 

 stratified, alternating beds of sand, gravel, and clay, are frequently met with 

 in wells and borings. The sand and gravel beds, generally make up but a very 

 small part of the total thickness, though sometimes single beds attain a very 

 considerable thickness, as, for instance, at Chenoa, in the northern part of 

 McLean county, where a boring for coal passed through a bed of sand and 

 gravel thirty feet in thickness, overlaid by forty-five feet of the usual clays of 

 this formation. Occasionally, also, a bed of black earth or vegetable mould, 

 still containing pieces of wood, trunks of trees, leaves, etc., only partially 

 decayed, is met with, and a bed of quicksand, containing fossil land or fresh 

 water shells of existing species. The following section of the Drift, afforded 

 by a shaft sunk in the city of Bloomington, is of especial interest as showing 

 both of these conditions at unusual depths. The shaft was sunk by the Bloom- 

 ington Coal Mining company, near the track of the Chicago and St. Louis rail- 

 road, about half a mile north of the depot: 



FEET. 



1. Surface soil, and brown clay 10 



2. Blue clay ._ 40 



3. " Gravelly hard pan " 60 



4. Black mould, with pieces of wood, etc "..... 13 



5. Hard-pan and clay 89 



6. Black mould, etc 6 



8. Blue clay 34 



8. Quicksand, buff and drab in color, and containing fossil shells 2 



9. Clay shale, (Coal Measures.) 



Total 254 



Another shaft, a little over a mile distant from this one, passed through 

 materially the same succession of strata, with only local variations in the thick- 



