164 GEOLOGY OF ILLINOIS. 



The geological formations in this district, consist of the Quaternary deposits, 

 the Loess and Drift, and the Coal Measures, which alone of the older forma- 

 tions, underlie the surface beds of clay, gravel, etc., in these counties. The 

 Loess forms the bluffs along the Illinois and Sangamon bottoms, in Cass county, 

 and also appears in the bluffs of the Sangamon river, and Salt creek, to some 

 extent, in Menard county, though it does not appear as prominently in the land- 

 scape as farther west. Its general features here are the same as in the other 

 river counties, and it forms the same bald bluffs, that are seen in other localities 

 along the Illinois and Mississippi rivers. The material here is an ash or buff 

 colored marly sand, containing fossil fresh water shells of existing species. 

 The thickness of the formation is considerable, some sixty or seventy feet 

 immediately at the bluffs, but it rapidly thins out in the back country, in many 

 places disappearing entirely within a very short distance. It appears to extend 

 the farthest inland along the Sangamon river in Cass county, north of the town 

 of Virginia, and several good sections of this deposit may be seen in the cuts 

 on the Peoria, Pekin and Jacksonville railroad, between that place and Chand- 

 lerville. Along the upper course of the Sangamon, in Menard county, this 

 formation is scarcely to be seen at any point, and may perhaps be said to cease 

 entirely along this stream, within the limits of the county. 



The Drift deposits in this district consist of brown, yellow, and blue clays, 

 with boulders, while sand and gravel seams are of frequent occurrence 

 amid the mass. The thickness will probably range, over the whole district, 

 between forty and one hundred feet; of this, only an estimate can be made in 

 most cases, as shafts and wells of sufficient depth, and other opportunities of 

 obtaining any exact knowledge in regard to this particular, are rarely met with 

 over a greater portion of this region. At Sweet-water, in Menard county, it 

 was found to be one hundred and ten feet from the surface to the uppermost 

 bed of rock, and the boring presented the following section. 



FEET. 



1. Surface soil and brown clay 40 



2. Quicksand 11 



3. Blue clay , , 59 



In the eastern part of section 2, township 17, range 6, near the village of 

 Athens, a shaft commenced at the bottom of a ravine, which cuts down some 

 forty or fifty feet below the general level of the country, was sunk eighty-six 

 feet without striking a bed of rock, and at the depth of sixty-five feet, pieces 

 of coniferous wood, in a tolerable state of preservation were taken out. Many 

 large boulders, which had to be removed by blasting, were also met with, some 

 of them of granite, indicating by their material a remote northern origin, but 

 more were fragments of the underlying Coal Measure limestone, and sandstone, 

 containing many of their characteristic fossils and showing, by their compara- 



