302 GEOLOGY OF ILLINOIS. 



Surface Geology. 



The surface deposits of this county comprise the usual subdivisions of the 

 Quaternary, Alluvium, Loess and Drift. The most extensive alluvial deposit 

 is that of the Mississippi bottom. This extends along the whole western bor- 

 der of the county, with a varied width of from two to five miles. Of this, 

 that portion which is situated in the northwest, and extends as far south as 

 New Boston, is much cut up by swamps, lakes and bays. Much of this land 

 is comparatively low, and valuable chiefly for meadow and grazing. 



Through a large portion of these bottom lands, there are one or more low 

 ridges of sand. The soil of this sandy portion is of but little value, there being 

 but few seasons when it is wet enough to produce full crops. In other por- 

 tions, the soil is a deep black loam and very productive. Narrow alluvial 

 belts are also found along nearly all the water courses, the soil of which is very 

 dark colored, but more or less intermingled with sand and pebbles. 



Loess. This deposit is found capping the Mississippi bluffs, and attains a 

 variable thickness of from ten to forty feet. It is a calcareous marl of light 

 brown or buff color, and generally contains great numbers of bleached fresh 

 water shells, mostly of species existing in the streams of the adjoining region. 



Drift. The deposits of this subdivision comprise a series of brown and blue 

 clays, locally intermingled with sand, gravel and small pebbles, which are 

 spread over the entire surface of the uplands, and underlying the Loess, where 

 both are present. Some large boulders of igneous or metamorphic rocks lie 

 scattered in the valleys of the water courses, but they are not numerous. In 

 section 9, township 13, range 4, and forming a portion of the bluff of Pope 

 creek, there are heavy beds of a sandy marl, containing some recent shells. 

 Two genera were recognized among the specimens obtained here, Limnea and 

 Succinea. Some fifteen or twenty feet of these beds were exposed, and they 

 appear to underlie the yellow clays of the Drift, which form the subsoil.* 



The older geological formations exposed in this county, belong to the Coal 

 Measures and the Kinderkook group. 



Coal Measures. Nearly all the stratified rocks exposed in this county belong 

 to the Coal Measures, and include the lower portion from coal No. 3 (?) of the 

 Illinois section, to near the base of this formation. They comprise various 



* It is probable the beds of sandy marl here referred to, are equivalent to the Post Tertiary 

 beds of stratified sands, clays, etc., mentioned in the preceding chapters as occurring in Mc- 

 Lean, Tazewell, Adams, and some other counties, and though underlying, and consequently 

 older than the Drift, they have as yet afforded no fossil molluscs of extinct species. 



A. H. W. 



