44 GEOLOGY OF ILLINOIS. 



the uplands. The bottom lands north of Quincy, towards the Hancock county 

 line, are intersected with numerous bayous, and in the northeast corner of the 

 county, one of these widens into a lake four or five miles in length, by about 

 two in width, known as Lima Lake. The bottom lands in this part of the 

 county, are mostly too wet for cultivation, but below Quincy, they are rather 

 higher, and afford some fine farming lands, especially along the foot of the 

 bluffs, where a considerable area is above the high water level of the river. 



The geological formations exposed in this county, comprise the Lower Car- 

 boniferous limestone series, about three hundred feet in thickness, about one 

 hundred feet of the lower part of the Coal Measures, and the Quaternary and 

 Post Tertiary deposits of more recent age, which unconformably overlie all the 

 others. The following section will show the thickness and relative position of 

 the formations exposed in this county : 



FEET. 



C Alluvium and Loess 30 to 40 



Drift clay, with gravel and boulders 80 " 90 



Quaternary -{ Post Tertiary soil 2 " 6 



I Brown clay 6 



[ Tough blue clay 20 



SBeds of sandstone, sandy, and argillaceous shale, with 

 bands of limestone, bituminous shale and fire clay, with 



two or three seams of coal 100 



(St. Louis group 40 to 50 

 Keokuk group 80 " 100 

 Burlington Limestone 100 



Kinderhook group, partly exposed 50 



The Quaternary system properly includes all the deposits, both stratified and 

 unstratified, that are of more recent origin than the Pliocene Tertiary. In this 

 county, we find a series of beds, comprising an aggregate thickness of about 

 one hundred and sixty feet, which properly belong to this system. They include 

 the surface soil and subsoil, on the uplands, and the alluvial deposits of the 

 river valleys, the Loess, which is largely developed along the bluffs of the Mis- 

 sissippi, the Drift proper, including all the thick beds of unstratified clay and 

 gravel, enclosing boulders of large size, and lastly, an ancient Post Tertiary 

 soil and subordinate clays, usually distinctly stratified, and without boulders, 

 which rest immediately upon the stratified rocks. 



The soil at different localities, rests upon, and is, in part, derived from each 

 of these subdivisions of the Quaternary system, and consequently varies con- 

 siderably in its general appearance and productive qualities, in accordance with 

 the character of the beds on which it rests, and from which it has been mainly 

 formed. 



The Alluvial deposits of the Mississippi valley, consist of partially stratified 

 sands, alternating with dark bluish gray, or chocolate brown clays, deposited 



