76 GEOLOGY OF ILLINOIS. 



On the eastern border of the county, there is a belt of alluvial bottoms 

 skirting the Illinois river, from a half mile to about four miles in width. Some 

 portions of this bottom land is above the high-water level of the river, and 

 these lands are very productive, while other portions are subject to annual over- 

 flow from the river floods, and are of little value at the present time for agri- 

 cultural purposes. A considerable portion of this is bottom prairie, but there 

 is usually a belt of heavy timber skirting the river, and also the small streams 

 by which the bottoms are intersected. The timber on these low lands comprise 

 cotton-wood, sycamore, soft-maple, ash, elm, hickory, pecan, Spanish oak, 

 swamp white oak, pin oak, black walnut, hackberry, buckeye, honey-locust, 

 paw-paw, horn-beam, willow, etc. There are also narrow belts of bottom land 

 on some of the larger creeks in this county, as on Crooked creek and Sugar 

 creek, but these seldom exceed a half mile in width, and are covered with a 

 heavy growth of timber, embracing most of the varieties mentioned as occur- 

 ring in the Illinois river bottoms, with the addition of white walnut, sugar- 

 maple, linden, white oak, etc. 



The general surface level of the uplands in this county, ranges from two to 

 three hundred feet above the level of the Illinois river, and the river bluffs 

 often rise abruptly to the height of two hundred feet or more above the bot- 

 toms, but exhibit none of the bold limestone escarpments, so conspicuous on 

 the lower course of the river, where the Lower Carboniferous limestones are 

 the prevailing formations. 



G eo logy. 



The geological structure of this county, like that of Brown, includes the 

 Quaternary system, the lower portion of the Coal Measures, and the upper 

 divisions of the Lower Carboniferous limestones, but differs from that in an 

 additional thickness of the Coal Measures sufficient to bring in another coal 

 seam, No. 4, which is not found in any county south of this, on the west side of 

 the Illinois river. The following section exhibits the formations to be found 

 in this county, in their relative order of superposition and thickness: 



FEET. 



Quaternary, comprising Alluvium, Loess and Drift 100 



Coal Measures 200 to 250 



St. Louis group 30 " 40 



Keokukgroup 60 " 70 



The three lower groups belong properly to what are called stratified rocks, 

 that is, to those that have been formed in regular strata or layers, and also to 

 that division of geological time termed paleozoic, because the embedded fossils 

 represent only ancient forms of animal and vegetable life, while the upper di- 

 vision belongs to the most recent geological age, and the fossils which it con- 



