714 THE ILLINOIS GLACIAL LOBE. 



glacial map, PL VI. Aside from these few ridges, the drift has a plane 

 surface and the drift filling is sufficient to nearly conceal preglacial valleys. 

 With the exception of a narrow strip along the Illinois-Mississippi divide, 

 in the central and northeastern parts of the county, the surface is generally 

 much eroded. In the vicinity of the Mississippi bluffs there is a thick 

 deposit of loess, in places reaching 50 or 60 feet, but within 5 or 6 miles 

 back from the brow of the bluffs the thickness decreases to 10 feet or less. 

 The loess affords quick absorption for the rainfall and supplies moisture to 

 the crops in seasons of drought. 



The glacial drift is largely till. It is underlain in places by deposits 

 of preglacial sand, first brought to notice by Worthen and later examined in 

 more detail by Salisbury and the writer. The heaviest deposits noted are 

 in the vicinity of Mendon, in the northwest part of the county, but deposits 

 of considerable depth also occur along the Illinois-Mississippi divide in the 

 southeast part of the county. In the former situation their elevation is 

 150 to 200 feet above the Mississippi River, and in the latter nearly 300 

 feet above that stream, or on about the highest rock surface known within 

 the limits of the county. The thickness of the deposits and their relation 

 to wells is set forth in the discussion below. 



The wells of this county are usually obtained at moderate depths in the 

 drift. In nearly every township, however, several deep wells have been 

 sunk. Records of 59 such wells were collected which show an average 

 depth of about 63 feet. The rock was struck in 36 wells at an average 

 depth of 61 feet. In the remaining 23 wells the average depth is 65 feet. 

 From these wells it appears probable that the average thickness for the 

 county is not less than 65 feet. None of the well records were obtained in 

 the Mississippi bottoms, which occupy about 100 square miles of the county 

 and in which the drift is probably more than 100 feet in averag-e depth, for 

 the valley floor of the preglacial river was cut to a level fully 100 feet 

 below the present stream. 



INDIVIDUAL WELLS. 



The public water supply for the city of Quincy is pumped from the 

 Mississippi River, but there are numerous private wells in the city, ranging 

 in depth from 90 to 200 feet. These wells are mainly through limestone. 

 In parts of the city 30 feet or more of loess is penetrated before the lime- 

 stone is entered. 



