556 THE ILLINOIS GLACIAL LOBE. 



Between this absorbing area and the wells is a district in which the porous 

 bed is overlain by limestone or semiporous rock, and also by drift beds, 

 which afford much opportunity for absorption of water. These overlying 

 beds, however, have altitudes fully as great as portions of the absorbing 

 area, and hence, when filled with water, the downward pressure equals or 

 exceeds that of the upward pressure of water from the porous beds, and 

 thus they prevent escape as effectually as a series of impervious beds. The 

 variability of head displayed by wells in northern Illinois which obtain 

 their main supply from the St. Peters formation is probably largely due to 

 the influx of water from overlying beds in the district between the fountain 

 head and the well. The main absorbing areas for the Potsdam and St. 

 Peter formations are shown in PI. XXII, which is compiled chiefly from 

 State geological maps and was first published by the writer in the Seven- 

 teenth Annual Report of this Survey. 



A topographic map of the St. Peter sandstone, showing also the dis- 

 tribution of artesian wells and deep borings and of the principal areas 

 where flowing wells are obtained from the drift, is presented in Pi. XXIII, 

 a map already published in the Seventeenth Annual Report. The three 

 sections in figs. 20, 21, and 22 illustrate the above statements concerning 

 the attitude of the rock formations and correspond to the lines A — A, B — B, 

 and C — C on PI. XXIII. The data used in figs. 7 and 8 are largely 

 obtained from records of wells collected by the writer, but some of the 

 data are from a rej)ort by Daniel W. Mead, bearing- upon the hydrogeo- 

 logy of the Mississippi Basin. 1 The data in fig. 9 are largely based upon 

 a special study by Prof. J. A. Udden of a line leading from Rock Island 

 eastward across Illinois, which was made for the Illinois Board of World's 

 Fair Commissioners. 2 It brings out clearly the monoclinal fold separating 

 the two blocks of eastward-dipping strata just noted. For a more complete 

 discussion of artesian-well conditions in Illinois, reference may be made to 

 the report by the writer contained in the Seventeenth Annual Report of 

 this Survey. Many records of wells appear in the detailed discussion 

 below. 



1 Hydrogeology of the Upper Mississippi Valley and some of the adjoining Territory, by Daniel 

 W. Mead, C. E. : Jour. Assoc. Eng. Soc, Vol. XIII, No. 7, July, 1894. 68 pages, with 6 maps. 

 - See Final Report Illinois Board of World's Fair Commissioners, 1895, pp. 115-151. 



