636 THE ILLINOIS GLACIAL LOBE. 



here discussed are distributed with some uniformity over the entire comity, 

 except the portion southwest of Vermilion River, where very few records 

 have been obtained. 



The upper portion of the drift, and indeed the deposit penetrated by 

 nearly the entire depth of the wells throughout the county, is blue till. 

 There are, however, small areas in which the drift contains a larger amount 

 of sand and gravel than till, and thin beds of gravel or sand appear to be 

 quite generally present in the till. These afford water for the shallow wells 

 on the farms and in villages. The public wells in cities and villages and the 

 stock wells on the farms are usually sunk to considerable depth. 



On the north border of the county, near Earlville, many flowing wells 

 have been obtained from the drift, usually at depths of 25 to 50 feet, or 

 even less, but occasionally of greater depth. Strong hydrostatic pressure 

 is exhibited by wells situated on the lower portion of the slopes of the 

 large moraines in the northwestern and in the eastern part of the county. 

 This pressure is in all probability due to absorption of the water on 

 more elevated portions of the moraine. The flowing well district near 

 Earlville is thus related to the moraine on the northwest. A few other 

 flowing wells occur in the county in valleys or lowland tracts, which are 

 probably supplied from the neighboring higher land. 



Man} T artesian wells are obtained from the St. Peter sandstone, Lower 

 Magnesian limestones and sandstone, and the Potsdam sandstone along the 

 borders of the Illinois River, and occasionally at points on the uplands, as 

 appears in the discussion below. 



INDIVIDUAL WELLS. 



The city of Mendota, in the northwest part of the county, obtains its 

 public water supply from wells about 400 feet in depth sunk to the St, 

 Peter sandstone. A portion of the water may be derived from higher 

 horizons. The head is sufficient to bring the water within 40 feet of the 

 surface, or to about 710 feet above tide, which is somewhat higher than the 

 head for the St. Peter in this region. The private wells at Mendota are 

 usually obtained at depths of but 12 to 20 feet in beds of sand or gravel 

 between the yellow and blue tills. A few deep wells have been sunk in 

 the vicinity of the city, which show the drift to be 160 to 200 feet in depth. 

 An old soil is frequently passed through in the lower part of the drift at a 

 depth of 100 feet or more. The till above the soil is usually of a blue 



