WELLS OP COOK COUNTY, ILLINOIS. 583 



in a westward direction. Wells along this valley reach a level 115 to 125 

 feet below the lake before encountering rock. 1 



In the northern part of the county the rock surface appears to be gen- 

 erally lower than in the southern, though the drift surface is higher. The 

 thickness of drift is found to be in places 250 or even 300 feet, while the 

 average probably exceeds 200 feet, Well sections reaching the rock have 

 been obtained in all parts of the county at frequent intervals, except in 

 the portion occupied by the Valparaiso moraine on the southwest border, 

 or in about 800 square miles of the 960 embraced in the county. Sixty-two 

 wells are found to show an average of 83 feet to rock, while 47 fail to enter 

 rock at an average depth of 79 feet. The drift is largely a compact 

 till, except in the northwest part of the county, where thick deposits of 

 sand and gravel are frequently found beneath a sheet of blue till. Beds of 

 sand or gravel are, however, associated with the till in sufficient amount to 

 furnish a fair supply of water for wells. In a few instances wells in the 

 northwest part of the county have penetrated a black soil below till at 

 depths in some cases of over 100 feet. It is thought that the drift above 

 such soils should be referred chiefly to the Wisconsin stage of glaciation. 

 The soil may, however, in some instances be found below the Iowan drift. 



INDIVIDUAL WELLS. 



In the city of Chicago several sources have been drawn upon for wells, 

 but the public water supply is pumped from Lake Michigan. Since the drift 

 is generallv thin, there are only small areas where wells or rather where 

 pure water may be obtained above the rock. The north part of the city, 

 however, obtained water from the drift in the early days of settlement. 

 The danger of contamination of such wells by sewage or otherwise is so 

 great that their use is largely discontinued. 



The Lockport (Niagara) limestone, which underlies the city, has been 

 drawn upon for water from the early days of settlement, but the use of water 

 from this source is decreasing, in part because of danger from contamina- 

 tion, and in part because of a sulphurous odor which often characterizes 

 the water. 



Artesian wells have come into extensive use within the past thirty 



'The position of this valley was noted first hy Mr. Samuel Artingstall about 1886, while city 

 engineer of Chicago. 



