552 THE ILLINOIS GLACIAL LOBE. 



but in seasons when the rainfall is deficient many of them fail, or yield but 

 a small amount of water. The wells here recorded are generally those 

 which afford a supply of water in seasons of drought as well as in rainy 

 seasons. 



The well records have been obtained largely from the drillers. In 

 some cases there is uncertainty as to the reliability of such records, but as 

 a rule the drillers are sufficiently careful in their observations to warrant 

 the acceptance of their statements. Records have, in many cases, been 

 obtained from the owners of the wells, who were present during their 

 excavation and often assisted in making the wells. The village wells have 

 usually been recorded carefully during the excavation or drilling, and thus 

 have a value above those whose records are given from memory. The 

 writer has rejected such well records as are thought to be unreliable, and 

 has endeavored to reduce errors to a minimum. It is thought that the 

 records here presented contain few serious errors, and that the general 

 exposition of the subject and general conditions for obtaining water are set 

 forth with a fair degree of accuracy. The published well records in the 

 Geology of Illinois are also included in this report and duly accredited. 



THE GEOLOGIC FORMATIONS. 



Since many of the wells enter rock, a map of the geologic formations 

 is here introduced (PI. XXI) which was prepared by the writer to accom- 

 pany a paper in the Seventeenth Annual Report. It follows in the main 

 Prof. A. H. Worthen's map of Illinois, published in 1875, to accompany 

 Vol. VI of the Geology of Illinois, but a few corrections have been made 

 in northern Illinois based upon observations by Mr. Oscar Hershey and the 

 writer. The portion covering western Indiana is based upon Dr. A. J. 

 Phinney's map of Indiana, published by this Survey in 1890. 1 The lime- 

 stone formations of the northern and western parts of Illinois and the 

 sandstones and sandy shales of the southern and southeastern parts are 

 extensively utilized as a source for ordinary wells for household use. The 

 limestones are also a source for artesian wells or deep wells having strong 

 hvdrostatic pressure. A few artesian wells obtain water from the sand- 

 stones of southern Illinois. In northern and western Illinois the St. Peter 

 and Potsdam sandstones are the sources of numerous artesian wells. The 



1 Eleventh Ann. Rept. U. S. Geol. Survey, PI. LXIII. 



