GEOLOGICAL AND NATURAL HISTORY SURVEY. 31 



Structure. —The drift of the Valparaiso sheet, together with 

 that of the Early Wisconsin, which underlies it, consists mainly 

 of a soft blue till, easily penetrated by the spade or auger. With 

 the till there are local deposits of sand or gravel, which collect the 

 water that supplies wells. There are. also a few places in which 

 the deposits of sand and gravel occupy nearly the entire drift 

 section. In following the moraine through Northeastern Illinois 

 there will be found occasional townships in which the till, which 

 is a poor source for water because of its compact matrix, is not ac- 

 companied by a sufficient amount of sand or gravel to furnish water 

 for strong wells, while in adjoining townships no difficulty may 

 be experienced in obtaining water. It is more often found neces- 

 sary to drill wells to great depths in eastern and northern Du 

 Page, northwestern Cook and southern Lake Counties than else- 

 where along the moraine in Illinois, many wells in that district 

 being more than 150 feet in depth. In southern Du Page County, 

 between the east and west branches of Du Page River, the drift 

 to a depth of fifty to sixty feet or more (the depth reached by the 

 deep wells) is mainly sand and gravel, this being an unusually 

 gravelly part of *he moraine. In Will County wells seldom reach 

 a depth of 100 feet because of the abundance of water-bearing sand 

 or gravel at less depth. 



In collecting well records there is, I find upon examining my 

 notes, a larger proportion showing exceptional than normal con- 

 ditions, it being natural to pass over the usual section after it is 

 once noted without collecting further records. This shiould be 

 borne in mind when examining the specific well records given 

 below. Scores of wells were found which show a section about as 

 follows: At surface ten to twelve feet of pebbly yellow till, be- 

 neath whidh is a soft blue till extending to the water bed of sand 

 or gravel at depths ranging from twenty-five feet to 150 feet or 

 more. If a well is carried to a depth much beyond 150 feet it is 

 usual to pass out of the blue till into a hard brown till, unless rock 

 is reached. This hard till presumably belongs to one of the ear- 

 liest invasions of the ice. In a few cases great depths of sand 

 have been found beneath the blue till, as noted below. The sec- 

 tions of wells 'here given include the deepest v\^ich came to my 

 notice. 



