690 THE ILLINOIS GLACIAL LOBE. 



Mackinaw River, which enters the Illinois just below Pekin. The northern 

 part of the county is drained by Farm Creek, which enters the Illinois oppo- 

 site Peoria. About one-fourth of the county is situated in the Illinois River 

 valley, in the northward continuation of the basin referred to in the descrip- 

 tion of Mason County. The uplands have an extreme altitude of about 450 

 feet above the Illinois River and a general elevation of nearly 300 feet. 

 The Blooming-ton morainic system crosses the northeast part of the county 

 and the Shelbyville or outer moraine of the Wisconsin drift leads north- 

 westward Through the central portion. Between these moraines there is a 

 narrow plane tract scarcely 5 miles in average width. The portion outside 

 the Wisconsin drift is mainly within the valley of the Illinois River, but a 

 narrow strip of upland is found between the moraine and river bluff south 

 from Pekin. 



There are rock outcrops at a few points along the east bluff of the Illi- 

 nois River, but the well borings distributed widely over the county indicate 

 that the drift is a heavy deposit. Twenty borings which do not reach rock 

 have an average depth of 135 feet, while four which reach rock penetrate 

 an average thickness of 247 feet of drift. It is probable that the average 

 for the county is not less than 200 feet. 



In the Illinois Valley the drift is largely gravel and sand, but on the 

 uplands there is a heavy sheet of soft blue till deposited at the Wisconsin 

 stage of glaciation. The deep wells frequently enter a hard till near the 

 bottom, which is presumably* the deposit of an earlier stage of glaciation, 

 and contemporaneous with the sheet of drift found outside the limits of the 

 Wisconsin drift. Wells are often obtained at depths of but 25 or 30 feet, 

 and the depth rarely exceeds 75 feet. 



INDIVIDUAL WKLLS. 



The public water supply of Washington is obtained from wells driven 

 in the drift to a depth of 50 feet or more, the supply coming from sand 

 below till. This sand is usually entered at about 30 feet. A well at 

 Andrew's mill, in Washington, 227 feet in depth, is thought to have entered 

 rock 2 feet. There was a change from soft to hard till at about 150 feet. 

 A well at the Milburn stock farm, near Washington, 236 feet in depth, did 

 not reach rock. It was mainly through till. Some inflammable gas was 

 encountered near the bottom. A well at William Kiel's, about 4 miles west 



