484 THE ILLINOIS GLACIAL LOBE. 



County, where a bulky moraine which forms the west border of the Wis- 

 consin drift makes further tracing difficult. The valley has been so incom- 

 pletely filled outside the limits of the Wisconsin drift that its rock bluffs rise 

 perceptibly above the general level of the filling along the valley. But 

 within the limits of the Wisconsin drift the valley has not only been com- 

 pletely filled but the general level of the drift surface rises 100 feet or more 

 above its bluffs. Its course can be known, therefore, only through data 

 obtained by deep borings. These indicate that it leads south westward 

 across eastern Lee and northeastern Bureau counties to Princeton, and 

 thence southward to the bend of the Illinois River at Hennepin. 



The breadth of the valley, in the portion exposed to view, averages 

 about 3 miles, though it in places reaches nearly 5 miles. Its rock bottom 

 appears to be somewhat lower than that of the preglacial valley occupied 

 by the neighboring portion of the Mississippi. Chamberlin reports a 

 boring at Lake Koshkonong, Wisconsin, which failed to reach rock at an 

 elevation only 450 feet above tide. The rock was reached on the Missis- 

 sippi in the same latitude at about 490 feet. Borings at Princeton, Peru, 

 Bureau Junction, and Putnam, Illinois, enter rock at about 340 feet above 

 tide, while borings on the Mississippi at Fort Madison, Iowa, which is some- 

 what farther south, reach rock at about 365 feet, though one boring- failed 

 to reach the rock at that elevation. No borings have been made between 

 Lake Koshkonong and Princeton which are calculated to test the depth 

 of the preglacial valley. Borings at Janesville, Wisconsin, and Rockford, 

 Illinois, made at the foot of the west bluff, enter rock at about 530 feet 

 above tide. They can scarcely be supposed to represent the depth along 

 the middle of the valley opposite these cities. Several wells have been 

 sunk in eastern and southern Lee County and in northeastern Bureau 

 County, Illinois, which fail to enter rock, though they terminate at an eleva- 

 tion less than 500 feet above tide. From the data at hand it appears that 

 the rock bottom descends from 450 feet or less above tide in southern 

 Wisconsin to 340 feet above tide at the bend of the Illinois, 90 to 100 miles 

 farther south. 



Several of the large preglacial ti'ibutaries of the old valley may be 

 traced readily in the district lying outside the limits of the Wisconsin drift, 

 The Pecatonica River, which is the largest of the present tributaries, is 





