DRIFT SHEETS OF NORTHWESTERN ILLINOIS 



271 



Significant phenomena. — -Within the limits of the Belvidere lobe 

 the drainage lines are poorly developed, the surface is undulating 

 and considerable portions have a decidedly glacial aspect; the loess 

 is thin, many crystallines and a few limestone bowlders occur on 

 the surface, the drift materials show moderate oxidation and are 

 leached to a shallow depth. An average of 120 borings showed 2.1 

 feet of soil and loess-like silt over 1.9 feet of leached till with calcare- 

 ous till below, making a total of 4.0 feet of leached material (see 

 Table I). One suggestive case of superposed tills occurs ^ mile 



TABLE I 



Summary of the Data on Leaching 



Areas 



Till 



Total 



Average* 



Bloomington drift (56 borings) . . , 



Belvidere lobe (120 borings) 



Green River lobe (119 borings) . . 

 lowan drift in Iowa (146 borings) 

 Illinoian drift (150 borings) 



1-9, 

 1-3 

 3-5, 

 4.2 



3-7 

 4 



* Of the 120 borings in the Belvidere lobe, less than 6 per cent did not pass through the leached 

 zone; and of the iig borings made in the Green River lobe, 4.2 per cent did not; of the 146 borings made 

 in the lowan drift area of Iowa, 10 per cent did not; and of the 150 borings made in the Illinoian area, 

 23 per cent did not. 



Of the borings in the Illinoian area which showed 5 feet or less of leaching, 74 per cent occur in the 

 headwater region of northeastern Boone County and in the Kyte River basin of Ogle County where the 

 dissection is slight and the ground-water table high. 



northwest of Irene station, where the Illinois Central Railroad cuts 

 through the upland. Here two glacial tills are separated by fossil- 

 iferous loess and fossihferous silts and sands with some suggestion 

 of old vegetation, but neither the fossU content nor the vegetation 

 excludes the possibility of a retreat and readvance of the same ice- 

 sheet. This exposure was also observed and recorded by Leverett.^ 

 At or near the margin of the lobe some drainage changes occurred. 

 The two most important ones were the diversion of the Rock River 

 from its old course past Stillman VaUey to the present gorges 

 upstream from Byron, and the translocation of the Kishwaukee 

 from its old course at Harrisville to the gorge across the divide 

 east of New Milford. Two others having a bearing on the direction 

 of ice movement will be mentioned later. 



U.S. Geological Survey Monograph XXXVIII (1898), p. 138. 



