106 THE ILLINOIS GLACIAL LOBE. 



soil had been leached to a depth of several feet, a feature which testifies to 

 the lapse of a considerable period between the deposition of the two till 

 sheets. This black soil between the tills is also penetrated in wells in 

 western Hancock County. 



A possible third line of evidence of the extension of the Keewatin ice 

 sheet into western Illinois is found in the presence of laminated clays in a 

 buried preglacial valley in central Adams County. These clays are con- 

 jectured to be due to an obstruction of the lower course of the valley in 

 western Adams County by the Keewatin ice sheet. The obstruction might 

 have formed a lake in which these deposits were laid down contempora- 

 neously with Kansan till. 



The lapse of a long interval between the deposition of the Kansan till 

 and the Illinoian is clearly shown in southeastern Iowa, not only by the 

 presence of a soil and leached subsoil between two sheets, but by the 

 markedly greater erosion of the Kansan than of the Illinoian sheet. This 

 erosion is manifest to the trained observer on passing from one sheet to 

 the other. In the district occupied by the Kansan the erosion is so great 

 that only narrow remnants of the original drift plain are preserved along 

 the water partings. But in the district occupied by the Illinoian more than 

 half the original drift plain is preserved, and that, too, on the immediate 

 borders of the Mississippi, where conditions for erosion are more favorable 

 than in the area to the west which is occupied by the eroded Kansan sheet. 

 The great contrast in amount of erosion supports strongly the view that a 

 longer interval elapsed between the Kansan and Illinoian glaciations than 

 between the Illinoian and the present time. 



Attention is called, in the discussion of the Illinoian drift border, to an 

 instance of its filling a valley in Des Moines County, Iowa, that had been 

 cut in the Kansan drift. This valley apparently had a depth of 50 feet 

 below the bordering plains^ but no data are available concerning its width. 

 It appears from a study of the terraces on valleys cut in the Kansan drift 

 of southeastern Iowa, that the erosion in the interval between the Kansan 

 and Illinoian stages of glaciation was such as to form broad shallow valleys 

 rather than narrow deep ones. The large valleys appear to have been cut 

 to a depth of but 50 or 60 feet, though they had a width of one or two 

 miles. 



