RELATIONS OF THE ILLINOIAN DRIFT SHEET. 105 



PLEISTOCENE DEPOSITS BENEATH THE ILLINOIAN TILL SHEET. 



Although the Illinoian till sheet throughout much of the region under 

 discussion may rest immediately on the rock, there are, in places, deposits 

 separating it from the rock formations. The deposits are principally of two 

 classes— namely, the Kansan till, with perhaps a somewhat distinct pre- 

 Illinoian till, and valley silts or sands. 



KANSAN TILL. 



Attention has been called to the till of southeastern Iowa and western 

 Illinois, which underlies the Illinoian drift and which is referred to the 

 Kansan. The full extent of this sheet of till in western Illinois is not 

 determined. Its eastern border comes to the Mississippi Valley from the 

 south near Hannibal, Missouri, and it probably continues northward into 

 Illinois across Adams and Hancock counties. It probably also extends into 

 counties east of the Mississippi farther north, although this has not been so 

 clearly determined. That it extended into western Adams and Hancock 

 counties is shown by at least two lines of evidence. The first line of 

 evidence is that furnished by striation. An exposure of a striated ledge 

 with a bearing S. 65° E. appears on the south side of "Wagner's Creek, about 

 4 miles above Hamilton, Illinois, and 2 miles east of the Mississippi. This, 

 so far as known, is the only instance yet discovered of striation produced 

 by the Keewatin ice field east of the Mississippi and south of the Wisconsin 

 Driftless Area. The striae reported above in the vicinity of Burlington, 

 Iowa, are in some cases situated on the brow of the west bluff of the 

 Mississippi, showing that the eastward movement extended at least to the 

 Mississippi Valley. 



The second line of evidence of the extension of the Keewatin ice sheet 

 into western Illinois is furnished by till deposits separated from the over- 

 lying Illinoian by a soil and carrying other evidences of greater age than 

 the Illinoian. In the discussion of the border of the- Illinoian drift in Han- 

 cock and Adams counties, Illinois, attention was called to the occurrence of 

 a heavy sheet of blue till similar in structure to that found beneath the 

 Illinoian west of the Mississippi, and to a few exposures near the line ot 

 Hancock and Adams counties of a black soil separating the till from the 

 overlying Illinoian. In these exposures it was found that the till under the 



