CHAPTER X. 

 THE LATE WISCONSIN DRIFT SHEETS. 



BASIS FOR SEPARATION FROM THE EARLY WISCONSIN. 



The necessity for separating - the moraines of this later drift into two 

 series was brought out by Chamberlin's early studies in eastern Illinois and 

 western Indiana. He discovered that the very bowldery morainic belts of 

 Benton and Warren counties, Indiana, pass directly across the line of the 

 earlier moraines which lead up to them from the west. The fact was also 

 noted that the earlier series of moraines are not markedly older than the 

 later series. In view of this freshness of the drift of the earlier series and 

 of certain obscure tracts in the Kankakee Basin, he makes the following- 

 statement: 1 



The drift of this area [Kankakee Basin] bears undoubted evidence of being 

 recent, and, though this is in considerable part due, superficially, to aqueous agencies, 

 it seems to me probable that the region will prove to have been largely, possibly com- 

 pletely, covered by ice in the earliest stage of the second glacial epoch. It is not, 

 however, traversed by conspicuous moraines, at least not by any as well developed 

 as those outlined. Low-ridged belts of subdued morainic aspect have been observed 

 at numerous points, but their relations have not yet been traced out. 



A similar qualifying remark may here be made concerning a considerable area 

 in northern Illinois, outside the moraine described in this paper. The freshness of its 

 drift and the nnsculptured contour of its surface bear evidence of recent origin. 



Subsequent studies by Chamberlin and his associates have brought out 

 more clearly the evidence that the bowldery moraines are out of harmony 

 in trend with the moraines of the early Wisconsin series. They have also 

 shown that the bowldery moraines present a somewhat fresher surface con- 

 tour than the moraines overridden by them. It is upon the basis of these 

 differences in trend and in freshness of contour that the removal from the 

 early Wisconsin has been made. As yet no soil or leached or weathered 

 zone has been found separating the drift of the two series, and it still seems 



1 Third Annual Report U. S. Geol. Survey, p. 331. 



317 



