IOWA ACADEMY OF SCIENCES. 81 



THE WEATHERED ZONE (YARMOUTH) BETWEEN 



THE ILLINOIAN AND KANSAN TILL 



SHEETS. 



BY FRANK LEVERETT, DENMARK, IOWA. 



PRELIMINARY STATEMENT. 



The full extent of overlap of the Illinoian upon the Kansan 

 has not been determined. It is certain that a sheet of Kansan 

 drift underlies the Illinoian throughout its extent in south- 

 eastern Iowa and in all probability it continues some distance 

 eastward into western Illinois in the section between Rock 

 Island and Quincy. 



There may be a sheet of Kansan age formed by the Illinois 

 glacial lobe. The available data, however, do not place this 

 beyond question. Occasional wells in central Illinois are 

 reported to have passed through a black soil at some distance 

 below the Illinoian till. But so far as the writer is aware no 

 exposures of such a soil have ever been discovered. Professor 

 Salisbury has collected data in southeastern Illinois and south- 

 western Indiana which support the view that there may be two 

 distinct drift sheets in that region. It is his opinion that the 

 upper or Illinoian sheet extends farther south than the lower 

 sheet (1). Whether the lower sheet is of Kansan age is still a 

 matter for conjecture. It also is still an open question whether 

 the drift on the east border of the driftless area in north- 

 western Illinois and southwestern Wisconsin is of Illinoian 

 age or of earlier date. In view of these uncertainties the Yar- 

 mouth weathered zone is restricted in this discussion to the 

 region where the Illinoian sheet of the Illinois lobe overlaps 

 the Kansan sheet of an ice lobe lying farther west. 



Numerous exposures of a soil and weathered zone have been 

 observed at the junction of the Illinoian and the Kansan till 



(1) Oommunclated to the writer 

 6 Lla. Acad. Sc, Vol. v.] [May 2, 1898.1 



