THE WEATHERED ZONE (YARMOUTH) BETWEEN 

 THE ILLINOIAN AND KANSAN TILL SHEETS. 1 



PRELIMINARY STATEMENT. 



The full extent of the 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 

 southeastern Iowa, and in all probability it continues some dis- 

 tance 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. 2 Whether the lower sheet is of Kansan age is still a mat- 

 ter for conjecture. It also is still an open question whether the 

 drift of the east border of the driftless area in northwestern Illi- 

 nois and southwestern Wisconsin is of Illinoian age or of earlier 

 date. In view of these uncertainties this discussion of the Yar- 

 mouth weathered zone is restricted to the region where the Illi- 

 noian 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 

 sheets in the region of overlap between Davenport, Iowa, and 



1 Read before the Iowa Academy of Sciences, December 1897. 



2 Communicated to the writer. 



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