THE WEATHERED ZONE 173 



Whiteside and Carroll counties, 111. It becomes a difficult matter, 

 therefore, to decide upon the position of the margin of the 

 Illinoian drift in any of these counties. It is also not fully 

 decided whether it reaches to the border of the driftless area in 

 Jo Daviess and northwestern Carroll counties, 111 , and in south- 

 western Wisconsin. The balance of probabilities, however, 

 seems to favor its extending to the driftless area. 



The Illinoian till sheet overlaps a few miles the Kansan till 

 sheet of the western lobe from the latitude of Hannibal, Mo., 

 northward to the vicinity of the southern point of the driftless 

 area. In this region of overlap a weathered zone is developed 

 between the Illinoian and Kansan till sheets at the level of the 

 outlying Kansan surface as indicated below. 



Introduction of the name Illinoian. — The tracing of this south- 

 western border of the Illinois lobe was begun by the writer in 

 the autumn of 1892, and carried as far north as Hancock county, 

 111., that season. No opportunity to continue the study was 

 afforded until the spring of 1894, when the mapping of the 

 border was carried from Lee county, Iowa, northward to Scott 

 county. The greater part of the data presented in this paper, 

 and conclusive evidence of a long interval between the deposi- 

 tion of the till sheets now known as the Kansan and Illinoian, and 

 also the evidence that the Illinoian is much older than the 

 Iowan had been obtained as early as June 1894. The writer 

 then began to use the name Illinoian in correspondence, but it 

 seemed best to defer its introduction into literature until oppor- 

 tunity had been afforded other geologists to examine it. In 

 August 1896 Professor T. C. Chamberlin and Dr. H. F. Bain 

 were conducted by the writer to some of the exposures in south- 

 eastern Iowa which show the soil above and below the sheet 

 formed by the Illinois lobe, and each recognized the need for a 

 distinctive name for this drift sheet. The name was accordingly 

 soon introduced into geological literature by Professor Chamber- 

 lin. 1 



Other interpretations. — At the ninth annual meeting of the Iowa 



'See editorial Jour. Geol., October-November 1896, pp. 872-876. 



