THE WEATHERED ZONE 175 



to this evidence there is found an abandoned river channel in 

 the district immediately west of the limits of the Illinoian drift 

 which carried southward the drainage outside the Illinois ice 

 lobe. The banks of this channel are well defined, and the chan- 

 nel evidently has not been filled by the drift of any subsequent 

 invasion. 



Extent of the Iowan loess. — By the term Iowan loess is meant 

 that sheet of loess which connects at the north with the Iowan 

 till sheet. A till sheet of Iowan age has been found in northern 

 Illinois as well as in eastern Iowa and it probably covers the 

 greater part of the northern half of Illinois. It is, however, 

 covered by the Wisconsin till sheet from Bureau county, Illinois, 

 east and south. How much of Indiana and Ohio was covered 

 by the Iowan ice invasion has not been determined. The Iowan 

 till certainly does not extend as far south as the Wisconsin in 

 those states. The loess forms a heavy deposit along the border 

 of the Mississippi and Illinois valleys, but is comparatively thin 

 in the region east of the Illinois, its average thickness being 

 scarcely ten feet. A silt tentatively correlated with the loess 

 covers the Illinoian till sheet wherever exposed outside the 

 Wisconsin from the Illinois River eastward to central Ohio. 

 The Sangamon weathered zone between the loess and the Illi- 

 noian till sheet is found from central Ohio westward to south- 

 eastern Iowa, i. e., to the limits of the Illinoian till sheet. The 

 Iowan loess extends also over the Kansan till sheet of southern 

 Iowa and adjacent portions of Missouri, Kansas and Nebraska, but 

 this loess is separated from the underlying till by a much longer 

 interval than that between the loess and the Illinoian till sheet, an 

 interval comprising two interglacial stages and one glacial stage. 



Applicatio?i of Buchanan. — At the tenth annual meeting of the 

 Iowa Academy Professor Samuel Calvin, after describing certain 

 gravel deposits in northeastern Iowa, introduced the term Buch- 

 anan as a name for an interglacial stage following the Kansan/ 

 and made the following statement concerning the origin and age 

 of the deposits : 



1 Proc. Iowa Acad, of Sciences, Vol. Ill, 1896, pp. 56-60. 



