GENERAL ASPECTS OF ILLINOIAN DRIFT SHEET. 25 



Eleventh Annual Report of this Survey and in earlier papers, is separated 

 from the underlying drift of eastern Iowa by a distinct soil horizon. It is 

 found that a notable interval occurs between the deposition of the Illinoian 

 sheet of drift and the deposition of the main sheet of loess of the Mississippi 

 Basin, an interval indicated by erosion, weathering, and the formation of a 

 soil horizon. But this sheet of loess seems to be intimately connected with 

 the Iowan drift sheet, as shown by McGree, a relation which subsequent 

 studies by several independent investigators fully confirms. It appears, 

 therefore, that the Illinoian drift sheet is separated by a notable interval from 

 the Iowan sheet as well as from the underlying sheets. Furthermore, a till 

 sheet referred to the Iowan stage is present in northern Illinois which falls 

 far short of reaching the limits of the Illinoian till sheet. Like the Iowan 

 sheet of eastern Iowa, it is found to be contemporaneous with the main loess 

 deposition. The interval between the Illinoian till sheet and the Iowan 

 loess and till has recently been named the Sangamon; 1 it is discussed in 

 some detail below. A deposit termed gumbo, which lies between the 

 Illinoian till and the Iowan loess, is also described. 



Culmination of the Illinois lobe at the Illinoian stage. At the time Of the deposition of the 



Illinoian drift sheet the southwestward movement from Labrador and the 

 heights south of Hudson Bay appears to have reached a limit in western 

 and southern Illinois beyond any earlier invasion. It is not as yet certain, 

 though it is quite probable, that the Illinoian sheet extends beyond all older 

 sheets in northwestern Illinois and southern Wisconsin. In the absence of 

 positive evidence to the contrary the present paper discusses the western 

 and southern limits of the drift of the Illinois lobe as a single line, occupied 

 at the Illinoian stage throughout its entire length by the ice lobe. Its 

 extent may be seen' by reference to PL VI. 



GENERAL ASPECTS OF THE ILLINOIAIST DRIFT SHEET. 



Extent of its exposures. — The Illinoian sheet is extensively exposed to view 

 outside the Iowan and Wisconsin sheets in Illinois and southwestern Indiana. 

 In western and southern Illinois and in southwestern Indiana it constitutes 

 the surface sheet (aside from a thin loess coating) over the entire interval 

 between the glacial boundary and the outer moraine of the Wisconsin series. 



1 In a paper presented by the writer at the twelfth annual meeting of the Iowa Academy of 

 Sciences : Proc. Iowa Acad. Sci., Vol. V, 1898, pp. 71-80; also Jour. Geol., Vol. VI, 1898, pp. 171-181. 



