192 THE ILLINOIS GLACIAL LOBE. 



SECTION I. SIIEEBYVIJLLE MORAINIC SYSTEM. 



EXTENT OF THE SHELBYVILLE SHEET. 



The outer border of the Shelbyville sheet is found in the Shelbyville 

 moraine, an outline of whose position is given below. This moraine is but 

 the thickened edge of a drift sheet, which not only covers the territory 

 occupied by the moraine and the plain between it and the next succeeding 

 morainic belt, but apparently extends many miles to the north beneath the 

 later moraines, for the fresh drift which characterizes this moraine is found 

 to extend below the base of the later moraines. As shown by Chamberlin 

 in his discussion of the drift of North America in the last edition of Geikie's 

 Great Ice Age, there is an imbricate arrangement of drift sheets by which 

 the later to some extent overlap the earlier. This is found to be true of 

 the several divisions of the Wisconsin sheet as well as of the main divi- 

 sions of the drift, known as the Wisconsin, Iowan, Illinoian, etc., though 

 the extent of overlapping is much less than in the main sheets. As already 

 remarked, the several moraines appear to have been formed by readvances 

 of the ice of more or less consequence rather than by simple halts during 

 its recession. 



SHELBYVILLE MORAINE. 



The oldest moraine of the Wisconsin drift, so far as recognized, is one 

 exposed to view in central and eastern Illinois and western Indiana, but 

 which is overridden by later moraines in districts farther north and east. 

 This has for some years been known as the Shelbyville moraine, the name 

 being derived from a city in central Illinois which is situated on the extreme 

 southwest point of the morainic loop. If we may judge from the amount of 

 erosion and weathering, it is somewhat older than the Altamont moraine of 

 the " Minnesota Valley glacier" 1 as well as the outer Wisconsin moraine in 

 the eastern United States. 



The thickened edge of this drift sheet has usually a breadth of several 

 miles, and merges gradually into the plain on the inner border, so that it 

 becomes difficult to limit the extent of the moraine. The crest usually is 

 found within 1 or 2 miles of the outer border, and is, as a rule, much nearer 

 the outer than the inner border of the thickened portion of the drift sheet. 



1 See Third Ann. Rept. U. S. Geol. Survey, p. 388. 



