GENERAL ASPECTS OF ILLINOIAN DRIFT SHEET. 29 



with a gummy or very adhesive clay, often several feet in depth. This 

 clay is found at frequent intervals throughout the entire area in which the 

 Illinoian sheet is exposed to view outside the limits of later till sheets, 

 and has also been found under the later till sheets at some distance 

 back from their margin in the central portion of Illinois. It occurs as far 

 west as the writer has made examinations in Iowa, a distance of 50 

 miles or more beyond the limits of the Illinoian till sheet, and is known 

 to be present over extensive areas in northern Missouri. Its extent and im- 

 portance seem to have been unrecognized prior to the present survey. It is 

 not such a continuous deposit as the overlying loess, there being many 

 places where the loess rests directly upon typical till. It is most conspicu- 

 ous in the vicinity of the Mississippi Valley and in the southern portion of 

 Illinois as far south as the glacial boundary. The region in which it occurs 

 ranges in elevation from 400 feet up to fully 800 feet above tide. Possibly 

 much higher elevations are attained by it in passing westward from the 

 Mississippi. Like the loess, it seems to be independent of contour lines in 

 its distribution. 



The color of this clay varies from ash or light gray to nearly black. 

 The black portions are heavily charged with humus and in places present 

 the appearance of a swamp muck. It is in this clay that the black soil so 

 often seen at the base of the loess is usually developed. Where the loess 

 rests directly upon till the soil formed beneath it is usualty of a reddish- 

 brown color. 



This gummy clay contains a few small pebbles. They seldom exceed 

 a half inch in diameter and are far less numerous than in the typical till. 

 It often bears a striking resemblance to the "gumbo" of the Illinois and 

 Mississippi River flood plains, which is deposited by flooded streams in the 

 portions of the flood plains where there is but little current. It, however, 

 contains more and larger pebbles than the gumbo of these flood plains. 



The origin of this deposit, and its time relations compared with the 

 overlying loess and the underlying till, are questions of prime importance, 

 but as yet no satisfactory conclusions have been reached. There is much 

 in the appearance of the deposit to support the hypothesis of aqueous 

 deposition. The pebbles may, perhaps, have been derived from neighbor- 

 ing prominent points in the till during the progress of a submergence. The 

 action of floating ice may be postulated as an accompaniment of submergence 



