180 THE ILLINOIS GLACIAL LOBE. 



The streams of southern Illinois are flowing in broad shallow troughs 

 in which a rise of 30 or 40 feet would cause an expansion of the stream to 

 a width of several miles. Such is notably the case in the valleys of the 

 Kaskaskia, Big Muddy, Saline, Little Wabash, Bon Pas, and Embarras 

 rivers. The same is true of the lower course of the Wabash and of a por- 

 tion of White River below Worthington and of the lower course of East 

 White River. The valleys of western Illinois and southeastern Iowa were 

 apparently, at the time of the loess deposition, cut down only to a level 

 50 to 100 feet above the present level of the streams, as is shown by the 

 level of the lowest loess-capped terraces. This, however, is 50 to 100 feet 

 below neighboring uplands. A rise similar to that which would cause 

 extensive flooding in southern Illinois would not carry them beyond the 

 limits of their present valley bottoms. Unless, therefore, a rise much 

 greater than 30 or 40 feet be assumed, the glacial waters would have 

 been confined to the immediate channels of the larger streams, and, as 

 urged by Chamberlin, the sweeping ground of silt for the winds to work 

 upon would have been too restricted to supply the great mantle of silt 

 covering the neighboring uplands. The problem here puts both the 

 fluvial and seolian hypotheses to severe test. It seems necessary to 

 grant a rise of water sufficient to overspread the neighboring uplands in 

 order to afford a sufficient sweeping ground to supply material for the 

 portion of the loess mantle found along- the divides. In southeastern Iowa 

 and neighboring portions of Illinois and Missouri a rise of 100 to 150 feet 

 would be necessary to cause a wide expansion of the glacial streams. 

 However, with the assumed low altitude of the loess-covered region, and 

 resulting low gradient of the streams, a depth of 100 feet of water over the 

 main valleys may be within the limits of probability. A rise of this 

 amount in the southern Illinois district would carry the waters over the 

 greater part of the interfluvial districts. 



The evidence relied upon to demonstrate the agency of water in con- 

 nection with the deposition of the loess and associated silts is the presence 

 of material too coarse to have been transported by wind, and the occur- 

 rence of water-laid beds of sand, silt, or coarser material. Such evidence 

 must be weighed very carefully to guard against including redeposited 

 material, in which the loess and the coarser material have been brought 

 down from higher ground. It may not in all cases be possible to decide 

 whether there has been a redeposition. But the present writer feels 



