THE VALPARAISO MORAINIC SYSTEM. 377 



has fallen to 580 feet and at Channahon to 570 feet, a fall of 50 feet in a 

 distance of about 20 miles. There is a corresponding fall on the lower 

 Dupage, the surface of the gravel in the vicinity of Plainfield being about 

 610 feet, at Grhiton 590 feet, and at Channahon 570 feet, 



The deposits along' the Dupage and Des Plaines rivers consist in the 

 main of coarse gravel and cobble, much of the finer material having been 

 swept away by the strong current. Excellent exposures are to be seen in 

 the gravel pits near Plainfield on the Dupage, and in numerous small 

 gravel pits along the border of the Des Plaines both above and below 

 Joliet. One of .the largest on the Des Plaines is found in an island-like 

 remnant of the old terrace known as Joliet Mound, about 2 miles below 

 the city of Joliet, on the west side of the river. At the southeast end of 

 this mound the following section is exposed : 



Section of Joliet Mound, Will County, Illinois. 



Feet. 



Surface coating of silty clay 1-4 



Coarse gravel and cobble 10-12 



Sandy gravel of medium coarseness, cemented in places 25-30 



Fine sand or loam 4 



Blue pebbleless clay, laminated, calcareous 8-10 



Bowlder bed, containing clay balls and a sandy clay matrix, extending to river level on east 

 side of mound, but underlain at slight depth by limestone at west side 5-20 



It is thought that the bowlder bed may be a result of interglacial erosion 

 of a till sheet, The blue pebbleless clay which overlies it is apparently a 

 still-water deposit, formed perhaps before the ice sheet had reached such a 

 stage of melting as to produce vigorous drainage. The upper member 

 appears to indicate a deposit by a stream whose vigor was greatest toward 

 the close of deposition, for at the top the cobble is swept almost free from 

 sand. The change, however, may have been brought about by shifting of 

 the main current of the stream, the coarse material being deposited over 

 portions of the bed which had before been outside the main current, As 

 only occasional remnants of this gravel filling are preserved, it is impossible 

 to determine the original variations in coarseness in a section passing across 

 the valley. 



The deposits of gravel along this valley terminate very abruptly at 

 the south, near Channahon, there being only a fine sand on the plain in the 

 Morris Basin at the head of the Illinois. This feature is apparently due to 

 the presence of a lake in the low country about the head of the Illinois 



