426 • THE ILLINOIS GLACIAL LOBE. 



the solid rock conies to the surface. Below Lemont the bare rock forms 

 much of the floor as far as Joliet. From Joliet to the head of the Illinois 

 perhaps half the floor is covered with deposits of drift and river debris, so 

 that tin- distance to rock is not known. The remainder is either bare rock 

 or rock with a very thin deposit of coarse river debris, with a liberal sup- 

 ply of bowlders of Canadian derivation. In the Morris Basin the rock is 

 largely shale. This has been eroded in places by the current, and the hol- 

 lows have been filled with sand. From the Morris Basin to the bend of 

 the Illinois the rock floor, mainly sandstone, is generally swept clean. The 

 St. Peter sandstone of this section is of such a texture as to break up rap- 

 idly into its constituent grains, and these, as fast as they were set free, 

 would have been carried by the strong current down to the lower Illinois, 

 and probably on into the Mississippi. The lower Illinois has only sand and 

 silt in its bottoms. This section is now in process of silting up, the current 

 being too sluggish to carry away the material brought in from the upper 

 portion of the stream. 



Accumulations of bowlders should be mentioned in connection with 

 the river debris. The most conspicuous accumulation noted is that on the 

 borders of the Sag outlet, just east of the point where it enters the Valpa- 

 raiso morainic system and northeast of the village of Worth. An area of 

 perhaps a square mile is so thickly strewn that one might almost step from 

 stone to stone over its entire extent. There are, it is estimated, more than 

 !,000 bowlders per acre. Surface bowlders are not rare in other portions of 

 the old lake bottom where sand deposits are thin or wanting, there being, 

 perhaps, 200 per square mile on the part of the lake bottom where till is 

 exposed. There seems, however, to be a tendency to aggregation at the 

 entrance to the old outlets. This feature suggests that floating ice has been 

 influential in their distribution, though there may have been a large number 

 brought by the ice sheet, the head of the outlets being near the inner border 

 of the Valparaiso morainic system. 



Some very large bowlders have been found along the Drainage Canal. 

 The large ones occur in most abundance where the Valparaiso system is 

 crossed by the lake outlet. Bowlders are also very numerous for a few 

 miles above the juncti< >n < >f the Des Plaines with the Kankakee. They seldom 

 reach the large size which bowlders in the Valparaiso system present. 



