GEOLOGICAL AND NATURAL HISTORY SURVEY. G3 



to the surface. (See Figs, i and 2.) Below Lemoiit the bare rock 

 forms much of the floor as far as JoHet. From JoHet to the head of 

 the Ihinois perhaps half the floor is covered with deposits of drift 

 and river debris, so that the 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 supply of Ijowlders of Canadian 

 derivation. In the Morris basin the rock is largely shale. This 

 has been eroded in places by the current and the hollows filled with 

 sand and gravel. 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 rapidly 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. 



In connection with the river debris should be mentioned 

 accumulations of bowlders. The most conspicuous accumulation 

 noted is that on the borders of the Sag outlet, just east of the point 

 where it enters the moraine 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 1,000 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, 100 

 per square mile on the part of the lake bottom, where till is ex- 

 posed. There seems, however, to be a tendency to aggregation 

 at the entrance to the old outlets. This feature suggests that float- 

 ing 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 moraine. 



Some very large bowlders have been found along the Drain- 

 age Canal, two of which are shown in the accompanying view 

 (Fig. i). The large ones occur in most abundance where the 

 Valparaiso moraine crosses the outlet. Bowlders are very numer- 

 ous for a few miles above the junction of the Des Plaines with the 

 Kankakee, both in the lake outlet and the districts south. The 

 cause for their abundance there is not satisfactorily determined. 

 They seldom reach the large size which bowlders in the Valparaiso 

 moraine present. 



