40 CHICAGO ACADEMY OF SCIENCES. 



Du Page 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 fouiid in an island-like remnant of the old 

 terrace known as "Joliet Mound," about two 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: 



1. Surface coating of silty clay i to 4 feet 



2. Coarse gravel and cobble .-....•• 10 to 12 



3. Sandy gravel of medium coarseness, cemented in places. 25 to 30 



4. Fine sand or loam • • . . • 4 



5. Blue, pebbleless clay, laminated, calcareous- • 8 to 10 



6. 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 to 20 



It is thought that the bowlder belt is 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 b}^ a 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. 



Th.e deposits of gravel along this valley terminate very ab- 

 ruptly 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 body of still water 

 in the low country about the head of the Illinois River. The 

 region is covered by sandy deposits up to a level about 575 feet 

 A. T., on the border of which there are in places well defined beach 

 lines. This lake had a western outlet through the Marseilles 

 moraine down the Illinois. Its beaches are traceable a short dis- 

 tance beyond Morris. Further west the bod}^ of water became 

 narrowed to the width of the Illinois valley and changed to an 

 eroding agent. 



It is probable that each of the small tributaries of the Kankakee, 

 which head in the Valparaiso moraine, and also eastern tributaries 

 of the Des Plaines, were lines for escape of glacial waters, but the 

 writer has not examined the valleys to determine the efifect of such 



