86 



A NATURALIST IN THE GREAT LAKES REGION 



Cemetery and it is named the Rose Hill Bar. It runs north and 

 east through Rogers Park and Evanston, where for six miles it is 

 followed by " Ridge Avenue." It rises some twenty feet above 

 the surrounding plain. The outlet to the north of Mount Forest 

 Island was apparently partially blocked, possibly by the Summit 

 sand spit and the others that were formed offshore, so that at 

 this Calumet stage the main outflow was through the present 

 Lake Calumet region, past the south end of Blue Island along 

 the outlet to the south of Mount Forest Island. The great 



FIG. 54. When the outlet of Lake Chicago was chiefly to the south of Mount 

 Forest Island the swift river current, carrying rocks, wore grooves and potholes 

 in the limestone bed. They are seen here where excavations for the new canal 

 freshly expose the bed rock. 



potholes in the bedrock here, uncovered in the construction of 

 the Calumet branch of the Drainage Canal, worn by rocks in 

 the grip of eddies, indicate a large and rapid river (Fig. 54). 



The rock barrier in the bed of the outletting river at Lock- 

 port which held the lake at the Calumet level could not stop the 

 lowering of the valley floor indefinitely. The obstruction was 

 made up of the inclined strata of Niagara limestone against 

 which, near Joliet, lay much softer shale. The river had no 

 difficulty in cutting away these shales, but the limestone was a 

 more difficult proposition. However, the limestone strata were 

 so inclined that the river rushed down the incline, which was at 



