PAPERS OX GEOLOGY AND GEOGRAPHY 409 



Joliet. Marl and saud and clay adulterate the peat in cer- 

 tain layers, while in others it is almost pure peat. The 

 material above the limestone measures in all upwards of 

 20 feet in thickness and contains shells all the way 

 through, the water shells largely in the lower levels and 

 the land shells in the upper strata." 



The lower strata containing the marl probably repre- 

 sent a stage when the valley was largely filled with water 

 from the Chicago outlet and there was little territory 

 for land mollusks to occupy. Later, when the outlet be- 

 came reduced to a narrower river, or perhaps during 

 some of the low water intervals between the different 

 lake levels, land mollusks came and took possession of 

 territory above the river. It is probable that the lower 

 strata which contain such an abundance of fresh water 

 mollusks represent the earlier stages of Glacial Lake 

 Chicago, perhaps as early as the Calumet or even follow- 

 ing the Glenwood, for many hardy mollusks, such as 

 many of the species represented are, probably followed 

 the ice very closely, and there is no reason why the water 

 at Joliet, 30 or 40 miles away from the ice, could not have 

 supported some kind of a snail fauna. A huge boulder 

 overlying a bed of Unios in the bed of Wilmette Bay at- 

 tests the presence of icebergs in Glacial Lake Chicago 

 when an abundant fauna flourished at Chicago. (Life 

 of the Pleistocene, Plate VIII). We must consider, I 

 think, that this life above the limestone in the Joliet 

 quarry represents the biota that migrated up the Illinois 

 and Desplaines rivers and reached Glacial Lake Chicago 

 by way of the outlet via Joliet and Lemont. This time, 

 therefore, could not be later than the Toleston, and was 

 more likely during late Glenwood time, for a consider- 

 able fauna has been found in Chicago that lies between 

 the Glenwood and Calumet stages (Bowmanville Low 

 Water Stage). Of this biota, at least a dozen species are 

 the same as those at Joliet. It is probable that these 

 different stages could be worked out if sections were 

 available, such as were studied during the excavation of 

 the large drainage canals in Chicago. At Lemont, above 

 Joliet, sections made with a post-hole auger showed 



