THE LIFE OF GLACIAL LAKE CHICAGO 73 



''kettle-holes" of considerable size. Several of the elevations of the ground 

 moraine are of such height, from nine to fifteen feet, that they may have formed 

 little islands in the shallow lake (see plate XXXIV). A notable example is 

 near Lincoln Avenue, where the boalder clay of the ground moraine is exposed 

 on the surface for nearly half a mile. If the level of the lake was as low as the 

 present level, as has been suggested by Leverett, these depressions might have 

 contained ponds or pools, similar to many which may now be found in the Sko- 

 kie marsh area. The presence of such mollusks as Anodonta, Lymnaea, Amni- 

 cola, and Campeloma rather disprove this theory, however, as these species live 

 in lakes or ponds usually having connection with a lake or river of some size. 

 The species of mollusks contained in these lower deposits, together with the 

 stratigraphical character of the deposits, both lacustrine and terrestrial, point 

 to the conclusion that they were formed in shallow water, not exceeding ten 

 feet in depth, which was protected from the violence of the waves on the east 

 by a low sand bar, subsequently removed by the rising of the water preceding 

 the Calumet stage. 



b. Other Deposits Belonging to this Stage 



A marl deposit was observed near the Ogden ditch, on Austin Avenue, 

 which probably may be correlated with the deposits previously mentioned. 

 It underlies five feet of peat and silt (the latter with naiads) and contains the 

 following species of mollusks: 



* Pisidium tenuissimum * Valvata tricarinata unicarinata 



medianum Physa warreniana 



* Musculium truncatum * " walker I 



* " secure Planorbis antrosus 

 Amnicola limosa " campanulatus 



" lustrica " parvus 



* Galba obrussa decampi 



It will be noted that six species and one race are found in this deposit which 

 are absent from the two deposits previously discussed. These are indicated 

 by an asterisk in the list. 



The richness of the molluscan fauna of the Bowmanville low water stage is 

 indicated in the following table (species and races treated alike) : 



Anodonta 2 



Proptera 1 



Lampsilis 1 



Sphaerium 7 



Musculium 2 



Pisidium 6 



A mnicola 3 



Valvata 2 



Goniobasis 



1 



Campeloma 



2 



Physa 



3 



Ancylus 



2 



Segmentina 



1 



Planorbis 



6 



Galba 



3 



Lymnaea 



1 



Total species 



43 



