PLEISTOCENE MOLLUSC A FROM ILLINOIS 59 



From the character of the deposit and its relation to the underlying 

 material, it is clear that it is post-Marseilles in age, and has evidently been 

 laid down in a somewhat limited depression, which was probably connected 

 with the main Illinois Valley through a narrow outlet three or four miles to the 

 southwest. This depression presumably constituted merely an area of over- 

 flow for the older Illinois River, and still bears somewhat of the same relation to 

 the present stream. The deposit of marl is in places less than three inches in 

 thickness, but probably exceeds a foot at the maximum. The areal extent is 

 not known, but judging from the topographic relation the basin in which it 

 was deposited covers two or three miles. It does not seem probable, however, 

 that the marl deposit is even as extensive as this. 



From Mr. Culver's description the deposit M^ould seem to be 

 related to the old glacial outlet from Glacial Lake Chicago, when 

 that body of water v^as at one of its high stages, possibly the 

 Calumet stage. During the Glenwood and Calumet stages the 

 Illinois Valley was well filled with water and every little cove, inlet, 

 or depression near the valley margin was filled with water and 

 formed ideal habitats for fresh-water mollusks such as are now 

 found in the deposit under discussion. Similar marl beds are 

 known from Joliet and are now being studied. The species in the 

 Morris deposit are mostly identical with those found in the Chicago 

 basin (Baker, 1920J), and it is to be presumed that the latter area 

 was supplied with life from the Illinois Valley. It is not impossible 

 for the Morris deposit to be pre-Lake Chicago in age, as the mol- 

 lusks in the deposit could easily have lived during the Glenwood 

 stage of Lake Chicago. 



DISCUSSION OF SPECIES 



Pisidium tenuissimum calcareum Sterki. An abundant species 

 occurring also in the deposits at Urbana. 



Pisidium compressum Prime. This species is evidently rare in 

 this deposit, only a single valve being found in picking over a half- 

 pint of material. 



Amnicola leightoni Baker. Abundant. This Amnicola appears 

 to be pecuHar to glacial deposits. Some of the Amnicola recorded 

 from Pleistocene deposits in the Chicago basin (Baker, 1920&) are 

 this species and not Amnicola limosa, although specimens believed 

 to be limosa occur. It is recorded from Ohio (Baker, 1920a, p. 448) , 



