150 LIFE OF THE PLEISTOCENE 



artesian well borings. The lacustrine deposits rest on the till and are from 

 200 to 300 feet in thickness. The sequence of strata is as follows: 



Surface soil 



Clay 



Sandy clay, containing trees and mollusks 



Gravel 



Clay and stones 



Clay 



Till 



• 



The upper clay varies from 40 to 60 feet in thickness and contains frag- 

 ments of mollusks and pieces of reeds and wood; logs were encountered in two 

 cases at 38-39 and 33 feet; the sand and gravel varies from 20 to 70 feet and 

 contain plant fragments, mollusks and many logs, several of good size; beneath 

 the coarse material is a second layer of clay of considerable thickness, which 

 overlies the true till. Prof. Penhallow identified the wood as follows: from 

 south well, 30-35 feet in clay, Pinus rigida; from well No. 1, 50 feet, in gravel, 

 Larix americana. Dr. Dall identified the molluscan genera as Valvata, Planor- 

 bis, Amnicola, Pisidium, and Sphaerium. 



The evidence, according to Tarr, seems conclusive that these sands and 

 gravels were either shallow-water, lake-margin deposits, or else stream-made 

 land deposits, and that they were succeeded by lake conditions. To the writer 

 they seem like lake-margin deposits, such as may be found on the southern 

 shore of Cayuga Lake, where a great stretch of beach is covered with fragments 

 of lake debris, consisting of mollusks, crayfish, fish, and hundreds of logs of 

 all sizes. Like the Huron -Erie basin to the west, these deposits mark fluctua- 

 tions in the level of the lake, and their interpretation would perhaps be as 

 interesting as that already worked out at Chicago, and other places. 



Six species of fresh water shells have been reported from New York City 

 which are referable to late postglacial time. The deposit from which thev 

 came is thus described: 95 "The shells here mentioned were found at what is now 

 17 1st street and Morris Avenue, Borough of the Bronx, New York City. The 

 swamp which is situated at this point lies in a long, narrow, anticlinal valley 

 which has been eroded in limestone. When the street, now known as Morris 

 Avenue, was filled in across the swamp, the peaty deposit, which had accumu- 

 lated here, was forced up to heights of several feet on either side of it. This 

 caused the peat to crack in all directions and revealed numerous pockets which 

 were full of small shells. " The species recognized are noted below. 



Amnicola limosa Planorbis antrosus ( = bicarinatus) 



Valvata tricarinata " parvus 



Physa heterostropha Pisidium variabile 



85 Humphreys, Nautilus, XXIII, p. 10. 



