H. W. Shimer — Post- Glacial History of Boston. Ill 



mean a submergence of this region of at least 33 pins 10, or 

 43 feet ; and if the peat was formed far above sea-level it would 

 mean a so much greater submergence. 



Following the submergence occurred the deposition of the 

 black silt, with which were enclosed shells and other records 

 of the life then living in these waters. This record is of pri- 

 mary interest because of the evidence it furnishes as to a 

 warmer period between the presence of glaciers here and the 

 present. That the climate of Boston has become slightly colder 

 since the time of the maximum development of this fauna is 

 shown in the change in the present distribution, especially of 

 the mollusks, for all are still living. Of the sixty some species 

 noted from the localities given, about half no longer occur 

 north of Cape Cod, or only rarely in sheltered areas, but find 

 their perfect environment farther south. These forms belong 

 to the Virginian fauna, which is typically developed from 

 Cape Cod to Cape H.atteras, though some of the species also 

 thrive northwards in a few protected places. Ganong* men- 

 tions nine such areas, including the Gulf of St. Lawrence, Oak 

 Bay, New Brunswick, Casco Bay, Maine, and Massachusetts 

 Bay. Between the retreat of the ice from this coast and the 

 present time a period must have occurred during which the 

 waters were as warm as those from Cape Cod to Cape Hatteras, 

 and during which this Virginian fauna migrated northward. 

 This was followed by a gradual refrigeration of the waters 

 sufficient to prevent the breeding of many of the species except 

 within a few areas protected enough to raise the temperature 

 of the air and water sufficiently during the summer, or breed- 

 ing season, for the development of the young. The following 

 are some of the principal post-glacial fossils of Boston occur- 

 ring thus rarely or not at all to the north of Cape Cod, — Ostrea 

 virgi?iica, Yenus mercenaria, Peeten gibbvs borealis, Laevi- 

 cardium mortoni, Triforis nigrocinchis, Mulinia lateralis, 

 and Yitrinella. That this refrigeration continued during 

 early colonial days is shown by the later disappearance from 

 the vicinity of Boston of such species as the then abundant 

 oyster, and the great reduction of many others both in number 

 and in size. 



Most sections through the black silt in the Back Bay area 

 show a more fossiliferous lower portion and an upper portion 

 with comparatively few fossils. This difference may be corre- 

 lated with the partial closure of Back Bay by the tidal build- 

 ing of Boston Neck ; the consequent reduction in tidal scour 

 would then cause a more rapid accumulation of sediment within 

 the Bay. 



*W. F. Ganong, Trans. Eoy. Soc. Can., vol. viii, Sec. 4, pp. 167-185, 

 1890. 



