440 H. W. Shimer — Post- Glacial Historxj of Boston. 



bring them from New York. And even now individuals of 

 enormous size are occasionally brought from both these places, 

 and probably might be found by special search, at any time." 

 The cause of this great numerical reduction since colonial days 

 is said to be a very severe cold spell about 1780 in which the 

 sea bottom was covered with ice, thus preventing the oysters 

 from getting air. 



Another factor which aided in the destruction of some of 

 these species, especially the oyster, from the Back Bay region 

 was the gradual obliteration of Boston as an island by the 

 formation of a neck uniting it with the mainland to the south. 

 Even during late colonial days heavy seas washed over this 

 neck into the Back Bay. Oysters need a clean substratum, 

 such as gravel, or other shells, to which the young, the spat, 

 may attach themselves, otherwise they will perish ; and the 

 opening across Boston neck would give the tidal currents extra 

 strength with which to cleanse this partially enclosed region 

 from the river muds ; but that this was never so exposed to the 

 action of waves as at City Point is shown hy the occurrence of 

 the surf-clam (Mactra solidissima) at the latter place 'only. 



Many plantings of the oyster spat in its old home in the 

 Charles River during recent days have resulted merely in the 

 death of the spat. 



Conclusions. 



The deposition of the blue clay took place probably in a 

 body of fresh water, since no remains of animal life are appar- 

 ent in it. The clay itself, derived from a near-by melting gla- 

 cier, is the so-called glacial flour, — the material ground from 

 the rocky floor by the stones held firmly in the advancing ice. 

 A few unidentified pieces of wood were noted in this clay. 

 After the glacier had melted away from this region, the area 

 was subjected to erosion by running water as evidenced by the 

 gullies in the surface of the clay as well as in the sand plains 

 (fossil deltas deposited by glacial streams). At the Longwood 

 Bridge, Brookline, the sand-plain was eroded to a depth of 37 

 feet. During and subsequent to this erosive period, or at 

 least the latter part of it, fresh-water peat was broadly devel- 

 oped. The land was at this time sinking with reference to sea- 

 level, and continued its downward motion until the sea invaded 

 the area under discussion. The evidence that the peat in Back 

 Bay furnishes in regard to this downward movement is as 

 follows : — 



The bottom of the peat at Fairfield Street is 23 feet below 

 low tide, at Exeter Street 15'5 feet, at Church Street 33 feet, 

 and at Charles Street it is 27 feet. 



With a height of tide of 10 feet, as it was in the Charles 

 River before the construction of the tide-water dam, it would 



