POST-GLACIAL HISTORY OF BOSTON. 457 



so-called glacial flour, — the material ground from its rocky floor by 

 the stones lield firmly in the base of the advancing ice. A few uni- 

 dentified pieces of wood were noted in this clay. 



2. After the glacier had melted away from this region, the earth 

 was exposed to the air so that the upper layers of the clay were hardened 

 through oxidation. During this time the region was subjected to ero- 

 sion by running water as evidenced, in Back Bay, by the gullies in the 

 surface of the clay. At the Longwood Bridge, Brookline, a sand-plain 

 (a fossil delta deposited by glacial streams) was eroded to a depth of 

 37 feet. 



3. During this erosive period, or at least during the latter part of 

 it, fresh-water peat was broadly developed. The majority of sections, 

 deep enough to penetrate the glacial clay or sands, encounter this 

 peat immediately above the glacial sediment. 



4. Subsequent to the deposition of a variable thickness of peat 

 the land sank with reference to sea-level and a large portion of this 

 region was submerged beneath the ocean. This period of submergence 

 has extended to the present except where man has willed otherwise. 

 During this time occurred the deposition of the black mud, in which 

 were enclosed the shells and other records of the life then living in these 

 waters. 



The evidence that the peat in Back Bay furnishes in regard to the 

 extent of this downward movement of the land 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 

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

 mean a submergence of this region of at least 33 plus 10, or 43 feet; 

 and if the peat was formed far above sea-level it would mean a so 

 much greater submergence. 



The shells enclosed in the mud deposited upon the peat since its 

 submergence beneath the sea give evidence of two climatic periods, — 

 an earlier period (4a) warmer than the present and a later colder 

 period (4b) extending to the present. In the Back Bay region, where 

 alone our sections were sufficiently detailed to give exact information 

 upon this point, the warmer period ends suddenly. In 4a the shells 

 are very abundant, making up, in places, one-half of the deposited 

 mass. In 4b the shells are comparatively rare. Yet there is little, 

 if any, gradation between the two. 



4a. The majority of the fossils noted in the Tabular List of Species 

 are from this lower bed. A comparison of these, especially the shells, 



