24 



BIOLOGICAL REPORT 31 



and the main channels in the bay mouth (30.5-42.7 

 m). It has been suggested that the bottom topogra- 

 phy reflects glaciation; deep channels in the bay 

 mouth extend into the bay (through shoaling) and 

 continue into the 1 5 .2-m contour and the greatly 

 extended 1 2.2-m contour toward the bay head. This 

 greater channel structure may reflect late glacial ero- 

 sion (Hough 1 940). To a lesser extent, the outwash 

 channels in the western shore (e.g.. New Bedford) 

 continue offshore (Driscoll and Brandon 1973) be- 

 cause of their drowning after formation. Once the 

 flooding of the Buzzards Bay basin commenced, 

 the now subtidal sediments were subject to reworking 

 and transport. 



2.3. Sediments of 

 Buzzards Bay 



The surficial deposits within the Buzzards Bay 

 watershed appear to be predominately Pleistocene 

 in origin. Although deposits of some pre-Pleis- 

 tocene sediments have been reported ( Woodworm 

 and Wigglesworth 1 934), these have not been con- 

 firmed. It is therefore thought that the earlier Ter- 

 tiary and Cretacean strata are not apparent (or ac- 

 tive) in the present system (Moore 1 963). The tex- 

 ture of the glacial drift is coarse with sand size par- 

 ticles most abundant and with little silt and clay 

 (Hough 1 940), and gravels and rocks are common 

 within the moraines. The thickness of the Pleistocene 

 deposits is of course variable, but appears to ex- 

 tend to the bedrock (e.g., Dedham granodiorite). 

 Emery (1969) reported that a subtidal boring in 

 Woods Hole encountered granodiorite at 83 m be- 

 low mean low water (under 8 1 m of clean sand and 

 2 m of water) although basement may be nearer to 

 the surface (47 m) to the northeast (Oldale and Turtle 

 1964). 



The most abundant rocks in the Buzzards Bay 

 moraine exposed to bay waters in the southern por- 

 tion are gneiss and granites (Hough 1 940; Driscoll 

 and Brandon 1973). The source of these granites 

 appears to be most likely the Dedham granodiorite 

 and associated rocks from the region adjacent to 

 the Boston Basin with apparently some southern 

 Maine diorite and possibly contributions from 



northeastern Massachusetts. "Thus it would appear 

 that ice moving southward from southern Maine and 

 southeastern New Hampshire across the eastern 

 margin of Massachusetts could have gathered all of 

 the diverse materials found in the Buzzards Bay Mo- 

 raine" (Mather et al. 1 942: 1 143), and in the reces- 

 sional moraines as well. On the western shore, in 

 addition to the glacial transport, Dedham grandiorite 

 can been seen in outcrops (Emerson 1917). The 

 glacial drift and to a much lesser extent this exposed 

 granodiorite adjacent to the bay primarily consti- 

 tute the source of "new" sediments to the bay 

 bottom (Hough 1940; Moore 1963). 



The initial source of bay sediments was the same 

 as the surrounding upland until flooding by the ocean, 

 at which time biogenic and water-transported de- 

 posits began to form, and reworking and sorting of 

 the sediments began to take place. A minerology 

 study (Hough 1 940) found quartz to be the domi- 

 nant mineral in all samples, and feldspars were sec- 

 ond in abundance. The feldsparthic sands are di- 

 rectly related to the erosion of Buzzards Bay sys- 

 tem glacial debris. In the deeper waters there is an 

 abundance of clays, micas, fine-grained quartz, and 

 feldspar. 



Tidal and wind-driven currents are the most im- 

 portant source of energy for sediment transport and 

 sorting within Buzzards Bay. These currents result 

 from the protection offered by Cape Cod and par- 

 ticularly the Elizabeth Islands, which prevents long- 

 period ocean waves from entering the bay (Moore 

 1 963). In addition to providing a mechanism to al- 

 ter basin sediments, the flooding of the basin al- 

 lowed for erosion of shoreline deposits by wave 

 action. To date, erosion of headlands and island 

 shores has cut them back many meters. Coupled 

 with longshore transport, the curvature of the coast 

 has been somewhat reduced, and some embayment 

 openings have been restricted by bay mouth bars 

 and, if shallow enough, have been filled with wet- 

 lands (for example. Great Sippewissett Salt Marsh 

 in West Falmouth). Overall, however, the change 

 in the shoreline has been modest due to the abun- 

 dant boulders in the glacial drift areas, which form a 

 pavement and retard erosion (Hough 1 940). 



