30 BULLETIN OF THE BUREAU OF FISHERIES. 



submerged land topography," and not as the creation of currents acting upon shifting 

 sands. Sandy beaches are common upon the ocean shores of Marthas Vineyard and 

 Nantucket, where the surf is heavy and erosion is known to be progressing rapidly. 

 Elsewhere within our region stones and gravel are a characteristic feature of the shore 

 line. Commonly, this coarser material extends down the beach to low-tide mark or 

 beyond, being succeeded by a gently sloping sand flat, more or less interspersed with 

 scattered stones and boulders. In places where the shores are not too steep the stony 

 belt gives place on its landward side to a sandy beach of varying breadth, or the littoral 

 zonation may at times be even more complex. On the other hand, there are many 

 tracts of shore where this phenomenon is not manifest at all, the entire shore and the 

 adjacent sea bottom, so far as visible, being wholly stony. Mud, largely of organic 

 origin, occurs in abundance in bays and inclosed waters which are not swept by tidal 

 currents. 



At certain points within our area preglacial formations have become exposed. 

 As the most conspicuous instances of this we may cite the cliffs of colored clay at Gay 

 Head and the outcroppings of granitic rock in the vicinity of New Bedford Harbor. These 

 last represent a formation "which probably in large part constitutes the foundation rocks 

 beneath the sea and under the islands which lie to the north of Marthas Vineyard." 

 (Shaler, 1888, p. 323.) This formation is the probable source, according to Shaler, of 

 the glacial bowlders of Marthas Vineyard. Passing reference may be made here to 

 Shaler's hypothesis that Buzzards Bay and Vineyard Sound each represents the sub- 

 merged valley of a former river. It does not lie within the province of this report, 

 however, to consider the various problems relating to local geology." 



As regards bottom characters. Vineyard Sound and Buzzards Bay stand in striking 

 contrast to one another. In the former, stones, gravel, and sand predominate; in the 

 latter, mud. These differences are very readily explained. Vineyard Sound is con- 

 stantly swept by strong tidal currents, which prevent the accumulation of fine deposits 

 except in sheltered bays, such as Tarpaulin Cove and Menemsha Bight. Buzzards Bay, 

 on the other hand, being open only at the lower end, is not subjected to such a thorough 

 scouring by the tides (see p. 37), and here, therefore, large deposits of mud occur, as, 

 indeed, they do at all points on the sea bottom off shore at depths which are beyond the 

 influence of currents. Moreover, there open into Buzzards Bay a number of rather 

 large estuaries, which doubtless furnish much of the material which becomes deposited 

 as mud. It has been shown that silt so fine as to remain for a long period in suspension 

 in fresh water is soon precipitated when mixed with sea water. (Allen, 1899, p. 380.) 

 Thus it is evident that a considerable part of the suspended material from the brackish- 

 water estuaries which empty into the northern and western parts of Buzzards Bay must 

 settle to the bottom before it can be transported to any great distance. 



One of the data recorded at each dredging station was the nature of the bottom 

 so far as revealed by the sample brought up. The classification was a very rough one, 

 and it must be freely confessed that it could have been greatly improved. The follow- 

 ing were the principal ingredients recognized: (i) Sand; (2) gravel (referred to as 

 "pebbles" when fine); (3) stones; (4) shells; (5) mud. These ingredients occurred 

 singly or in almost any combination. 



a The reader is referred to Shaler's two papers already cited. 



