by present-day gtorm wave action (see discussions 

 of this region by Dana, 1875; Lindenkohl, 1885, 

 1891 ; Shepard and Cohee, 1936 ; Stetson, 1938b, 

 1949; Lougee, 1953; Elliott et al., 1955; Garrison 

 and McMaster, 1966; and Uchupi, 1968). Below 

 about 35 fm. the surface appears to be dominated 

 by deltaic and alliivial deposition rather than by 

 erosion. 



As Garrison and McMaster (1966) pointed out, 

 two major directions of past drainage are apparent 

 on the present shelf surface south of Long Is- 

 land; one (north of about lat. 40°20' N.) is east- 

 ward into Block Channel, and the other east and 

 southeastward into a large embayment (well de- 

 fined between about 40 and 45 fm. on charts 

 0807N-52 and 0808N-54) near lat. 40°00' N. and 

 long. 72° 10-15' W. 

 The Block Channel 



The Block Channel was discovered during the 

 same surveys of 1842-44 that found the Hudson 

 Channel. This broad and shallow channel extends 

 some 70 nautical miles across the shelf, from inside 

 Block Island Sound to its delta at the shelf break 

 (charts 0807N-51, and 0808N-51 and -52). Block 

 Channel has several minor tributaries entering 

 from the west and two major tributaries entering 

 from the east— one from Rhode Island Sound and 

 Buzzards Bay, and the other from the area south 

 of Nantucket Sound. The Channel and its Ehode 

 Island Sound tributary contain what are appar- 

 ently well-developed tidal deltas between about 

 23 and 26 fm. 



Dana (1863) suggested that the Block Channel 

 had been eroded by the Connecticut River. Gar- 

 rison and McMaster (1966) considered it to have 

 been the main trunk for southern New England 

 drainage during late "Wisconsin and Holocene 

 times, and Krause (1966) suggested that its delta 

 was forming throughout the Pleistocene. These 

 authors noted that the delta's present form was 

 reached during the early Holocene when sea level 

 was about 45 fm. below the present one. This level 

 is similar to the depth of formation of about 43 f m. 

 proposed by Veatch and Smith (1939) for the 

 Hudson Delta. 



Surficial sediments in the Block Channel consist 

 of about 16 inches (41 cm.) of fine fluvial and estu- 

 arine sands and silt, probably of Holocene age. 

 These overlie clean medium sands of Wisconsin 

 age and of fluvial origin. The upper 1 inch (2 cm.) 



56 



or so is sandy silt, with a very high water content ; 

 it is probably late Holocene or modern sediment 

 (McMaster and Garrison, 1966; Garrison and 

 McMaster, 1966). 



Terrains Between the Block Channel and the 

 Great South Channel 



The sea floor east of the Block Chaimel (exclu- 

 sive of Nantucket Shoals) is zoned much the 

 same as to the west, although it is very much 

 smoother, contains fewer stream channels and other 

 well-defined offshore features, and is partly covered 

 by considerably different sediments (charts 

 0807N-51, and 0808N-51 and -52). 



Above about 20 fm. the bottom is rough and is 

 composed of Wisconsin glacial outwash and 

 morainal deposits modified by stream erosion, by 

 the late Holocene transgression, and by the 

 present surf-zone. Between about 20 and 35 f m. the 

 shelf is of very low relief and appears to be an 

 alluvial plain modified by a few transgressive 

 features. 



The surficial sediments on this iilain are fine to 

 coarse sands, which Gan'ison and McMaster 

 (1966) considered to be pre-Holocene fluvial de- 

 posits later reworked by the Holocene transgres- 

 sion. To the east, around the margin of Nantucket 

 Shoals, these authors believed these fluvial sands to 

 be covered by fine sand derived from the Shoals 

 during the late Holocene or present. 



The silty region south of Martha's Vineyard. — 

 Below about 30 to 35 fm. evidence of stream ero- 

 sion is sparse, the bottom is very smooth, and the 

 surface sediments change to sandy silt (chart 

 0808N-52). This region is unique because it is the 

 only extensive muddy deposit on the entire East 

 Coast Continental Shelf that is not associated with 

 a marked depression. It was first mentioned by 

 Pourtales (1870). 



Lindenkohl (1885) considered this muddy area 

 to be a region of Tertiary outcrop that had not 

 been covered by Pleistocene deposits. Shepard and 

 Cohee (1936) thought that the silt was a modern 

 deposit derived from Georges Bank. Stetson 

 ( 1938b) also thought that silt was now being added 

 to older sand deposits in the area, and Chamberlin 

 and Stearns (1963) have suggested a current eddy 

 to account for this deposition. Garrison and Mc- 

 Master (1966) noted that the northern edge of the 

 silt deposit is strongly intermixed with older al- 

 luvial sands and that the eastern edge appears to 



U.S. FISH AND WILDLIFE SERVICE 



