Stellwagen Bank Final EIS and Management Plan 



Page 15 



broad area between Stellwagen Bank and Jeffreys 

 Ledge (and east of Tillies Bank) is also covered by 

 sand mixed with small amounts of gravel. The sand 

 cover extends from Stellwagen Bank southward into 

 the current-swept channel between the southern 

 edge of Stellwagen Bank and the northern tip of 

 Cape Cod. From this channel, a cover of silty- 

 clayey sand extends westward and northward into 

 the southern portion of Stellwagen Basin. 



Sand is the predominant sediment for the inner 

 shelf off Cape Cod. The sand is likely derived from 

 the reworked sandy deposits of Cape Cod. In 

 deeper waters, sandy deposits give way to silty- 

 clayey sand; in the center of Stellwagen Basin, sandy 

 cover gives way to sand-silt-clay bordering clayey 

 silt. 



Broad bathymetric features such as Stellwagen, 

 and other banks and basins, relate to sediment type, 

 whereas smaller topographic featiues such as 

 hillocks, knobs, and swales in rugged areas 

 bordering the Massachusetts coastline, have little 

 relation to sediment types. These latter types of 

 areas exhibit a large variety of sediment types, and 

 lateral changes from one type to another are rapid. 



Sediment types in basins are affected by nearby 

 sources of coarse-grained sediment. Tillies Basin, 

 for example, is a small narrow depression 

 surrounded by shallow banks and ledges, which are 

 covered with coarse-grained sediment. This coarse 

 "debris" is apparently easily moved into the adjacent 

 Tillies Basin, as evidenced by the presence of sand 

 in Basin floor sediments. In Stellwagen Basin and 

 Cape Cod Bay generally, it is also possible that 

 nearby coarse-grained glacial deposits provide a 

 source for the coarse sediments foimd in these 

 areas. 



The highest concentrations of gravel in this 

 general area are found on Jeffreys Ledge; the 

 inshore shelf between Cohasset and Plymouth; and 

 an area east of Stellwagen Bank. Minor amounts of 

 gravel are associated with sand on Stellwagen Bank, 

 and also with till-like deposits foimd at Fishing 

 Ledge in Cape Cod Bay. 



As mentioned above, gravel deposits were most 

 likely transported to the Cape Cod-Cape Ann area 



by glaciers. Associated with many sediment types, 

 gravel occurs in different water current regimes. It 

 forms a lag veneer with sand, and marks a late 

 stage of ice deposition. Hence, gravel materials 

 may provide a crude guide for detecting the waning 

 stages of ice retreat from the offshore area. 

 Assuming the basic theory of gravel's glacial 

 deposition and of gravel's indication of ice retreat, 

 then both Stellwagen Bank and Jeffreys Ledge may 

 actually be offshore moraines and outwash, which 

 have been reworked during post-glacial rises in sea 

 level (CampbeU, 1987). 



Sand dominates the inshore shelf, shallow banks 

 (such as Stellwagen and Jeffreys), and the deep 

 water area east of Tillies Bank. Sand forms an 

 irregular belt of deposits stretching southward from 

 Jeffreys Ledge to Cape Cod. Although sand floors 

 deep as well as shallow areas, it is particularly 

 abimdant aroimd the periphery of Cape Cod Bay, 

 and along parts of the Massachusetts coastal shelf. 



The distribution of sand also provides a guide to 

 water currents. Currents are particularly strong on 

 Stellwagen Bank and in the channel between the 

 Bank and the tip of Cape Cod. The irmer shelf also 

 is an area of strong coastal currents and wave 

 action; and if sand is available as on Cape Cod, the 

 contiguous Bay sediments contain abundant sand. 

 Areas of sand also are foimd next to banks 

 composed in part of glacial deposits, such as 

 Jeffreys Ledge. Sand deposited by currents 

 apparently settles the bottom of the inner shelf 

 north of Cape Ann, where bathymetric contours are 

 widely spaced. 



b. Bathvmetrv 



The sea floor of the general area encompassing 

 Cape Cod to Cape Ann is dominated by two broad 

 ridges, Stellwagen Bank and Jeffreys Ledge, located 

 to its north. Stellwagen Bank extends some 24.85 

 miles (40 km) in a northwest direction between 

 Cape Cod and Cape Ann, and occiu-s at depths of 

 less than 50 meters (164 ft.). Jeffreys Ledge 

 extends northeast from Cape Arm at depths less 

 than 60 meters (196.8 ft.). A third, much smaller, 

 and completely dissected bank known as Tillies 

 Bank, is located between these two larger banks, 

 and is oriented in roughly a north-south direction. 



