and then southeast to front Nantasket Beach and generally enclose the 

 expanse of low gentle hills and flats off Nantasket Beach (area U3 in 

 Fig. 5). 



e. Unclassified Areas . Three areas (U in Fig. 5) do not fit any of 

 the previously described topographic elements, and in many respects, are 

 topographically dissimilar to one another. Areas Ul and U3 are the most 

 alike; both lie adjacent to the coast, are relatively shallow, contain 

 expanses of level bottom, and are characterized elsewhere by an irreg- 

 ular bottom topography, especially inside the 30-foot depth contour. 

 Area U3 is further characterized by many low relief, broadly rounded hills 

 and swales interspersed with low rocky outcrops and areas of level bottom. 



Area U2 lies well offshore, and has flat to irregular topography with 

 isolated hills, numerous mounds, and a low extensive platformlike feature 

 which projects as a salient well into the level bottom area A2 (Fig. 5). 



2. Shallow Subbottom Structure . 



a. General . In a study of the sedimentary framework of the western 

 Gulf of Maine and southeastern Massachusetts waters, Oldale, Uchupi, and 

 Prada (1973) inferred the existence of four discontinuous sedimentary 

 units overlying the basement complex. These units were defined largely 

 on the basis of seismic reflection data and consist of (a) inferred 

 Coastal Plain deposits of Late Cretaceous to early Pleistocene age, (b) 

 Pleistocene glacial drift, (c) glaciomarine and marine deposits of 

 Pleistocene and Holocene age, and (d) Pleistocene glaciolacustrine deposits. 



The basement complex rocks were judged to be crystalline and sediment*, 

 tary rocks of pre-Cretaceous age. Since the sediment units are discon- 

 tinuous, Oldale, Uchupi and Prada (1973) found the basement exposed in 

 some places and overlain elsewhere by one or more of the sediment bodies. 



Oldale, Uchupi, and Prada (1973) found only two of the sediment units 

 within the study area. One, a glacial moraine, occurs in only one place 

 where it forms a large submarine, hi 11 lying in the northeast part of the 

 study area and centered at approximately 42° 27.5'N., 70° 41.0'W. (Fig. 4). 

 The other, and the only sediment unit mapped by these authors, comprises 

 marine deposits of Pleistocene and Holocene age. This unit outcrops 

 throughout the area alternating with outcrops of the basement unit. Oldale, 

 Uchupi, and Prada called it the transparent layer due to its characteristic 

 acoustic transparency. Typically, they found this unit partly or com- 

 pletely filling topographic lows in the basement unit or in other sediment 

 units. Internal reflectors within the transparent unit were numerous in 

 some places and absent in others. The transparent layer of Oldale, Uchupi, 

 and Prada can be identified on seismic reflection profiles obtained for 

 this study. 



b. Acoustic Basement . An acoustic basement containing no internal 

 reflectors is evident on ICONS records. This basement lies under the 

 transparent unit and, in places, may crop out on the sea floor to create 



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