surface often exceeds the limited penetration of the CERC seismic rec- 

 ords, observations of bedrock are limited to eastern Long Island in the 

 Gardiners Bay region. The physical character of the bedrock is different 

 from the overlying sediment so the acoustic contrast is easily recognized 

 on the seismic records as darker lilies. Samples of bedrock soil were never 

 recovered in any of the cores; however, surface outcrops on the Connecticut 

 mainland to the north and logs from deepwater wells on eastern Long Island 

 confirm that the bedrock is a massive, crystalline, metamorphic rock of 

 granitic composition, which exhibits a regional southeast dip of about 

 1 on 55. The bedrock surface in eastern Long Island Sound shows consid- 

 erable local relief which is the result of massive glacial carving within 

 previously existing drainage channels. These conclusions agree with 

 similar evidence of massive local downcutting reported by Grim, Drake, 

 and Heirtzler (1970) for Long Island Sound, and by McMaster and Ashraf 

 (1973) for the inner New England shelf from eastern Long Island to 

 Martha's Vineyard. 



b. Coastal Plain Strata . The second geologic unit within the Long 

 Island shelf consists of Coastal Plain sedimentary strata of late Creta- 

 ceous or early Tertiary age. These consist of semiconsolidated strata 

 composed of glauconitic sand and gravel, poorly sorted sand, and in some 

 instances, fine sand, silt, and clay. They directly overlie the bedrock 

 surface and according to well boring and seismic data, apparently onlap 

 the basement along an east-west line extending through Long Island Sound 

 (Garrison, 1970) and east past Orient Point and Block Island to Martha's 

 Vineyard (McMaster and Ashraf, 1973). Coastal Plain strata dip and pro- 

 gressively thicken to the southeast: under Rockaway Beach the strata are 

 almost 1,000 feet (305 meters) thick; under Fire Island Beach (Fig. 3), 

 about 1,800 feet thick (549 meters); and under Montauk, nearly 800 feet 

 (244 meters) thick (McMaster and Ashraf, 1973). Because the seismic 

 records used in this study are limited in depth resolution to about 300 

 feet (91.4 meters), Gardiners Bay is the only area where Coastal Plain 

 strata in contact with underlying bedrock may be seen on the records 

 (Fig. 6). 



Lithologic variation within the Coastal Plain stratigraphic sequence 

 is limited; therefore, there is little acoustic contrast between strata. 

 Consequently, mappable Coastal Plain reflectors from the seismic records 

 are difficult to identify. However, the reflectors show a low angle, 

 monoclinal, southeast dip with no apparent breaks or dislocations result- 

 ing from faulting or slumping, or any evidence of ice-shove deformation. 

 The only structural deformation evident on any of the seismic records is 

 shown in Figure 7. The reflectors on line F show that Coastal Plain 

 strata have been folded into a single asymmetric anticline and subse- 

 quently truncated by erosion at the sea floor. The strike of the anti- 

 cline is oriented generally north-south and the folding with depth is 

 apparently symmetrical to at least -200 feet (-61 meters) MSL, the 

 maximum depth of record resolution. Strata at the crest of the anti- 

 cline and along the eastern flank crop out on the sea floor and form 

 an erosional cuesta. To the west, closer to the Hudson Channel, the 

 erosion surface on Coastal Plain strata is covered by about 50 feet 



28 



