second-order magnitude compared to the larger and better defined Hudson 

 and Block Channels. The 60-, 90-, and 120-foot shelf contours (Fig. 5) 

 widen to the southeast and show a pronounced ridge and swale morphologic 

 fabric with a northwest-southeast crestal orientation. These crenulations 

 in the contour lines (ridge and swale morphology) are especially evident 

 shoreward of the 120-foot (36.6 meters) depth contour, whereas seaward 

 they are more widely spaced and more subdued. This bathymetric fabric 

 for the Long Island shelf shows marked contrast in orientation to the 

 ridge and swale morphology on the New Jersey shelf west of the Hudson 

 Channel. The New Jersey shelf fabric is oriented northeast-southwest, 

 almost normal to Long Island fabric. Several authors have expressed 

 different hypotheses explaining the origins of ridge and swale morphology 

 and the contrasting shelf fabrics on either side of the Hudson Channel. 

 McKinney and Friedman (1970) suggested that present shelf topography is 

 relict from Pleistocene subaerial exposure during lowered sea level, and 

 that the ridges and swales are the remainder of an intricate fluvial 

 drainage system. Swift, et al. (1972) and Duane, et al. (1972), attribu- 

 ted this morphology to interaction of relict shelf sediment with modern 

 hydraulic forces on the sea floor. If this were true, then the features 

 are modern in the sense that they are presently being actively modified 

 by sea floor currents. Detailed discussions are provided by Knott and 

 Hoskins (1968), Uchupi (1968), McKinney and Friedman (1970), Swift, et al. 

 (1972), Duane, et al. (1972), and Stubblefield, et al . (1975). 



Eastern Long Island contains several flat-bottomed basins (Fig. 5) 

 which exhibit minor relief and resemble ancient lakes. These basins are 

 presently interconnected and filled with brackish water as a result of 

 Holocene submergence. The larger basins lie between the northern Harbor 

 Hill and southern Ronkonkoma Moraines. Great Peconic Bay is the western- 

 most basin in the area and consists of numerous smaller embayments. 

 Gardiners Bay lies between Orient Point and Gardiners Island, exhibits 

 a flat sea floor to a depth of 60 feet (18.3 meters), and then deepens 

 abruptly on the eastern side. Between Orient Point and Plum Island a 

 northwest-trending steep-sided trough, called "Plum Gut," extends about 

 150 feet (45.7 meters) deep (Figs. 5 and 6). To the east, another trough 

 exceeds 300 feet (91.4 meters) in water depth and separates Plum Island 

 from Fishers Island. These overdeepened waterways were apparently scoured 

 by late Wisconsin glaciers or fluvial agents. Since the Holocene trans- 

 gression, the waterways have been swept by high-velocity tidal currents 

 which flow between Block Island and Long Island Sounds, preventing signifi- 

 cant sedimentation. 



2. Shallow Subbottom Structure and Stratigraphy . 



Four different geologic units were found to characterize the shallow 

 subbottom shelf of Long Island on the basis of the seismic records which 

 allow a maximum of about 300 feet of resolution of the sea floor, and 

 sediment descriptions from the cores and borings. 



a. Bedrock . The deepest and most obscure unit in the study area is 

 the basement or bedrock surface. Because subsea elevation of the basement 



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