(15.2 meters) of Quaternary sediments. Because this anticlinal structure 

 is close to strike alinement with the Tertiary-Cretaceous formational con- 

 tact on New Jersey, and also with the position of the Shrewsbury Rocks 

 shoals off the New Jersey coast (suggested by Williams and Duane (1974) 

 as areas where Coastal Plain strata crop out on the sea floor) , the anti- 

 cline in Figure 7 is thought to be either proximal to the contact or a 

 flexure involving Monmouth Group strata or Tertiary strata. The struc- 

 tural condition shown on the right side of Figure 7 where Coastal Plain 

 strata have been evenly truncated at the sea floor and thinly covered by 

 loose residual detritus, and on the left side of the figure where the 

 Coastal Plain erosion surface is more deeply buried beneath variable 

 thicknesses of Quaternary sediment, seems characteristic of the general 

 subbottom structure for much of the Long Island inner shelf. Acoustic 

 reflectors on the seismic records and presence of the diagnostic mineral 

 glauconite in many of the cores suggest the northern limit of the erosional 

 cuesta of Coastal Plain strata cropping out on the sea floor extends from 

 the northern New Jersey shoreface at Shrewsburg River, offshore in a north- 

 east direction and apparently intersects the Long Island coast near Fire 

 Island. The cuesta underlies the Long Island mainland and McMaster and 

 Ashraf (1973) showed that the cuesta continues eastward from Plum Island 

 across Block Island and Rhode Island Sounds toward Cape Cod. The Coastal 

 Plain erosion surface landward of the cuesta has been scoured in some areas 

 to several hundred feet below sea level and subsequently covered by younger 

 sediments averaging 50 feet (15.2 meters) in thickness except for the ances- 

 tral river channels where sediment fill is sometimes greater than 300 feet 

 (91.4 meters) thick. 



c. Pleistocene Sediments . Examination and study of the 41 offshore 

 Nassau and Suffolk County sewer outfall cores (Fig. 8) have provided the 

 means of extending Long Island stratigraphy from the mainland across the 

 Long Island shoreface and inner shelf. These cores, located along three 

 nearly shore-normal transects seaward to -108 feet (-33 meters) MSL, show 

 the sea floor consists of gray and tan, fine to medium, clean quartzose 

 sand and gravel, overlying similar sand and dark gray clay and silt. 

 Glauconite is a relatively common mineral in many of the cores both at 

 the surface and with depth which suggests that either the Gardiners Clay 

 Formation or the Monmouth Group, the only two highly glauconitic units 

 in the Long Island Coastal Plain sequence, is present in the shallow sub- 

 subbottom. The 15 cores along the Nassau County transect (Fig. 9) extend 

 about 2.5 miles (4 kilometers) offshore from Tobay Beach and several con- 

 contain 15 to 20 feet (4.6 to 6 meters) of late Pleistocene and Holocene 

 silt- clay and sand sediments overlying a clay-silt surface at -66 feet 

 (-20 meters) MSL which correlates in elevation and stratigraphy with the 

 Gardiners Clay (Fig. 4, profile A-A'). Cores 241, 249, and 251 appear 

 to have penetrated the Gardiners (average thickness 10 feet or 3 meters) 

 and continued into underlying Upper Cretaceous strata of the highly glau- 

 conitic Monmouth Group, or Pleistocene sediments derived from the Monmouth. 

 The 26 cores along the two Suffolk County sewer outfall transects (Fig. 9) 

 show a similar stratigraphic sequence as the Nassau County cores, except 

 the Gardiners Clay is apparently thicker and its surface is lower in eleva- 

 tion indicating a slope or dip to the east. The Suffolk County west core 



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