strata. These dip and thicken to the southeast. Long Island originated 

 as a glacial depositional land mass made up o£ the Harbor Hill and 

 Ronkonkoma Moraines and seaward-sloping outwash plains which coalesce 

 to the south coast and extend onto the shelf. Shelf sediments are pri- 

 marily stratified blanketlike deposits of quartzose sand and gravel which 

 are discontinuous in areal extent. Most of the shelf sediments are relict 

 Pleistocene outwash deposits; however, parts of the west-central shelf 

 contain residual material from Coastal Plain strata which crop out on the 

 sea floor. Also, C-^'* radiometric-age dates from six cores show that 

 Holocene barrier is land- lagoon sedimentary sequences comprise minor parts 

 of the shelf, primarily east-central shoreface parts of the shelf and the 

 regions around Gardiners Bay and Block Island Sound. 



About 15 major buried drainage channels have been found to transect 

 the Long Island mainland and adjacent shelf. Channel thalweg depths range 

 from -100 to -700 feet (-30.5 to -213.4 meters) MSL and channel widths 

 from hundreds of feet to several miles. Most of the channels at the Long 

 Island north shore exhibit evidence of significant overdeepening by 

 Pleistocene glacial lobes which selectively followed preexisting stream 

 channels. The channels are filled with assorted Pleistocene sediments 

 and influence much of the present Long Island topography, but fluvial 

 and marine processes have obliterated any bathymetric clues to their 

 existence on the shelf surface, except for the Hudson and Block Channels. 



Fine to medium quartzose sand is the primary sediment type on the 

 Long Island Atlantic shelf with secondary quantities of coarse sand and 

 gravel; in the Gardiners Bay-Block Island region mixtures of fine-grain 

 sandy silts and clays predominate. 



Large quantities of sand suitable for beach restoration and nourish^ 

 ment are present on the Atlantic shelf in water depths suitable for 

 recovery by present dredge technology. Sea floor relief is fairly 

 regular and the sand occurs as evenly bedded, flat- lying, blanket 

 deposits. An estimated 8 billion cubic yards (6 billion cubic meters) 

 of sand is available for recovery. 



78 



