complete absence cannot be verified since much of the southeastern part of the Island is 

 relatively unexplored. Tertiary b eds undoubtedly occur offshore to the southeast. 



In contrast to earlier reports, the Pleistocene deposits are here divided into only three 

 groups. Each of these is essentially a hydrologic unit and is distinctive enough to be recognized 

 in well logs and samples. The oldest fluvio-glacial deposit, the Jameco gravel, is separated from 

 the upper Pleistocene outwash by the Gardiners clay, an inter-glacial deposit. The Manetto 

 gravel is not included as a separate unit in this report because of the difficulty in recognizing 

 the formation in well logs or samples. Further, the Manetto does not appear to be of hydro- 

 logic importance as it is of limited horizontal distribution and probably is hydrologically 

 connected with other Pleistocene deposits wherever it occurs. 



DESCRIPTION OF THE CORRELATION UNITS AND THE 

 CRITERIA USED IN THEIR IDENTIFICATION 



Bedrock 



The bedrock floor of Long Island consists predominantly of pre-Cambrian schists and 

 gneisses. Locally, as in northeastern Kings and northwestern Queens Counties, there are 

 occurencies of granodiorite. A narrow band of limestone cuts across the promontory pro- 

 jecting into the East River near well Q 375 in Astoria. This belt of limestone continues 

 beneath the East River between Welfare Island and northwestern Queens County. It is shown 

 on Berkey's geologic map of New York City (19). Outside of this small area it is not likely 

 that limestone occurs in any significant amount. The metamorphic rocks have been invaded 

 in places by pegmatitic and granitic instrusions such as was encountered at well Q 1030 in 

 Rockaway Park, Queens. There bedrock was found to be a normal granite showing no signs 

 of metamorphism (67). It has been thought by some geologists that diabase intrusions and 

 sandstones of Triassic age might possibly extend from the mainland of Connecticut over to 

 Long Island but no reliable occurrences of Triassic rocks have been observed anywhere on 

 Long Island. 



The greatest concentration of bedrock data occurs in the northwestern part of the Island 



where hundreds of bedrock test borings were made in connection with pre-construction studies 



for City of New York Water Tunnels 1 and 2. The amount of bedrock information decreases 



T-apidly to the east, so that in Suffolk County, which includes almost two-thirds of the land 



area of Long Island, there are only about 5 wells that are known to have been drilled to bedrock. 



A decaj'ed or weathered zone is usually encountered over the bedrock exc ept in the nor th- 

 western corner of the Island where glacial scouring has removed most of these deposits. In 

 most ol^her places, 'the residual weathered deposits have been preserved by a mantle of 

 Cretaceous sediments. The zone of decay appears to range from 5 feet to as much as 100 feet. 

 It commonly consists of red, gray, yellow, white, green, or mottled colored clay ; or sandy clay 

 with partially decayed rock and mineral fragments. Where good core samples are available 

 a definite graduation from sound rock to an almost pure clay can be observed. Frequently some 

 doubt exists as to whether the clay immediately overlying bedrock is the lower part of the 

 Lloyd sand member of the Raritan formation or is actually weathered rock. If samples are 

 available, a geologist can usually determine the age of the material in question. Decayed 

 bedrock usually contains some indication of the original minerals of which the rock was 

 composed such as angular, ragged quartz grains and pieces of garnet, biotite, amphibole, 

 pyroxene, feldspar or the altered equivalents of these minerals. In some instances, the residual 

 clays appear to have been reworked and redeposited. These deposits exhibit a finely laminated 

 structure indicating subaqueous deposition. In these instances, the clays should be considered 

 as__Cretaceous in age. 



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