an important freshwater aquifer in the Long Island area. The Clay Member 

 directly overlies the Lloyd Sand and is composed primarily of variegated 

 silts and clays. Fragments of lignitic material, pyrite, and iron-oxide 

 nodules are throughout. Deep borings indicate the Clay Member varies in 

 thickness from 30 to 300 feet (9.2 to 91.4 meters), with thicker values 

 in southern Long Island. Because of the physical properties of silt and 

 clay minerals, the Clay Member generally exhibits low permeability and 

 acts as an aquiclude by effectively capping the Lloyd Sand and preventing 

 the interchange of water between the Lloyd Sand and the overlying Magothy 

 Formation. The Clay Member also retards natural freshwater recharging of 

 the Lloyd Sand by reducing downward percolation of ground water, except 

 in northwestern Long Island where glacial processes have eroded the Clay 

 Member and the permeable Harbor Hill Moraine lies directly on the Lloyd 

 Sand. 



The Magothy Formation, chiefly a nonmarine deposit, was considered 

 the uppermost Cretaceous unit underlying the Long Island mainland until 

 1965 (Perlmutter and Todd, 1965) . Boring logs reveal its surface is 

 highly irregular and in places gaps exist where the Magothy has been 

 removed by erosion. Removal of Magothy by erosion is especially evident 

 near the buried ancestral stream channels which characterize northwestern 

 parts of Long Island. Lubke (1964) reported that the Magothy exhibits 

 relief of more than 500 feet (152.4 meters) in northern parts of Suffolk 

 County. Magothy strata crop out in places along the north shore; Fuller 

 (1914) reported several locations where the strata exhibit broad folds 

 and small displacement faults as the result of glacial ice pressure. 

 Most of the topographic highs on the Long Island north shore peninsulas 

 correspond in position with the Harbor Hill Moraine, but Magothy material 

 forms the core for many of the high areas. Upper parts of the Magothy 

 Formation are composed of stratified fine to medium quartz sand, with 

 interbedded lenses of clay and silt. Sandy gravel strata are present 

 but they are generally thin and limited in areal extent. The lower part 

 of the Magothy is significantly coarser in texture and marked by a greater 

 abundance of gravelly layers, e.g., the base of the Magothy in many areas 

 consists of thick gravel layers intercalated with fine sands. Because of 

 the nature of this basal zone, it is the most important source of fresh- 

 water within the formation. 



Examination of the microfauna and mineralogy from sediment from 31 

 deep cores in southern Suffolk County by Perlmutter and Todd (1965) 

 (Fig. 4) has led to reevaluation of Long Island stratigraphy, and has 

 enabled correlations to be made with similar stratigraphic sequences in 

 New Jersey (Table 2). Perlmutter and Todd (1965) combine the Magothy 

 Formation and the Matawan Group as an undifferentiated unit which they 

 correlate with the Matawan Group of New Jersey studied by Owens and Minard 

 (1960). In upper parts of the post-Raritan Cretaceous deposit, Perlmutter 

 and Todd (1965) identified highly glauconitic marine strata which, based 

 on mineral composition and micropaleontologic content, are correlated with 

 the Monmouth Group of New Jersey. Figure 4 shows Monmouth sediments are 

 apparently limited to the extreme southern coast and offshore shelf of 



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