GENERAL DESCRIPTION OF THE ISLAND AND SURROUNDINGS 



Along the Atlantic coast line of the United States, there is a geologic phenomenon known 

 as the continental shelf. This consists of a vast deposit of sands, gravels and clays, with a 

 wedge-shaped section, resting upon rock and feathering out to nothing at the landward edge. 

 The shelf itself is approximately 150 miles wide, sloping gently from the inward margin to a 

 point where the ocean is about 600 feet deep ; thence it plunges in a distance of about 40 miles 

 to a depth of 6000 feet and better (GW-2 — Figures 4 and 7) . A large percentage of the southern 

 portion of this shelf is exposed at slight elevation, composing the so-called tidewater sections 

 in the southern states. At the north, the exposed portion comprises much of eastern Maryland, 

 all of Delaware, South Jersey, Long Island and the New England islands to the east. In other 

 words, appearances are that the northeasterly end of the shelf has settled. , 



Long Island is the unsubmerged portion of the inner edge of the continental shelf fronting 

 the Westchester county and Connecticut shorelines and separated from them by the depression 

 of Long Island sound. It is underlain by a rock surface, sloping gently to the southeast, and 

 that rock is exposed in the extreme northwesterly corner of the island. Over most of the area 

 there are deep deposits of sand, gravel and clay resting oh this rock floor and as yet uncon- 

 solidated. Also along this margin of the shelf the glaciers have deposited a vast accumulation 

 of drift which covers practically all of Long Island and extends seaward of it. 



In these unconsolidated materials the geologists have identified more or less finally a 

 number of formations and assigned appropriate names to them as set forth in Wallace de 

 Laguna's report, these being Cretaceous formations subdivided from the rock upwards into 

 Lloyd sand member of the Raritan, clay member of the Raritan and Magothy ( ?) . On top of 

 these there are a number of glacial deposits representing advances of the ice and interglacial 

 eras. At the extreme west end these consist of Jameco with overlying Gardiners clay; on the 

 north shore of the island there is the Manetto, the Manhasset ; over all is the Wisconsin. These 

 subdivisions seem to be subject to change and the chronological sequence of the beds still 

 to be in doubt. 



In these formations there are a number of water-bearing beds or aquifers which are of 

 such extent and continuity as to be of economic importance, and it is the mapping of these 

 aquifers with which this report primarily is concerned. These are Lloyd, two or three in the 

 lower Magothy, Jameco and the so-called Upper Glacial, that is glacial deposits of an age later 

 than the Gardiners clay. 



These geologic terms have been more or less loosely used by well drillers, waterworks 

 officials and engineers until they have passed into the speech of the people but not always with 

 exact geologic accuracy. In such speech, the larger subdivisions are ignored and the average 

 person would not know whether Jameco and Gardiners clay were Pleistocene or Cretaceous, 



As this is not a geologic report but is written for engineers, well drillers, similar practical 

 users and the common man, it has seemed best to adhere to the common rather than the scien- 

 tific usage; thus Lloyd and Raritan are used for the Lloyd and clay members respectively of 

 the Raritan. Magothy is not queried, and certain specific and well-known aquifers in it are 

 distinguished as Magothy I, II and III. Jameco and Gardiners clay are spoken of as entities 

 without reference to their Pleistocene classification. The rest is spoken of as Glacial or Upper 

 Glacial, Pleistocene being a little too difficult for ordinary speech. This Upper Glacial is also 

 held to include Recent on the principle that it may be interglacial after all. 



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