U.S. Geological Survey

Cover image from wrir02-4284 (click for enlargement, 276 KB) Hydrogeologic framework of the North Fork and surrounding areas, Long Island, New York

by Christopher E. Schubert, Richard G. Bova, and Paul E. Misut

Water-Resources Investigations Report 02-4284


Ground water on the North Fork of Long Island is the sole source of drinking water, but the supply is vulnerable to saltwater intrusion and upconing in response to heavy pumping. Information on the area’s hydrogeologic framework is needed to analyze the effects of pumping and drought on ground-water levels and the position of the freshwater-saltwater interface. This will enable water-resource managers and water-supply purveyors to evaluate a wide range of water-supply scenarios to safely meet water-use demands. The extent and thickness of hydrogeologic units and position of the freshwater-saltwater interface were interpreted from previous work and from exploratory drilling during this study. 

The fresh ground-water reservoir on the North Fork consists of four principal freshwater flow systems (referred to as Long Island mainland, Cutchogue, Greenport, and Orient) within a sequence of unconsolidated Pleistocene and Late Cretaceous deposits. A thick glacial-lake-clay unit appears to truncate underlying deposits in three buried valleys beneath the northern shore of the North Fork. Similar glacial-lake deposits beneath eastern and east-central Long Island Sound previously were inferred to be younger than the surficial glacial deposits exposed along the northern shore of Long Island. Close similarities in thickness and upper-surface altitude between the glacial-lake-clay unit on the North Fork and the glacial-lake deposits in Long Island Sound indicate, however, that the two are correlated at least along the North Fork shore. 

The Matawan Group and Magothy Formation, undifferentiated, is the uppermost Cretaceous unit on the North Fork and constitutes the Magothy aquifer. The upper surface of this unit contains a series of prominent erosional features that can be traced beneath Long Island Sound and the North Fork. Northwest-trending buried ridges extend several miles offshore from areas southeast of Rocky Point and Horton Point. A promontory in the irregular, north-facing cuesta slope extends offshore from an area southwest of Mattituck Creek and James Creek. Buried valleys that trend generally southeastward beneath Long Island Sound extend onshore northeast of Hashamomuck Pond and east of Goldsmith Inlet. 

An undifferentiated Pleistocene confining layer, the lower confining unit, consists of apparently contiguous units of glacial-lake, marine, and nonmarine clay. This unit is more than 200 feet thick in buried valleys filled with glacial-lake clay along the northern shore, but elsewhere on the North Fork, it is generally less than 50 feet thick and presumably represents an erosional remnant of marine clay. Its upper surface is generally 75 feet or more below sea level where it overlies buried valleys, and is generally 100 feet or less below sea level in areas where marine clay has been identified. 

A younger unit of glacial-lake deposits, the upper confining unit, is a local confining layer and underlies a sequence of late Pleistocene moraine and outwash deposits. This unit is thickest (more than 45 feet thick) beneath two lowland areas--near Mattituck Creek and James Creek, and near Hashamomuck Pond--but pinches out close to the northern and southern shores and is locally absent in inland areas of the North Fork. Its upper-surface altitude generally rises to near sea level toward the southern shore. 

Freshwater in the Orient flow system is limited to the upper glacial aquifer above the top of the lower confining unit. The upper confining unit substantially impedes the downward flow of freshwater in inland parts of the Greenport flow system. Deep freshwater within the lower confining unit in the east-central part of the Cutchogue flow system probably is residual from an interval of lower sea level. The upper confining unit is absent or only a few feet thick in the west-central part of the Cutchogue flow system and does not substantially impede the downward flow of freshwater, but the lower confining unit probably impedes the downward flow of freshwater within a southeast-trending buried valley in this area. 

Citation: Schubert, C.E., Bova, R.G., and Misut, P.E., 2004, Hydrogeologic framework of the North Fork and surrounding areas, Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 02-4284, 23 p., 4 plates, 1:24,000 scale.

Download full PDF report including 4 plates   [Full report including 4 plates, Acrobat PDF (4.3M)]

Download text-only PDF report without plates   [Text-only report without plates, Acrobat PDF (1.6M)]

Download Plate 1 (PDF format)   [Plate 1 - Map of study area showing locations of vertical sections and associated boreholes and wells, and vertical sections B-B´ through E-E´ showing hydrogeologic units in the North Fork study area, Long Island, N.Y., orig. size 22"x42", Acrobat PDF (408K)]

Download Plate 2 (PDF format)   [Plate 2 - Map of North Fork study area showing altitude of bedrock surface and of upper surface of Cretaceous hydrogeologic units: (A) bedrock; (B) Lloyd aquifer; (C) Raritan confining unit; and (D) Magothy aquifer, orig. size 28"x27", Acrobat PDF (473K)]

Download Plate 3 (PDF format)   [Plate 3 - Map of North Fork study area showing Pleistocene confining units: (A) thickness of lower confining unit; (B) upper-surface altitude of lower confining unit; (C) thickness of upper confining unit; and (D) upper-surface altitude of upper confining unit, orig. size 27"x26", Acrobat PDF (461K)]

Download Plate 4 (PDF format)   [Plate 4 - Map of North Fork study area showing surficial Pleistocene units and extent of fresh ground water: (A) surficial hydrogeologic units and water-table altitude in March-April 1994; (B) altitude of base of freshwater above lower confining unit; and (C) altitude of freshwater-saltwater interface below upper surface of lower confining unit, orig. size 15"x39", Acrobat PDF (367K)]

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