Bedrock below Queens and Kings Counties is comprised of crystalline metamorphic rocks (gneisses and schists) that are similar to those found in Connecticut. The original basement rocks are believed to have been early Paleozoic (Cambro-Ordovician) to Precambrian granite or sandstone more than 400 million years old. These rocks were crystallized by heat and pressure during folding and faulting caused by tectonic forces during early Paleozoic time (200-300 million years ago).

The bedrock surface below Suffolk County is tilted southeast to south at a slope of approximately 50 to 70 feet per mile. It is, therefore, closest to land surface (subcropping) in northwest Queens and Brooklyn, and deepest along the South Shore (over 700 feet deep at the western part of Fire Island). In many places, the upper surface of the bedrock is weathered to a residual clay. Since the water bearing capacity of the unit is extremely low, the bedrock surface is considered to be the bottom of the groundwater reservoir.


The sediments comprising the Raritan Formation lie on the bedrock surface and are believed to have been derived from stream erosion of areas to the north and west during late Cretaceous time (60-100 million years ago). The formation is made up of a lower sand and gravel member (Lloyd Sand) and upper clay member (Raritan clay).

The Lloyd Sand Member has a moderate overall hydraulic conductivity and consists of sand and gravel interbeds, with occasional lenses of clay and silt. The Lloyd's beds are about parallel to the bedrock surface below. Its upper surface lies about 200 feet below sea level in northwest Queens, and over 900 feet below sea level at Rockaway. The unit is believed to terminate somewhere close to the North Shore beneath Long Island Sound, and is not found in Connecticut. The thickness of the Lloyd increases from north to south; it is about 100 feet thick in central Queens, and over 350 feet thick at Rockaway.

Clay Member of the Raritan Formation

The clay member of the Raritan Formation (Raritan clay) overlies the Lloyd Sand Member throughout Suffolk County. In some locations, however, the clay has been eroded, and glacial deposits overlie the Lloyd, thus providing good hydraulic conductivity between the glacial deposits and the Lloyd aquifer. The Raritan clay, although composed mainly of clay and silt, does contain some sand and gravel beds and lenses; overall, however, the hydraulic conductivity of the clay member is low, and it confines the water in the Lloyd aquifer.

The Raritan clay parallels the Lloyd Sand Member and terminates just offshore in Long Island Sound. The surface of the clay member lies between 0 and 100 feet below sea level in northwest Queens, and about 700 feet below sea level at Rockaway. Clay member thicknesses range between 50 and 100 feet in the northern areas, and reach nearly 300 feet in the western part of Fire Island.


The Magothy Formation - Matawan Group undifferentiated (informally "Magothy") is composed of river delta sediments that were deposited on top of the Raritan Formation during the late Cretaceous after a period of erosion. It consists of highly permeable quartzose sand and gravel deposits with interbeds and lenses of clay and silt that may have local hydrologic significance.

The Magothy was eroded during the time period between the end of Cretaceous and the Pleistocene. The surface was scoured by glaciers meltwaters that also shaped the Magothy's surface, creating north-south valleys. Unlike the upper surfaces of bedrock and the members of the Raritan Formation, the highly eroded upper surface of the Magothy does not exhibit any distinctive tilt to the southeast, although bedding planes within the formation have this orientation. Because the upper surface is so irregular, the thickness of the Magothy varies; however, the thickness generally increases from north to south, with the greatest thickness (around 1,000 feet) found along the South Shore.

Gardiners Clay

The Gardiners Clay is a shallow marine or brackish-water deposit of late Pleistocene age. It is typically grayish-green to gray; the variation in color is due to the content of minerals such as glauconite. The unit contains some beds and lenses of sand and silt, but its overall hydraulic conductivity is low, making it a confining layer for underlying aquifer formations, particularly the Magothy.

The Gardiners Clay is found along most of the south shore. Its northern extent varies from 3 to 5 miles inland and is indented by long, narrow north-south channels, which indicate the effects of erosion by glacial meltwater streams and areas of nondeposi-tion. The upper surface of the unit ranges in altitude from 40 to 120 feet below sea level. The thickness of the unit increases southward toward the barrier island, reaching thicknesses of over 100 feet.

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