2. Geologic Overview



2.1 Introduction

This section contains a description of the surficial and subsurface geology of the Central Pine Barrens. It contains a description of the different geologic time periods and events that shaped the Central Pine Barrens from bedrock to land surface. It utilized data collected for preparation of the Suffolk County Comprehensive Water Resources Management Plan.

The geologic formations of Suffolk County, including the Central Pine Barrens Area, consist of thick deposits of unconsolidated, water-bearing sediments resting upon a relatively impermeable, crystalline bedrock surface. The sequence of events that shaped Suffolk County's geology is not known with certainty, but it probably began with the formation of the original basement rocks in early Paleozoic to Precambrian time more than 400 million years ago. These were heated and compressed (metamorphosed) by folding and faulting to produce a rugged, mountainous topography. During the subsequent period ending with the Late Cretaceous Epoch, 100 million years ago, erosion reduced the land to a nearly planar surface that gently tilted to the southeast.

During the Late Cretaceous Epoch (60-100 million years ago), streams brought sediments from the north and west to the Long Island area on the continental margin, forming a permeable sand layer (Lloyd Sand Member of the Raritan Formation) and overlying clay layer (clay member of the Raritan Formation) upon the bedrock surface. After a short period of erosion or non-deposition, thick, permeable beds of river-delta clay, sand, and gravel were deposited on the Raritan Formation; these deposits comprise the Magothy aquifer. Toward the close of late Cretaceous time (approximately 60 million years ago), a sand and clay unit (Monmouth Group) of low permeability was deposited in shallow marine waters in the area that now constitutes Suffolk's south shore.

A long period of non-deposition, or possibly deposition followed by erosion, occurred after Cretaceous time. Geologic activities during this time left few sedimentary traces, but streams flowing across Long Island cut some deep valleys into the Magothy. It was not until late Pleistocene (Wisconsinan) glaciation (some 20-200 thousand years ago) that there were any significant additions to Long Island's geologic record. At that time, valleys were filled, and the older deposits were almost completely buried by glacial deposits.

Prior to the southward movement of the Pleistocene ice sheets to Long Island, an extensive clay unit (Gardiners Clay) was deposited in shallow marine and brackish waters along the shores of what is now Suffolk County. This unit rested upon the Magothy and Monmouth Group, and acted as a confining layer. The northern portions of the Gardiners Clay were subsequently eroded by advancing ice and glacial meltwaters. Consequently, Gardiners Clay beds are now found only in the south shore area.

The Pleistocene glaciation created the hilly Ronkonkoma moraine along Suffolk's "spine" and South Fork, and the Harbor Hill moraine along the North Shore and North Fork (Figure 2-1). The Ronkonkoma moraine runs down the middle of the Central Pine Barrens Area in an east-west direction while the Harbor Hills moraine is located to the north. Erosion of these morainal deposits (as the glacier melted away from Long Island) created extensive outwash plains of sand and gravel in the intermorainal area and south to the Atlantic Ocean. These highly permeable deposits comprise the upper glacial aquifer and represent the majority of Suffolk's surficial sediments. Some local confining clay units (e.g., the Smithtown clay) were also formed from glacial materials in intermorainal lakes and tidal lagoons. Since the end of glaciation about 12,000 years ago, Holocene beach and marsh deposits have been formed along the marine edge, and within stream corridors and ponds, such as the Peconic River.
 

2.2 Surficial Geology

2.2.1 Unconsolidated Deposits

The sequence of stratigraphic units below Suffolk County and the Central Pine Barrens Area is presented in Figure 2-2 and can be summarized, in ascending order, as follows:

Early Paleozoic to Precambrian Bedrock: impermeable, crystalline basement rock more than 400 million years old.

Late Cretaceous Deposits: deltaic clays, sands, and gravels deposited by streams along the continental margin or as marine sediments comprising the Lloyd Sand (aquifer), Raritan clay (confining unit), Magothy Formation (aquifer), and Monmouth Group (confining unit); (60-100 million years old).

Pleistocene (Wisconsinan) Deposits: marine clays and various glacial materials (till, outwash sand and gravel, intermorainal clay) comprising the Gardiners Clay, upper glacial (aquifer), and Smithtown clay (20-200 thousand years old).

Holocene Deposits: recent beach and marsh deposits less than 12 thousand years old.

Several geologic cross sections through the Central Pine Barrens Area as contained in the U.S. Geological Survey (USGS) Atlas HA-501, Hydrogeology of Suffolk County, Long Island, New York are presented in Figure 2-3 (see Figure 2-3 for key). These figures are included to illustrate the areal extent and thickness of the various hydrogeologic units corresponding to the geologic units listed in Figure 2-2. A detailed description of each of the geologic units is presented in the following subsections.

2.2.2 Bedrock

Bedrock below Suffolk County 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 (within 500-600 feet) in northwest Huntington and at Orient, and deepest along the South Shore (over 2,000 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.

2.2.3 Lloyd Sand Member of the Raritan Formation

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 400 feet below sea level in northwest Huntington and at Orient, and over 1,500 feet below sea level at western Fire Island. 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 200 feet thick in northwest Huntington, 100 feet thick at Orient, and over 500 feet thick at western Fire Island.

Figure 2-1: Surficial Geology, Hydrogeology of Suffolk County, New York
Figure 2-2: Suffolk County Stratigraphy and Hydrogeologic Units
Figure 2-3: Geologic Cross Sections Through the Central Pine Barrens
(Please see the printed version of the Plan for these figures.)

2.2.4 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 300 and 400 feet below sea level in northwest Huntington and at Orient, and about 1,100-1,300 feet below sea level at western Fire Island. 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.

2.2.5 Magothy Formation - Matawan Group Undifferentiated

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, particularly in northwest Huntington, where it is completely eroded. Glacial meltwaters also shaped the Magothy's surface, creating north-south valleys that are now buried below the areas of Huntington-Deer Park, Saint James-Ronkonkoma-Sayville, and Miller Place-Selden. In addition, the Connecticut River is believed to have created the buried valley that runs through the vicinity of Orient-Amagansett during a period of lowered sea level.

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 thicknesses (around 1,000 feet) found along the South Shore.

2.2.6 Monmouth Group

The Monmouth Group is the youngest Cretaceous unit. It was deposited in a shallow marine environment and consists of interbedded clay, silt, and sand, giving the unit a low overall permeability. The Monmouth contains much of the mineral glauconite, which gives the unit a dark greenish color, and is the basis for the hydrogeologic unit's name--Monmouth greensand.

The Monmouth Group overlies the Magothy Formation along most of the South Shore, except for portions of the South Fork. Post Cretaceous erosion by glacial meltwater streams is evident on the surface of the unit, but it appears to have been spared the glacial scouring seen in other Cretaceous formations located further north. The upper surface ranges from about 70 to 165 feet below sea level, and has been found in thicknesses up to 200 feet.

2.2.7 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 nondeposition. 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.
 

2.3 Glacial Deposits

Continental glaciers of Wisconsinan age (20 to 86 thousand years ago) brought to Long Island the materials that now comprise nearly all of its surficial sediments. Glacial material was deposited in two terminal moraines: the Ronkonkoma moraine, which forms Suffolk's "spine" and South Fork, and the Harbor Hill moraine, which runs along the North Shore and forms the North Fork. Some of the original glacial material (till) can still be seen along the north shore of the South Fork, at Montauk, and on Shelter Island, where it acts to retard the downward movement of recharge.

Most of the glacial material was reworked by meltwater to form large, sandy outwash plain deposits south of, and between, the two moraines. These highly permeable, stratified sand and gravel deposits filled in the valleys eroded on the surface of the Magothy (although some filling may have occurred prior to the ice sheet's advance to Long Island).

The glacial deposits can reach thicknesses of up to 700 feet (e.g., in the "Ronkonkoma Basin"). They generally overlie Magothy deposits, except in areas of the North Shore where the Magothy was scoured away by glaciers, and in areas of the South Shore where the Gardiners Clay or Monmouth Group intervene.

Clay materials were also eroded from the moraines and deposited in freshwater lakes or shallow marine lagoons. An example of such a clay is the Smithtown clay unit, located in northeast Huntington, northern Smithtown, and northeast Brookhaven. It occurs within the sequence of outwash deposits and ranges in depth from 90 feet above sea level to 150 feet below sea level. Its thickness ranges up to 170 feet. Because of its overall low conductivity, the clay acts as a local confining unit between upper and lower portions of the glacial aquifer.
 

2.4 Recent Deposits

Beach and marsh deposits of Holocene age are found primarily along the shoreline, and within stream corridors and ponds. Beach deposits consist primarily of sands eroded from the outwash plain or bluffs by wind, runoff, and wave action. Marsh deposits consist of mud and peat, which accumulate along streambeds, in ponds, and in tidal marshes and shoals. These deposits are generally thin, but may be of local hydrogeologic significance (e.g., on the barrier islands).
 

2.5 Topographic Relief

The elevations within the Central Pine Barrens area range from mean sea level where the study area borders Flanders Bay, to a high of 295 feet at Bald Hill, which is on the Ronkonkoma Moraine just southwest of the Eastern Campus of Suffolk County Community College (SCCC) south of Riverhead. Generally, elevations are lowest in the areas where recent geologic deposits are found and highest in the moraine areas as noted in Figure 2-3

Since the Peconic River is a drainage basin between the Harbor Hills Moraine to the north and the Ronkonkoma Moraine to the south, the river and its tributaries are the low points in the watershed area. The elevation of the river goes from mean sea level at Flanders Bay, rising in a westerly direction to a high of approximately 40 feet in the Peasys Pond area and approximately 80 feet in the wetland area west of William Floyd Parkway, which comprises the headwaters of the Peconic River. North and west of the Peconic River, elevations generally rise to the Harbor Hills Moraine where they can exceed over 200 feet above mean sea level in many places. South and west of the river, elevations generally rise to the Ronkonkoma Moraine where they exceed over 250 feet in many places. South of the Ronkonkoma Moraine elevations decrease along the outwash plains within the Central Pine Barrens area.

Similarly, the Carmans River originates in the western Pine Barrens area at Middle Island and cuts through the Ronkonkoma Moraine starting at an elevation of approximately 70 feet, and flows in a southerly direction through Southaven Park to Bellport Bay, which is also at mean sea level.

2.5.1 Slopes

Slopes within the area of the Central Pine Barrens where outwash plains and recent deposits can be found are generally even to gently rolling, and range from 0 to 15%. The moraine areas are very hilly and uneven containing slopes that range from 15 to 35% in many areas.

2.5.2 Land Forms

In addition to common glacial features which include moraines, outwash plains and recent geologic deposits, kettle holes, kames and swale areas can be found in or adjacent to the moraine areas in the Central Pine Barrens. A kettle hole is a depression in glacial drift formed by the melting of a detached block of stagnant ice that was buried in the drift. It often contains a lake or swamp. Many of the ponds within the Central Pine Barrens area are kettle holes. A classic example of a kettle hole can be found directly north of the Eastern Campus of SCCC.

Kames are mounds, knobs or short irregular ridges left by the glaciers that consist of stratified, poorly sorted sand and gravel, and at some locations they are overlain by a thin ablation till. A representative example of a kame can be found south of County Road 111, Port Jefferson-Westhampton Road, approximately a half mile southeast of its intersection with the Long Island Expressway.

Swale areas occur when 2 steeply sided hill areas converge on one another leaving a steep sloped gully or ravine. Many of these are found throughout the moraine areas. A good example is found south of Birch Creek in the Flanders area at Sears Bellows County Park.

Kame-and-kettle topography, also known as knob-and- kettle topography, is an undulating landscape in which a disordered assemblage of knolls, mounds, or ridges of glacial drift is interspersed with irregular depressions, pits, or kettles that are commonly undrained and may contain swamps or ponds. A representative example of this type of topography can be found in the southeastern corner of the Central Pine Barrens area in a section known as Henry's Hollow.
 

2.6 Bibliography: Geologic Overview

Flint, Richard Foster. Glacial and Pleistocene Geology. New York: John Wiley & Sons, Inc., 1957.

Jensen, H.M. and Julian Soren. "Hydrogeology of Suffolk County, Long Island, New York." Department of the Interior, U.S. Geological Survey, Atlas HA-501, 1974.

Nemickas, Bronius and Edward J. Koszlaka, "Geohydrologic Appraisal of Water Resources of the South Fork, Long Island, New York." Department of the Interior, U.S. Geological Survey, Water Supply Paper 2073, 1982.

Suffolk County Department of Health Services' Staff, "Geology," Suffolk County Comprehensive Water Resources Management Plan, Volume 1, 5-1- 5-14. Dvirka and Bartilucci Consulting Engineers and Malcolm Pirnie Inc, 1987.