Explore Geology
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Fire Island

National Seashore

New York

cover of park brochure

park geology subheading
Photo of Fire Island National Seashore
Fire Island National Seashore, New York

Fire Island is located at the terminal moraine of the Laurentian ice sheet. The glacier began its retreat 8,000-12,000 years ago, and residual material supplies the garnet and magnetite sand found on the island. The sediments comprising Fire Island National Seashore and the south shore of Long Island are largely derived from reworking of Pleistocene sediments. These sediments were either deposited directly by the Laurentide glacial event or from glacial drift deposited offshore during a period of sea level rise in the Holocene period of the last 18,000 years. Similar material may be found in the Ronkonkama moraine exposed near Montauk Point about 100 km to the east. The occurrence of the heavy mineral fraction in the otherwise silica and feldspar dominated sediment lends support to a glacial source. Glauconite is absent from the glacial till and outwash sediments off of the eastern two-thirds of Long Island, but is abundant off unglaciated New Jersey about 50 miles to the southwest. Glauconite is almost exclusively of marine origin. It is a mica mineral formed authigenically either from the alteration of biotite or from an aluminosilicate gel. Magnetite and feldspar are common on Long Island but not in New Jersey (Williams and Meisburger, 1987).

Sea level rise during deglaciation slowed about 9,000 years ago and may have led to the development of ancestral barriers (Schwab, et al., 2000). A decrease in the rate of sea level rise about 4,000 years ago favored more growth of barrier islands. Continued rise in relative sea level forced these islands to migrate onshore, especially to the east where the offshore slope is steeper and inlets are more frequent. As a result, the larger barrier lagoon is located to the west, and smaller lagoons are found to the east. At the same time, obliquely incident waves created a net littoral drift of sediment to the west, resulting in westward spit extension and inlet migration.

At present the central portion of the barrier is the oldest, about 1200 years old (Leatherman and Allen, 1985). Inlet dominated transgression is more frequent at the eastern part of the island with salt marshes occurring on old flood tidal deltas. A prograding spit with recurved dunes occurs to the west. Democrat Point was “stabilized” by a terminal jetty in 1940, but the adjacent Fire Island Inlet requires dredging to provide navigational access. The regional longshore sediment transport rate has been estimated at 70,000 m3 per year from the headlands, increasing to roughly 200,000 m3 per year along the barrier islands and inlets east of Fire Island. Sediment transport increases to 400,000 m3 per year at Fire Island Inlet. The local increase along the western half of Fire Island has been attributed to the shedding of sediment from the remnants of a Cretaceous-age outcrop offshore of Watch Hill near the center of Fire Island (Schwab et al., 2000). Onshore flux is manifested by oblique sand ridges 10-30 meters deep. These ridges appear to be associated with patterns of shoreline change on the order of decades and with dune erosion during very large storm events (Allen and LaBash, 1997; Schwab, et al., 2000).

The area is very well studied in terms of geological evolution, albeit with contentious issues of timing and relict offshore expressions (Taney, 1961; Leatherman and Allen, 1985; Williams and Meisburger, 1987; Rampino and Saunders, 1981; Paneogtou and Leatherman, 1986). Fire Island has rich history of inlet development with one of the best sets of digital data in the nation describing shoreline change over the past century (Leatherman and Allen, 1985). Since 1993, this has been augmented by GPS surveys of shoreline position at annual to seasonal intervals (with one storm impact) along the full length of the island. Historically Fire Island has always been bounded on the west by Democrat Point and by Fire Island Inlet. Fire Island Inlet migrated about 64 meters per year westward until construction of a jetty.

Old Inlet was open from about 1750 to nearly 1830 in the present Wilderness Area, about 15 kilometers west of the present eastern end (Figure 2). From 1830 to 1932, the barrier chain was a continuous spit until Moriches Inlet formed. During this period there was a major advance in the shoreline near Old Inlet. What once was the old foredune now is a secondary dune. The formation of Moriches Inlet led to the interruption of longshore sediment transport and the development of flood tidal shoals. The inlet migrated westward a few kilometers and started to close by 1950. By 1953 it was reopened by dredging, and a jetty was constructed for navigational access. The inlet then shifted into an ebb-dominated regime accompanied by the growth of an ebb-tidal delta. Shoreline change between 1933 and 1979 shows the sediment deficit effect of the inlet. 120 meters of erosion occurred downdrift of the inlet which then transitioned to no net change about 14 kilometers downdrift. The center of the island again prograded resulting in major sediment accumulation at the Democrat Point jetty.

Dune crestline positions were mapped in 1976, 1981, 1986, and in late November and late December 1992. The mapping identified a series of nested hierarchies of crestal change responding to both natural and anthropogenic effects on dune morphology (Psuty and Allen, 1993). The data are vertically controlled by annual topographic surveys of 28 profiles between Kismet (to the west) and Watch Hill (Figure 2) and by detailed 3-D surveys of dunal change just west of Kismet and in front of the Talisman-Barrett Beach area (Allen et al., 2001).

Recent comparisons of GPS surveys with NOAA “T” Sheets and aerial photography updated work by Leatherman and Allen (1985) documenting recent evolution of the island (Allen et al., 2001). After accumulating a volume of about 2.3 million cubic meters of sediment between 1986 and 1994, the ebb-tidal delta off of Moriches Inlet began bypassing sediment to Fire Island. About 2 kilometers downdrift, the beach accreted 120 meters since 1979. An erosional shadow still existed up to about 15 kilometers west of the inlet. The center of the island still accreted but the eastern half displayed very high frequency variability. The western half of the island possessed a pattern of very large “wave forms” of erosion and accretion at about 6 kilometer intervals in front of the resident communities.

These spatially periodic features of shoreline change are not only related to natural dispersion of sediment inputs but also to offshore controls upon incident wave energy and cross-shore sediment transport (Gravens, 1999). Allen and Psuty (1987) argued that the development of gaps in the prominent longshore bar and trough morphology are contributing factors to localized beach and dune erosion. Schwab, et al. (2000) suggest that the offshore ridges may also be linked to the erosion.

The December 1992 storm had over 10 high tides with greater than 0.3 meter storm surge as measured nearby at Sandy Hook. The storm has a recurrence interval of about 25 years, but a very long duration. Local cells of erosion migrate westward yielding a waxing and waning episode to shoreline change direction. These erosion cells have a persistence time of months to possibly years.

From 1870 to1979, there was an island-wide mean rate of shoreline retreat of 0.44 meters per year. Spatial variability at both long and short time intervals dominates the simple central tendency. This confounds engineers, planners, and others attempting to remedy impacts of coastal retreat on the communities surrounding Great South Bay behind Fire Island. For decades there has existed a conflict between the Department of the Interior and the Army Corps of Engineers as to how to provide storm damage protection along the south shore of Long Island.

In 1978 the Council on Environmental Quality ordered a Reformulation Study which led to a flurry of research in the early 1980s. During the 1990s, the NPS continued research on shoreline change and breaching threats. In response to storm damages in the early 1990s, the New York Planning District of the ACE initiated an “interim” project of beach nourishment and dune rebuilding in 1996. This led to an announcement by the NPCA in April 2001 that Fire Island is one of the nation’s “10 most threatened parks.”

Geologic resources are unique features of the dynamic coastal environment. But as changes occur, so does the utilization of the resource both by humans and by many other species. Special attention is given to those species that have Threatened and Endangered status. On the beach this includes piping plover, seabeach amaranth, the Northeastern tiger beetle, and various other listed species that utilize the offshore area of Fire Island and/or are migratory species using the offshore or island habitat .

Principal competitors for habitat space of these species are human recreational activities aggravated by the fact that Fire Island is the only developed barrier island in the nation without a central roadway. The beach is the highway for all vehicular access, including federal and county safety patrols, utility suppliers, maintenance of power and telephones, local contractors, local residential traffic and 4-wheel drive school buses. The beach is utilized far beyond its role as an energy buffering function of sand vs. waves. Dunes are more than just repositories of aeolian sediment taken from the beach. They also provide flood protection for the island and protect the mainland from flooding overwash and breaching events. At Fire Island the geological resources are functionally tied into the culture of modern, coastal development in America. Although other coastal parks are also impacted by development, none have the proximity to one of the world's largest urban areas as does Fire Island.

Littoral Processes
Littoral processes include the interaction of waves, currents, winds, tides, sediments, and other materials near the shoreline. Littoral currents flow either parallel to the shoreline (e.g., longshore currents) or perpendicular to the shoreline (rip currents or undertow). Littoral currents along Fire Island generally run in an east-west direction. Together with waves, winds, and tides, littoral currents transport coastal materials towards and away from beaches. Such materials, collectively referred to as littoral drift, include sand, gravel, other sediments, and organic material. Littoral transport is the movement of littoral drift in the littoral zone by waves and currents. Depending on the rate and direction of littoral transport, beaches erode, accrete, or remain relatively stable.
Waves are the primary cause of sediment transport in the littoral zone and are the principal cause of most shoreline change. A variety of factors influence the direction and energy of waves, including wind and water depth. In shallower waters, the energy of waves is dissipated through friction with bottom sediments and additional energy is lost as waves break on shorelines or other objects. In general, waves that approach shore through deeper water or channels retain greater energy that is spent in closer proximity to the shore. When greater energy is expended by waves in the littoral zone, erosive forces increase the transport of littoral drift.

Structures that extend perpendicular to shorelines interfere with natural littoral processes and sediment transport. For example, groins are constructed to control or modify littoral transport. Such structures block the nearshore movement of littoral materials and cause “up-current” beaches to accrete. Although groins may increase deposition on up-current beaches, they effectively steal sediments from down-current beaches, intensifying erosion in those areas.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A photo album for this park can be found here.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

General information about the park's education and intrepretive programs is available on the park's education web page.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://www2.nature.nps.gov/geology/parks/fiis/index.cfm   I  Email: Webmaster
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