The Geology of Fire Island National Seashore: A Young Barrier Island Off Long Island

Fire Island National Seashore protects a 42 km (26 mi) stretch of Fire Island, a barrier island separated from Long Island by the Great South Bay in Suffolk County, New York. Its approximately 7,923 hectares (19,580 acres) include sandy beaches, dunes, intertidal scrub, maritime forest and wetlands, as well as the Fire Island Lighthouse and the William Floyd Estate — once home to a signer of the Declaration of Independence, preserving 250 years of history.

Visitors swim, boat, beachcomb, fish, camp, hike, and watch birds, wildlife and stars.

Young, loose ground

Fire Island sits in the Atlantic Coastal Plain, in its Embayed section, and its geology is young: loose and partly consolidated gravel, sand, silt and mud. Most of the island is recent dune sand, beach deposits, and swamp and marsh deposits.

The oldest rocks in the seashore are ice-age: Pleistocene outwash, about 30,000 years old, washed out from glacial ice along the Ronkonkoma Drift, with outcrops of the Manhasset Formation at the William Floyd Estate on the Long Island mainland.

A landscape in motion

Wind, waves, tides and human activity work on this foundation, together shaping and rearranging the seashore’s habitats — beaches and dunes, scrub, forest and marsh.

A sand spit at Fire Island. Image from NPS’s page.

Mapping the seashore

This page draws on the NPS Geodiversity Atlas, a product of the Geologic Resources Inventory, which makes digital geologic maps and reports for more than 270 natural resource parks to support education, geoconservation and the joint management of living and non-living resources. A Soil Resources Inventory has also been completed for Fire Island; both are available through the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Fire Island National Seashore, New York," National Park Service; a work of the United States government in the public domain. The source dates the seashore’s establishment to 1984 but says the William Floyd Estate was added to it in 1978, so neither date is given here.