COASTAL GEOLOGY OF FIRE ISLAND NATIONAL SEASHORE

Fire Island and the other barriers islands comprising about two-thirds of the south shore of Long Island, NY began to form about 8,000 years ago, well seaward of their present locations, as the rate of sea-level rise began to slow, and waves and currents began reworking glacial-fluvial sediments deposited by the Laurentide Ice Sheet (21,000-10,000 years before present). Today, the sandy south shore barriers extend from the western end of Long Island to Southampton and are broken by six tidal inlets. Historically, the inlets of the south shore migrate to the west in response to the dominant southeast waves, resulting in a net westerly longshore sediment transport current (Williams, 1976).

Fire Island National Seashore is located on one of the central barriers of Long Island's south shore (Figure 1A). Fire Island is a long narrow barrier island about 50 km (32 miles) in length and varies in width from 200 m to 1 km (~0.1 and 0.6 miles). Twenty-six miles of Fire Island were designated as National Seashore in 1964 in an effort to preserve "unspoiled areas of great natural beauty…" (Figure 1B). Much of the western half of the Long Island coastal region including western Fire Island has a complex offshore geologic framework that is controlled by outcrops of semi-consolidated Cretaceous coastal plain strata offshore the Watch Hill region of Fire Island. This more stable portion of Fire Island is thought to be supplied with sediment being transported onshore from sand ridges associated with Cretaceous and Quaternary sediment outcrops (Williams and Meisburger, 1987). In contrast, east of Watch Hill the shoreline is experiencing a more rapid rate of shoreline retreat which is attributed to the lack of an inner shelf source of sand-size sediment (Schwab and others., 2000).

Three major storms in the 20th century severely impacted the south shore of Long Island. One such storm was the Hurricane of 1938, which cost an estimated $6.2 million ($15 billion --1998 adjusted) and opened 12 new inlets across the barrier islands of the south shore (Francis, 1988). A major extratropical nor'easter storm (the "Ash Wednesday" storm) in March, 1962 resulted in 50 washovers and a new inlet in Westhampton (http://chl.wes.army.mil/shore/ashwednesday.htm). Another major nor’easter in December 1992 created two breaches east of Moriches Inlet and destroyed more than 100 homes. Storm impacts compounded by sea-level rise are not directly addressed in this report because the long-term effects of storms on coastal change are uncertain. However, it is important to acknowledge major events that have and will continue to contribute to geomorphologic change along the south shore.