2 Geologic Setting and Morphologic 

 Development 



Geologic Setting 



Long Island is the largest island adjoining the continental United States. It is 190 km long 

 and extends from the Narrows at the entrance to New York Harbor eastward to Montauk Point, 

 due south of the Connecticut-Rhode Island boundary. The island is part of the Atlantic Coastal 

 Plain, with basement of Cretaceous age rock and some older metamorphic rocks that outcrop in 

 the extreme west near Long Island City. Coastal plain deposits are exposed only in the western 

 part of the island. Most of both the surficial and the underlying materials are Pleistocene 

 morainal and outwash accumulations associated with continental glaciers (Fuller 1914). 



Two morainal ridges run the length of Long Island, with the southern one, the Ronkonkoma, 

 extending to Montauk Point. Most of the north shore of the island facing Long Island Sound 

 consists of bluffs 10 to 30 m high. South of the southern ridge, an outwash plain of fine gravel 

 and sand stretches southward for 1 to 15 km to the Atlantic Ocean (Fuller 1914). Off the south 

 shore, a more or less continuous barrier encloses broad, shallow Jamaica, Hempstead, Great 

 South, Moriches, and Shinnecock Bays. Coney Island, once the westernmost extension of the 

 barrier chain, is now artificially attached to the mainland. At present, six inlets provide access to 

 the bays (Table 1). 



The barrier ends at Southampton, and from there to Montauk Point, the coast follows a nearly 

 straight line intersecting old headlands and crossing old bays. One of the bays, Mecox, is 

 occasionally open to the Atlantic via an intermittent inlet. The exposed bluffs along this eastern 

 portion of the south coast are generally considered to be the source of sediment that feeds the 

 development of the barrier beaches to the west. The direction of longshore drift is predominantly 

 westward along the entire south shore, but local reversals occur near the inlets. Although the 

 dominant westward drift has been recognized for decades, McCormick and Toscano (1981), 

 Williams and Meisberger (1987), and even Fuller (1914) proposed that some sediment may be 

 moving onshore from the shelf to augment that moved by longshore currents. Almost no sand is 

 delivered to the coast by streams. 



Chapter 2 Geologic Setting and Morphologic Development 5 



