SEDIMENT FRACTIONATION AND DISPERSAL ON A RECEDING CLIFFED SHORELINE, CAUMSETT STATE PARK, HUNTINGTON N.Y

A detailed cliff, beach, and offshore study at Caumsett State Park in Huntington, New York, has given a detailed picture of the erosion and dispersal of the variegated sediments exposed in the cliffs. The cliffs are composed of Pleistocene glacial sediments that unconformably overlie Cretaceous coastal plain sediments. Storm waves undermine the cliffs, causing erosion by slumping in the Pleistocene sections and mudflows in the Cretaceous sections. Erosion is aided by permeability changes along the unconformity. Reworking by waves between storms results in a lag beach deposit of rounded pebbles and cobbles. Finer silts and clays are deposited into Long Island Sound.

The Caumsett cliffs are oriented NW-SE, perpendicular to the dominant NE storm winds. During storms, this results in a strong long shore current that moves sand and gravel along the shoreline. In the initial part of the storm, some of the sand is moved, through washover fans, across the marsh behind the beach. However, most of the sand and the gravel are moved westward by storm accelerated longshore drift, to build a massive recurved spit into Cold Spring Harbor. Size analyses show a steady grain size reduction westward along the spit. Well-developed “a” axis imbrication in some spit gravels indicates high velocity longshore transport. Offshore cores show that continuing sea level rise has generated a nearshore facies consisting of massive cliff-derived boulders in a lower lag of rounded pebbles and cobbles overlain by silts and clays deposited from suspension in storms.