COMPARATIVE GPR STUDY OF PLEISTOCENE AND MODERN PARABOLIC DUNES, LONG ISLAND

Long Island has two remarkable sets of parabolic dunes – the Grandifolia dunes of the north shore which formed proglacially in the latest Pleistocene, and the modern Walking Dunes on the south fork. Ground-penetrating radar (GPR) imaging the subsurface has made it possible to study these two ecologically sensitive sites in great detail without causing environmental damage.

The Grandifolia dunes trend south-southeast, with their relief stabilized by an ancient dwarf beech forest. A ridge we studied consists of a roughly 12 m thick layer of aeolian sand atop glacial till. Internal bed geometry is consistent with deposition in response to glacier-dominated prevailing winds blowing from the NNW. With sea level much lower at that time, such a wind would lead to accumulation of sand from across the dry, glacially reworked, and friable sediment of present-day Long Island Sound. These dunes provide a revealing window into conditions on Long Island during the latest phases of the last glaciation. The Walking Dunes of Hither Hills are modern parabolic dunes, actively overrunning a forest. GPR reveals numerous small buried trees within an internal structure characteristic of parabolic dunes. Varying orientations of imaged bedding planes illustrate complexities in the growth histories of these dunes. GPR imaging below the base of the dunes shows evidence of reworked glacial sediments and past erosional surfaces below present-day sea level. Aerial images dating back to 1930 allow us to track the motion of the youngest of these dunes, which currently migrate at rates of 1 to 3 m/yr. High-frequency (high resolution) 3-D GPR imaging on the crest of the most rapidly evolving of these dunes reveals a complex, asymmetrical pattern of growth, characterized by changing slip surface orientations and migration of the main blow-out. The older dunes are stabilized to varying degrees by vegetation. Like their Pleistocene analogs, the Walking Dunes grow in response to prevailing N to NW wind, though the modern dunes apparently feed upon the mobilization of shoreline-eroded glacial sediments by winds channeled between offshore islands. The presence of other parabolic dune forms, to the west, across Napeague Bay, suggests that the Walking Dune system may have once been part of larger dune field at a time when sea level was lower.