GPR INVESTIGATION OF GLACIOTECTONICS, HITHER HILLS, LONG ISLAND

Glaciotectonism describes processes whereby a glacier deforms pro- and/or subglacial sediments into a variety of landforms, through a variety of dynamic processes such as thrusting, folding, and distributed shear. The moraines of Long Island display many examples of glaciotectonism. Hither Hills State Park, located on the Ronkonkoma Moraine in eastern Long Island, is an area of ENE-trending hills similar in appearance to features elsewhere which have been attributed to glaciotectonics. The consistent trend and “washboard” arrangement of these hills, as well as the glacial origin of Long Island, lead us to hypothesize that they are the product of intermittent (perhaps seasonal) glacial readvances during overall retreat. At Hither Hills, we have carried out extensive Ground Penetrating Radar surveys at a variety of frequencies (25-500 MHz) to discern the subsurface structure of the hills. This allows us to observe shallow fine (decimeter)-scale structure and stratigraphy while placing them in a broader context of up to 20-30 m depth. The goal of these surveys is to probe for coherent, dipping reflectors that could indicate the folding and thrusting of previously flat-lying strata into hills via glacial (re)advance and correlate them with physical stratigraphic data (deformation observed in a nearby eroded shoreline and stratigraphy revealed in monitoring wells). Initial results from these radar surveys indicate the presence of multiple reflectors and layers that appear in places to follow topography, indicating an underlying structural rather than erosional/depositional source for many of the hills. Combined with stratigraphic and morphological observations, these surveys present strong evidence that late Quaternary glaciotectonic processes caused these features at and near the front of a glacier.