THE ISLAND EDGE: OLD MAPS AND NEW GEOPHYSICAL DATA FROM NANTUCKET COAST SHED LIGHT ON ITS DYNAMIC PAST

The highly dynamic, paraglacial coastline of Nantucket Island, Massachusetts presents an opportunity to study the modes of coastal change along a spectrum of closely spaced geomorphic settings. Ground-penetrating radar (GPR) surveys of a suite of coastal landforms along the exposed sections of the island reveal subsurface signatures of geomorphic elements and sedimentary patterns that are largely masked by coastal dunes, vegetation, or development. The combination of geophysical data and historical shoreline change rates also allow quantitative assessment of the geometry and periodicity of paleo-shoreline features. Along the northeast side of the island, the formation and evolution of the historical Haulover inlet suggest a time-averaged channel migration rate of 75 m/year. Based on this value, the strongest northward-dipping GPR reflections produced by a migrating channel suggest fortnightly events, possibly of tidal origin. Geophysical surveys over a prograded strandplain of Low Beach on the southeast corner of the island imaged multiple 8-10-m thick clinoforms of beach and nearshore deposits, which have beachface gradients of 5-6° and show signs of erosional truncation. Comparison of historical shoreline positions indicates that this broad strandplain evolved as an oscillating salient. At Surfside, along the south shore of this strandplain, prograding beach and dune sections comprise a traveling foreland that has migrated alongshore and abuts a relict glacial bluff. In a shore-normal GPR record, the buried erosional face of the bluff has a 12° slope and extends to a depth of at least 8 m. This paleo-scarp is likely a part of a regional retreat terrace. Shore-parallel images of the Miacomet Pond barrier reveal a buried proglacial valley (width: 120 m; depth: 5.5 m), which shows infilling from both sides of the valley, a trend consistent with the bidirectional longshore transport system. The upper part of the valley fill sequence contains a 2-m-deep ephemeral paleo-channel fill, which corresponds to documented inlets that open in response to both storm surges from the ocean side and increased pond levels inside. This study illustrates the value of integrating historical and geological data to offer a more complete picture of Holocene coastal dynamics along island shorelines.