SPRING SAPPING ORIGIN OF THE ENIGMATIC RELICT VALLEYS OF CAPE-COD ANDMARTHAS-VINEYARD AND NANTUCKET ISLANDS, MASSACHUSETTS

Steep-sided, flat-floored linear valleys that lack well developed trib utaries and end in amphitheater-like heads are eroded on the outwash p lains of Cape Cod, Martha's Vineyard and Nantucket Island. The valleys are restricted from the mid to the distal ends of the outwash plains and show no connection to possible water sources at the updip end of t he plains. Their distribution and morphology lead us to propose that t hey were eroded by groundwater seeps fed by proglacial lakes (the high hydrostatic heads of the lakes led to the elevation of the water tabl e) dammed by the outwash plains and associated moraines. The valleys o n Cape Cod were initiated by seeps along the foreset surfaces of sandy deltas emplaced iii lakes in Nantucket Sound and Cape Cod Bay after t hese lakes drained. Those on Martha's Vineyard and Nantucket islands w ere either eroded by seeps at the distal ends of outwash plain wedges emplaced atop the subareal continental shelf South of the islands or a long the foreset Surfaces of sandy deltas emplaced on a lake behind a peripheral crustal bulge south of the glacial front. Valley erosion te rminated after the lakes were drained and the water table dropped.