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Figure 2. Map of end moraines and submerged ridges in southern New York and New England with study-area location.

The sea floor in Rhode Island Sound, located off southeastern Rhode Island and Massachusetts, between Block Island and Martha's Vineyard, exhibits a record of glacial fluctuations and marine transgression. Glacial moraines in southern New England and New York (fig. 2) mark the southernmost extent of the Laurentide Ice Sheet along the Ronkonkoma-Nantucket terminal moraine, which was deposited about 20,000 to 24,000 years ago (Uchupi and others, 2001). The position of the ice sheet at about 18,000 years ago is marked by the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine, which crosses through the eastern part of the study area (Uchupi and others, 2001). As the ice sheet melted and retreated northward, proglacial lakes formed between the ice sheet and end moraines. Glacial lakes covered much of present-day Long Island Sound, Block Island Sound, Narragansett Bay, and Rhode Island Sound and deposited sediments tens of meters to more than 100 meters (m) thick (Needell and others, 1983; Lewis and DiGiacomo-Cohen, 2000; Uchupi and others, 2001). The glacial lakes eventually drained through a network of channels, leaving the area subaerially exposed before sea level rose and flooded the sounds and bay. As sea level rose, waves and currents reworked the glacial and glaciolacustrine sediments, while Holocene estuarine and marine sediments were deposited (Needell and others, 1983).