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Figure 2. Map of end moraines and submerged ridges associated with the offshore extensions of these moraines in southern New York and New England along with the study area.

Setting

Block Island Sound is located south of Rhode Island and south and east of Fishers Island, New York. It is bordered on the west by Long Island, N.Y., and the entrance to Long Island Sound, and on the east by Block Island and Rhode Island Sound (fig. 2). The sea floor of Block Island Sound, like those of Long Island and Rhode Island Sounds, reflects the glacial history and subsequent marine transgression in the area. The southernmost extent of the Laurentide Ice Sheet is marked by the Ronkonkoma-Block Island-Nantucket terminal moraine deposited 25,000 to 28,100 years ago (fig. 2; Stone and Borns, 1986; Ridge, 2003; Balco, 2011). A retreated position of the southern edge of the ice sheet is marked by the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay end moraine, which was constructed around 20,400 to 22,400 years ago (Balco, 2011). As the ice retreated further northward, meltwater formed glacial lakes between the ice front and end moraines, covering parts of present-day Long Island, Block Island, and Rhode Island Sounds, where glaciolacustrine sediments tens to hundreds of meters thick were deposited (Needell and others, 1983; Lewis and DiGiacomo-Cohen, 2000; Boothroyd, 2009). These glacial lakes eventually drained, subaerially exposing the land and allowing rivers to cut valleys across the region before eustatic sea-level rise. As the shoreline transgressed northward, glacial sediments were eroded and transported, leaving a coarse-grained lag, and transitional and modern marine sediments were deposited (Knebel and others, 1982; Needell and Lewis, 1984).

Strong tidal currents reaching up to 2.7 meters per second in constricted areas, such as The Race, influence the sea floor in western Block Island Sound (White and White, 2012). Modern marine conditions include erosion and coarse bedload transport in high-energy environments, or sorting and reworking of the sea floor and deposition of marine sediments in low-energy environments.