Long Island Sound, covering almost 3,400 square kilometers of the region between Long Island, New York, and the Connecticut mainland, was studied using 700 kilometers of high-resolution seismic profiles and 75 vibratory cores to determine the geologic character and Quaternary history and evolution of the Sound, as well as to assess- the resource potential of sand and gravel in seafloor deposits.The subbottom consists of igneous and metamorphic bedrock that crops out close to the Connecticut shore and slopes south to depths of -250 meters at the Long Island north shore. The bedrock surface under the Sound is highly irregular and exhibits relief on the order of ±30 meters. There are numerous buried river channels that generally trend north-south in the western Sound, and apparently project south under Long Island; a large east-west channel along the Long Island north shore that underlies eastern Long Island and Gardiners Bay, projects south to possibly join the Block Island Shelf channel. Many channels, deeply scoured by Pleistocene glaciers, project south from present-day rivers along the Connecticut shore. The deepest channel at -244 meters MSL underlies New Haven Harbor and trends southwest past a bedrock high at Stratford Shoal and then projects south under the Long Island mainland. Cretaceous strata overlie the bedrock in isolated areas of the middle of the Sound and exhibit considerable surface relief due to erosion by multiple glacial advances. Pleistocene sediments consist of thick accumulations of very firm varvelike silts and clays filling and overlying the bedrock surface throughout much of the central Sound, as well as moderately to poorly sorted sands and gravels up to boulder size that were placed as discontinuous recessional moraine segments, glacial outwash heads, and fluvial deltas. The presence of these glacial depositional features on the seabed from the western end of the Sound to about the Connecticut River longitude is evidence for one or more readvances of the late Wisconsin glacier subsequent to deposition of the Harbor Hill Moraine rimming the Long Island north shore. Holocene sediments consist primarily of organic sandy muds that are accumulating in low energy environments. The primary sources of these finegrained materials are coastal erosion of glacial debris, riverine inputs, and landward transport of fines from adjacent Block Island Sound and the inner shelf. Fourteen isolated shoal features have been identified as offering the highest potential as sources of beach-fill sand and construction aggregate. Based on available data, a conservative estimate is that 189 million cubic meters of sand and gravel is available in water depths not exceeding 20 meters if the latest dredging technology were used.