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In 1957, Holocene marine concretions and mud clasts containing post-glacial fossils of marine invertebrates and land plants were dredged from offshore at Prospect Beach, West Haven, Connecticut, as part of a local beach sand-fill project. Indurated, calcareous concretions, dated at 4530 to 5330 BP, yielded a rich fauna, similar to today's, of bivalves (dominated by Argopecten and Anomia), gastropods (dominated by Bittium and Nassarius), and crustaceans, indicating accumulation in a shallow, subtidal environment perhaps 0.3 to 2 m (1 to 6 ft) below mean low water. Megafloral plant remains belonging to eight taxa—including Fagus grandifolia (American beech), two species of Betula (birch), Carya cf. C. glabra (pignut hickory), and Picea cf. P. rubens (red spruce)—indicate the presence of a mixed hardwood forest growing under climatic conditions similar to, or at most only slightly cooler than, those of today. Less numerous, noncalcareous, undated mud clasts are probably younger and contain invertebrates suggestive of life in a protected lagoon. Incorporated peat layers indicate that the sediments accumulated in a protected, shallow water, low energy, subtidal or intertidal environment.