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The northern Atlantic coastal plain forms the western margin of the Baltimore Canyon Trough, a large sedimentary basin that underlies the continental shelf along the Middle Atlantic states (Fig. 1). The coastal plain narrows northeastwardly from Virginia to Long Island where it plunges beneath the Atlantic Ocean; small exposures of coastal plain sediments occur on Block, Marthas Vineyard, and Nantucket islands east of Long Island.

Deposition in the coastal region is related to the development of the Baltimore Canyon Trough, which took place during the postrift phase of the opening of the Atlantic Ocean. The coastal plain is composed of unconsolidated and semi-consolidated sediments of Cretaceous and Cenozoic age. Sediments of late Jurassic age possibly lie beneath the eastern edge of the coastal plain but this has not been clearly documented. In outcrop, the coastal plain is divided into an inner belt of Cretaceous and early Tertiary formations and an outer belt of younger Tertiary and Quaternary formations. The coastal plain sediments thicken eastwardly into the Baltimore Canyon Trough as a series of basin fills that vary in thickness, along strike. In general the section thins northeastwardly. Near Salisbury, Maryland, the sedimentary section is approximately 2165 m in thickness, whereas at Long Island the thickness is less than 625 m. The variation in thickness along strike is related to structural highs and lows of the underlying basement rocks.

Sedimentation began on the coastal plain during Early Cretaceous (Neocomian) or possibly Late Jurassic time with deposition of fluvial sands, gravels, and variegated clays. The Lower Cretaceous stratigraphic sequence is composed almost entirely of sediments of continental (fluvial) origin. In the distal downdip parts of the coastal plain, marine fossils (molluscs, dinoflagellates, and foraminifers) occur in the Lower Cretaceous section in some wells. This suggests that the coastal plain was influenced from time to time by marginal marine incursions.

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