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Studies of the barrier ridges and beaches along the presently transgressing mid-Atlantic coastal area have been used to formulate morphologic and vertical sequence models for transgressive beaches. Each type of barrier is a response to the topography being inundated in the ongoing transgression as well as to wave and current conditions. Four major types of sand-gravel barriers may be identified in coastal Delaware: (1) bay-mouth barriers, (2) beach against highlands, (3) estuarine barriers, and (4) spit complex.
The bay-mouth barriers, between enclosed lagoons and the open Atlantic Ocean, are characterized by a vertical transgressive sequence of sedimentary environments identical with the horizontal sequence in the direction of the transgression. In the beach against highland barrier, beach-bern system impinges on low-lying Pleistocene highlands (20-30 ft) which are being eroded and provide a partial source of sediment to the beach system. Estuarine barriers occur as long arcuate shorelines of large estuaries such as Delaware Bay. A barrier of sand and gravel is formed with small width (30 ft) and thickness (< 10 ft) but extreme length (50-75 mi). The internal structure of these thin but extremely long sand barriers is complex and mainly comprised of washover features. A modified version o the estuarine barriers is found in places near the leading edge of transgression on the landward shorelines of lagoons. The spit-dune-barrier-marsh tract includes all of the expected coastal environment sediments of a normal barrier-lagoonal area in addition to typical spit-accretion sand and gravel. However, vertical sequences are disrupted and not in order.
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