Continuous seismic-reflection profiles reveal a south-southeast-dipping Fall Zone surface on which deposits of probable Late Cretaceous age occur. A distinct cuesta and inner lowland, which mark the inshore edge of these deposits, are traceable across the sounds between Long Island and the Martha's Vineyard area. Post-Cretaceous erosion cycles produced a well-defined drainage system on the Fall Zone surface and coastal plain formations. The locations of many of the linear depressions on the present bottom surface of the sounds are closely related to these post-Cretaceous valleys. Glacial events have had a minimal effect on this inner shelf although locally thick Pleistocene deposits may be present in the inner lowland. Finally, thin accumulations of post-glacial sediments overlie a distinctive erosion surface of late Wisconsin or early Holocene age.

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