Abstract

The southern Virginia shelf is traversed by a shelf valley which may constitute the retreat path of the ancestral James River during the late Holocene sea level rise. It is flanked by levee-like margins (shoal-retreat massifs) which are the retreat paths of littoral drift depostional centers that were maintained on the sides of the retreating estuary mouth. This transgressive nearshore marine topography has been heavily modified as Holocene sea level rise has continued; the water column has deepened, and the shoreline has receded. Strong south-trending currents associated with "northeaster" storms have redistributed the surficial sand. They have impressed patterns of transverse sand waves, current-parallel lineations, and large-scale, nearly current-parallel sand ridges. The sand ridges are a variety of large-scale bedform whose genesis is still not adequately understood. The ridges on the Virginia Beach Massif appear to have been incised into the massif as successive segments became exposed to open shelf flows, while the False Cape ridges appear to have formed by a somewhat different scheme of ridge growth and detachment during erosional retreat of the shoreface. However, both ridge sets have a number of significant characteristics in common. Trough talwegs and crestlines climb to the south in both sets. Both sets make northward-opening angles of 20-35 degrees with the shoreline. For both sets the landward flanks are more coarse grained and more gently inclined than the seaward flanks, perhaps as a consequence of a cross-ridge component of flow. The same or very similar hydraulic mechanisms appear to have been involved in their respective schemes of formation.

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