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Stanley R. Riggs, William J. Cleary and Stephen W. SnyderFigure 8. Map of the Onslow Beach showing the 1) location ofdrill holes used in Figure 7, 2) location (middle arrow) ofcoastal inversion that takes place where the Oligocene rockridges intersect the island, 3) plot of average annual rates ofshoreline recession from Benton et al. (1993) along OnslowBeach. The southern portion of the island is experiencingsevere shoreline recession whereas the northern pint is experiencingaccretion and dune ridge development. The area ofmost severe erosion adjacent to New River Inlet is largely adirect response to inlet modification.to 6 m/yr.Crowson (1980) believes that active bioerosion of therock scarps represent a major source and supply of 'new sediment'to the adjacent beaches (Fig. 5). Abundant gravel, upto boulder-size grains, is derived from the rock scarps andlower shoreface and delivered to the beach during stormswhere it is rapidly broken down to sand-sized grains in thesurf zone.Figure 9. Oblique aerial photograph looking northeast fromNew River Inlet to Onslow Beach (A). The approximate locationand orientation of the submerged Oligocene rock ridgesare indicated on the photo (B); notice the major change orientationof Onslow Beach northeast of the intersection of theseridges with the barrier island (C).Non-Headland Dominated ShorefacesTransgressive ShorefacesLarge segments of the NC barrier islands are underlainby deposits of estuarine peat and mud, often with in situ treestumps. These semi-indurated, back-barrier sediments cropout in the surf zone along major portions of Onslow Bay barrierislands, particularly during winter storms when much ofthe sand is transported off the beach and is stored in offshorebars. These fossil estuarine units were overrun by barrierisland systems as they migrated upward and landward inresponse to the general Holocene transgression. Obviously,these sediments have very different compositions, densities,cohesiveness, and resistance to erosion and transport thannormal beach sands. Consequently, their occurrence in theshoreface will affect the beach width, shoreface profile, andrates of shoreline recession.Discontinuous zones of 1,200 to 1,700 year old peathave been mapped along major portions of both MasonboroIsland (Fig. 1) .At the northeast end of Topsail Island, a 0.5m thick peat crops out periodically in the surf zone and canbe traced laterally around New River Inlet to a modern backbarriersalt marsh. Underlying the peat is a compact grayclay of unknown thickness. Storm erosion produces largeboulders (up to 0.7 m) of peat and clay, along with Oligocenerock fragments from the offshore scarps; these gravels representa significant input of 'new' post-storm beach sediment.Regressive ShorefacesOnly local and relatively small segments of the NorthCarolina shoreline are presently characterized by regressiveshoreface conditions. These areas generally occur on theflanks of headlands and represent temporary episodes ofcoastal progradation that usually alternate with episodes oflonger-term truncation as the headland recedes. However,during episodes of regression, these shorefaces are relativelystable, are characterized by progradational geometries, beachridge accretion, dune ridge development, and have the potentialfor approximating the idealized "profile of equilibrium".In Onslow Bay, Cape Lookout, Shackelford Banks, and BaldHead Island contain local and often temporary examples ofthis type of shoreface system (Fig. 1).Shoreface regression also takes place on a small scale onbeaches adjacent to some inlets. This process depends uponthe individual inlet processes and will develop only if there isan adequate sand supply. The result is the progradation of theshoreface adjacent to the inlet to produce the classic 'drumstick'barrier of Hayes (1976). The drumstick portion of thebarrier is characterized by wide beaches, continuous duneridges, multiple recurved accretionary beach ridges, andshoreline accretion. The northeastern end of Onslow Beach,adjacent to Browns Inlet, displays these characteristics(Cleary and Hosier, 1987). Similar inlet influenced barriersegments include Figure Eight Island downdrift of Rich Inlet36