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Stanley R. Riggs, William J. Cleary and Stephen W. SnyderOnslow Bay are highly migratory (Cleary and Hosier, 1979).They conclude that over 70% of the barrier island lengthsouth of New River Inlet has inlet channel-dominated shorefacesas a result of inlets migrating along the island duringthe last several centuries. For example, most of the coastalarea of Shell, Figure Eight, and Coke Islands have historicallybeen occupied by inlets (Fig. 11) .The result is a shorefacesystem that is underlain by channel sands and gravelsthat formed in response to a series of rapidly migratingHolocene inlets. Steel (1980) drilled and mapped a minimumof eleven relict inlets in the Holocene record beneath BogueBanks.Channel-Dominated Valley-Fill ShorefacesPortions of barrier islands that are within the valleys ofmajor Piedmont drainage systems (Fig. 2) have shorefacesthat are characterized by complex sediment sequencesdeposited in ancient paleofluvial valleys (Pearson, 1979;Hine and Snyder, 1985; Riggs et al., 1992; Snyder et al.,1994). These drainage systems have repeatedly incisedthemselves into their valleys and associated valley fill duringeach Pleistocene glacial episode when sea-level occupiedlowstand positions. During the subsequent transgression,these large channel complexes were systematically backfilledwith a new vertical succession of fluvial and estuarinesediments. The upper estuarine sediments generally are composedof fine-grained, muddy sediments. As the shorefacerecedes in response to ongoing transgression, the portion thatis underlain by estuarine valley fill, erodes rapidly relative tothe adjacent headland dominated shorefaces (Fig. 2).CONCLUSIONSAlong continental margins with limited sand supplies,such as the U.S. Atlantic coast, the shoreface is not an infinitelythick pile of sand. Rather, it is a thin, variable, andtemporal accumulation of sand superimposed or perchedupon a pre-existing and highly dissected geologic framework.Holocene sea-level rise has produced a modern transgressivebarrier island, estuarine, and fluvial sequence ofcoastal sediments that are being deposited unconformablyover irregularly preserved remnants of pre-existing stratigraphicsequences consisting of many sediment and rockunits of variable ages, origins, and compositions. It is thecomplex variability in this underlying geologic framework,in consort with the physical dynamics of each specificcoastal system, that ultimately determines the 1) threedimensionalshoreface morphology, 2) composition and textureof beach sediments, and 3) shoreline recession rates.Based upon the pre-Modern geologic framework, thereare six general categories of shoreface systems that occuralong the Onslow Bay coast of North Carolina. Headlandsare morphological features that rise above the active ravinementsurface and are composed of semi-indurated to indurated,Pleistocene or older sediment units. 1. Subaerialheadlands are characterized by the active incisement of awavecut platform and cliff into Pleistocene or older stratigraphicunits with an associated perched beach. 2. Submarineheadlands are submerged morphological featurescomposed of Pleistocene or older stratigraphic units thathave been incorporated into the modern shoreface and uponwhich the barrier-estuarine system is perched. Ancient sedimentscrop out on the eroding shoreface and commonlyoccur on the inner shelf as bathymetric highs seaward of themodern shoreface and thus, modify incoming waves and currents.Nonheadland shorefaces are dominated by Holoceneprocesses and sediments and can be divided into four generalsubclasses. 3. Transgressive shorefaces are composed ofcompact peat and mud that extend from the modern estuaries,under the barrier sands, and crop out within the surf zoneand upper shoreface. The steep shoreface is often characterizedby an irregular geometry and the beach is dominated bydiscontinuous and highly scarped dune ridges with abundantwashover fans. 4. Regressive shorefaces are composed ofunconsolidated sand and occur along barrier segments thathave adequate sediment supplies and are often associatedwith inlets, headlands, and cape structures. The beaches aredominated by progradational geometries with accretionarybeach ridges. Channel-dominated shorefaces consist of twodistinctive types of systems. 5. Inlet-fi!l shorefaces are composedof unconsolidated sand and gravel sediments with clinoforminfill geometry that forms in response to activelymigrating inlets. 6. Va!ley-fi!l shorefaces have historicallybeen occupied by large paleofluvial drainage systems andare underlain by thick accumulations of fluvial- estuarinechannel-fill sediments in response to deglaciation andHolocene sea-Ievel rise. Valley-fill sediments are typicallycomposed of fine-grained sediment and therefore will erodedifferently than inlet- fill sands and gravels. The sedimentsderived from valley-fill erosion are often not compatible withthe dynamics of the adjacent beach systems. Consequently,valley-fill shorefaces are often characterized by lower slopesand sediment deficient beaches that are actively eroding.Thus, the basic structural, stratigraphic, and geomorphiccharacteristics of the pre-barrier land surface interacts in acomplex way with modern coastal processes to determinethe barrier beachmorphology and shoreface dynamics. Eachbarrier beach and shoreface are total products of their geologicheritage; the signature of their geologic history controlsand influences the present morphology, shoreface dynamics,and rates of shoreline recession. Consequently, the conceptof a common equilibrium profile for all shorefaces is neitherrealistic nor adequate when considering detailed processesalong any given coastal segment. It is imperative that societylearn to live with and manage our complex shorelines. Inorder to do this, we must understand the detailed geologicframework underlying the shoreface and the inner shelf, as38