Anatomy of a shore face ridge system, False Cape, Virginia

Marine Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

ANATOMY OF A SHORE FACE RIDGE SYSTEM, FALSE CAPE, VIRGINIA DONALD J. P. SWIFT, BARRY HOLLIDAY, NICHOLAS AVIGNONE AND GERALD SHIDELER Institute of Oceanography, Old Dominion University Norfolk, Va. (U.S.A.) (Received March 19, 1971) (Resubmitted June 2, 1971)

ABSTRACT

SWIFT, D. J. P., HOLLIDAY,B., AVIGNONE,N. and SHIDELER,G., 1972. Anatomy of a shore face ridge system, False Cape, Virginia. Marine Geol., 12: 59-84. The Middle Atlantic Shelf of North America is a broad sand plain, characterized by a subdued ridge and swale topography. Some ridges extend into or merge with the shore face. Three such ridges at False Cape, Virginia, trending southwest, have second order ridges on their flanks. Systematic asymmetry of secondary ridge crests away from major troughs, toward major ridges, indicates that the ridges are maintained by coast-parallel currents forming a pattern of helicoidal flow cells. Median diameters decrease to the southwest along the ridge system. In addition, grain size varies with topography. Troughs tend to be floored with a primary lag deposit of coarse, pebbly sand. Wave-winnowed crests consist of a secondary lag deposit of well-sorted, medium- to fine-grained sand. Flanks are fine- to very fine-grained sand winnowed out of the crests. Vibracores and pinger probe records reveal a three-fold stratigraphy. A basal unit consists of clayey fine-grained sand of probable Late Sangamon-Early Wisconsin age. An intermediate unit of relatively watery mud has yielded a radiocarbon date of 25,700 ± 800. The uppermost unit is the modern sand sheet into which the ridges are molded. The innermost ridge and trough appear to be actively forming in response to southtrending coastal "currents," and the shelf tide during tropical and extra-tropical storms. The ridges appear to be moving westward, but it is not clear whether their longterm movement is more or less rapid than the rate at which the beach is retrograding. In any case, Bruun coastal retreat appears to be occurring, with sea floor aggradation at the expense of the adjacent shore face. In this manner the retreating coast is generating a shelf "relict" sand blanket with a ridge and swale topography. Coastal sediment movement has a southward component for fine sand through the ridge system, and may have, for coarser sand, a return north along the beach, by littoral drift. The ridge system may be moving south in response to headward erosion of the troughs. INTRODUCTION SHEPARD (1963, p.213) n o t e d that the M i d d l e A t l a n t i c Shelf o f N o r t h A m e r i c a is a b r o a d sand plain characterized by a subdued ridge an d swale t o p o g r a p h y " c o m p a r a b l e to the barrier islands and their inner l ag o o n s w h i ch extend al o n g m u c h o f the present coast south o f N e w Y o r k . " M o r e recently, UCHUPI 0 9 6 8 , p.217, fig.14) has m a p p e d this t o p o g r a p h y . H e describes the ridges as: " l i n e a r sand b o d i e s . . .

[ w h i c h ] are a b o u t 4 k m wide an d tens o f kilometers long, Marine GeoL, 12 (1972) 59-84

60

O. J. P. S W I F T ET A L .

are symmetrical and broad in cross section, and have side slopes of 1:' and 2°. '' He notes that in Long Bay between Cape Fear and Cape R o m a i n . . . "[they] have a complex orientation, diverging like the spokes of a wheel," while in the Georgia Bight, they rotate in the opposite sense. He comments that 'if the origin suggested by Shepard . . . is true, there has been a drastic change in the alignment of the shore line in recent times." He suggests that, instead, the ridges are now forming during intense storms and are remaining inactive between storms. An examination of UcnuPl'S map (1968, fig.14) indicates that in addition to the elongate, broadly spaced ridges of the open shelf, there are zones and clusters ot closely spaced ridges on the shore face. The 1200 series of U.S. Coast and Geodetic Survey maps reveals considerable detail in these shore face ridge systems. Three categories occur. Ridge systems radiating from the throats of Chesapeake and Delaware Bays and from the mouths of the Georgia-South Carolina estuaries comprise 1 0 ~ of the shore face between Chincoteague Inlet, Maryland, and St. Johns River, Florida. These systems may be related to the tidal regime (JORDAN, t962; LUDWICK, 1970). A second category consists of arcuate, seaward-convex ridge systems on shoals off the cuspate forelands of the Carolina coast, and off other, less prominent cuspate tbrelands. Ridge systems off forelands occupy 11 ~ of the same coastal sector. Since tidal currents are weak off many of the forelands, these ridge systems may be responses to the fair weather wave regime, to storm surge currents, or both (TANNER, 1960a, b; EL ASHRY and WANLESS, 1968). The most enigmatic variety of shore face morphology is the oblique-trending ridge systems which tie to the shore face in depths as shoal as 12 ft. They occupy 40 ~ of the southern Atlantic shore face. SANDERS(1962) who examined the False Cape, Virginia, ridge system suggested that these ridges were relict Late Pleistocene beach ridges. MOODY (1964), however, has carefully mapped a similar system off Bethany Beach, Delaware, before and after the great Ash Wednesday storm of 1962, and noted the ridges had moved as much as 120 ft. in the interim. He speculated the ridges were large-scale, hydraulic bedforms, whose continued evolution during a period of sea level rise and shoreline recession gave rise to the offshore ridge systems. It is clear that any account of the shaping of the Atlantic Shell surface by the advancing Holocene shoreline must undertake to explain these ubiquitous morphologic elements. In order to evaluate the hypotheses outlined above we have initiated a study of the shore face ridge system at False Cape, Virginia. Our analysis of the bathymetry, petrography, and stratigraphy is reported in this paper. A study of the hydraulic regime is currently in progress and will be reported in a later, complimentary paper. E V O L U T I O N OF T H E V I R G I N I A COAST

CURRAY (1969) has suggested that most modern barrier coasts were initiated by the Late Holocene reduction in the rate of sea level rise, of 4,000 to 7,000 B.P. Marine Geol., 12 (1972) 59-84

ANATOMY OF A SHORE FACE RIDGE SYSTEM

61

This concept is applicable to the Virginia coast to the extent that NEWMAN and MUNSART (1968) have assigned a minimum date of 5,500 B.P. to the initiation of the barrier system of Virginia's Eastern Shore peninsula, to the north of the study area. PmRCE and COLQUHOUN (1970) report that the barrier system between capes Henry and Hatteras was initiated as a reoccupied Late Pleistocene barrier system. A study of old maps extending back to the middle of the 19th century suggests that the southern Virginia coast is retreating at an average rate of 9 inches per year (Felton, unpublished report, Norfolk District Corps of Engineers). However, stratigraphic evidence suggests a higher rate. An intertidal tree stump from a stump field on the foreshore at False Cape has yielded a date of 725 __+ 70 years (OAKS, 1964). At a distance of 1.7 nautical miles to the west a living forest emerges from beneath the dunes. Here the mean rate of retrogression of the dunes has been 17.3 inches per year, with a total retreat on the order of 1.3 nautical miles since the lower limit for the initiation of the coast of 5,500 years ago. BATHYMETRY

The False Cape area was mapped with an Edo depth sounder, using a Cubic Autotape Precision Navigation system (see Fig.2). Profiles normal to shore were spaced at approximately 0.5 nautical mile intervals. Depth data was corrected for the state of the tide as determined from the Coast and Geodetic Survey tide tables, but not for water density, since the data was collected within a space of two days, and salinity and temperature gradients across the area were negligible. Cubic Autotape, a circular navigation system, is theoretically accurate to within three feet, where the circular distance envelopes cross at right angles. Circles in our distance grid crossed at high angles except along the baseline very close to shore. Here, within the 20 ft. isobath, accuracy deteriorated rapidly with proximity to shore. Elsewhere, our horizontal accuracy was practically limited by knowledge of the positions of the shore beacons, and map reproduction techniques to _ 15 ft. Our depth data, limited by wave noise on the records, is probably accurate to _ 0.5 ft. The regional bathymetric map (Fig.l) indicates that the False Cape Ridge System lies athwart a gentle nose in the shore face, which here extends 4 nautical miles seaward. Our detailed map (Fig.2; see also Fig.3), shows that the shore face may be divided into an upper, steeper sector, intersecting with a lower, gentler sector at 30 ft, which in turn intersects with the shelf floor at 55 ft. Two of the ridges emerge from the upper, steeper shore face, and traverse the lower shore face, while a third outermost ridge, paralleling the other two, rests directly on the inner shelf floor. Unlike the other two, it is not attached to the shore face. The three ridges shall be henceforth described as A ridge, B ridge, and C ridge, starting with the most landward ridge. Maximum relief on the ridges is 20 ft. Slopes of Marine Geol., 12 (1972) 59-84

62

D. J. P. SWIFT ET AL.

Fig.l. Bathymetry of the southern Virginia inner shelf (Modified from PAYNE, 1970). Isolated highs with bases at 60 ft. or above are stippled. The prominent east-west lineation in the northwest quadrant is an artifact of mapping. The south-trending Spit is Currituck Spit. Its gentle seaward convexity is False Cape. The False Cape study area is outlined.

ridge flanks are 2 ° or less, while ridge spacing is approximately l nautical mile. The troughs are poorly defined. They are generally flat-floored and much broader than the narrowly rounded crests. The amplitude of the three main ridges decreases to the north. Ridge A emerges from the shore face as 2 distinct subridges, and a third intermediate subridge, plus others less well defined, appears to the north. Ridges B and C likewise become diffuse, and develop secondary crests to the north (see Fig.2 and 3). While most ridge and subridge crests are nearly symmetrical, such asymmetry as exists follows a pattern of divergence from the two major troughs, the AB trough and the BC trough, with the steeper slopes of secondary crests facing away from these major troughs, towards major crests. This asymmetry pattern is indicated by arrows in Fig.2. Superimposition of the 1922 and 1969 maps suggests that within this time span, the ridge crests have at many points moved landwards, with displacements Marine Geol., 12 0972) 59 84

63

ANATOMY OF A SHORE FACE RIDGE SYSTEM

36" 37'

36" 52' 75.54, %

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PROFILE

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Fig.2. Bathymetry of the False Cape study area. Letters designate the three major ridge complexes. See Fig.1 for location.

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Fig.3. Physiographic sketch of the False Cape Ridge system. Letters designate 3 major ridge complexes. The sketching technique does not always resolve crest asymmetry. The northsouth horizontal distance has been exaggerated for clarity. Marine Geol., 12 (1972) 59-84

64

D. J. P. SWIFT ET AL.

locally in excess of 150 yards (Fig.4). Fig.4 should be interpreted with caution. While we used a high-accuracy electronic navigation system, it was only as accurate as the location of our transponders which was based ultimately on U.S.G.S. quadrangle maps. The earlier Coast and Geodetic Survey used range towers and horizontal sextant angles. The accuracy of horizontal sextant angles is impressive

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in the hands of experienced users, but even such persons experience difficulty at distances in excess o t 2 miles. Furthermore our compilation is subject to cumulative error in transfering data from between maps of differing scales. Consequently we would assess the observed movement of the ridges as being at the limit of our resolution. We note however, that the skippers of the vessels "Range Recoverer" and "Eastward" also concluded that at least B and C ridges had moved inland by approximately the amount indicated in Fig.4, on the basis of radar ranges. This suggests that if systematic error exists it is as old as the 1922 survey. We also note that the apparent movement of ridge A occurs in an area where our fixes were based on orthogonal ranges, and the Coast and Geodetic Survey fixes were within optimum sextant range. Fig.4 likewise shows a significant pattern of vertical changes, in which the upper and lower shore face have generally undergone erosion, while the sea floor, from the BC trough axis seaward, has generally undergone deposition. Fig.5, a net change map based on superposition of the two primary maps, has too large Marine Geol., 12 (1972) 59-84

ANATOMYOF A SHOREFACE RIDGE SYSTEM

65

a contour interval to resolve this two-fold pattern. It reflects mainly the apparent shoreward advance of the ridges, but reveals an interesting anomaly on the upper shore face not fully indicated by the profiles; namely that of up to 10 ft. of aggradation on the upper shore face south of ridges A and B.

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Fig.5. Net vertical change in the False Cape study area between 1922 and 1969, and subaerial geomorphology of the barrier spit. Data from Felton's study of shoreline changes of the south Virginia coast (unpublished manuscript, Norfolk District Corps of Engineers) indicated that during approximately the same interval, the adjacent beach prograded up to 120 ft., while the beach to the north, with ridge A, receded up to 80 ft. The U.S.G.S. topographic quandrangle map reveals an extensive dune field west of the beach in this sector (Fig.5). SURFICIAL SEDIMENTS General

Data on surficial sediments was gathered by grab-sampling with a Shipek sampler along the bathymetric profiles. The samples and bathymetric profiles were collected during the same three-day period, using the same electronic naviMarine Geol., 12 (1972) 59-84

66

D. J. P. S W I F T ET AL.

gation system. Sample spacing was 0.5 nautical miles. Size frequency distributions were determined by means of a modified Woods Hole Rapid Sediment Analyser, (SANFOR3 and SWIFT, 1971). In addition, cores up to 30 ft. long were collected with an Alpine vibracorer along a transect across the southern portion of the study area. The cores were split longitudinally. One half of the upper sand unit of each core was sampled at 3-inch intervals for size analysis, and the other half was impregnated according to the method of Bt3RG~R et al. (1969). The area was surveyed with an EG and G dual channel system composed of a side scan sonar and a 5 kh pinger probe. The southern coring transect was also the site of ten diving stations. These were occupied on a monthly basis, weather and shiptime permitting. Aspects of the surficial sediments presented below will allow us to make inferences concerning the genesis of the ridges in a succeeding section. Grain size variation: the shore face Fig.6 reveals a regional grain size gradient, marked by the presence of very fine-grained sand in the southwest portion of the area, and its absence in the northeast. The area of fine-grained sand is mainly coincident with the shore face

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Fig.6. Median diameter map of False Cape study area, in relation to topography. Note ephemeral mud lenses (M), and localization of gravel (G) and hard clay substrate (CL) in troughs. Marine Geol., 12 (1972) 59-84

ANATOMY OF A SHOREFACE RIDGE SYSTEM

67

above the ridges, plus a further area of very fine-grained sand on the seaward flank of the south end of ridge B. Regional aspects of the southern Virginia shore face have been described elsewhere (SWIFT et al., 1971b and in press), and the shore face at False Cape differs only in detail. The berm here consists of medium- to fine-grained sand (SWIFT et al., 1971a), the finest beach sand between Cape Henry and Cape Hatteras. A beach core (FCC-A, Fig.7 and 8), exhibited well-developed, very thin to thin bedding (thickness terms from INGRAM, 1954). The uppermost 6 inches was a FCC - A

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68

D. J. P. SWIFT ET AL.

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Marine Geol.~ 12 (1972) 59-84

ANATOMY OF A SHORE FACE RIDGE SYSTEM

69

Several clay and shelly sand strata are present near the base of the core while the rest of the core contains several percent of fine to very fine pebble-sized shell fragments. Many of these are recognizable as the dwarf razor clam, Solen viridens. The median diameter curve for these cores (Fig.7) suggest that sedimentation units 6 inches to 7 ft. thick are present. These tend to have a basal part that fines upward through as much as half a phi unit, a central section of constant grain size, and a somewhat coarser upper few inches. Dives to this station throughout the year have indicated that suspensions of very fine-grained sand and clay are present for several days after storms, and that this turbidity is nearly continuous throughout the winter, to the extent that there is usually a total blackout on the sea floor. Consequently the sedimentation units are believed to be storm-generated sequences, deposited from dense suspensions drifting down slope from the surf zone, as has been described by PASSEGAet al. (1967) and CooI((1969). In addition to the upward-fining sedimentation units, the core exhibits an overall grain size trend from a fine base of 3.62 ~b through a coarse maximum of 3.00 ~b half way up the core, to a fine top of 3.28 ~b. This trend may reflect the varying proximity of the nearest shore-face ridge during progradation of shore face. Grain size variation, ridge and swale topography General. The relationship of grain size to topography in this province is complex. Ridge crests are mantled by a fine- to medium-grained sand while the shore face, ridge flanks, and margins of some troughs are floored by fine- to very fine-grained sands. However, the axes of troughs are generally floored by pebbly, medium to coarse-grained sands. These appear to comprise a thin, discontinuous mantle over a compact, clayey, pre-Recent substrate. The substrate is locally exposed, has been observed by divers, and has been sampled by the grab-sampler (Fig.5). Clay pebbles from the substrate are present in the sand veneer. Fig.6 does not completely reveal the relationship of grain size and topography, since sample spacing is close to the spacing of ridge crests, and in the ridge A area is in fact greater than ridge spacing. A better idea of the sympathetic variation of grain size and topography is afforded by Fig.9, where the two are directly correlated, except in the "Anomalous" AB trough. A comparison of graphic mean with graphic inclusive standard deviation (FOLK and WARD, 1957) shows that the coarse sands of the trough axes and crests become better sorted as they become coarser (Fig. 10). This relationship is characteristic of lag deposits subject to winnowing, since the more prolonged or intense the winnowing, the fewer and coarser are the grains of the residual deposit. The fine sands of the ridge flanks shore face become better sorted as they become finer. This relationship is characteristic of sediment in transit from an area undergoing winnowing. As the sand departs from the high energy zone, it abandons successively finer fractions from the coarse tail of its size frequency distribution. Marine Geol., 12 (1972) 59-84

70

D.J.P. SWIFTET AL° 12

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ridge crests and flanks may be viewed as a response to the fair weather hydraulic regime of long-period, low amplitude swells, with associated strongly asymmetric bottom surge. In fact, the ridge crests and seaward flanks must comprise a set of hydraulic environments whose response to wave surge is similar to (though less intense than) that of the shore face. The medium-to-fine-grained lag sands of the ridge crests are similar to those of the surf zone. The seaward flanks of the ridges are fine- to very-fine grained sands, which decrease in grain size seaward. They have presumably been winnowed out of the ridge crests. Like shore face sands, they may be deposited as a fall-out from clouds of suspended fine sand drifting down slope (CooK, 1969). Flank sands, like shore face sands, are subject to further size-fractionation by asymmetric wave surge and downslope creep (SWIFT et al., in press). SCUBA dives to ridge crests on calm summer days suggest that they are indeed zones of active sediment movement and unmixing. On such days, bottom wave surge on ridge crests has been measured at up to 0.1 knots. The current usually contains an unidirectional component. Short-crested, combined flow marks (Fig.1 l) up to 4 inches in amplitude develop in the medium-grained sand Marine Geol., 12 (1972) 59 84

ANATOMY OF A SHORE FACE RIDGE SYSTEM

71

and migrate slowly across the ridge crest. During the more intense wave climate of winter, dives to ridge crests must be conducted by feel, sincc the dense sediment suspension of the lowest 6 ft. generates a total blackout. The side-scan sonar records suggest that these ripples are ubiquitous. A fine striation is almost universally present on the records (Fig.12B). Various other frequencies of striations are best explained as artifacts, but this variety trends

Fig.11. A clear day on B ridge. Combined flow ripples in foreground have an amplitude of 4--6 inches. Object in background is a Newton ripple profiler. consistently northeast-southwest, regardless of the ship's course. The striations are interpreted as wave ripples, generated by the southeast wave train observed during the day of profiling. In some areas the striations are overlaid with a flecked pattern (Fig.12B). These may be saturated returns from short-crested ripples, similar to the "sea clutter" seen near the center of ship's radar screen. The role of fair-weather oscillation ripples has probably been underestimated as a mechanism of sediment transport. KEULEGAN(1948) and SCOTT (1954) have shown that sand may be size-fractionated by such ripples, with the finer sand transported seaward and the coarser sand transported landward, at rates related to the rate of ripple migration. The textural asymmetry of ridge flanks with landward flank sands coarser than seaward flank sands (see Fig.10), and also the apparent landward migration of the ridges themselves, may be related to this phenomenon. Some lower flank and trough stations were characterized by sand with an Marine GeoL, 12 (1972) 59-74

72

O. J. P. SWIFT ET AL.

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Marine Geol., 12 (1972) 59-84

ANATOMY OF A SHORE FACE RIDGE SYSTEM

73

excess of 15 ~o by dry weight of clay-sized material, or by layers of soft mud. The latter were rarely more than 6 inches thick, since the grab sampler generally bit through them. The layers may be ephemeral or seasonal, since sampling was conducted during the quiet midsummer period. During the same period, a series of diving stations were maintained on a transect across the southern sector of the study area. At the 18 It. shore face station, the concrete anchor block sank into the sand over a period of a month, undermined by the scour of wave surge. By the end of the second month, a dimple in the sand bottom 12 ft. across by 1 ft. deep marked the site. The sand within the dimple was notably muddy. By the end of the third month, the dimple was filled with a mud lens, and covered in turn by a few inches of sand. By the subsequent April, the pinger marking the site was buried beneath at least 2 ft. of sand. Trough textures as a storm response. The surficial sediments of the ridges appear to be best explained as responses to fair-weather wave surge. The thin ephemeral patches of soft mud found in some troughs fit this pattern, but many other features of troughs do not. Trough currents more intense than those observed during fairweather cruises would generally be required to produce the scoured surfaces of stiff clay, and the thin gravels and pebbly lags that mantle them. Core FCC-3 (Fig.7 and 8) indicates that these gravels extend at least under B ridge as a basal gravel. The side-scan sonar records revealed further indications of strong currents. In AB trough the dark, highly reflective pattern of coarse trough sand is resolved into a vague banding parallel to the trough axis (Fig.12A), as though currentfashioned longitudinal windrows of sand were present over the substrate. Elsewhere in AB trough a current-swept ledge of pre-Recent material may be seen (Fig. 12C). Dives to troughs during periods of northerly winds may have revealed conditions approaching those which have shaped the troughs; irregularly lingoid current ripples with amplitudes up to 6 inches have been observed migrating up the axes of AB and A troughs, in response to south-trending bottom currents, that in A trough approached 0.4 knots (averaged from 0 to 6 inches off the bottom by a Bendix Q-18 current meter). Internal structures o f ridges indicating storm activity. While the surficial grain size

distribution of the ridges appear to reflect mainly fair-weather wave surge, cores reveal the presence of internal cross-stratified horizons, and suggest that the ridges, like the troughs, are subjected to periods of intense currents. Core FCC-2A (Fig.7 and 8) was collected by divers from the crest of B ridge. It consists of a medium-grained, well-sorted, dusky yellowish-orange sand with abundant valves of immature Spisula. Two upward-fining sedimentation units of approximately 1 foot thickness are apparent. They are cross-stratified with opposite senses. The large-scale fair-weather ripples observed by divers at this station are Marine GeoL, 12 (1972) 59-84

74

D. J. P. SWIFT ET AL.

rarely more than hall the amplitude of these cross-strata sets, and their migration, alternately burying and uncovering the feet of the current meter rack, appeared to have no net deposition associated with it. The buried cross-strata sets are more easily explained as the product of storm surge currents, generating bedforms that behave more nearly as unidirectional sand waves. The basal sections of cores FCC-3 and FCC-5 (Fig.7 and 8) from the seaward flanks of B and C ridges respectively, are massive- to vaguely-laminated sequences of essentially invariant grain size, apparently deposited under uniform conditions. If the ridges have moved significantly landward, these would be landward flank deposits. The upper sections consist of upward fining sedimentation units, with shell lags at their bases. The upper unit of FCC-5 is of particular interest, because it seems to comprise the record of a single hydraulic event. A basal shell gravel is presumably a lag generated at high current velocities, perhaps in the upper flow regime. The succeeding 15 inches is cross-stratified, apparently reflecting deposition by advancing sand waves. The remaining, upward-fining, obscurelylaminated subunit presumable was deposited as the current continued to decay. This stratum possibly represents an event similar to the great Ash Wednesday storm of 1962, which profoundly modified the coastline and floor of the inner Atlantic shelf', False Cape Ridge System as a resultant response. Thus surficial sand distribution

over the ridges appears to reflect fair-weather wave surge, whose main effect is to degrade the ridges and size fractionate its surficial sand. However the textures and structures of the troughs, and also the internal structures of the ridges appear to reflect periods of more intense currents which erode the troughs, and perhaps restore the crests to their original relief. But before further assessing substrate response to this alternating hydraulic regime, it is necessary to consider the stratigraphic framework of the study area. STRATIGRAPHY The stratigraphy of the study area, as revealed by the pinger probe records and the vibracores (Fig.13) is three-fold. A basal layer consists of stiff, clayey, greenish-gray (5GY4/1) fine sand. At core station 3, the uppcr foot was stained black with an apparent "humate" precipitate (SWANSON and PALACAS, 1965; PIERCE and COLQUnOUN, 1970), that may have perculated down from a former soil profile. A yard and a half below this zone, the sediment of core 3 becomes a coarse-grained, yellowish-gray, pebbly sand. The coarse sand is not seen at the same horizon of cores 4 and 5. An analysis performed by Geochron Inc. indicated an age in excess of 37,000 radiocarbon years. We tentatively correlate this basal unit with the mud fiat and sand ridge facies of the Sandbridge Formation, the nearest subaerial unit that has been Marine Geol., 12 (1972) 59-84

ANATOMY OF A SHORE FACE RIDGE SYSTEM

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assigned a Pleistocene age (OAKS and COCH, 1963; OAKS, 1964). The Sandbridge is apparently a regressive chenier-like sequence of Late Sangamon or Early Wisconsin age (N.K. Coch, personal communication, 1970). A thorium-ionium date on some fossil corals yield ages of 50,000--70,000 years, but there are some problems with the date (N.K. Coch, personal communication, 1970). If our correlation is valid, the basal layer in core 3, would have penetrated the flank of one of the beach ridges, capped by the B horizon of a soil zone. Overlying this basal unit lies a discontinuous, relatively watery mud, penetrated in cores 1, 2, and 5. Its uppermost and lowermost 3 ft. are interbedded with well-sorted, muddy, fine to very fine sand, and in cores 2 and 3, it is underlain by a basal gravel. The mud is light brownish-gray (5YR3/1). The unit is 0 to 22 ft. thick. A radiocarbon date has been obtained from a seaward outlier of the middle mud layer. Eight feet from near the base of the unit in core 5 was sieved by Geochron Laboratories to obtain enough shell material for analysis. This section, from 67 ft. below sea level, dated at 25,700 4- 800 radiocarbon years old. Similar dates have been obtained from surficial oolites on the North Carolina shelf (MILLIMANand EMERY, 1968), and are believed to reflect the regression of the sea associated with the end of the mid-Wisconsin Interstadial. The most landward Marine GeoL, 12 (1972) 59-84

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and stratigraphically highest portion of the main mass of the middle mud unit has yielded a date of 20,400 + 850 radiocarbon years B.P. It was necessary to analyse the total carbonate of 8 ft. of sample to obtain enough carbon for the date. Possibly this main portion of the unit also dates from the mid-Wisconsin interstadial. It is also possible that the unit is a Holocene lagoonal mud overridden by the retreating barrier, and that the date results from analysing Holocene shell material admixed with finer Pleistocene carbonate. The uppermost stratigraphic unit consists of medium dark gray, very fine-grained sand to yellowish-gray or orange-brown medium-grained sand. It is 0 to 20 ft. thick, and is continuous with the surficial sediment. A single large articulated Mercenaria value from 6 ft. below the surface of the upper most sand unit at core station 3 has yielded a radiocarbon age of 4,220 _+ 140 B.P.; hence the unit appears to be of Holocene age. THE HYDRAULIC REGIME

It has been suggested that the ridges of the upper sand layer are in fact beach ridges relict from the Pleistocene (SANDERS, 1962). While our present radiocarbon control is not definitive, it suggests that the ridges are instead modern. Furthermore, we note that at least ridge A lies within the zone "swept" by the retreating barrier since its formation (see discussion of regional setting). Further morphologic and petrographic data, as synthesized below, provide evidence of a circumstantial nature, which contradicts the relict interpretation, and suggests that the ridges are instead large-scale modern bedforms. Three distinct mechanisms have been proposed for the genesis of such largescale submarine sand ridge systems. They are not necessarily mutually exclusive, and they are probably not the only mechanism capable of generating these bedforms. The mechanisms are: (1) ebb-flood channel systems; (2) spiral flow systems; and (3) rotary tidal current systems. Ebb-flood channel systems were first described by CORNISH (1901) who noted that in British estuaries, tidal channels with a greater flood discharge tend to interdigitate with ebb-dominant channels. VAN VEEN (1936) has described additional ebb-flood channel systems molded into the sand sheet that floors the Southern Bight of the North Sea. Here the channels are less prominent than the sand shoals that separate them, and Van Veen pointed out these ridges may be regarded as circulating sand cells, as a consequence of the opposing sense of the residual tidal currents on their flanks. ROBINSON(1966) has noted ebb-flood channel systems on the Anglian coast of Great Britain which tend to approach the shoreline obliquely, as do the False Cape Ridges. LtJoWICK (1970) has recently demonstrated that the mouth of Chesapeake Bay, adjacent to the False Cape area, is floored by a complex, interdigitating pattern of ebb and flood channels, separated by sand ridges. Perusal of these papers suggests that when ebb-flood channel systems Marine Geol., 12 (1972) 59-84

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77

develop, tidal currents of a knot or greater maximum surface intensity are generally present, and an abundant supply of sediment is available. The helical flow theory (CASEV, 1935; PRANDTL, 1952; KOLAR, 1955; TANNER, 1963b) starts with the premise that "secondary flow" (velocity component normal to main flow) resulting in helical flow lines is inherent in turbulent fluid flow (NEMENYI, 1946). TANNER (1963b) notes that helical flow on a large scale "at Froude numbers less than I and Reynolds numbers over 100, is common in moving fluids. Retardation of part of a moving parcel of air or water, perhaps because of roughness, results in veering and overturn (BAGNOLD, 1941). The current tends to bend to the decelerated side. The slower part of the fluid rises and the faster part sinks (also note SHEN, 1961). A general spiral develops which is damped out in a map distance equal to some tens of times the water depth (TANNER, 1962). The velocity gradient, from the bed toward the thread of maximum current motion, is steep on the fast, descending, outer side, and is gentle on the slow, ascending inner side (KENNEDY and FULTON, 1 9 6 1 ) . . . these effects do not depend on the presence or absence of a mobile bed of any kind(TANNER, 1960C)... In as much as the descending part of the current is significantly faster than the ascending part, and furthermore inasmuch as the velocity gradient is much steeper where the water descends, active scour occurs here." The sinking along one flank of the helical cell, and the rising water on the other flank entrain the adjacent waters in such a way that a sequence of symnz_etrical vortex couplets are created. Alternating zones of scour and accretion would be expected on the bottom, resulting in a series of ridges and troughs whose dimensions would be a function of the dimensions of the cells of spiral flow. HOUBOLT (1968) notes that most of the North Sea sand ridges are planoconvex lenses in profile; they appear to have been swept up out of the adjacent troughs. Houbolt's study is unique in that it contains the only published field evidence for large-scale spiral flow. He has noted zones of divergence over North Sea ridges, as indicated by flotsam-free lines on the sea surface, separating sectors of different wave roughness. Zones of convergence appeared over troughs, marked by foam and flotsam. He describes the North Sea ridges as frequently asymmetrical in cross section and locally sigmoidal in plan view, indicating that the spiral flow mechanism is operating in conjunction with residual ebb-flood current systems. SMITH (1968, 1969) has presented a theory for the maintenance of largescale sand ridges parallel to the long axis of rotary tidal currents. He notes that a longitudinal sand ridge can grow if rotary tidal currents have their maximum parallel to the ridge because the sand body is at a small angle of attack to the flow throughout most of the high-velocity part of the cycle. Under such circumstances, the cross-shoal component of the flow can be considered in two dimensional terms, as driven by the cross-shoal pressure gradient. He has developed (1968) a mathematical treatment of such a two-dimensional cross-flow. He points out that if inertia is considered, high velocity water will tend to move close to the bed as Marine GeoL, 12 (1972) 59-84

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the water flows over the up-current slope of a sand wave. Thus maximum bottom shear and erosion will occur on the up-current side, while deposition will occur on the surface of reduced shear, on the crest and lee slope. A reversing cross flow, coupled with an adequate sand supply, will thus permit a sand ridge to grow upward into the zone of wave agitation, until equilibrium between tidal construction and wave destruction is attained. Consideration of these mechanisms indicates that they are not mutually exclusive; indeed, since tidal residuals are ubiquitous in coastal waters, the first two cannot operate independently, and if the tidal stream is rotary, all three mechanisms must operate simultaneously. The cases described in the literature, however, are associated with tidal currents with surface maximums in excess of 1 knot. In order to apply these hypotheses to the False Cape Ridge system, it is necessary to consider the hydraulic regime of the area. Waves from the northeast predominate (18~o). Four percent of the waves are over 12 ft. high, 1 8 ~ are 6-12 ft. high, 59 ~ are 0-6 ft. high, and calm prevails 19 ~o of the time (unpublished data, Norfolk District Corps of Engineers). Wave-length figures are not available, hence it is difficult to predict the effect of these waves on the bottom. However, during SCUBA dives we have observed active, rippled bottoms, out to 30 ft. when surtace waves were less than 3 ft. high. Bottoms in the deeper parts of the study area (up to 60 ft.) were also rippled, but these deeper ripples were generally covered with a brown, algal film. Presumably the sea floor in all portions of the study area are mobilized during the 22 ~o of the time when waves are over 6 ft. high. Tidal currents are the next most obvious source of hydraulic energy input. In the course of ongoing hydraulic studies, we have recorded mid-tide velocities integrated from 0 to 6 inches off the bottom, of 0.01 to 0.20 knots. The velocity fields that we measured probably include components of permanent coastal current. While we are presently engaged in a quantitative assessment of the fairweather hydraulic regime at False Cape, it is by no means clear that this investigation will lead us directly to an hydraulic process model for the maintenance of the ridges. A major unresolved problem of shelf sedimentation that must be considered is the relative roles of fair weather versus storm hydraulic regimes. The fair-weather currents described above fall considerably below those reported in studies of active sand ridge topographies, but there is some reason to believe that much higher velocities may be attained in the study area during the passage of occasional intense storms, both of tropical and extra-tropical origin. These storms are accompanied by "storm tides"; abnormally high tides amplified by wind and wave set-up, and by the "inverted barometer effect," or upward bulge of water beneath the low pressure area. Tidal currents associated with them are likewise amplified. The normal tide range at Virginia Beach, Virginia is 4 ft., and the maximum surface flood tide velocities are 0.5 knot at the south end (tide and current tables, Atlantic Coast of North America, U.S. Coast and Geodetic Survey). The Norfolk District Marine Geol., 12 (1972) 59-84

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Corps of Engineers (unpublished manuscript) has predicted probable average occurrences of storm tide heights (above mean sea level) of 9 ft. once every 150 years, 10 ft. once every 3 centuries, and 11 ft. once a millenium. The associated currents would presumably be likewise augmented. In addition, wind drift currents ranging from one twentieth to one fiftieth of surface air speeds would, in shallow water, impinge on the bottom. Hurricane Camille crossed the Mississippi-Louisiana Coast with winds in excess of 200 miles per hour (TANNER, 1970); it would have generated surface currents of at least 4 knots. Thus, it is possible to suggest that False Cape topography is a response to the storm hydraulic regime, or perhaps a resultant response to the storm and fair weather regimes. SEDIMENTOLOGIC INFERENCES CONCERNING THE HYDRAULIC REGIME

There are a variety of clues in the geomorphic, stratigraphic, and petrographic data presented above, concerning the nature of hydraulic regime responsible for the False Cape Ridge system. The most readily exploited characteristic is probably the systematic variations in transverse symmetry of the first and second order ridges, as indicated in Fig.3. They are consistent with the spiral flow mechanism and suggest two major circulatory half-cell couplets are present, with descending limbs over the AB and BC troughs, and ascending limbs over B and C ridges, respectively. A second geomorphic clue lies in the fact that all but C ridge terminate in the shore face. The resulting channels open to the north, as is the case for all of the oblique trending, beach-tied ridge systems of the Atlantic Coast, regardless of the local direction of net littoral drift. Both tropical and extratropical storms tend in this region to move north, seaward of the shoreline. Thus the coast is regularly subjected to intense northeast winds which pile up water against the coast, and push it south along the shoreline (DuNN and MILLER, 1960; HAYES and BOOTHROYO, 1969). Hence it is possible to hypothesize that the beach-tied ridges are at least in part ebb channel systems, developed under storm conditions, in response to the phase lag between the stormamplified tidal wave moving westward across the shelf, and the "wind tide" moving south along the shore face. This hypothesis is most readily applied to A ridge and A trough. The most intense current in the area has been observed in this relatively steep-walled "slot," during a northeast wind which increased slowly through the day to 15-20 knots. Bottom currents of up to 0.4 knots, integrated from 0 to 6 inches off the bottom were recorded on a Bendix Q-18 meter. The crest of A ridge began to develop surf, and indeed during such periods must exhibit some of the response characteristics of a wave-maintained bar. The resulting discharge of water over the ridge would have added to the observed current. Flocks of sea birds appeared at the head of the "slot," apparently hunting fish being swept over the head of the ridge. The current was not appreciably affected by the turn of the tide. Marine Geol., 12 (1972) 59-84

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The grain size distribution tends to support both ot the suggested flow patterns. As noted the sympathetic variation of grain size with topography appears to be a resultant response to alternating storm and fair weather conditions. In the Southern Bight of the North Sea, HOVBOLT(1968) has shown that coarse lags develop in the troughs beneath descending limbs of helicoidal cells, while the ridges have finer sands on their crests. The False Cape ridges however, undergo long periods of quiescence, when their crests would be winnowed and would develop secondary lags. The regional textural gradient, with accretion of much fine sand south of the ridges since 1922, supports the ebb channel mechanism suggested above. The fine sand normally present on the shore face appears to be a fallout resulting from the winnowing action of the surf (see previous discussion). This zone has been truncated north of the base of A ridge by the strong currents of the "slot." Fine sand swept out of this trough has been redeposited on the broad shore face to the south. There may be a return of coarse sand along the crest of A ridge, since the predominant drift is to the north, and A ridge would thus serve as a natural groin during period of southerly wind, diverting littoral drift seaward along its crest. EVOLUTION OF THE RIDGE SYSTEM

Our topographic time series indicates that at least the outer ridge has moved landward more rapidly than the beach since 1922. This is probably a result of the shoreward asymmetry of wave surge on the ridge crests, resulting in shoreward spillover of sand. However, we believe that the distances involved are close to the practical limit of resolution of our technique, and in any case the time base (47 years) is too short to effectively establish secular coastal changes. Hence it is possible to support as an alternative hypothesis, that in general the beach has been retrograding more rapidly than the ridges. If so, the ridges would be slowly "let down" on the sea floor, as intermittent storms cut into the axes of the westwardmoving troughs. The ridges would then form an evolutionary sequence with A ridge presently being formed as an ebb channel, B and C ridges still maintained by spiral flow, and C ridge having finally broken its contact with the shore face, to become part of the shelf-floor ridge and swale topography. This model for the evolution of the ridges implies southward migration of the ridge system along the beach, by headward erosion of the troughs. Thus the ridge system would "sweep" south along the shore face and in the process, contribute to its retreat by a sort of plowshare action. The series of ridges trending obliquely north-northeast across the shelf from the study area (see Fig.l) may consist of ridges abandoned by the False Cape hydraulic system as it moved west and south along the retreating coast.

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Our interpretation of the relative and absolute movements of the False Cape ridge system is highly speculative, and must remain so, until very close stratigraphic control is obtained for the areas to the west and north. Indeed, its past movements may be ultimately unresolvable. However, the abundant evidence for trough erosion, and the indication that the troughs are moving landward, show that shoreface ridge systems are important agents of shore face erosion, and must be considered in any analysis of the evolution of the Holocene Atlantic Shelf. Bruun's rule of coastal erosion (a rise in sea level results in shore face erosion and an equal volume aggradation of the adjacent sea floor; BRUON, 1962; SCHWARTZ, 1965) seems to be partly applicable to the False Cape sector. The northern and central cross sections (Fig.4) reveal this relationship, which, however, breaks down for the southern cross section, in consequence of the net southward sediment transport. The classic concepts of modern shelf floor sediment facies (nearshore modern sediments versus offshore relict sediments; EMERY, 1968; SWIFT, 1970) have survived this scrutiny of the False Cape sector, but in modified form. The nearshore modern sands appear as the "surf fallout" draped over the shore face (see previous discussion). The "relict" sands of the shelf floor appear as the primary lag deposits of the troughs and the secondary lag deposits of the ridge crests. Flank sands are less readily assimilated into the modern-relict dichotomy since they are often indistinguishable from the nearshore modern sands. There is a good reason for this; they, too, comprise a "fallout"; here from winnowing of the crests. Both facies have both modern and relict attributes. Both are relict in the sense of being essentially autochthonous, and both are modern in the sense of having adjusted to the local hydraulic environment. Bruun's rule essentially describes the generation of the "relict" sediment facies; it is formed by aggradation of the inner shelf floor as the shore face is wasted away and retreats. It overlies a compound disconformity, resulting partly from subaerial weathering during the Late Wisconsin low stand of the sea, and partly from Holocene submarine erosion, as the shore face retreats. Our examination of the special case of shore face retreat at False Cape allows us to view the ridge system as a processing mill which treats the sediment and adjusts its textures to the storm-generated currents which intermittently sweep the sea floor. ACKNOWLEDGEMENTS

Our study was supported by the Coastal Engineering Research Center, contract DACW-72-69-C-0016, and National Science Foundation Grant GA-13837. The Office of Marine Geology, U.S. Geological Survey, provided funds for vibracoring. Dr. Robert Sheridan of the University of Delaware graciously consented to include our coring operation in his cruise (cruise E-18B-70) on board the Duke Research Vessel "Eastward". The "Eastward", and the cooperative oceanographic Marine GeoL, 12 (1972) 59-84

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p r o g r a m of D u k e M a r i n e L a b o r a t o r y is supported by N a t i o n a l Science F o u n d a t i o n G r a n t GB-17545. The Edgerton, G e r m e n h a u s e n a n d Grier pinger probe survey was conducted for us by Lt. K. R. Bitting of the United States A r m y Mobility E q u i p m e n t Comm a n d , F o r t Belvoir, Virginia. O u r study has benefited by discussions with D a v i d D u a n e of the Coastal Engineering Research Center; J o h n Schlee of the Office of Marine Geology, U.S. Geological Survey; J. C. Ludwick, Institute of Oceanography, Old D o m i n i o n University; Jack Pierce, Division of Sedimentology, S m i t h s o n i a n I n s t i t u t i o n ; John Sanders, B a r n a r d College; a n d J o h n Kraft a n d Charles Dill, University of Delaware. However, the interpretations expressed in this paper are the responsibility of the authors. REFERENCES BAGNOLD,R. A., 1941. The Physics of Blown Sand andDesert Dunes. Methuen, London, 245 pp. BRUUN, P., 1962. Sea level rise as a cause of shore erosion. J. Waterways Harbors Div., Am. Soc. Civ. Eng., 88: 117-130. BURGER, J. A., KLEIN, G. de V. and SANDERS,J. E., 1969. A field technique for making epoxy relief-peels in sandy sediments saturated with salt water. J. Sediment. Petrol., 39: 338-341. CASEY, H. J., 1935. Ober gescheibelewengung Mitteilungen der Wasserbau auf Schiffau, Berlin. Dissertation, Tech. Hochschule, Berlin, 87 pp. COOK, D. O., 1969. Sand Transport by Shoaling Waves. Thesis, Univ. Southern California, Los Angeles, Calif., 148 pp., unpublished. CORNISH, V., 1901. On sandwaves in tidal currents. Geogr. J., 18: 170-201. CURRAY, J. R., 1969. Shore zone sand bodies: barriers, cheniers and beach ridges. In: D. J. STANLEY (Editor), The New Concepts of Continental Margin Sedimentation. American Geological Institute, Washington, D.C., pp.JC-2-1-JC-2-19. DUNN, G. E. and MILLER, B. I., 1960. Atlantic Hurricanes. Louisiana State Univ. Press, Baton Rouge, La., 377 pp. EL ASHRY,M. T. and WANLESS,H. R., 1968. Photo interpretation of shoreline changes between Capes Hatteras and Fear (North Carolina). Marine Geol., 6: 347-380. EMERY, K. O., 1968. Relict sediments on continental shelves of world. Bull. Am. Assoc. Pet. Geologists, 52: 445~,64. FOLK, R. L. and WARD, W. C., 1957. Brazos River Bar: A study in the significante of grain size parameters. J. Sediment. Petrol., 27: 3-27. HARRISON,W., BREHMER,M. L. and SXONE,R. B., 1964. Nearshore tidal and non tidal currents, Virginia Beach, Virginia. Coastal Eng. Res. Cent. Tech. Rep., 5: 1-20. HAYES, M. O. and BOOTHROYD,J. C., 1969. Storms as modifying agents in the coastal environment. In: M. O. HAYES (Editor), Coastal Environments, Northeast Massachusetts and New Hampshire. Coastal Res. Group Contrib. No. l, Geol. Dept., Univ. Massachusetts, Amherst, Mass. pp.245-246. HOUaOLT, J. J. H. C., 1968. Recent sediment in the southern bight of the North Sea. Geol. M~inbouw, 47: 245-273. INGRAM, R. L., 1954. Terminology for the thickness of stratification and parting units in sedimentary rocks. Geol. Soc. Am. Bull., 65: 937-938. JORDAN, G. F., 1962. Large submarine sandwaves. Science, 136: 839-848. KENNEDY, R. J. and FULTON,J. F., 1961. On the effect of secondary currents upon the capacity of a straight, open channel. A S M E - E I C Hydraul. Conf. Pap. 61-EIC-I : 7 pp. KEULEGAN,G. H., 1948. An experimental study of submarine sand bars. Beach Erosion Board, Tech. Rep., 3:42 pp. Marine Geol., 12 (1972) 59-84

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KOLAR, V., 1955. Spiral Motion of Fluids. Rozpravy Ceskoslovenske Akademic Ved., Prague, 118 pp. LANGFELDER, J., STRAFFORD,D. and AMEIN, M., 1968. A Reconnaissance of Coastal Erosion in North Carolina. Dept. Civil Engineering, North Carolina State University at Raleigh, Raleigh, N.C., 127 pp. LUDWICK, J. C., 1970. Sandwaves and tidal channels in the entrance to Chesapeake Bay. Va. J. Sci., 21: 178-184. MILLIMAN,J. D. and EMERY, K. O., 1968. Sea levels during the past 35,000 years: Science, 162: 1121-1123. MooDY, D. W., 1964. Coastal Morphology and Processes in Relation to the Development of Submarine Sand Ridges off Bethany Beach, Delaware. Thesis, Johns Hopkins University, Baltimore, Md., 167 pp., unpublished. NEMENYI, P. F., 1946. Discussion of transportation of suspended sediment by water. Trans. Am. Soc. Civ. Eng., 111:116. NEWMAN, W. S. and MtmSART, C. A., 1968. Geology of the Wachapreague Lagoon, Eastern Shore, Virginia. Marine GeoL, 6: 81-105. OAKS, JR., R. Q., 1964. Post-Miocene stratigraphy and morphology, outer Coastal Plain, southeastern Virginia. Off. Nay. Res. Tech. Rep., 5: 1-240. OAKS, R. Q. and COCH, N. K., 1963. Pleistocene sea levels, southeastern Virginia. Science, 140: 979-983. PASSEGA,R., RIZZINI, m. and BORGHETTI,G., 1967. Transport of sediment by waves, Adriaict coastal shelf, Italy. Bull. Am. Assoc. Pet. Geologists, 51: 1304-1319. PAYNE, L. H., 1970. Sediments and Morphology of the Continental Shelf off Southeastern Virginia. Thesis, Columbia Univ., New York, N.Y., 70 pp., unpublished. PIERCE, J. W. and COLQUHOUN,D. J., 1970. Configuration of Holocene primary barrier chain, outer Banks, North Carolina. Southeast. Geol., 11." 231-236. PRANDTL, L . 1952. Essentials of Fluid Dynamics. Blackie, London, 432 pp. ROBINSON,A. H. W., 1966. Residual currents in relationship to shoreline evolution. Marine GeoL, 4: 57-58. SANDERS, J. E., 1962. North-south trending submarine ridge composed of coarse sand off False Cape, Virginia. Bull. Am. Assoc. Pet. Geologists, 46:278 (abstr.). SANFORD, R. B. and SWIFT, D. J. P., 1971. Comparison of sieving and settling techniques for size analysis, using a Benthos Rapid Sediment Analyser. Sedimentology. 17: in press. SCHWARTZ, M. L., 1965. Laboratory study of sea level rise as a cause of shore erosion. J. GeoL, 75: 76-92. SCOTT, T., 1954. Sand movement by waves. Beach Erosion Board, Tech. Memo., 48:37 pp. SHEN, H. W., 1961. A study on meandering and other bed patterns in straight alluvial channels. Univ. Calif. Berkeley, Water Res. Cent. Contrib., 33:688 pp. SHEPARD, F. P., 1963. Submarine Geology. Harper and Row, New York, N.Y., 567 pp. SHIDELER,G., JOHNSON,G., SWIFT,D. J. P. and HOLLIDAY,B., in press. A standard section for the submarine Quaternary of the inner Virginia shell GeoL Soc. Am. Bull. SMITH, J. D., 1968. The Dynamics of Sand Waves and Sand Ridges. Thesis, Univ. Chicago, Chicago, Ill., 63 pp., unpublished. SMITH, J. D., 1969. Geomorphology of a sand ridge. J. Geol., 77: 39-55. SWANSON, V. E. and PALACAS,J. G., 1965. Humate in coastal sands of northwest Florida, U.S. Geol. Surv. Bull., 1214-B: 34 pp. SWIFT, D. J. P., 1968. Shore face erosion and transgressive stratigraphy. J. Geol., 39: 18-33. SWIFT, D. J. P., 1969. Processes and products on the inner shell In: D. J. STANLEY(Editor), The New Concepts of Continental Margin Sedimentation. American Geological Institute, Washington, D.C., pp.DS-4-1-DS-4-46. SWIFT, D. J. P., 1970. Quaternary shelves and the return to grade. Marine GeoL, 8: 5-30. SWIFT, D. J. P., STANLEY,D. J. and CURRAY, J. R., 1971a. Relict sediments: a reconsideration. J. Geol., 79: 322-346. SWIFT, D. J. P., SANFORD,R. B., DILL, JR., C. E. and AVIGNONE,N. F., 1971b. Textural differentiation on the shore face during erosional retreat of an unconsolidated coast, Cape Henry to Cape Hatteras, western North Atlantic Shelf. Sedimentology, 16: 221-250. Marine Geol., 12 (1972) 59-84

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