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JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 34, No . 4 , PP . 86 4-8 74FIGS. I -5 , DECEMBER, 1964

S I G N I F I C A N C E O F S K E W N E S S I N R E C E N T S E D I M E N T S ,W E S T E R N P A M L I C O S O U N D , N O R T H C A R O L I N A 1

D A V I D B . D U A N E 2Detro i t , Michigan 48226A B S T R A C T

A p p l i ca t i o n o f t h e m e t h o d o f m o men t s t o s i ze d i s t r ib u t i o n ch a rac te r i s ti c s o f R ecen t sed i men t s i n w es t e rnPam hco Sound, Nor th Ca rol ina , and v ic in i ty indicate that th e s ign of skewness i s envi ron me ntal ly sensi t ive . Awinnowing act ion produced by flu id media i s the mechanism producing negat ive skewness. Sedim ents of beaches,t h e l i t t o ra l zo n e , and tidal inlets are negat ively skewed. S ediments from a shel tered lagoon fi ll ing wi th sedi -men t a ry ma t e r i a l a re d o m i n an t l y p o s i t i v e ly sk ew ed . W h ere w i n n o w i n g ac t io n can b e sh o w n t o b e o p e ra t i v ein term i t ten t ly , sediments are characterized by local d i fferences in s ign of skewness. The sensi t iv i ty of skewnessto several environments in the western Pamlico Sound area s t rongly supports s imi lar conclusions reached byMason and Folk (1958) and Friedman (1961) who studied d i fferent areas.Pre l i mi n a ry s t u d ie s o f t h e co mp ar i son o f re su l t s o b t a i n ed u s i n g t h e same d a t a i n d i f fe ren t fo rmu l ae i n d i ca tethe s ign of skewness i s reproducib le. As a reproduc ib le p aram eter, skewness s ign should be d i rect ly appl icableto s tudies of Recent sediments e lsewhere , and used wi th o ther cri teria i t should be valuable in the in terpr eta t ionof paleoenvi ronme nts , part icu larly in those sediments w here effects of d iagenesis are negl ig ib le or non-ex isten t .

INTRODUCTIONP r i m a r y s t r u c t u r e s i n s e d i m e n t s a n d f a c ie sr e l a t io n s o f v a r i o u s l i t h o t o p e s a r e v a l u a b l e t o o l su s e d i n i n t e r p r e t i n g s e d i m e n t a r y e n v i r o n m e n t sa s r e c e n t ly s h o w n b y L a n e ( 1 9 6 3) . W h e r e i t i sn o t p o s s i b le t o v i s u a l l y e x a m i n e s t r u c t u r e s a n df a c ie s r e la t i o n s h i p s o f s e d i m e n t s t h a t c r o p o u to v e r m a n y t e n s o f m i l e s i t is n e c e s s a ry t o s e a r c hf o r o t h e r c r i te r i a b y w h i c h t o i n t e r p r e t s e d i m e n -t a r y e n v i r o n m e n t s . A b i l i t y to d i s t in g u i s h e n v i -r o n m e n t s h a s l o n g b e e n r e c o g n i z e d t o b e o f e c o -n o m i c i m p o r t a n c e p a r t i c u l a r ly i n t h e p e t r o l e u mi Man uscrip t re ceived February 14 , 1964; rev isedMarch 27 , 1964.U . S . A rmy Co rp s o f En g i n ee rs , Lak e Su rv ey .This paper i s a condensat ion of a report to R. H.Benson, Unive rsi ty of Kansas, who as principal in-v es t i g a t o r o f N a t io n a l Sc ien ce F o u n d a t i o n G ran t N o .G-7152 arranged for and d i rected field operat ions.He, S. Grossman, and D. Rochna assis ted in the col -lect ion of samples and fie ld data . H. A. Ire land of theU n i v e rs i t y o f K an sas su p e rv i sed t h e l ab o ra t o ry w o rkfor the orig inal report and together w i th R. H . Bensoned i t ed t h i s man u sc r i p t . G u i d an ce i n p ro g rammi n gdata for s ta t i s t ica l analyses was provided by F. W.Pre s t o n, U n i v e rs i t y o f K an sas ; fu n d s w e re mad eav a i l ab le b y t h e U n i v e rs i t y o f K an sas Co m p u t a t i o nCen t e r G ran t N o . 0 2 5 4 . O t h e r K an sas U n i v e rs i t ygrants an d the Shel l Oi l Com pany F el lowship pro-v ided funds for expenses during various s tages of theorig inal s tudy.Memb ers o f t h e In s t i t u t e o f F i sh e r i e s o f t h e U n i -versi ty of North Carol ina freely exchanged ideas andfurnished certa in equipment .Th e Co m man d an t o f t h e Co as t G u a rd S t a t i o n a tFo r t Maco n , N o r t h Ca ro l in a , an d t h e Ch i e f o f S t af f o f

t h e M ar i n e Co rp s A i r S t a ti o n a t Ch e r ry Po i n t , N o r t hCarol ina p laced personnel and equipment a t our d is-posal.The author expresses s incere thanks to a l l theabove persons.

i n d u s t r y . C o n s e q u e n t l y t h e s e a r c h f o r c r i t e r iac a p a b l e o f d if f e r e n t i a t in g s e d i m e n t a r y e n v i r o n -m e n t s f r o m s m a l l i s o l a t e d s a m p l e s , e i t h e r o u t -c r o p o r s u b s u r fa c e c o r e s , a t t r a c t s m u c h o f th ea t t e n t i o n o f s e d i m e n t a r y p e t r o l o g is t s a n d p h y s i -c a l s t r a t i g r a p h e r s . I n r e c e n t p a p e r s F o l k a n dW a r d ( t 9 5 7 ) , M a s o n a n d F o l k ( 1 9 58 ) , a n d F r i e d -m a n ( 19 6 1) h a v e s t a t e d t h a t g r a i n s i z e p a r a m e -t e r s , p a r t i c u l a r l y s o r t i n g , s t a n d a r d d e v i a t i o n ,s k e w n e s s , a n d k u r t o s i s , w e r e u s e f u l c r i t e r i a f o rd i s t i n g u i s h i n g b e t w e e n b e a c h , d u n e , a n d r i v e rs a n d s. H o w e v e r , t h e t h e s i s t h a t g r a i n s i ze p a r a m -e t e r s a r e e n v i r o n m e n t a l l y s e n s i t i v e i s s e v e r e l yq u e s t i o n e d b y S h e p a r d a n d Y o u n g ( 1 96 1 ) w h oc o n c l u d e d t h a t g r a i n s i ze p a r a m e t e r s w o u l d n o tp e r m i t d i s t i n g u is h i n g b e t w e e n b e a c h a n d d u n es a n d s .

I n a b r o a d l y b a s e d s t u d y o f th e R e c e n t s e d i -m e n t s i n t h e w e s t e r n p a r t o f t h e P a m l i c o S o u n da r e a o f N o r t h C a r o l i n a , t h is a u t h o r h a s f o u n ds e v e r a l c h a r a c t e r i s t i c s o f t h e s e d i m e n t s u s e f u l i nh e l p i n g t o d is t i n gu i s h v a r i o u s s e d i m e n t a r y e n v i -r o n m e n t s o f t h a t r e g i o n ( D u a n e , 1 9 62 ) . A m o n gt h e u s e f u l c r i t e r i a i s t h e s i g n o f s k e w n e s s . I n v i e wo f t h e c o n t r o v e r s y c o n c e r n i n g t h e u s e f u l n e s s o tg r a i n s iz e p a r a m e t e r s a s e n v i r o n m e n t a l l y s e n s i -t i v e c r i te r i a , t h e a u t h o r i s p r e s e n t i n g h i s f i n d i n g so f t h e s i g n i f i c a n c e o f g r a i n s i z e p a r a m e t e r s o f t h eR e c e n t s e d i m e n t s o f a b a r r e d e s t u a r i n e a n dl a g o o n a l s y s t e m o n t h e N o r t h C a r o l i n a c o a s t i na n e f f o r t t o a i d c o n s t r u c t i v e l y i n t h e r e s o l u t i o n o ft h e c o n t r o v e r s y .DESCRIPTION OF AREA

P a m l i c o S o u n d w h i c h is a p p r o x i m a t e l y 6 0m i l e s l o n g w i t h a m a x i m u m w i d t h o f 26 m i l e s,r e p r e s e n t s t h e l a r g e s t e m b a y m e n t f o r m e d b e h i n d

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SIGNIFICANCE OF SKEWNESS IN RECENT SEDIM ENTS 865

7 7 "

Woehington

Bern b u d

I s lom~

CherryP o i n t

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I ] Shockle oed ~ 1 ~ 7 7

l l IFIG. 1.--Area of investigation showing location of sample collecting stations. Map derived from basemap of N.S.F. grant (;-7152, and published with permission of R. H. Benson, principal investigator.

the barrier beaches present along the AtlanticCoast of the United States from Long Island toFlorida. Specifically the stu dy area comprises thearea in, and adjacent to, the western part ofPamlico Sound (hereafter referred to as westernPamlico Sound) and the much smaller CoreSound, shallow bodies of water separated fromthe Atlantic Ocean by the "Outer Banks," aseries of low-lying, narrow elongate barrierbeaches and islands (Fig. 1). From northeast tosouthwest these "Outer Banks" are namedOeracoke Island, Portsmouth Island, and CoreBanks. Shackleford Banks extends in a westerlydirection and is situated near the southern end ofCore Banks and is separated from it by narrowBarden Inlet.

In the western Pamlico Sound area three inletsbreach the northeast and southwest trendingbarrier beach. The largest, and only permanentone of the inlets is Ocracoke, which lies betweenPortsmouth and Ocracoke Islands. Swash andDrum Inlets are semi-permanent. During largestorms additional inlets may form but such aretemporary features eventually closed by littoraldrift aided by other sedimentation processes.Water oscillating through these inlets under theinfluence of flood and ebb tides moves clasticmaterial first in one direction and then in theopposite direction depositing sediments in theshape of a delta on both the sound and oceansides of the opening. In this manner the tidaldeltas are formed.

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866 D A V I D B . D U A N EThe Neuse and Pamlico River systems whichfeed western Pamlico Sound drain the piedmontarea of North Carolina and the Coastal Plain.Though the watershed is large and mainly re-sponsible for the fresh-water acqusitions of west-ern Pamlico Sound, it annually contributes lessfreshwater than the volume of the Sound (Roe-

lofs and Bumpus, 1953). Volume of sedimentcontributed to the Sound by the two river sys-tems is low, and composed predominantly ofclay-size materi al (Duane, 1962).Sediment of the study area ranges from clay-size material to medium sand with clay-sizematerial being deposited in parts of the estuaries,the deeper parts of the Sound, and to some ex-tent, the marshes. Fine to medium quartz sand isthe dominant sediment type in the westernPamlico Sound area, comprising the bulk of thebeach sediment and a large proportion of thesediment in the Sound. Sands of the beaches arebeing carried into the adjacent sounds resultingin a gradual decrease of water depth. Stronglunar, solar, and wind-tides then concentratethese sediments in marsh areas creating shallowflats (Benson, et al., 1961).Terrain adjacent to Pamlico Sound is low inrelief and is characterized by swamps and saltmarshes as well as drowned valleys. This low-lying area between the Atlantic Ocean and thePiedmont is covered by Recent, Pleistocene, andMiocene sediments composed of sands, clays,marls, and gravels which contribute to the sedi-ment in the study area. However, the source ofmost of the sediment in western Pamlico Soundis from the direction of the barrier beaches andfrom the barrier itself (Duane, 1962). Sand forthe barrier beaches is derived from southwesterlylittoral drift carrying sand from the shoals offCape Hatteras and from coarse debris carriedfrom the continental shelf by storms and strongwinds.

PROCEDURESSamples for this study were gathered in con-junction with samples for a study of the micro-fauna by Grossman (1961) and Grossman andBenson (1963), and a physiographic and sedi-ment study of Core Banks by Rochna (1961).(Samples on the south shore of the Neuse Estu-ary were supplied by J. DuBar, then of the Uni-versity of Houston.) Sample stations in westernPamlieo Sound and contiguous water bodieswere spaced to gather material from environ-ments ranging from fresh-water to normal-marine, and from areas where the sediment type

ranged from mud to sand (Fig. I). Both core andgra b samples were collected. Where the waterwas too deep, or the sediments too coarse for

retention or penetration by the simple handoperated coring apparatus, a Peterson grabsampler was used. Samples of the barrier barwere made perpendicular to the long axis of thebar at intervals from the littoral zone just sea-ward of the surf zone to the lagoon behind thebarrier. To minimize operator error and to stand-ardize data as much as possible, several samplescollected by Rochna (1961) were reanalysed bythe present author before being included in thisstudy.Grain size determinations of the samples weremade using ~-phi intervals. Raw samples wererinsed in distilled water and dispersed by addinga solution of Calgou. Aliquot s of the - 62 micron(4 phi) fraction and the -3.9 micron (9 phi)fraction were obtained and in a few instanceswhere the proportion of sand size material wasless than 95 percent of the whole sample a com-plete pipette analysis at ~-phi intervals wasmade. The author questions the validity of sizeanalyses of the fine fraction obtained by pipettemethod. Whitehouse, et al . , (1959) showed thatwhen clay minerals settle out of saline watersthey do so as aggregates, not individual grains,adding that aggregation is a reversible process.Therefore, in the process of complete disaggrega-tion of Recent sediments prior to pipettlng orsieving, the size characteristics of the originalfine particles that settled out of suspension maybe altered with the result that spurious datawould be obtained leading to inaccurate conclu-sions. For this reason few analyses were made ofsediments containing more than 5 percent finesof silt and clay.Sand size material obtained after calculationand removal of silt and clay fractions was sievedon Tyler screens with openings at -phi inter-vals from - 1 phi (2.0 mm) to 4 phi (.062 mm).Fractions retained on the sieves were weighedand recorded and combined where appropriatewith data from pipette analyses.The data accumulated were programmed onan IBM 650 computer to give mean size, stand-ard deviation, and skewness, by the method ofmoments. The open end in the mome nt measureswas arbitrarily closed at g-phi interval beyondthe last measured interval. In the computed datathe amount of sediment in this interval wasalways less than 5 percent, rarely greater than 3percent, and usually less than 1 percent.Determination of the values of the parametersreported here were made from the followingformulae:

mean, Xpk~ = 1/100 ~fMz,h~where X#i is mean grain size in phi units, f is thefrequency of the grains present in the various

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S I G N I F IC A N C E O F S K E W N E S S I N R E C E N T S E D I M E N T S 867grades, and ]zroi is the midpoin t in phi units ofeach grade; standard deviation, ova =

(M~hl -- X~,A,:)= i t ,- o o )and skewness, S ~ = o t a / 2 , (Krumbein and Pettl-john, 1938), where

Z f ( M n k l - - Xvhi) 3~ = ( 1 / l O O ) Ophl aR E S U L T S

A total of 136 samples were analysed in thisstatistical study. The mean of these samplesranges from a maximum of 0.7 phi (.62 mm) to aminimum of 6.2 phi (.014 mm) with the greatestdensity of readings occurring from 2.0 phi (.25mm) to 2.7 phi (.155 mm).The frequency distribution curve plotted forthe mean sizes of the sands, though polymodal,approaches that of a normal Gaussian curve, butis slightly skewed to the fine, or positive, side ofthe major mode. Areal distribution is randomwith the exception that samples in the littoralzone are slightly coarser than adjacent beacheswhich are themselves slightly coarser than sedi-ments in Core and Pamlico Sounds.Standard deviation values range from .32 phito a maximum 2.45 phi with 84 percent of thesamples analysed having a standard deviation ofless than 1 phi.Skewness values range from --1.59 to +1.60with 60 per cent of the samples being skewedtoward the coarser sizes (negative skewness).Most of the sediments analysed were stronglyskewed; only 15 percent of the samples werenearly symmetrical (--.10 to +.10).Standard deviation and phi-mean seem tohave no definitive distributional pattern orsignificance. When those parameters are plottedagainst one another, depth, and skewness, thepoints on the resulting graph are randomly dis-tributed. When plotted according to environ-ment, identifying samples from the tidal deltas,the open sound (unrestricted western PamlicoSound), estuaries, the littoral zone, and lagoon(Core Sound), the patter n is much th e same.This randomness reflects the general uniformityof the sediments in the western Pamlico Soundarea.Skewness, plot ted in the form of scatter dia-grams shows no obvious trends. However, whenskewness sign is plotted in map form accordingto environments a trend can be seen to emerge.Basically, sediments in the littoral zone, thebeaches, and the tidal inlets are negativelyskewed; sediments in Core Sound (the narrowelongate sound immediately back of the barrier

S IGN OF SKEWNESS0 P o l ; t I r e N I g o t b / e

. . . . I Z F o o t D e p t h . .

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' ~ . . " < i i i i : :: : . . : : : :~ . . . . ' . . . . . . . . . . .. . . . . . - . . . . . . . . .. .. .. .. .

: ' . :. . . . . . . . . . ,q . . . o ! "- . . " P / k"' " : ~ o " o % * ' ' . - : - ' ~ , ~ "

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FIG. 2.--Areal distribution of skewness. Note asso-ciation of negative skew (solid circles) with inlets andbeaches. Size and absolute position of the circles repre-sentkng stations have been altered somewhat forpresentation, but relative positions remain the sameas in Fig. 1.beach) are dominantly positively skewed whilesediments in Pamlico Sound are both negativelyand positively skewed (Fig. 2).

At the time samples /or this study were col-lected (June-July, 1959), dunes were not plenti-ful along the "Outer Banks" from OcracokeInlet south to Cape Lookout but some did occuralong the southern shore of Cape Lookout and afew remnants of dunes occurred locally on thesound side of Portsmouth Island. Dunes sampledat Cape Lookout (Sample 79b) and northeast ofthe study area at Cape Hatteras, were found tobe positively skewed.I N T E R P R E T A T I O N A N D S I G N I F I C A N C E

O F S K E W N E S S"Outer Banks"

As a result of their study of the sediments onMustang Island, Texas, Mason and Folk (1958)concluded that skewness is sensitive to environ-ment. They were able to differentiate beachesnearest the Gulf (negatively skewed) from dunes

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868 DA VID B. DUA NEand aeolian flats (positively skewed). Friedman(1961) concluded that for the most part dunesands are positively skewed whereas beach sandsare usually negatively skewed. The water turbu-lence caused by incoming waves and outgoingwash, characteristic of beach environments,winnow away the fines and skew the frequencycurve to the coarser sizes, or negative side. Dat apresented here describing the skew of littoralzone and beach sediments as negative, and dunesas positive, is considered evidence corroboratingFriedman's conclusions and expanding beyondprovincial boundaries the findings of Mason andFolk.Back of the beach toward the dunes on Mus-tang Island, Mason and Folk (1958) reported thesign of skewness to change from negative topositive. Back of the beach along the "OuterBanks" no such trend was found--perhaps be-cause the wind there is an erosive agent and nodunes have accumulated. It is judged that backof the beach the wind is continuing the winnow-ing process begun in the littoral zone, conse-quently the sediments on the barrier bar lackingfines will remain negat ively skewed.Samples 45b and 45c, on the southwestern endof Portsmouth Island are positively skewed, andtherefore are seemingly anomalous. Two possibleexplanations exist to account for this anomaly.One is that this is the region of the relict duneswhere the influence by dunes now destroyed onthe characteristics of the present sediments isstill evident. "Inherited" characteristics is thereason given by Friedman (1961) to account forthe anomalous positive skewness of beach sandon Padre Island, Texas. A second possible expla-nation is also predicated on the physiography ofPortsmouth Island. The southern region of theisland is generally awash a t high tide with waterswirling through little breaks in the berm andspreading out laterally in the back beach area.This swash is likely to be carrying the finer-grained sediment kept in suspension in the turbu-lent water of the surf zone. (Mean size of Sample45c, back beach, is 2.0 phi; Sample 45d, littoralzone, is 1.5 phi). Most of the swash remains inthe back beach area covering the sand withabout an inch of water which may soak in, butdoes not return to the breaker zone. Conse-quently a new accumulation of the finer-grainedsediment adds a tail a t the fine-grained end of thefrequency distribution curve skewing the curveto the right (positive skew). Of these two pos-sible explanations for the situation as it exists onPortsmouth Island, the author favors the latter.

Tidal DeltasIn the tidal delta of Ocracoke Inlet a fewpositively skewed samples exist in an environ-

ment characterized by sediments with negativeskew. Samples 77 and 78, due south from Ocra-coke Inlet, are positively skewed but also haveanomalously high values of standard deviationand mean size when compared to other sedimentsin the delta region. The positively skewed char-acter of these sediments is a real and significantdifference. These two samples lie to the south ofthe delta and are not under the direct influenceof the current surge associated with tidal flow. Itis possible that some of the finer grained material

carried southeastward along the axxs of the del tacomes under the influence of the southwest warddirection of longshore drift with a vector result-ant current carrying the finer grained sedimentsouthward. That this is the ease is indicated bythe fact that the clay-silt content of Samples 77and 78, 25 and 76 percent respectively, is anoma-lously high to the clay-silt content of other sam-ples in the tidal delta. No satisfactory explana-tion can be found to explain the anomalous posi-tive skew of Sample 73, nor for Samples 52, 59,and 61 which are on the sound side of OcracokeInlet. However, in spite of the above unexplain-able anomalies existing at Ocracoke Inlet, thedominant character of skewness of the sedimentsin the tidal delta environment is negative. Sam-ples from Barden Inlet, at the southern end ofCore Sound, and Drum Inlet are uniformlynegatively skewed.The association of negative skewness in thetidal delta environment coupled with similarlyskewed curves for samples from the litto ral zoneand the barrier beach indicates that sedimentswill have negative skewness where sedimentaryenvironments can supply an active winnowingagent. In the case of the tidal inlets the winnow-ing agents are judged to be tidal currents result-ing from solar and lunar phases as well as stormsand strong winds blowing for prolonged periodsof time. For Ocracoke Inlet the U. S. Coastal Pilot(Section D, 1948) predicts veloci ties of 52 era /sec. (1 knot) during flood tide and 41.5 cm/sec.at ebb tide. In the tidal channels Roelofs andBumpus (1953), using an Eckmaa CurrentMeter at depths of 9 and 30 feet, measured amaximum flood velocity of 137 cm/sec, and amaximum ebb velocity of 134 era/see. Averagevelocities measured were 92 and 80 cm/sec.respectively for flood and ebb. Hjulstr6m's dia-gram (1955) shows that the average veloci ty atflood-tide would be capable of eroding materialapproximately -2.8 phi (7 mm) in diameter, yetthe mean size of four samples (56, 66, 68 and 76)at the narrowest point in Ocracoke Inlet is 1.7phi (.3 mm). The max imum depth of water in thestudy area is 40 feet and occurs in OcracokeInlet, unquestionably the result of the erosivepowers of the tidal currents. That depth, which

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SIGNIFICANCE OF SKEWNESS IN RECENT SEDIMENTS 869will fluctuate somewhat depending on local con-ditions, probably represents the critical pointbetween current velocity and part icle size whichdetermines by-pass, erosion, or deposition. Thesestrong currents, effective in moving clastic mate-rial back and forth through the inlets, depositcoarser particles in the immediate vici nity of theinlet producing the shoals on both ocean andsound sides of the inlets and carry the "fines" tothe sound and ocean fringes of the delta andbeyond. Th:s are produced the negativelyskewed tidal delta sediments.

Sounds and Estuary MouthsIf a winnowing process as postulated for thelittoral zone, beaches, and tidal deltas is calledupon to explain the pattern of the sign of skew-ness for sediments in Core Sound, western Pam-lico Sound, and the mouths of the estuaries, thedistribution of negatively and positively skewedsamples should be a function of wave actionand/or currents. As Roelofs and Bumpus (1953)report finding no bottom currents within the

center of western Pamlico Sound and only aninsignificant hydraulic gradient in the estuaries,responsibility for winnowing action must lie inwave action.Turbidity of water and compressed nature ofwave trains observed in western Pamlico Soundindicate the wave form is active down to thesedlment-water interface. According to Hjul-strtim's diagram (1955) the minimum velocityneeded for erosion, 10.8 cm/sec., is capable oferoding a particle having a diameter of 1.09 phi(.48 ram). Using a velocity of 10.8 cm/sec, andassuming a wave period of 10 seconds (obser ved),the orbital diameter of a water particle at adepth of 6 feet is 1.12 feet (34.4 cm). Assumingfurther th at 6 feet is between one-ninth and two-ninths of the wave length, the surface wave willhave a height of 3 feet and a length of approxi-mately 36 feet (Bigelow and Edmondson, 1947).Calculated wave characteristics necessary forerosion are within the limits of wave phenomenaobserved during field operations. Once set inmotion, these medium-size sand particles could,by impact, force smaller particles into the milieuof the moving water. Smaller particles wouldthen remain in suspension until carried to deeperwater or until the motion of the water subsidedsomewhat permitting them to settle.Mean diameter of most of the analysed sedi-ments in western Pamlico Sound is 2 phi (.25mm). Consequently a velocity only slightlygreater than the minimum (10.8 cm/sec.) wouldbe required for erosion and subsequent trans-port. An orbital velocity of 14.4 cm/sec, at adepth of 5 feet, calculated from observed wavephenomena, is in excess of the veloci ty required

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FIG. 3.--Graph of shewness versus depth. Notethe slightly denser grouping on the negative side ofthe graph for samples from depths less than 6 feet.to lift fine sand from the bottom of the Sound.The author believes that a winnowing action asdescribed here is responsible for producing thenegatively skewed sediments in the shallows ofthe sounds and estuaries.When plotted against depth, skewness valuesare not preferentially grouped although there is asomewhat denser grouping on the negative sideof the graph for depths less than 6 feet (Fig. 3).Depth then cannot be considered as a singledominant factor for there are more negative thanpositive values of skewness for areas where wateris greater than 30 feet deep. Most of the sedi-ments in these deep areas are associated with th etidal delta of Ocracoke Inlet and its strong tidalcurrents.In western Pamlico Sound and at the entranceto the estuaries positively skewed sediments onthe shoals are about as abundant as those nega-tively skewed, but there are considerably morepositively skewed sediments in the deeper partsof the region and just off the shoals in deeperwater (Fig. 2). Strength of winds, direction,fetch, and duration will all act to influence thesize of waves which directly will effect the depthof wave action. The wind diagram (Fig. 2)graphically portrays the direction and frequencyof winds of varying velocities. Noting the varia-tion in wind character, it is an obvious conclu-sion that size of waves will vary, as will thereforethe depth at which the winnowing process is

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8 7 0 DAV ID B, D UA NEa c t i v e w h i c h u l t i m a t e l y a f f e c t s t h e s i g n o f s k e w -hes s .U n d e r t h e i n f l u e n c e o f a s o u t h e r l y w i n d , w a v e sw o u l d b e m o s t e f f e c t i v e g e o l o gi c a l a g e n t s o n t h en o r t h e r n s h o r e o f w e s t e rn P a m l i c o S o u n d , a n dl e a s t e f f e c t i v e o n t h e s o u t h e r n s h o r e i n t h e l e e o ft h e l a n d a n d i n t h e d e ep s . N o r t h o f B r a n t I s l a n dS h o a l i n t h e n o r t h e r n p a r t o f w e s t e r n P a m l i c oS o u n d a c o n c e n t r a t i o n o f n e g a t i v e ly s k e w e ds e d i m e n t s o c c u rs . D u r i n g t h e t i m e t h e s e d i m e n t sw e r e c o l l e c t e d ( J u n e - J u l y , 1 9 5 9) t h e w i n d w a s ,f o r t h e m o s t p a r t , f r o m t h e s o u t h , a d i r e c t i o nw h i c h p r o v i d e d u n l i m i t e d f e t c h . T h e e f f e c t i v e -n e s s of t h e w i n d a s a d r i v i n g f o r c e i s i n d i c a t e d b yt h e s a l t - w e d g e s h o w n o n F i g . 4 . I t i s t h e r e f o r e

c o n c l u d e d t h a t t h e o v e r a l l d i s tr i b u t i o n o f t h es i g n of s k e w n e s s i n w e s t e r n P a m l i c o S o u n d i s af u n c t i o n o f w i n n o w i n g p r o c e s s e s , t h e e f f e c t i v e -n e s s o f w h i c h i s a f u n c t i o n o f w i n d , w a v e s , a n dw a t e r d e p t h . I t i s a l s o n e c e s s a r y t o c o n c l u d e t h a tt h e s i g n o f s k e w n e s s a t a n y o n e l o c a t i o n i n t h eo p e n s o un d a n d e s t u a r y m o u t h s m a y v a r y f r o md a y t o d a y .C o r e S o u n d i s m u c h s h a l l o w e r t h a n w e s t e r nP a m l i c o S o u n d . I t is m o r e p r o t e c t e d f r o m w i n da n d s w e l l t h a n w e s t e r n P a m l i c o S o u n d , r e s u l t in gi n m o r e p e r i o d s o f calm w a t e r a s w e l l a s w a v e st h a t a r e g e n e r a l l y s m a l l e r a n d m o r e i r r e g u l a r .P e r i o d s o f c a l m p e r m i t t i n g m a t e r i a l i n s u s p e n -s i o n t o s e t t l e o u t , t o g e t h e r w i t h a l e s s e f f e c t i v e

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FIG. 4 . - - B o t t o m s a l i n i ty f o r t h e m o n t h o f J u n e , 1 95 9. N o t e t h e p r o m i n e n t s a lt - w e d g e e m e r g i n g f r o m C o r eS o u n d w h i c h t h e n c o n t i n u e s i n t o w e s t e r n P a m l i c o S o u n d d r i v e n b y p r e v a i li n g s o u t h e r l y w i n d s. M a p p u b l i s h e dwi th p e r m iss io n o f R . H . B en so n , p r in c ip a l i n v es t i g a to r , N . S . F . g r an t G- 7 1 5 2.

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SIGNIFTCA NCE OF SKE WNES S IN REC EN T SED IMEN TS 8 71

d e p t h o f w a v e a c t i o n c a n b e c a l l ed u p o n t o h e l pe x p l a i n t h e d o m i n a n c e o f p o s i t i v e l y s k e w e ds e d i m e n t s i n t h i s l a g o o n.T h e a u t h o r s t a t e d p r e v i o u s l y t h a t t h e w i n d isa n e r o s iv e a g e n t o n t h e b a r r i e r b e a c h e s , c o n t i n u -i n g t h e w i n n o w i n g p r o c e s s a b o v e w a t e r l e v e l .D u r i n g t h e o c c u r r e n c e o f s u s t a i n e d o n s h o r ew i n d s s o m e m a t e r i a l w i n n o w e d f r o m t h e b a r r i e rb e a c h w o u l d l i k e l y b e b l o w n i n t o a n d c o m e t or e s t i n C o r e S o u n d . D i u r n a l o n s h o r e b r e e z e s m a ya l s o b e e f f e c t i v e i n t h i s r o l e a n d f i n e s e d i m e n td e p o s i t e d o u t o f t h e s e w i n d s w o u l d g i v e p o s i t i v es k e w n e s s t o t h e f r e q u e n c y c u r v e . T h e b a r r i e rb e a c h e s , a n d i n p a r t i c u l a r C o r e B a n k s , a r e n a r -r o w a n d o n l y e x c e p t i o n a l ly m o r e t h a n 1 0 f e e ta b o v e m e a n s e a l ev e l. C o n s e q u e n t l y , w a s h o v e r sa r e n o t u n c o m m o n , a n d t h o u g h l es s c o m p e t e n ta g e n t s t h a n b r e a k e r s , d o t r a n s p o r t m a t e r i a l a n da r e d e p o s i t i o n a l a g e n t s ( F i g . 5 ).M e a n s iz e of t h e s e d i m e n t s i n C o r e S o u n d i sl es s t h a n t h a t f o r s e d i m e n t s i n t h e l i t t o ra l z o n e o ro n C o r e B a n k s ( D u a n e , 1 9 62 ) . R i v e r s a r e n o tc a r r y i n g s e d i m e n t i n t o C o r e S o u n d a n d t h e o n l yr e a d i l y a v a i l a b l e s o u r c e i s f r o m t h e b a r r i e rb e a c h . T h e c h a n g e i n m e a n g r a i n s i z e a n d t h ep r e s e n c e of w a s h o v e r s c a r s a n d f a n s a r e t o g e t h e ru n d e r s t o o d t o i n d i c a t e t h a t t h e b a r r i e r b e a c h i st h e s o u r c e of s e d i m e n t f o r C o r e S o u n d . D o m i -n a n c e o f p o s i t i v e ly s k e w e d s e d i m e n t s i l l u s t r a t i n gt h e a d d i t i o n o f f i n er m a t e r ia l , a n d d a t a f r o mh e a v y m i n e r a l d i s t r i b u t i o n ( D u a n e , 1 9 62 ) s u p -p o r t t h i s c o n t e n t i o n . N e t e r o s io n o f C o r e B a n k sr e p o r t e d b y t h e C o r p s o f E n g i n e e r s ( U . S . H o u s eo f R e p r e s e n t a t i v e s D o c . 7 6 3 , 19 4 3 ) w a s a t t r i b -u t e d b y t h e m t o m o v e m e n t o f s a n d l a n d w a r da c r o s s th e b a r r i e r a s w e ll a s s a n d m o v e m e n tp a r a l l e l t o t h e b a r r i e r .

I n s u m m a r y , t h e s i g n of s k e w n e s s d oe s s e e m t ob e a s i g n if i c a n t p a r a m e t e r , c o n t r a r y t o t h e c o n -c l u s io n s of S h e p a r d a n d Y o u n g ( 1 96 1 ). A l t h o u g ha p r e c i s e d e f i n i t i o n of e n v i r o n m e n t i s e l u s i v e , t h es i g n o f s k e w n e s s c a n b e r e l a t e d t o e n v i r o n m e n te n e r g y , a n d t h e re f o r e to e n v i r o n m e n t . W h e r ew i n n o w i n g i s a d o m i n a n t f o r ce , h e re e q u a t e d t oh i g h e n e r g y , a s i n t h e t i d a l i n l e t s , t h e l i t t o r a lz o n e , a n d b e a c h e s , a s w e l l a s m o s t o f t h e b a r r i e ri s la n d , t h e s e d i m e n t s a r e v e r y d o m i n a n t l y n e g a -t i v e l y s k ew e d . A r e a s c h a r a c t e r i z e d b y n o p a r t i c -u l a r d o m i n a n c e o f e i th e r p o s i t i v e o r n e g a t i v es k e w n e s s a r e r e gi o n s w h e r e w i n n o w i n g m a y b ee f f e c t iv e o n e d a y b u t n o t t h e n e x t . S u c h a r ej u d g e d t o i n d i c a t e a r e a s o f fl u c t u a t i n g e n e r g yl e v el s a s in t h e e s t u a r y m o u t h s a n d w e s t e r nP a m l i c o S o u n d . A r e a s w h e r e e n e r g y l e v e l s a r el o w a r e c h a r a c t e r i z e d b y p o s i t i v e s k e w n e s s , a s i nC o r e S o u n d a n d t h e d u n e s . A f u r t h e r i n d i c a t i o nb a s e d o n r e l a t i o n s h i p s in t h i s s t u d y a r e a i s t h a tn e g a t i v e l y s k e w e d c u r v e s a r e i n d i c a t i v e o f a r e a so f e r o s i o n o r n o n - d e p o s i t i o n , w h e r e a s p o s i t i v e l y

FIG. 5 . - - Ae r i a I p h o to g r ap h o f a p o r t i o n o f t h e" O u t e r B a n k s " o p p o s i t e A t l a n t i c , N o r t h C a r o l i n as h o w i n g w a s h o v e r s c a rs a n d w a s h o v e r d e l t a s. T h e t h i nd a r k b a n d r u n n i n g f ro m l e f t t o r i g h t i n t h e u p p e r l e f to f t h e p h o to i s a 6 - f oo t d r ed g ed ch an n e l t o t h e At l a n t i cC o a s t G u a r d S t a t i o n , n ow a b a n d o n e d . C o r e S o u n di s t o t h e l e f t o f t h e b a r r i e r an d th e At l an t i c Ocean i s t oth e r i g h t . U . S . C . & G. S . p h o to 5 8 W 1 5 7 4. Fu l l sca l ea t b o t to m l e f t o f p h o to i s 1 0 0 0 y a r d s .s k e w e d c u r v e s i n d i c a t e d e p o s i t io n . A m i x t u r e o fp o s i t i v el y a n d n e g a t i v e l y s k e w e d c u r v e s w o u l dt h e n i n d i c a t e a r e g i o n i n a s t a t e o f f lu x .

REPRODUCIBILITY OF RESULTSC o n c l u s i o n s d r a w n h e re , a n d a p p l i c a b l e t o

w e s t e r n P a m l i c o S o u n d a n d a d j a c e n t a r e a s a r eu s e fu l p e t r o g r a p h i c - s t r a t i g r a p h i c t o o ls o n l y i ft h e y a r e r e p r o d u c i b le . A s p o i n t e d o u t p r e v i o u s l yn o n e o f t h e p a r a m e t e r s s t u d i e d s e e m e d t o h a v ed i s t i n g u i s h i n g n u m e r i c a l v a l u e s i n d i c a t i n g t h a td i s t i n c t i v e n u m e r i c a l l i m i t s a re n o t e v e r y w h e r er e p r o d u c i b l e . V a l i d i t y o f c o n c l u s i o n s b a s e d o nc o m p a r i s o n s o f n u m e r i c a l v a l u e s a m o n g t h e s et y p e s o f s t u d i e s i s o p e n t o q u e s t i o n h o w e v e r ,b e c a u s e i n e a c h s t u d y d i f f e r e n t f o r m u l a e h a v eb e e n u s e d ( t a b l e 1 ) . H o w e v e r , f o r m u l a e u s e d i nt h i s p a p e r s h o w o n l y m i n o r d i f f e r e n c e s t o t h o s eu s e d b y F r i e d m a n ( 1 96 1 ).

I n a r e c e n t p a p e r o n s o r t i n g a n d t h e l og -n o r m a l i t y o f t h e g r a i n s i z e d i s t ri b u t i o n o f s a n d -s t o n e s F r i e d m a n ( 1 96 2 ) c o m p a r e s i n d e ta i l t h eg r a p h - s o l v e d s o r t i n g c o e f f i c ie n t s o f T r a s k ( 1 9 3 0 ),I n m a n ( 1 95 2 ), a n d F o l k a n d W a r d ( 1 95 7 ) w i t ht h e s t a n d a r d d e v i a t i o n , a m o m e n t m e a s u r e .F r i e d m a n c o n c l u d e s t h a t t h e m e a s u r e u s e d b y

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872 D A V I D B . D U A N ETABLE 1 --Comparison of formulae used by indicated authors for g raphic solution o f statistical parametersused in describing sediments

Trask* Inman Mason and Folk Shepard and YoungMean 4,50

4,75 - 4,25Standaxd deviation 2Skewness 4,25 + 4,75 - 24,50

4,16 + 4,84 4,16 + 4,50 + 4,84 16 + 842 3 2

684 -- 4,16 4,84 -- 4,16 -t- 4,95 - 4,5 84 -- 162 4 6.6 2~84+4,16--24,50 4,16+4,84--24,50 4,5+4,95--24 ,50 4,95+,65--24,50q4,84 -- 4,16 2(#84 -- 4,16) 2(4,95 -- 4,5) 4,84 -- A6

* Modified for use with phi units.

Folk and Ward correlates most closely with thestanda rd deviation. A study analysing the resultsobtained by using different graphical formulaewith results obtained by using mome nt measuresis in progress by this author. Preliminary com-parisons of the statistical measures of Trask(1930), Inman (1952), Mason and Folk (1958),and Shep ard and Youn g (1961) show numericalvariations for sediment uniformity measuressimilar to those noted by Friedman. Values forskewness of several samples are reproduced intable 2. Variations in numerical values are evi-dent. Note also that phi quartile skewness andInma n's skewness do not always agree with otherskewness signs; in all preliminary cases the for-mulae of Mason and Folk, and Shepard andYoung (all of which include a greater part of thefrequency curve) produce results that agree insign with the m oment methods. As the skewnessformulae used here differs from that used byFriedman (1961) only by the factor one-half,there is complete agreement in sign among th em.

Shepard and Young (1961) stated that skew-ness values in their study were too negative,probably due to settling tube method of sizeanalysis; Folk (1962) concurred. As describedpreviously the present study used the sievemethod in determ ining size frequency. Applica-tion of the raw data thus obtained to variousformulae for the calculation of skewness showsthe skewness sign to be consistent among themore sophisticated formulae which includes thatof Shepard and Young. Therefore it is judgedthat the settling tube produces a grain size distri-bution curve different from the distributioncurve determ ined by sieves, as Folk pointed out,and that comparisons between results of investi-gations using these two different techniques arenot valid.Agreem ent in skewness sign among the differ-ent but more sophisticated measures, whenapplied to sediments coming from several of theenviro nments encountered in this study, lendsconsiderable weight to the reproducibility of the

TABLE 2 . - - Va lues o f skewness o f several samples sho wing variations among formu lae o f different authorsTrask* Inman Folk and Ward Shepard and DuaneSam ple (1930) (1952) (1957) Young (1961) (1962)

29 (Pamlico Sound) - . 100 - . 256 - .250 - . 115 - . 15232 (Pamlico Sound) +.0 40 +.20 0 .-I-.470 +3.7 7 +1. 2834 (Pamlico Sound) - .400 - .319 - .268 - .379 - .28356 (Tidal Inle t) - .010 - .011 - .041 - . 135 - .52066 (Tidal Inlet) - .030 - . 110 - . 140 - .3 0 0 - .48079b (Dune) .000 - .013 + .008 + .053 + . 109(Cape HatterasDune) +.030 +.053 +. 109 +.453 +. 19988 (Core Sound) + .030 + .088 + .350 + .279 + .27088c (Core Soun d) + .060 + .063 + .091 + .280 .00091 (Core Sound) - .010 +.0 71 -.I.-.171 +. 57 6 + . 2 4 095a (Barr ier Island) - .010 - .060 - . 120 - .370 - .66995b (Barr ier Island) .000 - .073 - . 126 - .369 - .69095c (Beach) - .060 - . 147 - . 127 - . 170 - .21995d (Littoral Zone) - . 060 - . 234 - . 328 - . 850 - . 750* Modified for use with phi units.

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SIG NIFICA NCE O F SKEW NESS IN RECENT SED IM E NT S 8 7 3s i g n o f s k e w n e s s . A l s o , g e n e r a l a g r e e m e n t o f t h es i g n i f i c a n c e o f s k e w n e s s e x p r e s s e d i n t h i s s t u d yo f R e c e n t s e d i m e n t s t o o t h e r s t u d i e s o f R e c e n ts e d i m e n t s s t r e n g t h e n s t h e b e l ie f t h a t s k e w n e s s ise n v i r o n m e n t a l l y s e n s i t i v e . M o r e o v e r , i t is t h isa u t h o r ' s o p i n i o n t h a t , b e i n g r e p r o d u c i bl e , t h es i g n o f s k e w n e s s h a s m o r e p o t e n t i a l i n s e d i m e n ts t u d i e s t h a n n u m e r i c a l l i m i t s . U s e d w i t h o t h e rc r i t e r i a a v a i l a b l e f r o m a n a l y s i s o f s m a l l s a m p l e s( h e a v y - a n d l i g h t - m i n e r a l s t u d i e s, m i c r o p a l e o n -t o l o g y , a n d g e o m e t r y o f t h e s a n d b o d y ) , t h es k e w n e s s s i g n s h o u l d b e v a l u a b l e i n i n t e r p r e t a -t i on s o f p a l e o e n v i r o n m e n t s r e c o r d e d i n t h e s t r a t i -g r a p h i c c o l u m n , p a r t i c u l a r l y i n t h o s e s e d i m e n t sw h e r e e f f e c t s o f d i a g e n e s i s a r e n e g l i g i b l e o r n o n -e x i s t a n t .

C O N C L U S I O N SR e s u l t s o f th e a p p li c a ti o n o f t h e m o m e n t

m e t h o d o f s ta t is t i ca l a n a l y s i s t o R e c e n t s e d i -m e n t s i n w e s t e r n P a m l i c o S o u n d a n d v i c i ni t yi n d i c a t e t h a t , i n t e r m s o f it s s i g n , s k e w n e s s i se n v i r o n m e n t a l l y s e ns i ti v e. N e g a t i v e s k e w n e s s i sp r o d u c e d b y a w i n n o w i n g a c t i o n , t h e r e f o r e w h e r ea w i n n o w i n g p r o c e s s i s o p e r a t i v e a l l o f t h e t i m e ,a s a l o n g b e a c h e s , t h e l i t t o r a l z o n e , a n d t i d a li n l e t s , s e d i m e n t s w i l l b e n e g a t i v e l y s k e w e d .W h e r e w i n n o w i n g a c t i o n m a y b e a c t i v e o n l yp a r t o f t h e t i m e , n e i t h e r p o s i t i v e l y n o r n e g a t i v e l y

s k e w e d s e d i m e n t s a r e l i k e l y to b e d o m i n a n t f o ra n y l e n g t h o f t i m e a n d s u c h a n a r e a w o u l d b ec h a r a c t e r i z e d b y l o c a l d i f f e r e n c e s o f s ig n . T h es i g n o f s k e w n e s s i n t h e s h e l t e r e d l a g o o n ( C o r eS o u n d ) w h i c h i s f il l in g w i t h f i n e s e d i m e n t t r a n s -p o r t e d o v e r t h e b a r b y w i n d a n d w a v e s is d o m i -n a n t l y p o s i t iv e . D u n e s w e r e a ls o f o u n d t o h a v ep o s i t i v e s k e w n e s s . T h e s e n s i t i v i t y o f s k e w n e s s t ot h e s e v e r a l e n v i r o n m e n t s i n t h i s s t u d y a r e as t r o n g l y s u p p o r t s t h e f i n d in g s o f M a s o n a n dF o l k ( 1 9 5 8 ) a n d F r i e d m a n ( 1 9 6 1 ) .A g r e e m e n t in s k e w n e s s s ig n b e t w e e n t h e m o r es o p h i s t ic a t e d g r a p h i c a l m e a s u r e s a n d t h e m e t h o do f m o m e n t s w h e n a p p l i e d t o s e l e ct e d s a m p l e sf r o m s e v e r a l e n v i r o n m e n t s i n d i c a t e s t h a t t h es i g n o f s k e w n e s s i s r e p r o d u c i b l e a n d s h o u l d b ed i r e c t l y a p p l i c a b l e t o s t u d i e s o f R e c e n t s e d i -m e n t s e l s e w h e r e . U s e d w i t h o t h e r c r i t e r i a t h es k e w n e s s s i g n s h o u l d b e v a l u a b l e i n t h e i n t e r p r e -t a t i o n o f p a l e o e n v i r o n m e n t s r e c o r d e d i n t h es t r a t i g r a p h i e r e c o r d , p a r t i c u l a r l y i n t h o s e s e d i -m e n t s w h e r e e f f e c t s o f d i a g e n e s i s a r e n e g l i g i b l e o rn o n - e x i s te n t . B e c a u s e o n l y a s m a l l a m o u n t o fs e d i m e n t i s n e c e s s a ry f o r a n a l y si s , a n a m o u n tr e t r i e v a b l e i n s u b s u r f a c e c o r e s o r s m a l l o u t c r o p s ,t h i s m e a s u r e s h o u l d h e e s p e c i a l l y a p p l i c a b l e t ot h e p e t r o l e u m i n d u s t r y w h e r e e n v i r o n m e n t a li n t e r p r e t a t i o n s a r e i m p o r t a n t i n th e s e a r c h f o rc o m m e r c i a l h y d r o c a r b o n s .

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8 7 4 DA VID B. D UA NESttEPAR D, F . P . , AND YOUNG, F . , 1 9 61 , D i s t i n g u i sh in g b e tween b e ach an d d u n e san d s : Jo u r . Sed im en ta r yPetro logy , v . 31 , p . 196--214 .T ~ s K , P . D . , 1 9 30 , Mech an ica l an a ly s i s o f sed im e n t s b y cen t r i f u g e : Eco n . Geo lo g y , v . 2 5 , p . 5 8 1 - 5 9 9 .UNITED STATES C OASTAL PI LOT, 1 9 58 , A t l an t i c C o as t , S ec t io n D , C ap e He n r y to Key W e s t : U . S . G o v ' t . P r in t -i n g O~ ce , W ash in g to n , D . C . , 4 1 4 p .UNITED STATES HOUSE OF REPRESENTATIVES, Doc. 763 , 1943, 80 th Congres s, Se cond Session , No r th Ca ro l i nash o r e l i n e , b each e r o s io n s tu d y , 3 3 p .WHITEHOUSE, U. G. , JEFFREY, L. M. , AND DEBBRECHT, J . D. , 1960, Dif fe ren t ia l se t t l in g ten dencie s of c laym in e r a l s i n sa l i n e wa te r s , i n C lay s an d c l ay m in e r a l s , P r o c . of 7 th Na t l . C o n f . o n C lay s an d C lay Min e r a l s :

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