Bimodal Sediment Research Articles

Sand sheet dynamics and Quaternary landscape evolution of the Selima Sand Sheet, southern Egypt

The Selima Sand Sheet occupies more than 120,000 km 2 of the hyperarid, uninhabited Darb el-Arba’in Desert centered at the border of Egypt and Sudan at latitude 22° N, and is characterized by a featureless surface of lag granules and fine sand broken only by widely separated dune fields and giant ripples of varying height and wavelength. Monitoring of the largest of these chevron-shaped ripples using repeat orbital images and field surveys indicates migration rates of 500–1000 m/yr, accompanied by 0–2.0 cm erosion or deposition of the youngest sand sheet stratigraphic units. Beneath this active surface, several developmental stages of sand sheet sediments have undulatory upper contacts and varying degrees of pedogenic alteration. The younger stages retain their horizontal lamination and have cracking patterns indicative of past wetter conditions, while older stages have lost their laminar structure through pedogenesis. Historical remains in the desert as well as 14C and Uranium-series dating indicate that the younger strata of the sand sheet have a very low accumulation rate, despite the active movement of the surface. The lower strata were extensively modified during mid and late Pleistocene pluvials, resulting in an initial undulatory surface that set the stage for later accumulation of sand sheet. Below these Quaternary sediments lies irregular topography dissected by channels of mid-Tertiary drainage. The Selima Sand Sheet is neither the result of net aggradation nor degradation, but results from inheritance of an initial fluvial landscape increasingly modified during climatic cycles. Wet periods led to local drainage and deposition, while the increasingly severe arid periods of the late Pleistocene and Holocene resulted in deposition of the blanketing bimodal sediments of the sand sheet.

Critical shear stress for incipient motion of sand/gravel streambeds

Results of an experimental study of the incipient motion of streambeds composed of sand/gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments. The experiments were conducted in an 8 m long by 0.30 m wide glass‐walled tilting flume and an 18 m long by 0.80–1.10 m wide trapezoidal concrete channel. A reference transport method is used to define the beginning of bed material movement. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size (intergranular effects), mixture standard deviation (effect of the shape of grain‐size distribution), absolute value of median size (absolute size effect), and bed slope (effect of relative depth on overall flow resistance). The shear stress at incipient motion of median‐sized grains in mixtures is found to be the same as for uniform sediment of this size. The present findings are consistent with available flume and field data. A technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal/weakly bimodal sediment mixtures is proposed.

Experiments on bar formation in a straight flume: 2. Graded sediment

Laboratory experiments have been carried out in a large laboratory flume using a strongly bimodal sediment mixture in a range of flow conditions such that the initially flat bed of the flume became unstable enhancing the development of alternate bars. In order to elucidate the effect of grain sorting, the present experiments are compared with a previous set of experiments performed in the same flume under similar hydraulic conditions but using a nearly uniform sand with a mean geometric grain diameter equal to that characterizing present experiments. The comparison suggests that sediment heterogeneity may appreciably affect bed form characteristics. The development of smallscale (ripples) and mesoscale (dunes) sediment waves tended to be inhibited thus allowing a decrease in flow resistance. Owing to the bimodal character of the adopted mixture at low values of the bed shear stress a condition of partial transport was attained for which only the finer‐grained portion of the mixture was observed to move, while the coarsergrained fractions remained essentially immobile throughout the experimental run. However, complete mobilization of all size fractions was observed to occur in runs carried out at higher slopes (i.e., at higher bed shear stress) in order to generate an alternate bar pattern. Selective transport of individual grain size fractions, coupled with the characteristic bar topography pattern led to an intense longitudinal sorting which accreted the coarser particles on bar crests. Furthermore, bar migration caused, through scour and fill, a significant vertical sorting. As a consequence of these sorting processes and in accordance with previous experimental observations, bar height turns out to be invariably damped with respect to uniform sediment experiments. The trend exhibited by the wavelengths is less clear and suggests that in the present experiments sorting effects do not enhance the clear shortening of bar wavelengths typically observed in other series of flume experiments carried out with weakly bimodal mixtures.

Polycyclic aromatic hydrocarbon (PAH) source, sediment deposition patterns, and particle geochemistry as factors influencing PAH distribution coefficients in sediments of the Elizabeth River, VA, USA

Abstract Sediments and pore waters from two sites in the urbanized Elizabeth River, Virginia were sampled for levels of polycyclic aromatic hydrocarbons (PAHs). Pore water PAH concentrations were similar between Sites 1 and 2, despite sediment PAH concentrations being much greater at Site 2. Organic carbon-normalized distribution coefficients (KOC)obs for all PAHs were significantly higher at Site 2 compared to Site 1, but varied ∼2–3 orders of magnitude between the top and bottom of the core in the sediments at Site 1. PAH distribution between sediments and pore waters at Site 1 were most notably related to the bimodal sediment deposition patterns as depicted by X-radiograph depth profiles, and PAH isomer concentration ratios. Sedimentary organic carbon/nitrogen ratios, lignin-phenol concentrations, soot carbon, and particle surface area both before and after organic digestion, were also analyzed in order to assess the role of sediment geochemistry on PAH (KOC)obs. Different factors were determined to control particle surface area at each site, offering insight into explaining observed PAH distribution coefficients. At Site 1, sediment organic matter was inaccessible for digestion and perhaps PAH partitioning as well. Sorption of PAHs to mineral surfaces explains low values for PAH distribution coefficients observed in the deeper sediments in this core. At Site 2, large and invariant (KOC)obs may result from sediments comprised of particles (e.g., woody debris) infused with creosote and coated with organic matter sequestering PAHs within the particle matrix. Our results indicate there is significant heterogeneity in PAH distribution coefficients in estuarine sediments, which may be attributed to sediment deposition patterns, PAH source(s), and sediment geochemistry.

Turbulence and Secondary Flow over Sediment Stripes in Weakly Bimodal Bed Material

Longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic, spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction. This paper quantifies mean and turbulent flow structures over self-formed sediment stripes in a weakly bimodal sand and gravel mixture. Turbulence anisotropy generates two secondary circulation cells across the channel half-width, which produce a cross-stream perturbation in boundary shear stress. The interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline. Finer sediments are entrained from regions of high shear stress, transported laterally by the secondary flow, and deposited in regions of lower shear stress. Lateral changes in bed texture further enhance the near-bed secondary flow, which provides a positive feedback mechanism for stripe growth. In bimodal sediments, at shear stresses just above the entrainment threshold, stripes may replace lower-stage plane beds. At higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms.Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(1999)125:5(463)

Measuring and defining bimodal sediments: Problems and implications

Gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions. These can be quantified in several ways, giving wide differences in derived statistics and calculations. Problems associated with measuring, summarizing, and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition. No single index of bimodality serves all purposes; alternatives are suggested. Measured near‐bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume; this has implications for the abrupt downstream transition often found from gravel to sand bed.

Bimodal fluvial bed sediments: origin, spatial extent and processes

Historically fluvial channels have been broadly classified as either gravel or sand bed ded. However, a third type, bimodal, also exists. In this case significant quantities of both gravel and sand are present. Studies into bimodal sediments have concentrated on their origin, spatial extent and, more recently, the processes occurring within them. Given the advances in our knowledge of these complex processes it is appropriate to review the developments of research into bimodal sediments. Based on a synthesis of this research it is shown that bimodality may occur readily and be spatially widespread. The effects of the surface sorting prevalent within bimodal sediments on entrainment and transport processes are also explored. From this it is concluded that bimodal beds may represent a distinct threshold between gravel- and sand-bed states. This manifests itself as an abrupt downstream change from a gravel bed to a sand bed.

The Gravel-Sand Transition Along River Channels

ABSTRACT This paper discusses a hitherto neglected aspect of fluvial downstream fining: the transition from a gravel bed to a sand bed. From the few known examples and by additional study of new sites, some general features of such transitions are given. These transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope. We propose three possible causes: local base level control an excess of sand supply, and abrasion/breakdown of fine gravel. We further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed, and on this basis we suggest they may also aid in interpreting ancient sediment showing bimodality and rapid proximal-to-distal changes in grain size.

Fluvial transport of sand and gravel mixtures with bimodal size distributions

Initiation of motion and fractional transport rates of sediments with bimodal size distributions were measured in a laboratory channel and in a natural channel. Transport experiments were conducted with unimodal 100% sand and 100% gravel bed sediments and three bimodal mixtures of the two sediments: 10% gravel—90% sand, 25% gravel—75% sand, and 45% gravel—55% sand. In the bimodal sediment beds the critical shear stress for the initiation of motion showed very little change with grain size for the sand sizes, but increased with grain size for the gravel sizes. For low flow strengths, only the sand sizes were in transport, while for the highest flow strengths the bed load sediment size distribution approached that of the bed material.

Critical Shear Stress of Natural Sediments

The critical shear stress of individual fractions τci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture. For strongly bimodal sediments, τci increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes τci to deviate from size independence in the direction of unisize (Shields) values. A quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of τci is observed and to predict the variation of τci with grain size in bimodal sediments. Because properties of sediment grain‐size distributions other than bimodality appear to have little influence on τci the trends presented here for 14 unimodal and bimodal sediments may be quite general.

Experimental studies of particle packing and sintering behaviour of monosize and bimodal spherical silica powders

The packing arrangements and sintering characteristics of monosize and selected bimodal mixtures of spherical silica particles are reported. Gravitational sedimentation and simple drying from a droplet of suspension were used to form consolidated bodies of micron and sub-micron sized particles. Monosized sediments were composed of ordered domains of close packing, interspersed with regions of disordered packing. During the initial stages of sintering, preferential densification of the ordered domains was observed and this promoted pore growth at domain boundaries. Moving to bimodal mixtures containing at least 20 wt% of the minor component prevented the formation of ordered domains and allowed the production of sediments with a reduced spatial variation in packing density. The more even distribution of porosity in bimodal sediments led to more uniform sintered microstructures. Comparisons are made with the results of earlier work using computer simulation techniques to study packing geometry. Die Packungsanordnung und die Sintercharakteristika von sphärischen Siliziumdioxid-Teilchen mit einerseits einheitlicher Größe und andererseits bestimmten bimodalen Größenverteilungen werden beschrieben. Gravitationssedimentation und einfaches Trocknen von Suspensionstropfen wurden angewandt, um konsolidierte Körper aus Teilchen der Größenordnung weniger Mikrometer oder weniger herzustellen. Die Sedimente der Teilchen einheitlicher Größe bestanden aus geordneten Bereichen dichter Packung, zwischen denen Bereiche ungeordneter Packung angeordnet waren. In den Anfangsstadien des Sinterns konnte hevorzugtes Verdichten der geordneten Bereiche beobachtet werden, was zu Porenwachstum an den Bereichsgrenzen führte. Beim Übergang zu bimodalen Mischungen, die mindestens 20 Gew.% der kleineren Komponente enthielten, wurde die Bildung geordneter Bereiche vermieden. Auf diese Weise konnten Sedimente mit einer geringeren räumlichen Variation der Packungsdichte hergestellt werden. Die gleichmäßigere Verteilung der Porosität in bimodalen Sedimenten führte zu gleichförmigeren Gefügen. Um die Packungsgeometrie zu untersuchen, wurden die Ergebnisse mit früheren Arbeiten verglichen, indem Berechnungen mittels Rechnersimulationen durchgeführt wurden. Cet article présente le comportement au frittage d'empilements de particules sphériques de silice, soit de même dimension, soit en mélanges bimodaux, en fonction de leur arrangement topologique. Une technique consistant en une sédimentation et en un séchage d'une gouttellette de la suspension a été utilisée pour préparer des compacts de particules dont les tailles étaient de l'ordre du micron et submicroniques. Les sédiments de particules monodisperses etaient composés de domaines ordonnés d'empilement désordonné. Durant les phases initiales du frittage, on a constaté une densification préférentielle des domaines ordonnés. Ce phénomène s'accompagne d'une croissance des pores aux joints entre les domaines. Lorsque l'on examine les mélanges bimodaux contenant au moins 20% en poids de l'espèce mineure, il apparaît que la formation de domaines ordonnés est inhibée, ce qui autorise la production de sédiments dont la densité compactée, donc la porosité initiale, est beaucoup plus homogène. En conséquence, le frittage conduit à une microstructure plus uniforme. Enfin, les résultats sont comparés à ceux d'un travail précédent basé sur des techniques de simulation de la géométrie de compacts par ordinateur.

Gravel overpassing on humpback bars supplied with mixed sediment: examples from the Lower Old Red Sandstone, southern Britain

ABSTRACT The Lower Old Red Sandstone (Downton to ?Emsian) in southern Britain is a largely fluviatile sequence of increasing upward sand‐dominance. The highest beds at two groups of localities include many sedimentation units composed of gravelly foresets in depositional continuity with overlying topsets of parallel‐laminated sandstone. These units are thought to have been fashioned by humpback bars having a crest a considerable way upstream from the brink at the top of a gravelly slipface. The overall textural composition but internally segregated character of the bar units suggests that a bimodal sediment load of mixed sand and gravel was supplied at the upstream end of the bar, but that this load became texturally differentiated as it moved downstream.Differentiation is suggested to have occurred because the comparatively large and well‐rounded gravel particles behaved on the sandy topset as though on a smooth surface, and were transported under similar flow conditions to the sand, much of which eventually lodged on the topset instead of being passed on, like the gravel, to the slipface beyond. A quantitative model is outlined which justifies the proposed gravel overpassing. In terms of the control of sedimentary structures exerted by grain size under laboratory conditions, the association of cross‐bedding (gravel) with simultaneously formed parallel lamination (sand) seems to be a natural consequence of the efficient textural differentiation of the supplied load by the overpassing of the gravel component under a single flow condition.

Jointed Source Rock and Fluvial Gravels Controlled by Rosin's Law: A Grain-Size Study in Calabria, South Italy

The primary distributions of the untransported source rocks are unimodally distributed and very positively skewed; the average sand fraction is only 4%. Only 6% of the distributions are lognormal; 94% follow the Rosin distribution, which is typical for products of technical breakage and milling processes. Twelve of the 19 river-mounth sediments are bimodally distributed with a primary mode in the gravel range, a marked valley in the 2 mm range, and a secondary mode in the sand range. The seven rivers with unimodal sediments also have their modes in the gravel range. Both unimodal bimodal sediments consist of, on the average, 74% boulder and gravel, 25% sand, and 1% silt and clay. Surprisingly, 77% of the gravel modes of the river sediments also follow the Rosin distribution. Only the sand fractions of both the unimodal and bimodal sediments are lognormally distributed. The Rosin distribution of the Calabrian river gravel is interpreted as a source-specific primary feature; the lognormality of the sand mode, in contrast, as a transport-specific secondary feature. Since the jointed and weathered source rocks contain only 4% sand they cannot be the only source of the 25% sand in the river sediments. The sand must also come from other sources such as soil building and transport wear.--Modified journal abstract.

Sediment transport in a microtidal estuary: Matanzas River, Florida (U.S.A.)

Tidal channels and shoals at the mouth of the Matanzas River are the site of considerable sediment transport. Rates and patterns of transport were determined using fluorescent dyed sediment tracers which were introduced on three intertidal locations. Temporal sampling after introduction was at the surface and by coring. Transport rates and patterns were examined for both quartz and shell hash fragments in the distinctly bimodal sediment. Wave activity and time-velocity asymmetry of currents produced a net landward sediment transport over the channel-margin bar and flood delta. The combination of time-velocity asymmetry and deflection of currents by the ebb shield causes landward transport over the flood delta. Ebb transport of sediment dominates in the south flood channel. Minimum sediment transport rates vary with morphology, currents, and grain-size parameters, but these rates average 15–20 m over one tidal cycle. Wave activity enhances transport of shell hash. Vertical variation in tracer concentration consistently decreases with depth. Depth of disturbance values are typically 10–15 cm on the channel margin bar, but are variable elsewhere, being controlled by type and height of bedforms.

Origin of Bimodal Sands in Some Modern Environments

Grain settling velocity distributions of sediments from modern fluvial channel, eolian barchan dune, beach, and tidal flat environments of the United States and Mexico were analyzed using a photo-extinction settling tube. The results revealed the occurrence of bimodal distributions in several specific places: (1) trough cross stratification in fluvial channels, (2) the lower slip face, lower stoss side, and crest of barchan dunes, (3) eolian mega-ripples, (4) lower beach foreshores, (5) beach storm layers, and (6) inner parts of tidal fiats. Possible sorting mechanisms for generating bimodal characteristics of these deposits are suggested and are classified into three categories: a) Mixing of two sorting processes which have different sorting tendencies. For example, mixing of avalanche sorting and projection sorting on the dune slip face is responsible for bimodal sediments of types (1) and (2). b) Special hydraulic circumstances yielding high mobility of the coarse fraction; type (3) and (4) bimodal sands are examples. c) Unusual transportation events. Storm transportation of a coarse fraction into ordinarily coarse-sediment-free environments may be responsible for type (5) and (6) distributions. This study has demonstrated that bimodal distributions of grain settling velocity of sediments could be interpreted by simple and basic sorting processes.

The Texture of Compacted Bimodal Sediments

ABSTRACT The differences in texture between the angular matrix grains and the rounded pebbles of a bimodal sediment are normally related to differences in mode of transport. However, the difference may be greatly exaggerated by pressure-solution if the sediment has been subjected to compaction.

Structure and texture of a gravelly barrier island in the Fitzroy estuary, Western Australia, and the role of mangroves in the shore dynamics

In the stratification of the most coarse-textured barrier island of a system of sand and gravel barrier spits and islands on Point Torment peninsula, King Sound, W.A., foreslope beds due to swash-backwash on the beach face are unimportant. The main units are steeply-dipping backslope beds and topslope beds of gentle, chiefly landward inclination. Their bimodal sediments are poorly sorted for wave-built features. A very coarse mode usually at −1.5 to −2 ☎i; belongs to a population of well-rounded but platy, impure limestone, of local but unlocated source, whereas the fine mode, almost invariably 1.00 to 1.25 ☎i;, represents a population of quartz sand fed from low and sub-tidal estuarine shoals. The two populations are thought to derive from the surface creep and saltation cloud components of “traction carpets”. Swash of intermediate energy waves forms the topslope beds through percolation. Washover by waves of translation generated by occasional hurricanes erodes the top of the feature and deposits the backslope beds on the rear face in standing water at abnormally high levels through storm set-up. In both, unidirectional currents produce sediments with certain resemblances to fluvial “traction clog” deposits. Dated by underlying mangrove wood of 1200 B.P. in front and of 500 B.P. in the rear, this barrier probably retreated through mangrove swamp in the manner of some presently active members of the barrier system. The seaward mangrove swamps neither protect the embankments behind nor suffer seriously from the waves rolling the latter landward.

Studies on Sedimentology of Shell Beds on Bermudian Patch Reefs: ABSTRACT

The upper surfaces of two patch reefs on the northern edge of the Bermuda platform were observed to consist of coral-coralline algal rock mounds amid a complex system of low-lying channels containing coarse- to fine-grained calcareous detritus. The rock mounds on the larger of the two reefs are noticeably concentrated toward the periphery. Detailed mapping of a part of one of these reefs indicates that the distribution of Spondylus valves and other coarse detritus bears a strong relation to the shape and width of the intermound channels. Oddly, 79% of the shells lay with the concave side up. The apparent preference for this generally unstable orientation is attributed to the lofting of shells during transport and the preferential burial of shells lying concave-side-downwa d. The mode of the fine fractions of samples taken from shelly areas consistently is toward the right of the mode of samples taken from adjacent nonshelly areas. It thus appears that the shells act as baffles to permit the local deposition of fine sediment. It is concluded that shell-bed sedimentology reflects the existence of two wave-energy regimes throughout the year. In winter, powerful storm waves determine the distribution of shell beds; calmer seas prevailing in summer months promote the preferential deposition of fines among the shelly material. Further, the rock-sand configuration on larger patch reefs suggests that the reefs may concentrate unimodal sand toward the center and bimodal sediments in the more narrow, circuitous, shell-rich channels toward the periphery. End_of_Article - Last_Page 736------------

Petrography and Derivation of Jurassic-Cretaceous Clastic Rocks, Southern Rocky Mountains, Canada

Late Jurassic and early Cretaceous sediments in the southern Rocky Mountain region consist of three lithologic divisions; in ascending order these are: (1) the Fernie Formation, consisting mainly of shale of Jurassic age, (2) the Kootenay Formation, consisting of shale and coal-bearing beds with subordinate sandstone, at least partly of Jurassic age, and (3) the Blairmore Group, consisting of a basal conglomeratic member of unknown age overlain by sandstones with subordinate shales of Early Cretaceous (Aptian) age. The precise position of the Jurassic-Cretaceous contact remains controversial. Toward the west, sections appear to be most complete, and a hiatus, or unconformity, increases in magnitude toward the east. Thus in the westerly sections coal-bearing Kootenay beds grade into the coarse-grained Blairmore clastics by increase in frequency, thickness, and grain-size of the sandstone and conglomeratic units. In the easterly sections there is no such gradation, and conglomeratic beds of the Blairmore cut into and overstep siltstones and coal-bearing beds of the Kootenay Formation. In the Mount Allan section (the source of most of the analyzed material), the cliff-forming conglomerate is composed of a series of interdigitating lenses of conglomerate with sandstone, and shale with sandstone. The majority of rock types analyzed are lithic or rock-fragment sandstones and conglomerates, the components of which consist of clastic mineral and rock fragments with clays, and authigenic minerals. The mineral and rock fragments are grouped into four suites which apparently originate from four distinct sources: (1) the Metamorphic Suite, from the Selkirks metamorphic province, (2) the Stable Detritals Suite, reworked from a Paleozoic and possibly pre-Paleozoic source, (3) the Pyroclastics Suite, from the Pennsylvanian-Permian volcanic province, and (4) the Phosphorite-Bitumen-Carbonate Suite from the uppermost Paleozoic and lowermost Mesozoic sedimentary sequence. Uplift and erosion of the distributive province in the west apparently began at the close of Permian time and continued intermittently into Jurassic time. The proportions of the components of the four suites are related to the grain-size of the sediments (an observation which is confirmed by statistical analysis) and thus to abrasion and sorting during the entire period of erosion of the distributive province. Sedimentary differentiation of the clastic material is related to sorting (or lack of sorting) by the transporting agent during a geologically short period of time. Rapid alternation of high- and low-velocity transporting media caused scouring of underlying beds and, with loss of transporting power, these media deposited poorly sorted coarse-grained material and ultimately moderately sorted fine-grained material. There was no sorting after deposition to complete sedimentary differentiation. Depositional features, such as festoon- and cross-bedding, and poorly sorted, bimodal sediments indicate that the clastics were laid down in a continental or alluvial environment. Late Jurassic uplift in the west resulted in increased erosion and rate of transport, culminating in the deposition of the basal Blairmore conglomerate and followed by the quieter estuarine and flood-plain conditions of middle Blairmore time. The course of the various rivers and their tributaries shifted laterally, so that the gross effect achieved is of one, homogeneous conglomeratic unit extending from northern Alberta and British Columbia to the United States. The Kootenay-Blairmore clastics are thus transitional between one type of sedimentary environment in the Jurassic and another in the Cretaceous and are indicative of one of a series of orogenies in the western hinterland which began at the close of the Paleozoic Era and culminated during the Tertiary Laramide orogeny.

Brazos River bar [Texas]; a study in the significance of grain size parameters

A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.