Abstract

On the basis of particle shape the surface layers of some South Wales beaches are subdivided into four zones:--a large disc zone landward, typified by cobble sized discs, having on its seaward side the imbricate zone composed mainly of imbricate disc-shaped pebbles. Seaward of the imbricate zone lies the infill zone where spherical and rod shaped pebbles (drawn from a reservoir, underlying the large disc zone, and in which there are particles with a shape and size making them potentially capable of rapid seaward transport) infill a framework of spherical cobbles fringing the seaward margin. The spherical cobble framework is called the outer frame. Particle shapes are not so much made as used on these beaches; and particle shape differentiation is related to settling velocity, pivotability, and ability to filter through the porous gravels. Discs are not produced by a special feature of marine abrasion: the most oblate discs are found in areas least worked on by the sea. Composition is a function of particle size and shape; particle size and shape vary systematically across the beach; so composition and maturity indices are also seen to vary in a similar way. In the reworking of the boulder clays, which forms a source for much of these marine gravels, two processes are recognised: the post glacial weathering of the boulder clays and abrasion on the beach. Both are selective in that they affect the labile (in this case a subgreywacke) more than the stable (in this case, quartzites). Weathering and abrasion work to split greywackes into discs. At the same time destruction of this kind in always reducing the number of large and increasing the number of small particles, produces a size maturity correlation where the coarser grains are more mature than equivalent sizes in the original boulder clay, but possibly the finer are less mature than equivalent sizes in the original boulder clay. As abrasion further continues, (exemplified here on one beach) the composition shape function begins to disappear, and maturity greatly increases with size. Particle size parameters vary across the beach. Changes in standard deviation and skewness are, to a considerable extent, effected by either the removal from, or addition of small coarse modes to large fine. Size frequency and shape frequency are combined in an attempt to understand more fully the type and extent of sediment movement taking place in these gravels. The beaches are divided into two types: the one on breakdown building up to the seaward a succession of coarse spherical cobbles infilled with spherical and rod shaped pebbles; the other an alternation of beds containing spherical and rod-shaped grains with beds of disc-shaped grains.

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