Abstract

The density of V-shaped impact pits on the surface of quartz grains has been investigated for seven size fractions (- 1.0 to 2.0 phi ) in a single sand sample, collected from the trough of a megaripple in the intertidal zone on the south side of Selmah Bar, Cobequid Bay, Bay of Fundy. V density on each grain was measured in two square areas of different size, in the center of SEM photos taken at magnifications of 5,000 and 10,000. V density was found to be a function of technique of measurement (area, magnification) and grain size: it was larger for smaller areas, higher magnifications, and larger grain sizes. A high degree of precision in measuring V density can be achieved only by adopting closely standardize measurement procedures. V density increases from medium to very coarse sand grades, but the rate of V density increase is relatively low in the coarse sand grade. The coarse sand grade also corresponds to the point of equal overlap between intermittent suspension and traction grain-size populations. It is suggested that Vs are produced by grain impacts in a zone of high grain concentration close to the bed and that the low rate of V density increase at the boundary between the two grain-size populations can be attributed to a combination of the effects of sheltering of coarse sand by larger grains, and the reduced time that grains of coarse sand spend in suspension close to the bed.

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