Gravel-sized sediment has not shared in the rapid expansion of experimental sedimentology since 1960. In terms of the sedimentary structure hierarchy, it is considered that ‘rank 6’, and to a lesser extent ‘rank 5’, structures embrace the possible range of conglomerate features amenable to a laboratory study. Gravel fabric appears to be a more profitable line of inquiry in view of its common usage in paleocurrent studies, and because of the low preservation potential and/or difficulty in recognising small-scale bedforms.

The laboratory flows were steady-state and spanned the lower- and upper-flow regimes. Clasts isolated on a sandbed undergo combined rotation and translation in response to current-crescent scour. In general, motion is dependent both on the degree of form roughness associated with migrating sand bedforms as well as on the geometric properties of the clasts. Imbrication to moderate angles and current-normal orientation are the dominant responses. Optimum stability of imbricated clasts is attained at the regime transition where flows best approach a steady and two-dimensional character, and is described by a simple power function of projection area versus nominal diameter. The ellipsoidal tendency of the clasts was utilised in the calculation of projection areas.

Since the sandbed co-existing with gravel deposition would likely be in the transition or upper-flow regime, where a stable clast configuration is rapidly attained in response to vigorous current-crescent scour, it follows that the ‘equilibrium area’ concept is a potentially useful tool for the investigation of matrix-supported gravels.