Introduction

Grain size and shape are not truly independent variables for characterizing sedimentary particles. Nearly all measures of grain “size” are associated with some aspect of the form of the envelope enclosing the mass of the grain. This problem can be observed with the role of size–shape interaction on standard sieving for grain size distribution (Pang & Ridgway, 1983). The shape of a grain will affect its passage through a regular square sieve mesh such that spheres are more likely to pass through the mesh than other irregular grains given the same B axis and type of sieving (Ludwick & Henderson, 1968). Alternative methods of sizing (e.g., the use of a settling tube) cannot dispense with the problem of shape effects on a grain's settling rate (Cui & Komar, 1984), and electronic methods of sizing (e.g., Coulter Counter) assume a nominal particle shape.

Sedimentologists usually regard grain size as the most important parameter to be determined in grain characterization. Particle shape has often been dismissed from sediment analysis given that size (and hence mass) is more influential in sediment transport, and because of the practical reality that grain size appears easier to measure than grain shape. Other investigators, notably powder technologists, have been well aware of the need to relate these two aspects of grain structure, and a programme of reference materials including shape aspects for the calibration of sizing techniques is often employed (see, e.g., Scarlett, 1985).