THE DESCRIPTION AND REPRESENTATION OF PARTICLE-SHAPE

Despite widespread agreement on the value of shape analysis in the study of sediments, methods of representing and summarizing data have proved difficult to standardize. This paper shows that Sneed and Folk ternary diagrams provide an appropriate standard method of representing data and a rational basis for the choice of descriptive indices and analytical methods. Such diagrams have the unique quality of representing the variation of particle shapes without bias or distortion, unlike alternative diagrams (e.g. biaxial Zingg plots). Simple descriptive shape indices plot on Sneed and Folk diagrams as straight lines, and behavioural indices (e.g. sphericity) as curves. No index or pair of indices can be regarded as 'optimum' in all situations: indices should be chosen with reference to particular data distributions and specific research aims. The use of ternary diagrams in studies of clast shape is illustrated using data from Slettmarkbreen, Norway.