THE DESCRIPTION AND REPRESENTATION OF PARTICLE-SHAPE

Despite widespread agreement on the value of shape analysis in the stu dy of sediments, methods of representing and summarizing data have pro ved difficult to standardize. This paper shows that Sneed and Folk ter nary diagrams provide an appropriate standard method of representing d ata and a rational basis for the choice of descriptive indices and ana lytical methods. Such diagrams have the unique quality of representing the variation of particle shapes without bias or distortion, unlike a lternative diagrams (e.g. biaxial Zingg plots). Simple descriptive sha pe indices plot on Sneed and Folk diagrams as straight lines, and beha vioural indices (e.g. sphericity) as curves. No index or pair of indic es can be regarded as 'optimum' in all situations: indices should be c hosen with reference to particular data distributions and specific res earch aims. The use of ternary diagrams in studies of clast shape is i llustrated using data from Slettmarkbreen, Norway.