Rust, Brian R. Department of Geology, University of Ottawa, Ontario, Canada.
Last reviewed:November 2019
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An unconsolidated sedimentary aggregate containing more than 50% by weight of gravel-sized particles (mean diameter greater than 0.08 in. or 2 mm). The gravel-sized particles are termed the framework; those less than 0.08 in. (2 mm) in diameter are the matrix. There is an important distinction between framework-supported and matrix-supported gravels. The latter may possess a muddy matrix, in which case they are termed diamictons. Typically, diamictons are unstratified internally, contain subangular framework clasts, and are deposited by mass-flow processes such as debris flow or glacial-ice transport. Water-laid gravels are typically stratified or cross-stratified and are framework-supported, with subangular to rounded clasts in a sand matrix. Less commonly, they may be sand-matrix–supported, or they may lack matrix and then are termed openwork gravels. Water-worn gravel clasts tend to conform to the shape of triaxial ellipsoids and develop preferred orientation; with long axes normal to stream flow and intermediate axes dipping gently upstream.
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