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Roundness

During transport, the shape and outline of clasts continue to evolve as they are carried. Grains transported by currents such as wind, rivers, or waves, can move as suspended load, by saltation (jumping), and traction with the ground/bed. Bouncing, jumping, and drag of the grains produces friction between grains and with the ground. This erodes grains, progressively removing sharp edges in favor of more rounded, smoothed shapes. Roundness (or angularity) is, hence, a measure of how much sediments have been transported by a current. Roundness is different from sphericity, which is, on the other hand, a measure of the shape of a clast. The external outline of a clast can be:
very angular: sharp edges separated by deep incisions
angular: sharp edges separated by small incisions
sub-angular: incipient rounding of the most prominent edges
sub-rounded: rounded edges separated by incisions
rounded: well smoothed edges
well rounded: generally convex shape

Roundness is linked to the intensity and type of transport. In general, protracted transport produces rounded shapes. Some processes, like waves and rivers, can round grains very well, while others, like wind, are not so efficient. Gravity flows, such as debris flow, and glaciers transport materials in mass, leaving angular shapes intact. The degree of rounding also depends on composition: in sandstones, quartz grains, with higher hardness, tend to maintain a more angular shape compared to feldspars. High-energy transport can cause textural inversion, which happens when a rounded grain is broken to angular fragments.

Roundness. Graphics: Samuele Papeschi/GW.

References
Compton, R. R. (1962). Manual of field geology. Soil Science93(4), 295.
Dott, R. H. (1964). Wacke, graywacke and matrix; what approach to immature sandstone classification?. Journal of Sedimentary Research34(3), 625-632.
Folk, R. L. (1980). Petrology of sedimentary rocks. Hemphill publishing company.
Lewis, D. W., & McConchie, D. (2012). Analytical sedimentology. Springer Science & Business Media.
Powers, M. C. (1953). A new roundness scale for sedimentary particles. Journal of Sedimentary Research23(2), 117-119.
Wentworth, C. K. (1922). A scale of grade and class terms for clastic sediments. The journal of geology30(5), 377-392.
        

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