Rounding (sediment)

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Rounding, roundness or angularity are terms used to describe the shape of the corners on a particle (or clast) of sediment.^[1] Such a particle may be a grain of sand, a pebble, cobble or boulder. Although roundness can be numerically quantified, for practical reasons geologists typically use a simple visual chart with up to six categories of roundness:

Very angular: corners sharp and jagged
Angular
Sub-angular
Sub-rounded
Rounded
Well-rounded: corners completely rounded

Rounding of sediment particles can indicate the distance and time involved in the transportation of the sediment from the source area to where it is deposited.

Speed of rounding will depend partly on composition. For example, a soft clay pebble will obviously round much faster, and over a shorter distance of transport, than a more resistant quartz pebble. The rate of rounding is also affected by the grain size and energy conditions.