Roundness is the degree of smoothing due to abrasion of sedimentary particles. It is expressed as the ratio of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere. The roundness terms are less environmentally meaningful for grains in a limestone since some, such as ooids and peloids, are well rounded to begin with. Skeletal grains in a limestone should be checked to see if they are broken or their shape has been modified by abrasion.

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
Sub angular
Sub 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 on composition, hardness and mineral cleavage. For example, a soft claystone 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.



Pagina realizzata da Papeschi Samuele