Particle size

Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.

four gross divisions of the size scale are easy to use: clay, silt, sand and gravel.

Clay is microscopically small, smooth to the touch and teeth. Silt is gritty but invisible, with particles up to 1/16th millimeter in size. Sand is familiar, and gravel is anything larger than 2 millimeters. The coarse, medium and fine subdivisions of those categories can be learned too with a little practice. Particle size is the primary way of assigning the right name to siliciclastic rocks: shale, siltstone, sandstone and conglomerate.

The Wentworth scale was published in 1922 by Chester K. Wentworth, modifying an earlier scale by Johan A. Udden. Wentworth's grades and sizes were later supplemented by William Krumbein's phi or logarithmic scale, which transforms the millimeter number by taking the negative of its logarithm in base 2 to yield simple whole numbers. The size fraction larger than sand (granules, pebbles, cobbles and boulders) is collectively called gravel, and the size fraction smaller than sand (silt and clay) is collectively called mud.


The Wentworth scale