Particle size | A soil type classification term. Tom Stevenson (2011) explains how ‘the size of a rock determines its descriptive name. No handful of soil will contain particles of a uniform size, unless it has been commercially graded, of course, so all such descriptions can only be guesstimates, but it is worth noting what they should be, otherwise you will have nothing to base your guesstimates on. According to the Wentworth-Udden scale, they are: boulder (greater than 256mm), cobble (64mm–256mm), pebble (4mm–64mm), gravel (2mm–4mm), sand (1⁄16mm–2mm), silt (1⁄256mm–1⁄16mm) and clay (smaller than 1⁄256mm). Notice that even by this precise scale, Wentworth and Udden have allowed overlaps, thus a 1⁄16mm particle might either be sand or silt and, of course, sub-divisions are possible within each group, as there is such a thing as fine, medium, or coarse sand and even gritty silt.’
Tom Stevenson (2011) The Sotheby’s Wine Encyclopedia 5th Edition by Tom Stevenson (Dorling Kindersley, 2011), p.17-19.