cobble [part size] . (a) A rock fragment larger than a pebble and smaller than a boulder, having a diameter in the range of 64-256 mm (2.5-10 in., or -6 to -8 phi units) being somewhat rounded or otherwise modified by abrasion in the course of transport; in Great Britain, the range of 60-200 mm has been used. Also, a similar rock fragment rounded in place by weathering at or somewhat below the surface of the ground; e.g. a "cobble of exfoliation" or a "cobble of spheroidal weathering". See also: large cobble; small cobble. (b) A rock or mineral fragment in the soil, having a diameter in the range of 20-200 mm (Atterberg, 1905). In the U.S., the term is used for a soil particle having a diameter in the range of 75-250 mm (3-10 in.) (SSSA, 1965, p.333). Syn: cobblestone. (c) An engineering term for a particle having a diameter greater than 76 mm. (d) cobblestone.