In the late 1800s and early to mid-1900s, prior to the Udden–Wentworth scale's widespread adoption, size classifications tended to group all particles larger than 2 millimeters (0.079 in) together as gravel or stones. The Committee on Sedimentation of the US National Research Council has recommended that in situ cobbles be identified by their process of origination, if possible (e.g., cobbles by disintegration, by exfoliation, etc.). On the Udden–Wentworth scale, an unlithified fraction of cobbles is classified as gravel while a lithified sample primarily composed of cobbles is a conglomerate. On this scale, cobbles are larger than pebbles which measure 4–64 millimeters (0.16–2.52 in) in diameter and smaller than boulders, whose diameters range from 256–4,096 millimeters (10.1–161.3 in). This classification corresponds with the Udden–Wentworth size scale which defines cobbles as clasts with diameters from 64–256 millimeters (2.5–10.1 in).

Within the widely used Krumbein phi scale of grain sizes, cobbles are defined as clasts of rock ranging from −6 to −8 φ. Classifications Sandy conglomerate with cobbles in the Hazeva Formation (Miocene) of southern Israel Chester Wentworth referred to cobbles as cobble bowlders in his 1922 paper that would become the basis for the Udden–Wentworth scale. The term is further related to the German Kopf, meaning head. Cobblestone is a building material based on cobbles.Ĭobbles, also called cobblestones, derive their name from the word cob, meaning a rounded lump. A rock made predominantly of cobbles is termed a conglomerate. Other scales define a cobble's size differently. Clast of rock Beach cobbles ( Nash Point, Wales)Ī cobble (sometimes a cobblestone) is a clast of rock defined on the Udden–Wentworth scale as having a particle size of 64–256 millimeters (2.5–10.1 in), larger than a pebble and smaller than a boulder.

River rock vs gravel