Cobble (geology)

A 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. Other scales define a cobble's size in slightly different terms. A rock made predominantly of cobbles is termed a conglomerate. Cobblestone is a building material based on cobbles.

Cobbles, also called cobblestones, derive their name from the word , meaning a rounded lump. The term is further related to the German Kopf, meaning head. Chester Wentworth referred to cobbles as cobble bowlders [sic] in his 1922 paper that would become the basis for the Udden–Wentworth scale.

Within the widely used Krumbein phi scale of grain sizes, cobbles are defined as clasts of rock ranging from −6 to −8 φ. 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). 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). 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. 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.).

...
Wikipedia