Cobble (geology)

A rock hammer rests atop a variety of well-rounded gray cobbles.
Beach cobbles (Nash Point, Wales)

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 cob, meaning a rounded lump. The term is further related to the German Kopf, meaning head.[1] Chester Wentworth referred to cobbles as cobble bowlders [sic] in his 1922 paper that would become the basis for the Udden–Wentworth scale.[2]

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