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.

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

[3] 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.

If the till is water-laid, finer particles like sand and pebbles may be entirely washed away, leaving a deposit of only boulders and cobbles.

Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal striations on lateral facets.