Pebbles are the largest sizes of stone that can be transported by a river. Normally they move only in floods.

Pebbles are rounded because they are moved frequently. They are larger than gravel (above 6mm across) and smaller than boulders (above 256mm across). Boulders tend to be less rounded because they are not moved often. Large pebbles (above 64mm across) are sometimes called cobbles.

Pebbles often form a layer on the bed of a river in hilly and mountain areas. This layer is difficult for the river to move and so sand, and even mud, might be protected below it. For this reason a layer of pebbles is called bed armouring (after the protective armour on a medieval knight). You can see this in the picture below.