Attrition is the process where pieces of rock are transported through water and wear down the shore bed over time as a result of friction. Gravel or other small stones are often carried through a current and then come into contact with the sides and bottom of the water body.
A secondary effect of attrition is the breaking or refining of small pieces of rock. When carried through the water and into other rock, small rocks will be ground down or broken into smaller pieces of rock. On top of rocks, even small particles of sand can be carried through the water and cause attrition.