attrition

In earth science, the process by which particles of rock are rounded and gradually reduced in size by hitting one another as they are transported by rivers, wind, or the sea.

The rounding of particles is a good indication of how far they have been transported. This is particularly true for particles carried by rivers, which become more rounded and smaller as the distance downstream increases. Thus the load of a river is often large and jagged in its upper course, and smaller and rounder in its lower course.