A boulder is a piece of rounded or semirounded stone. There is no particular size for a boulder, and some are many metres across, while others are less than a metre. Even the smallest boulders can barely be moved by a single person. A boulder is a worn piece of rock. That makes it different from piece of cliff in a rockfall.
A boulder may be a large chunk of rock that has originally fallen off a cliff and been worked on by the waves, or might have fall ed from the edge of a river and been worn by river, or it might have been plucked from a bare rock during the Ice Age and made more round by being dragged along by the ice.
