In
geology, a
boulder is a
rock fragment with size greater than in diameter. Smaller pieces are called
cobbles and
pebbles, depending on their "
grain size". While a boulder may be small enough to move or roll manually, others are extremely massive. In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called
rocks or stones. The word
boulder is short for
boulder stone, from
Middle English bulderston or
Swedish bullersten.