Bowlder; Boulder

Part of speech: noun (n.)

1. A large stone, worn smooth or rounded by the action of water; a large pebble.

2. A mass of any rock, whether rounded or not, that has been transported by natural agencies from its native bed. See Drift. Bowlder clay, the unstratified clay deposit of the Glacial or Drift epoch, often containing large numbers of bowlders. -- Bowlder wall, a wall constructed of large stones or bowlders.