Bowlder

n.
  1. A large stone, worn smooth or rounded by the action of water; a large pebble.
  2. (Geol.) A mass of any rock, whether rounded or not, that has been transported by natural agencies from its native bed. See Drift.
    3. 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.