The Collaborative International Dictionary of English v.0.48:

Bowlder \Bowl"der\, Boulder \Boul"der\, n. [Cf. Sw. bullra to
   roar, rattle, Dan. buldre, dial. Sw. bullersteen larger kind
   of pebbles; perh. akin to E. bellow.]
   [1913 Webster]
   1. A large stone, worn smooth or rounded by the action of
      water; a large pebble.
      [1913 Webster]

   2. (Geol.) A mass of any rock, whether rounded or not, that
      has been transported by natural agencies from its native
      bed. See Drift.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bowlder
    n 1: a large smooth mass of rock detached from its place of
         origin [syn: boulder, bowlder]