The Collaborative International Dictionary of English v.0.48:

cartload \cartload\, cart load \cart load\n.
   as much as will fill or load a cart; the quantity that a cart
   holds. In excavating and carting sand, gravel, earth, etc.,
   one third of a cubic yard of the material before it is
   loosened is estimated to be a cart load.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cartload
    n 1: the quantity that a cart holds