A sledge is another word for sled or toboggan or really any device designed to travel over snow.
During expeditions to the poles in ye olden days, captain Scott etc when the sledge got stuck on deep ice ruts or snow ridges a hammer was used to whack it to help free it hence the term sledgehammer which has become a generic term for most heavy hammers whose shaft is over 34 inches.

sledge·ham·mer
â??noun 1. a large heavy hammer wielded with both hands.
â??verb (used with object), verb (used without object)

Hope this helps
NCM