chockstone

noun2

1. •
   A stone, ranging in size from a boulder to a pebble, which has become wedged in a vertical fissure or cleft.

   • In these coldest hours before dawn, from three until six, I take up my knife again and hack at the chockstone. I continue to make minimal but visible progress in the divot. — 2004
2. •
   A metal wedge or nut, threaded on a wire, used in rock climbing for protection by wedging it into a crack in the rock.