One skier unintentionally triggered a slough while attempting to ride an previously unskied gully. Skier made one slow turn and initiated a "slough-like pushalanche". Skier was being lightly pushed/carried down slope with his skis below him in a skier/traverse stance when his skies struck a hidden rock. At this point skier lost a ski, and fell and was subsequently pushed into a tree where the skier self arrested. Touring group proceeded to look for the ski and was unsuccessful. Skier exited with one ski. Skier unhurt.