We did Peter PAn a couple of years ago.   Built the base "rock" shapes out of plywood and interestingly shped 2 X8's stacked up.   Very sturdy and easy to walk on and climb.    Then we used crumpled up newspaper which we spray glued onto the "rocks"  wet some gauze sheets with plaster of paris impregnated in the fibers and covered the paper crumples.  Painted with some slightly different shades of gray and black paint and the effect was really great, from the first row of seats it looked real enough to cause some to ask how we managed the weight of the stone on stage.  My only caution would be that you need to make your climbing and walking lines so that the actors will not kick or step on the facing.   For the overhang on the lost boys cave we used some chicken wire bent and stapled in odd shapes and covered this with the plaster of paris gauze as well, worked well too and after drying is easy to paint.

    Good luck,

Charlie