Has anyone tried to build a cave/solid stone wall?  What materials do you recommend, I am trying to build one for our church production. I have thought of spray foam, seen ideas for mixing dry wall compound and Polymer based glue. Has any one tried one of these? Any help or ideas will be greatly appreciated.

There is another method that you may want to consider...You make a rough form of the downstage cave entrance out of cut plywood--make this an irregular shape, not square or rectangular. Make a bigger rough form for the upstage side exit of the cave.  Attach the two to two pieces of 2x4 that run on the floor between the two.    The entrance and exit don't need to be far spaced, even 2-3 feet apart look fine. You may need to brace the upright entrance and exit. Take chicken wire, bend it to fit over this opening to make a tunnel form but extend the tunnel.  Keep your chicken wire rough and bent occassionally to simulate rough rock--even pull sections back from the plane of the cave wall with a pliers to roughen it up.  Then cover with primed (medium grey color) fabric strips or rug pieces.    Cover the inside a little and outside as far as each will show.  If needed-- wire the fabric into place.  Or simply pull ends of strips through chicken wire to fasten them.  Now with a spray tank spray a splatter of dark grey color.  With a sponge or wadded up rage, sponge on light grey paint.

We  use plain old sheets of plywood with a comination of plaster/glue soaked cloth and newspaper to create three-d rocks then paint to give depth and value to each one, Although we haven't tried to make something fully naturalistic (were usually doing stone fireplaces, or stone walls that have a pattern of the rocks to them, they come out awesome with this method, it truly is how the paint is applied that gives the rocks their realistic look. Take a look at the attached photo to confirm, this fireplace was done in the way I described, fully out of paper mache, and glue/plaster dipped paper and fabric.