Once the spray foam has completely hardened, you can begin carving. As I mentioned in the previous step, be patient. The foam will expand a lot as it dries so you'll have plenty of area to carve on.
I used a fine-toothed pull saw to carve the faces of the rock. It worked well because it was long enough to cut wide faces. Work your way across the entire mountain cutting off the rounded "lava-like" sections of spray foam. Be careful not to cut too deep -- you don't want the chicken wire to show through.
I cut horizontal and vertical ridges into the rock face to simulate a real mountain.