Why not just make your foam the rocks? You just need three simple tools.
I like the look of carved foam and also use a light brushing of acetone / water in addition to the knife and hot iron to sculpt it However I don't like to use it in an area where kids can get to it. Many years ago after a divorce my layout ended up over in my brothers basement. One day the kids found that the mountains were soft and before anyone discovered their hijinks they had picked a big patch off. No great loss since we had decided that we would pull that layout apart and build an around the room layout but when we did we made the mountains from hydrocal and drywall mud in areas where we wanted to carve because hydrocal is so hard to carve. As to the original question hydrocal on roughed up foam holds fine and you could use it to glue small rocks to a vertical rock formation and hydrocal cures very fast so you don't have to hold the rock in place long. I still like carved foam for rock faces as long as little kids can't reach it. j