That's excellent, here's another idea that I came across recently. Cut a piece of fairly thick paper to the size you need and paint it with a dark grey poster paint or similar, then crumple it up - you may need to try a couple of times to get it right, you can vary the size of the relief created by the amount you crumple - then smooth it out and with an almost dry brush lightly brush over the surface with a lighter colour paint, or several different colours. The paper rockface can then be simply stuck in place and using the same method you describe with scatter etc to blend it in. The two methods will work well together I suspect.
I have inherited a large shed with a 12 foot by 5 foot baseboard already built. I am trying to design a suitable layout. I would like to include a harbour and a mine.