To achieve the natural look of rugged rock, just rip / tear small and large pieces of foam rubber.
Don’t even think about where to start tearing. Just tear in different directions.
You will be amazed at the various rock profiles that are formed. Cover foam rubber with one or two thin coats of compound, using a large paint brush.
This compound cover eliminates the sponge look and also gives a surface for painting and ground cover.
Fix pieces of foam rubber together using liquid nails or similar.