Rocks on the top of a dry stone wall seem to have more lichens than those lower down the wall, especially if the stone has a rough surface like this Old Devonian conglomerate (which would have originated in strata on the top of Rhossili Down). Here the fine black lichen outlines the larger protruding crystalline fragments in the stone.
This series of photographs was taken beside the path from Rhossili village towards the Worms Head Causeway.
Jessica's Nature Blog 