Rock, Lichen, Scissors

We have all played the game of rock paper scissors.  Paper covers rock, rock breaks scissors and scissors cut paper.  For the stone walls in the garden it is not paper that beats them, but lichens.  I like some of the lichens that grow on the walls.  I find them attractive.  The longer the walls stand out there in the sun the more lichens grow on them.  The lichens are in fact very slowly consuming the stones.

It's impossible not to notice the black lichen that is growing on the stone square.  I find it interesting to notice that where the stone is shaded the black lichen is absent.

Here a number of lichens are growing on the top of the wall.

Although the black lichen looks smooth from a distance, it is really quite pebbly in texture.  How the black lichen got started or where is came from I don't know.  I'm sure it has a name, but I don't know it.  Perhaps is the stone equivalent of a sunburn.