In the beginning, when the glaciers melted, there was nothing but bare rock. And lichens thrived there. Then came the grasses and flowers in newly weathered soils, followed by scrub and finally forest whose shade cast out the bare rock-loving lichens. Then came humans with their axes and fire, cutting and burning the forest to grow crops and graze animals that often wandered away. So their owners quarried rocks and built walls. And then the lichens - like this Cladonia macilenta (I think - correct me if I'm wrong) found a new habitat on the rocks that humans piled one on top of another, in an unconscious act of habitat creation. Full circle.
Under close scrutiny, lichens are extraordinarily beautiful organisms - an intimate amalgam of alga and fungus, with each different combination producing a lichen of a distinct shape, colour and texture. The granular columns here, rising from those grey-green scales,are called podetia. They're tipped with red apothecia that produce fungal spores that will blow away, land on a rock, germinate and - if they happen to have landed close enough to a compatible alga, form a new lichen, just as they did on bare, broken rock left by the glaciers.
More on walls and their associated wildlife here
More lichens here