The Faces of Lichen

It wasn’t so long ago that I thought these memorials were just dirty, worn away with time and the elements, including acid rain.
But I’ve been looking closer. At the lichens.
Tireless, long- and slow-growing lichens, lovers of stone. Well, at least these species. Others favor wood. Some grow on both wood and stone. Some favor other kinds of stone (these are limestones and marbles, I think). There are specialists and generalists among the lichens.
Lichens can corrode stone. They do this physically, by expanding into the rock, as well as chemically, by excreting acids.
(Berry or seed courtesy of a bird.)
(This one is two-faced.)
Up on their plinths, they’re too high to get that close to.