English: A lichen: Rhizocarpon oederi Several patches of this lichen were growing on a rock in a dry-stone wall. The top of the rock had a wishbone-shaped piece of rusty iron embedded in it (both the rock and the piece of metal are visible near the right-hand edge of the following photo: 1050079).

Rust-coloured lichens are often associated with iron-rich rocks, and the species shown here is "common on iron-rich siliceous rocks in upland areas" [F.S.Dobson, "Lichens - An Illustrated Guide to the British and Irish Species"]. In this case, the rock itself may be iron-rich, or the piece of rusty metal embedded in it may provide an iron-rich run-off.

Another species, Tremolecia atrata, is very similar in appearance and in habitat; both are rust-coloured, with black apothecia (the spore-producing discs), but those of T. atrata are smooth, while those of R. oederi can be seen, through a hand-lens, to have a wrinkled margin.

(See also 1037581 and 954017.)