English: The "Map Lichen" - Rhizocarpon geographicum. This lichen, a few centimetres across, was growing on the boulders of an ancient hill-fort. Like crazy paving, thin black lines divide its surface into smaller areas that are called 'areoles'; these black lines are part of the 'prothallus' (areas without algal cells). The larger black patches are the 'apothecia' (structures which produce spores).

This very distinctive patterning gives the lichen its common name: "because of the strong pattern of lines formed by the black prothallus running between the areoles and the black apothecia it is often known as the Map Lichen" [Frank S. Dobson, "Lichens - An Illustrated Guide to the British and Irish Species"]

Rocks will often be found covered in "mosaics" made up of this species and others, with boundaries clearly visible between them. This specimen has a yellowish surface because it is growing on a site fully exposed to sunlight (see 945680, growing at the same site). However, its colour varies according to the amount of exposure to light, another instance where the characteristics of a lichen can reveal details about its environment. This species is "yellowish green when exposed to strong mountain sunlight, but greenish where the light is weaker" [George Baron, "Understanding Lichens"]