Lecidea lithophila

Lecidea lithophila is a common crustose lichen on siliceous rocks in the uplands. The thallus is often cracked and stained rust-red from the iron oxide that it extracts from the iron-rich stone that it prefers. There are often large (up to 2mm), dark apothecia on its surface. Large siliceous boulders frquently become covered in numerous colonies making it identifiable at a distance. 

Listed by Alan Orange in Flora of Glamorgan (Wade et al, 1994).