Rhizocarpon reductum Th. Fr.
Cracked-areolate, greyish to mousey-brown thallus with thin black prothallus. Black apothecia with thin, persistent margin, initially inset later becoming slightly raised. Very common throughout Britain, on pebbles and other siliceous rocks
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species