Thin crustose thallus forming small grey or creamy-grey patches surrounded by a black prothallus. Round or irregular soralia with greyish-white or greyish, farinose or granular soredia. Found on siliceous rocks and pebbles, brick walls and gravestones

References: 
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species