A small, foliose species, with narrow, convex lobes with distinct white flecks (pseudocyphellae). Conspicuous, large soralia, usually rounded and convex, first appearing as grey-blue areas on the thallus surface. Marginal cilia absent. Extremely common and widespread on rocks, concrete and even asphalt

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
Ahti et al. editors (2002) Nordic Lichen Flora, Volume 2: Phyciaceae