Physcia caesia (Hoffm.) Fürnr.
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
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