Thallus foliose, green-grey or distinctly yellow-grey with peg-like isidia on the older lobe surfaces. Lower surface black. Apothecia with red-brown discs and pale, often undulate, crenulate margins. Common and widespread on siliceous rocks, often in exposed locations

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
Thell & Moberg, editors (2011) Nordic Lichen Flora, Volume 4: Parmeliaceae