Leprocaulon calcicola

A distinctive lichen in appearance and habitat; a bluish green leprose thallus lacking a true medulla, without projecting hyphae and growing on calcareous walls. With zeorin and usnic acid (K-, C-, KC+ yellow, Pd-). The KC+ yellow reaction distinguishes L. calcicola from other leprose British species, except for Lepraria ecorticata on acid rock, which also has a distinct pale yellow tinge. So far mainly found in East Anglica, but likely to be found elsewhere in the lowlands.

See Fungi of Great Britain and Ireland & Orange et al (2017) The Lichenologist 49: 183 - 188.