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(Map updated: 11 August 2009)

On the north-facing surfaces of old limestone churches and ancient castle walls in warm, sheltered spots around Ireland, look for this species growing like a white-grey crust in small coalescing patches up to several centimetres wide. The grey crust is covered in a uniform dusting of powdery soredia so that it has a minutely speckled appearance. A fingernail dragged across the thallus should scratch orange. Discs are absent. Reactions: C+ rose-red, K-. Very thinly scattered in Ireland.

Key characteristics

• Forming a thick, grey (sometimes chalky) crust on north-facing limestone walls of very old buildings
• Thallus surface is grey sorediate which scratches orange with a fingernail; C+ rose-red.

Original text submitted by Vince J. Giavarini