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(Map updated: 11 August 2009)

A very common lichen of hard, siliceous rocks, walls, monuments and pebbles; it is notably one of the first acid-rock lichens to puzzle the beginner because of the thallus colour which can vary from light grey to almost blue-green. To complicate matters further, in upland areas the thallus may become oxidised turning rusty-red. The smooth to slightly roughened surface is spotted with grey-blue soralia (floury-granular powder that is a mixture of fungal hyphae and algae), that sometimes form little piles which can join up to dominate the centre of the thallus. All chemical reactions are negative. Discs, when present, are black.

Key characteristics

• Relatively smooth, grey to bluish-grey thallus covered in heaps of tiny grey-blue soredia easy to see on the paler specimens
• The scattered soredia may join up to form a confluent surface crust.

Original text submitted by Vince J. Giavarini