Description: Thallus foliose, adnate, forming orbicular or irregular rosettes to 10 cm in diam. Lobes 1-2 mm broad, convex, stiff, contiguous or overlapping; center of thallus sometime with a few imbricate lobules. Upper surface pale grey to brownish, darkening or blackening especially on the lobules and towards the tips, shiny. Upper cortex paraplechtenchymatous, covered with an epinecral layer; medulla white, solid; lower cortex paraplechtenchymatous. Lower surface black, brown toward the tips, smooth to weakly and shallowly wrinkled, with scattered hapters. Apothecia rare, lecanorine, up to 10 mm diam., with a dark brown disc and a thin, entire to crenulate thalline margin. Epithecium brownish; hypothecium and hymenium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, broadly ellipsoid, 7-10 x 6-7 µm. Pycnidia numerous, immersed, the visible part black to grey-brown. Conidia bifusiform, 5-7 x c. 1 µm. Photobiont chlorococcoid. Spot tests: cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC+red, P+ orange, UV-. Chemistry: cortex with atranorin and chloroatranorin; medulla with physodic and protocetraric acids.
Note: on wind-exposed surfaces of acid siliceous rocks wetted by rain near or above treeline; less bound to situations with a long snow cover than B. intestiniformis.