Description: Thallus foliose, heteromerous, dorsiventral, adnate, forming up to 10 cm wide, orbicular or irregular rosettes. Lobes 1-2 mm broad, convex, stiff, contiguous or overlapping; center of thallus sometimes with a few imbricate lobules. Upper surface pale grey to brownish, darkening or blackening especially on the lobules and towards the tips, shiny. Lower surface black, brown toward the tips, smooth to weakly and shallowly wrinkled, with scattered hapters. Upper cortex paraplectenchymatous, the cell walls with Cetraria-type lichenan, with a non-pored epicortex; medulla white; lower cortex paraplectenchymatous. Apothecia rare, lecanorine, up to 10 mm across, with a dark brown disc and a thin, entire to crenulate thalline margin. Epithecium brownish; hypothecium and hymenium colourless. Asci 8-spored, clavate, the K/I+ blue tholus penetrated by a faintly amyloid apical cushion with parallel or diverging flanks, the wall K/I-, surrounded by a K/I+ blue outer layer, 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: upper cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC+red, P+ orange, UV-. Chemistry: upper 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.