Plants in cushions or mats, yellow-green, glaucous-green to dark green, sometimes almost black. Stems 1--1.2 (--1.5) cm, central strand weak. Stem leaves narrowly lanceolate to ovate lanceolate, 1--1.8 × 0.2--0.6 mm, keeled, not plicate, margin plane proximally, incurved distally, awn 0.3--0.8 mm, costal transverse section prominent, terete; distal laminal cells 2-stratose, bulging, marginal cells 2-stratose; medial laminal cells quadrate to short-rectangular, straight, thin-walled; basal juxtacostal laminal cells quadrate to short-rectangular, straight, thick-walled; basal marginal laminal cells quadrate to short-rectangular, straight, thick-walled, not hyaline. Perichaetial leaves not enlarged. Sexual condition dioicous. Seta straight, 2--3 mm. Capsule occasionally present, exserted, yellow to brown, ovate to oblong-ovate, exothecial cells quadrate to short-rectangular, thick-walled, stomates absent, annulus of 1 row, quadrate, thick-walled, operculum conic to mammilate with a short obtuse beak, peristome present, fully developed, split and perforated in distal half.

Exposed acidic granite and sandstone; 360--3300 m; Greenland; Alta., B.C.; Ariz., Calif., Col., Idaho, Mont., Nev., Oreg., S. Dak., Utah, Wash., Wyo.; Eurasia.

Grimmia alpestris has a distribution similar to that of G. montana, being widespread and common on acidic rock in the warm, dry, western interior of North America from southern British Columbia and Alberta down through to California and Colorado. The other two North American species with bulging laminal cells, G. caespiticia and G. sessitana, both have capsules with stomates and so capsulate specimens of G. alpestris are readily separated from these two species. H. C. Greven (2003) reported that the capsules of G. alpestris are brown, with thick exothecial cell walls, and are smooth when empty, whereas those of G. sessitana are yellowish to light brown, with thin-walled exothecial cells walled, and are striolate when empty. The incurved margins and uniformly 2-stratose distal lamina of G. alpestris contrasts with the plane or one recurved margin and 1-stratose distal juxtacostal laminal cells of G. sessitana. The dioicous sexuality of G. alpestris helps separate these species. Some specimens of Grimmia alpestris have leaves with streaks of thickened or multistratose cells; superficially these may appear to be plications. However, in transverse section they are readily separated from the truly plicate leaves of G. caespiticia. Further, G. alpestris leaves are never cucullate, while G. caespiticia usually are. Grimmia alpestris is best separated from G. montana by its bulging laminal cells and secondarily by its relatively uniform, quadrate to short-rectangular basal areolation. While G. donniana also has a uniform basal areolation, its cells are much longer and thin-walled, and its margins are plane throughout the length of the leaf.