Plants to 2.5 cm. Stem leaves loosely erect-appressed when dry, elongate-lanceolate to lanceolate, 1.7--3.5 mm, ± narrowly obtuse, rarely bluntly acute; margins revolute below, recurved to almost plane near apex, entire; distal laminal cells 7--13 µm wide, 2-stratose, rarely with a few 1-stratose areas, papillae always small, conical, 1--3 per cell; basal laminal cells rectangular to short-rectangular, grading to quadrate at margins, non-nodose. Sexual condition goniautoicous. Seta 0.5--1 mm. Capsule immersed when moist, about 1/2 emergent when dry, urn oblong, short-oblong, or oblong-ovate, 1--1.8 mm, strongly 8-ribbed 1/2 to entire length, rarely with 8 very short intermediate ribs; stomates immersed; peristome double; endostome segments 8, of 1 row of cells, finely longitudinally-striate; exostome teeth 8, sometimes irregularly split to 16, incurved until old, then spreading or rarely reflexed, coarsely striate-reticulate, or striate-papillose. Calyptra oblong, sparsely hairy, hairs papillose. Spores 10--17 µm.

Rocks, usually limestone or calcareous sandstone, sometimes granite, quartzite, or basalt, rarely on trunks of deciduous trees, open pine forest, spruce-fir forests or deciduous scrub oak-maple forests, especially common on vertical canyon walls and shaded cliff faces; 200--3,000 m; B.C.; Ariz., Calif., Colo., Idaho, Mont., Nev., N.Mex., S.Dak., Utah, Wyo. Restricted to western North America.

Orthotrichum hallii is distinguished by having leaves that are completely, or nearly so, 2-stratose in their distal portion, and oblong, 8-ribbed capsules that are emergent. The peristome teeth vary from ridged-striate to papillose-striate. The proximal portions of the teeth are often coarsely papillose with the distal portions having striae mixed with papillae in all proportions. The papillae are always small and conical.