Plants in flat patches, dark green to brownish black. Stems 1--2 cm. Stem leaves narrowly ovate lanceolate from an ovate base, 2--3 × 0.4--0.8 mm, both margins incurved, intermarginal bands absent, awn 0.1--0.5 mm, not decurrent, acute, costa narrow proximally; distal laminal cells 2--4 stratose, rounded, thick-walled; medial laminal cells quadrate, slightly thick-walled; basal juxtacostal laminal cells quadrate to short-rectangular, straight, thick lateral walls, green; basal marginal laminal cells quadrate, straight, thick transverse and thin lateral walls, green, hyaline. Perichaetial leaves not enlarged. Seta sigmoid, 3--4 mm. Capsule occasionally present, exserted, brown, oblong-ovoid, exothecial cells short-rectangular, thin-walled, stomates present, annulus of 2--3 rows, rectangular, thick-walled, operculum long-rostrate, peristome perforate in distal half, split in distal half. Calyptra cucullate.

Cracks and exposed faces of dry to periodically wet, acidic or calcareous rocks, commonly along streams or splash zones of lake shores; 20--600 m; Ont., N.S.; Ark., Conn., Del., Maine, Mass., Mich., Mo., N.J., N.Y., N.C., Pa., R.I., Tenn., Vt., Va.

Grimmia olneyi is endemic to eastern North America with its center of distribution being along the Appalachians from the New England states southward to North Carolina and eastern Tennessee. A disjunct population occurs west of the Appalachians on the Ozark Plateau of Missouri and Arkansas. Unlike Grimmia unicolor, G. olneyi is tolerant of calcareous rocks and is able to occupy drier sites. Because of its sinuose seta and somewhat wrinkled capsule, Grimmia olneyi is usually placed in subg. Rhabdogrimmia. However, the seta of G. olneyi is usually only somewhat sigmoid and is rarely arcuate. Further, its capsules only become wrinkled when dry, whereas the Rhabdogrimmia typically have plicate capsules whether dry or turgid. In fact, specimens of G. olneyi are most commonly misidentified as either G. ovalis or G. laevigata, species in subg. Litoneuron. The seta and capsule of G. olneyi are at the extremes of both subgenera. However, the general habit of the plants and their leaf structure suggests a close relationship with G. ovalis and G. unicolor.

Grimmia olneyi and G. laevigata have broadly overlapping distributions in eastern North America where many specimens of G. olneyi have been misidentified as G. laevigata. Typical specimens of G. olneyi are readily separated from G. laevigata. However, some leaves on a stem of G. laevigata may have defined ovate bases with narrowly decurrent awns. These specimens will resemble G. olneyi. But, G. olneyi always has a narrow costa, occupying no more than 1/4 to 1/3 of the basal lamina, while G. laevigata has a broad costa occupying at least 1/2 of the lamina and the costa grades gradually into the basal laminal cells. Further, G. olneyi has quadrate to short-rectangular basal juxtacostal cells, while those of G. laevigata are elongate, almost resembling costal cells. Grimmia olneyi most closely resembles G. ovalis. Both have ovate lanceolate leaves from an ovate base, a narrow distal lamina that is plane and channelled, ending in a narrowly attached, long and toothed awn. The costa is narrow proximally in both species. However, they are otherwise quite distinct. Aside from the sigmoid seta and slightly wrinkled dry capsule of G. olneyi, its basal juxtacostal laminal cells are shorter and straighter than G. ovalis and its basal marginal cells are quadrate while those of G. ovalis are rectangular. Geography alone has also been used to separate these species. As H. A. Crum and L. E. Anderson (1981) rejected all reports of G. ovalis from eastern North America, it seemed that G. ovalis was a western species while G. olneyi was eastern. However, we have seen a number of specimens of G. ovalis from eastern North America and thus using geography alone is not reliable.