Leaves and stems:

Leaves (fronds) are 3 to 8 inches long and to 2 inches wide, lance to egg-shaped in outline, once or twice compound typically with 4 pairs of leaflets (pinnae). Sterile fronds are thin with rounded to fan-shaped lobes and forked veins, with only the lowest pinnae sometimes twice compound.

Fertile fronds are larger and firmer than the sterile fronds, with larger, lance-oblong lobes bluntly pointed at the tips and edges curled under. The lowest pinnae are often more similar to those of sterile fronds.

Stems are brown in the lower half (or less), mostly green above, slender and smooth, lacking scales or with just a few pale scales on the lower stem. Leaves are scattered along a slender, brittle, creeping rhizome and are not clump-forming, though sometimes appear densely packed. Leaves wither and die back in late summer, the rhizome shriveling the next year after leaves emerge.

Spores:

The sori (group of spores) develop on fertile fronds starting in late spring and are found on the underside of a pinnae along the edge, the edge rolled around them so most are unexposed. The edging becomes a translucent white border around the pinnae.

Notes:

Slender Cliff Brake is a circumboreal species, preferring cool, moist, forested rocks and alpine habitats. While several references note the fronds grow erect, it's really a matter of the particular habitat and the angle of the rock cracks and crevices where it grows. Many we encountered were decidedly droopy and even upside down. Slender Cliff Brake is distinct from other ferns with its dissimilar sterile and fertile fronds, spores along the pinnae edge, rhizomatous growth, and the die-back starting in midsummer. While the sterile fronds may somewhat resemble some Cystoperis or Woodsia species, the spore pattern, stem coloration, presence of hairs or glands, and clump-forming growth should separate them from Slender Cliff Brake.