Moist cliffs occur on shaded (by trees or the cliff itself because of aspect), moist to seeping, mossy, vertical exposures of various rock types, usually sandstone and dolomite. A greater proportion of sandstone cliff sites tend to be moist, compared to limestone cliff sites, due to the potential for capillary action in sandstone to transport water essential for plant survival. Igneous (granite, basalt) and metamorphic (quartzite) rocks tend to be dry due to their impermeability, but in some situations, water moving through the ground above the bedrock cannot go through the rock and moves laterally until it finds a path to take it downward. There it will exit, often over the face of a cliff. Moist cliffs may be shaded due to a cool (e.g., north-facing) aspect, or receive shading from associated tree canopy above or below the cliff. Relative to dry cliffs, they are often found on micro-sites of very restricted spatial extent.
Common vascular plant species growing on these cliffs include columbine (Aquilegia canadensis), the fragile ferns (Cystopteris bulbifera and C. fragilis), wood ferns (Dryopteris spp.), rattlesnake-root (Prenanthes alba), and wild sarsaparilla (Aralia nudicaulis). The rare flora of these cliffs varies markedly in different parts of the state--Driftless Area cliffs might have northern monkshood (Aconitum noveboracense), those on Lake Superior, butterwort (Pinguicula vulgaris), and those in Door County, green spleenwort (Asplenium viride). Lichens, mosses, and ferns are important components of cliff habitats. Present knowledge of the distribution and status of many of these plant species, along with many invertebrate species, is limited.
