In most of Wisconsin the bedrock is buried beneath glacial materials that were deposited during the Pleistocene Ice Age. In glaciated regions, cliffs are associated with certain stretches of the Great Lakes coasts, stream-carved gorges, and the vestigial remnants of ancient, eroded mountain ranges and escarpments. In the "Driftless Area" of southwestern Wisconsin the mantle of glacial drift is absent and erosion has exposed sedimentary bedrock of Paleozoic Age at many locations, most often as a linear series of vertical cliffs.
Technically, a cliff is a geologic feature, not a plant community, however, they are important habitat for a number of plant and animal specialists. They can occur on virtually any rock type. Rock type, exposure, surrounding land cover and other factors create a wide variety of environmental conditions that may influence species composition. The presence or absence of fractures and other features that may hold soil particles and moisture, or the alternation of strata composed of different rock types that have different properties, can affect habitat suitability for plants and animals.
A greater proportion of limestone (dolomite) cliff sites tend to be dry compared to sandstone cliff sites, due to the potential for capillary action in sandstone to hold and slowly transport the water that is essential for plant survival. A soil profile is generally absent, or may occur as localized, usually thin deposits on ledges or in cracks. Dry cliffs may be influenced by aspect, local hydrology, or the proximity of waterbodies. Series of dry cliffs may include stretches or patches that are moist, and these often support additional species. The separation of "dry" from "moist" cliffs can be somewhat artificial, and the totality of the environment should be considered when assessing conservation values and opportunities.
Dry cliff communities occur on many different rock types and vary in species composition. Scattered pines, oaks, cedars, and drought-adapted shrubs such as bush honeysuckle (Diervilla lonicera) and huckleberry (Gaylussacia baccata) often occur on the margins of the exposed rock, or where mineral soil has accumulated on ledges or in fissures. Floristic homogeneity between cliffs is typically rather low, but representative herbs may include the ferns common polypody (Polypodium virginianum), smooth cliff brake (Pellaea glabella), rusty woodsia (Woodsia ilvensis), and northern fragile fern (Cystopteris fragilis), along with wild columbine (Aquilegia canadensis), harebell (Campanula rotundifolia), sand cress (Arabidopsis lyrata), sleepy catchfly (Silene antirrhina), pale corydalis (Capnoides sempervirens), and rock spike-moss (Selaginella rupestris). Dry cliffs are frequently colonized by crustose lichens, which may be the most common inhabitants of bare rock environments for decades or even centuries.
Plant species composition is strongly influenced by the plant community in the immediate vicinity of the cliff, but also includes bare rock specialists, among which are some of Wisconsin's most dramatic examples of disjunct species. An example of a disjunct species is the population of Lapland rose-bay (Rhododendron lapponicum) that grows on a sandstone cliff along the Wisconsin River in the Central Sand Plains. One other population of this species is known from Wisconsin, but the next closest population is on an Adirondack mountaintop in rural New York.
Cliffs are used for denning and roosting by mammals, for nesting and roosting by birds, as hibernacula by herptiles, and also provide suitable conditions for specialized invertebrates. Besides insects, the latter group includes several very rare terrestrial gastropods.
