Exposed, gently sloping, granitic outcrops of the Piedmont physiographic region support distinctive communities characterized by lichens and sparse vascular plants. This community group ranges from eastern Alabama to Virginia and is probably best represented in Georgia, with another noteworthy cluster in north-central North Carolina. Granitic Flatrocks in Virginia are restricted to a six-county area of the south-central Piedmont. Theyrange in elevation from about 70 to 100 m (230 to 320 ft) and occur on true granites and a range of related rocks such as granitic gneisses and granodiorites. Most examples are located on gentle slopes along streams, where the erosive power of water over time has worn rock surfaces smooth and created small, gravel-filled depressions.
Lichens (e.g., Xanthoparmelia conspersa, Cladonia caroliniana , and other Cladonia spp.) and the bryophyte (Grimmia laevigata) are dominant biota on granitic flatrocks, covering much of the exposed bedrock. However, vascular plants dominate locally in crevices, flats, and depressions where moisture and thin layers of detritus accumulate. Among the vascular plants, Small's stonecrop (Diamorpha smallii), Small's purslane (Portulaca smallii), and granite flatsedge (Cyperus granitophilus) are globally rare and endemic to these habitats. Other typical vascular plants include roundleaf fameflower (Phemeranthus teretifolius), Appalachian sandwort (Minuartia glabra), rock spikemoss (Bryodesma rupestre), common buttonweed (Diodia teres), glade rushfoil (Croton willdenowii), broomsedge (Andropogon virginicus var. virginicus), little bluestem (Schizachyrium scoparium var. scoparium), fork-tip three-awn grass (Aristida dichotoma), purple three-awn grass (Aristida purpurascens), common hair sedge (Bulbostylis capillaris), open-flower panic grass (Dichanthelium laxiflorum), and pineweed (Hypericum gentianoides). These small-patch communities often cover less than 0.4 ha (1 ac) and are considered globally rare. Major threats include invasive non-native weeds and quarrying.Reference: Belden (1998), Berg (1974), Fleming (2002a).
Click on the images below to open a larger image in a separate window.
Extensive moss and lichen mats (primarily Grimmia laevigata and Cladina mitis) on a large granitic flatrock in Brunswick County. Photo: Irvine Wilson / © DCR Natural Heritage.
Sparsely vegetated flatrocks of quartzofeldspathic gneiss along the Nottoway River downstream from The Falls, Lunenburg County. Gary P. Fleming / © DCR Natural Heritage.