Geological Age and Weathering
Geologists estimate that most of the granitic rocks that outcrop in the Piedmont of the southeastern United States are approximately 300-350 million years old. These outcrops were formed by the intrusion of molten granite into preexisting country rock at a depth of about sixteen kilometers below the surface. Over millions of years, erosion removed thousands of feet of overlying rock, exposing the more resistant bodies of granite.
Granite itself is often very heterogeneous in mineral composition and texture. Often neighboring outcrops that would appear to have developed as parts of the same rock actually occur on distinctly different types of rock intrusions. For example, Stone Mountain, Panola Mountain, and Arabia Mountain all occur within about fifteen miles of each other, east and southeast of Atlanta. Yet each represents a distinct and separate rock type: Stone Mountain granite, Panola granite, and Arabia Mountain gneiss, respectively.
Rock-rimmed pits that are deep enough to hold rainwater harbor a number of rare aquatic plants. Other depressions, which have accumulated a thin layer of soil and which lack a complete rim to retain water, support many unusual annual plants. Other distinctive habitats on the outcrops include deeper accumulations of soil over bare granite, seepage areas created by the slow discharge of water from patches of forest on the rocks, and the edges of nearby forests.
An unexpectedly large number of plant species occur only on Piedmont granite outcrops, which would seem to reflect their antiquity and isolation. Among others, this group includes the monospecific genera Diamorpha and Amphianthus.
Environmental conditions on granite outcrops are harsh and differ sharply from conditions in the adjacent forest. Temperatures on the outcrops typically are much higher than in the surrounding forest because of the high absorption of heat by the rocks and low evapotranspiration. Temperatures in excess of 120 degrees Fahrenheit are common at rock surfaces during the summer months.
Shallow mineral soil overlying impervious rock and sparse vegetation cover lead to extraordinarily high water runoff. It has been estimated that more than 95 percent of the annual precipitation in outcrop communities is lost as direct runoff. These conditions make the granite outcrops, despite their occurrence in a region that receives forty-five to fifty inches of rain per year, like islands of desert embedded in a sea of deciduous forest.
Unfortunately, local landowners and government agencies alike have viewed most of the granite outcrops in Georgia as relatively worthless land, since such property cannot be used for construction, agriculture, or grazing. Thus, outcrops have frequently been used as dump sites for household or construction waste. Many others have been destroyed by quarrying or have been covered by lake waters after the construction of a dam.
Some sites, however, have been protected, either because of their inaccessibility to humans or because of timely action on the part of governmental or private agencies. These sites include Stone Mountain State Park in DeKalb County, Heggie's Rock in Columbia County, Davidson-Arabia Mountain Park in DeKalb County, Little Kennesaw Mountain in Cobb County, and Panola Mountain State Conservation Park, which spans Rockdale, Henry, and Dekalb counties.
Madeline P. Burbanck and Robert B. Platt, "Granite Outcrop Communities of the Piedmont Plateau in Georgia," Ecology 45 (1964): 292-305.
Robert Edward Wyatt, Highlands Biological Station
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