Scattered though the eastern Appalachian Blue Ridge and Piedmont are many isolated bodies of ultramafic rock; all are to some degree metamorphosed and weathered. One such body is Soapstone Ridge in southeast Atlanta. Lying in south DeKalb County near the I-75/I-285 intersection, Soapstone Ridge seems to be an ancient body of deep ocean crust/mantle with its ultramafic rock altered to soapstone (steatite), pushed into place during assembly of Pangea. The base of this body shows thrust fault contact with underlying rock.
Soapstone is dirty talc; talc is metamorphosed ultramafic rock, usually serpentinite. Serpentinite is water-damaged peridotite. Peridotite forms near the crust-mantle boundary at mid-ocean ridges where the driving forces of plate tectonic motion create new ocean crust. When we see serpentinite on land, we are looking at a body of deep ocean crust that has been soaked in sea water, broken by tectonic collision, and wedged up from ocean depths. Serpentinite often occurs in association with other deep-ocean rocks: pillow basalts, diabase sheet dikes, and gabbros. This rock suite, collectively called an ophiolite, occurs in mountain chains worldwide. Taken together, this suite constitutes a vertical section of ocean crust; its presence on land is considered evidence of past activity at destructive tectonic margins (subduction zones).
Native Americans made bowls from Soapstone Ridge rock, as indigenous peoples have done with similar rock in Alaska and Nova Scotia. The Soapstone Ridge area may contain their artifacts and other archaeological evidence, as well as plant communities adapted to nutrient-poor serpentinite soils. Although the Soapstone Ridge area has been disturbed by recent development, presence of massive rock near the ground surface has inhibited large-scale construction.
For further information on Soapstone Ridge, ophiolites, and ultramafic plant communities, consult: