Let's see, now - I haven't added an installation to the Geology of Georgia series since May 2, 2010, some seventeen months ago, so I guess this still qualifies as a very "occasional" series. Not that there aren't other things to discuss, but then again there's no reason not to discuss the geology of the Peach State either.
In southwest DeKalb County, just to the south of I-285, Atlanta's Perimeter Highway, is Soapstone Ridge, a 25-square-mile structural complex with summit elevations over 1,000 feet above mean sea level. Soapstone Ridge is but one of the many high areas in an area referred to as the Winder Slope District of the Southern Piedmont Physiographic Province. The generally gently-rolling topography of the Winder Slope District is dissected by the headwaters of major streams draining to the Atlantic Ocean, and numerous dome-shaped, granitic mountains, such as Georgia's Stone Mountain, are located on the interfluves of the rivers.
Soapstone Ridge, although not dome-shaped and not granitic, is another of the District's interfluvial summits. The South River, a southeast-flowing tributary to the Ocmulgee River (which, in turn, flows to the Altamaha River, which, in turn, flows to the Atlantic), passes under I-285 in southwest DeKalb County. The high elevations of Soapstone Ridge divert the South River, and the river marks the northern boundary of Soapstone Ridge over much of its area.
Bedrock beneath Soapstone Ridge is a chaotic mixture of mafic and felsic rocks, consisting mainly of light-green-weathering amphibolites intimately interlayered with light-gray to nearly white gneisses and schists. Pods of very dense, resistant ultramafic rocks are also abundant, contributing to the rugged topography of Soapstone Ridge.
Mike Higgins and Bob Atkins gave the rock beneath Soapstone Ridge the appropriate-enough name "Soapstone Ridge Complex," and considered it to be a large ophiolitic sheet of mostly ultramafic rock (ophiolites are sequences of rock thought to represent oceanic crust). Higgins envisioned the Soapstone Ridge Complex as part of a large sheet of oceanic crust thrust up onto and above the crystalline rock of much of the Southern Piedmont, although only widely scattered remnants were now left due to either erosion or breakup of the sheet during transport and emplacement, or both. Other remnants of the sheet were thought to occur in Alabama as the Goodwater and Boyd's Creek ultramafic-mafic complexes, as well as the Doss Mountain amphibolite and Slaughters metagabbro.
However, subsequent work by Higgins and others showed that the bedrock beneath Soapstone Ridge was not all that unique, but part of another Georgia lithologic unit named the Paulding Volcanic-Plutonic Complex by (who else?) Mike Higgins. Like the "Soapstone Ridge Complex," the Paulding Volcanic-Plutonic Complex is essentially an all igneous unit, devoid of clastic metasedimentary rocks; however, an abundance of mafic and ultramafic blocks and slabs in the Soapstone Ridge exposure initially disguised its actual identity. The name "Soapstone Ridge Complex" was later formally abandoned by Higgins.
Interestingly, rocks of the Paulding Volcanic-Plutonic Complex at Soapstone Ridge are surrounded by Stonewall Gneiss, which forms a sheath of rocks around the Paulding, with the Soapstone Ridge rocks the "knife" in a large sheath of folded Stonewall Gneiss.
So there's that. For such dense rock that's so resistant to erosion, the Paulding rocks on Soapstone Ridge are covered by a surprisingly deep mantle of reddish-brown residual soil, easily excavated by a backhoe. Test pits within the material reached the 10-foot arm length of the backhoe without any indication of hard bedrock, yet the ridge still rises high above I-285 and the South River beyond.
In case you've never looked into ten-foot test pits dug into the Paulding Volcanic-Plutonic Complex on top of Soapstone Ridge before, like I did for much of today, here you go:
Curiously, most archeological sites in the Atlanta area are located at and around soapstones not in the Pauling (former "Soapstone Ridge Complex") but in the surrounding Stonewall Gneiss. Because of the refractive qualities of the soapstones within the Stonewall, and because of its ease in sculpting bowls, cooking utensils, and heat-retaining devices thought to have been used as bed-shelter-food warmers, most soapstone bodies within the Stonewall are archeological sites. Bowl fragmets can reportedly be found at many sites (I've never found one), and occasional bowl blanks can be found still attached to the rock.
Mike Higgins and Bob Atkins gave the rock beneath Soapstone Ridge the appropriate-enough name "Soapstone Ridge Complex," and considered it to be a large ophiolitic sheet of mostly ultramafic rock (ophiolites are sequences of rock thought to represent oceanic crust). Higgins envisioned the Soapstone Ridge Complex as part of a large sheet of oceanic crust thrust up onto and above the crystalline rock of much of the Southern Piedmont, although only widely scattered remnants were now left due to either erosion or breakup of the sheet during transport and emplacement, or both. Other remnants of the sheet were thought to occur in Alabama as the Goodwater and Boyd's Creek ultramafic-mafic complexes, as well as the Doss Mountain amphibolite and Slaughters metagabbro.
However, subsequent work by Higgins and others showed that the bedrock beneath Soapstone Ridge was not all that unique, but part of another Georgia lithologic unit named the Paulding Volcanic-Plutonic Complex by (who else?) Mike Higgins. Like the "Soapstone Ridge Complex," the Paulding Volcanic-Plutonic Complex is essentially an all igneous unit, devoid of clastic metasedimentary rocks; however, an abundance of mafic and ultramafic blocks and slabs in the Soapstone Ridge exposure initially disguised its actual identity. The name "Soapstone Ridge Complex" was later formally abandoned by Higgins.
Interestingly, rocks of the Paulding Volcanic-Plutonic Complex at Soapstone Ridge are surrounded by Stonewall Gneiss, which forms a sheath of rocks around the Paulding, with the Soapstone Ridge rocks the "knife" in a large sheath of folded Stonewall Gneiss.
So there's that. For such dense rock that's so resistant to erosion, the Paulding rocks on Soapstone Ridge are covered by a surprisingly deep mantle of reddish-brown residual soil, easily excavated by a backhoe. Test pits within the material reached the 10-foot arm length of the backhoe without any indication of hard bedrock, yet the ridge still rises high above I-285 and the South River beyond.
In case you've never looked into ten-foot test pits dug into the Paulding Volcanic-Plutonic Complex on top of Soapstone Ridge before, like I did for much of today, here you go:
Curiously, most archeological sites in the Atlanta area are located at and around soapstones not in the Pauling (former "Soapstone Ridge Complex") but in the surrounding Stonewall Gneiss. Because of the refractive qualities of the soapstones within the Stonewall, and because of its ease in sculpting bowls, cooking utensils, and heat-retaining devices thought to have been used as bed-shelter-food warmers, most soapstone bodies within the Stonewall are archeological sites. Bowl fragmets can reportedly be found at many sites (I've never found one), and occasional bowl blanks can be found still attached to the rock.
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