Clairmont Formation

Named for its exposure at I-85 and Clairmont Road, the Clairmont formation underlies part of the eastern Atlanta area. The Clairmont consists mostly of dark, contorted gneiss with inclusions of other rock types. Based on its composition, structure, and the variety of unrelated rock fragments bound in it, some geololgists have concluded that the Clairmont is a metamorphosed Paleozoic tectonic melange, possibly a subduction melange. And you thought those were all in California!

In the Atlanta area -- and possibly northeast and southwest of Atlanta -- the Clairmont formation lies closely southeast of the Brevard Zone. Recent mapping suggests that the Clairmont formation is a tectonized phase or facies of Stonewall gneiss, which resembles Lithonia gneiss, which may be both derived from Promised Land formation and parent material for Stone Mountain granite. Yes, this is confused (perhaps literally).


Exposures in the Atlanta area:
Boford Dam
Rock in the rubble pile below Buford Dam -- and in the bed of the Chattahoochee River below the dam -- is probably Clairmont formation.
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Ga. Hwy 20 at Suwanee Dam Road
Construction cut behind shopping center in the intersection's northeast quadrant exposes possible Clairmont formation.
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Clairmont Road and I-85
Type locality for the Clairmont formation. Look for a massive cut face in the southwest quadrant of the intersection, alongside the exit ramp from I-85 northbound. Nearly impossible to miss, even if you are not looking for it.
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Buford Highway and North Druid Hills Road
Cut face on the southbound side of Buford Highway in the north quadrant of its intersection with North Druid Hills Road. This exposure is relatively small, dirty, and close to traffic. (No photos.)
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Georgia Highway 13 (I-85 feeder) southbound near Piedmont Road
Cut face on right (west) along the feeder road to I-85 southbound, at the Armour Drive entrance ramp south of Piedmont Road.
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I-75 northbound near I-75/I-85 split
Cut face on right (east) at on-ramp just north of I-75/I-85 division departing midtown Atlanta. Structure at this site is particularly interesting. It is possible to stop here without being killed by traffic.
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I-85 entrance ramp to I-75 northbound
Cut face on right of entrance ramp from I-85 southbound to I-75 northbound, departing midtown Atlanta. You must be crazy to stop here. (No photos.)
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Ga. Hwy 400 southbound at Sidney Marcus Boulevard
Cut face on right (west) at exit ramp to Sidney Marcus Boulevard. This and other roadcuts on Ga. 400 southbound through the Brevard Zone are worth the 50-cent toll.
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Peachtree Center MARTA Station
Underground, where North Line MARTA trains stop beneath Peachtree Center in Atlanta. Regrettably, the passenger platform is centered between tracks, so you cannot inspect this exposure closely; also, cut faces are dulled by accumulation of airborne crud. Still, you should stop at Peachtree Center and ogle rocks while you wait for the next train. Also look at architectural facings of fossiliferous black marble.
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South Fulton Parkway
Roadcuts about 1/2 mile east of Cedar Grove Road and about 1-1/2 miles east of Georgia Highway 92 (Campbellton-Fairburn Road) expose rock strongly resembling Clairmont formation.

See photos.

For further information on the Clairmont formation, consult:

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