Stone Mountain, Georgia

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Stone Mountain is a famous granite monolith east of Atlanta. It's more an inselberg than a monadnock since it's the exfoliated remnant core of a considerably larger intrusion.

Kennesaw Mountain, northwest of Atlanta, was a major Civil War battlefield and is now a National Military Park, but Stone Mountain just doesn't offer any place to dig in, and anyway it was off the Union axis of advance.

Gutzon Borlum practiced for Mount Rushmore by carving Robert E. Lee, Jefferson Davis and Stonewall Jackson into the side of Stone Mountain.
Open air performances use the sculpture as a backdrop.
Exfoliation sheet in the granite. That granite was dated at 291 m.y. and was estimated to have been emplaced at a depth of 12 km.

Ar-39 studies suggest the granite was exhumed from a depth of about 11 km in the last 71 m.y., or about 0.15 mm/year or 155 m/m.y. This is actually a bit rapid for a tectonically quiet region.

Left and below: a crude foliation curves into an exfoliation joint.
 
Left and below: closeups of the granite.
 
View from the east.
Below: a stern-wheel sightseeing boat cruises the reservoir that encircles the southeast half of the mountain.
Above: zoned dikes in the granite.Below: the inscriptions have weathered at most a millimeter in over 100 years.
Some mathematically literate graffiti.
 
Worn footpath on the lower slopes of the mountain.

Below: weathered fractures.

Exfoliated slabs on the lower slope.
 
Rare Georgia gum tree. Beats sticking it to the rocks.
Exfoliated surface.
The railroad around the base of Stone Mountain at the entrance to the foot trail. Below: views of the mountain from the west and south.
Left and below: these strange discoidal structures are common on Stone Mountain.
This photo shows that the discoids dip into the face of the mountain. I suspect they are en echelon cracks opened by shear (top to the east). If it happened while the magma was still partially molten, filter-pressing could have squeezed magma into the cracks, possibly even as they were forming. Differential weathering produces the results we see today.
Close-up of discoid.
Remnant of a discoid
 
Left and below: erosional grooves.
A railing helps at the steepest part of the climb.
 

The Tramway

The lower tram terminal. The cable cars climb past the sculpture.
 
Upper tram terminal.
Left and below: views from the top of the tram.
 

Summit Views

Left and below: Atlanta skyline with the aura of progress.
 

Features on the Summit

Deep chemical weathering pits.
Below: where water trickles over the rock, chemical weathering makes grooves.
Left and below: faint foliation on weathered surfaces.
Exfoliated slab on the summit.
Large deep weathering pit. Shrimp live in some of these pits when they contain water. The eggs were probably originally blown by the wind or carried by birds. Once there, eggs can survive dry periods in crevices or lichens.
Exfoliated slab and erosional grooves.
The wedges embedded in the granite appear to be the cores of the odd discoids that occur all over the mountain.
Joints and weathering.
Weathered discoid.
A true geologic mystery is this giant pair of fossilized lips in granite. Pity they didn't know about this when they made the Blarney Stone.

I suspect this is differential weathering of one of the discoids.

Left and below: flow banding accentuated by weathering.
 
 

Goodness, Glaciers!

Glaciers in Georgia? These patches of grooved and polished rock certainly look glacial.

On the other hand, I get just a wee bit skeptical when the grooves also cut into asphalt. And the white line parallel to the grooves doesn't help any. Very few glaciers needed a white line to guide them.

These are almost certainly cable grooves created by stringing power line or lift cables. But in a notorious case, grooves made by logging cables were once mistaken for glacial grooves elsewhere in the southern Appalachians.

Cultural Features at Stone Mountain

Left and below: an old grist mill has been relocated to the park.
Left and below: a covered bridge now crosses the channel to a small island.
The east quarry: buildings all over the U.S. were faced in Stone Mountain granite.
The quarry worked exfoliation sheets, so the quarry had little effect on the overall appearance of the mountain.
Scarp left behind by quarrying an exfoliation sheet.
The blocks were split out by drilling a series of holes, then tearing along the dotted line.

An antebellum plantation house has been restored at Stone Mountain.
Outbuildings, including slave cabins, are part of the plantation.
Picture every movie you ever saw about dark, perverted things happening in a decaying old plantation house. Wouldn't this be a great set? It looked truly creepy sitting there in the woods (February 2004).

Apparently it was just parked because it wasn't there in November when I went back. I presume it's being restored. How they got it in or out of there I haven't a clue.

Whitney, J. A. and Jones, L. M., 1976, Age and origin of the Stone Mountain Granite, Lithonia district, Georgia, Geological Society of America Bulletin: Vol. 87, No. 7, pp. 1067�1077.

Atkins, R. L. and Joyce, L. G., 1980, Geologic Guide to Stone Mountain Park, Georgia Dept. of Natural Resources, Georgia Geologic Survey, Geologic Guide 4, 29 p.

Dallmeyer, R.D., 1978, Ar 40/Ar 39 Incremental release ages of hornblende and biotite across the Georgia Inner Piedmont: Their bearing on Late Paleozoic-Early Mesozoic tectonothermal history, American Journal of Science, v. 278, p. 124-129.

Dennison, J.M., 1999, Geomorphology of Stone Mountain, Georgia - Field Trip, in Whitney, J.A., Dennison, J, and Gore, P. J. W., Geology and Geomorphology of Stone Mountain, Georgia, Field Trip Guide, Geological Society of America, Southeastern Section Meeting, Athens, GA.

Grant, W. H., 1986, Structural and petrologic features of the Stone Mountain granite pluton, Georgia, Geological Society of America Centennial Field Guide - Southeastern Section, Locality 65, p. 285-290.

McConnell, K.I. and Abrams, C.E., 1984, Geology of the Greater Atlanta Region, Georgia Geologic Survey Bulletin 96.


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Created 27 October 2005, Last Update 01 July 2012

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