Setting and Regional Geology
 

The park is located right at the foot of a major topographic feature called the Blue Ridge escarpment.  This escarpment is where the slope rises quickly to the west, and it represents the boundary between the Piedmont and the Blue Ridge physiographic provinces.  In North Carolina, the Piedmont and Blue Ridge consist almost entirely of metamorphic and igneous rocks.  Most of these rocks were formed during a series of Paleozoic plate tectonic processes associated with the eventual creation of the supercontinent Pangaea and the formation of the Appalachian Mountains.  The rocks of Stone Mountain State Park are no exception: they belong to a large igneous pluton that was intruded as magma during the collision of continental plates.  Stone Mountain lies in the Blue Ridge geologic province, and is a pluton belonging to the Spruce Pine plutonic suite (Hatcher and Goldberg, 1991), a group of granitic intrusive bodies lying between Spruce Pine and Mount Airy, North Carolina.  This suite includes the alaskite and pegmatite bodies in the Spruce Pine mining district, that are mined for feldspar, quartz, and muscovite, and also contain garnet, uraninite, and beryl, including emerald.  The Mount Airy pluton is home to the largest open-face granite rock quarry in the world, which is mined for dimension stone, and provided stone for many historic builstructures including the Arlington Memorial Bridge in Washington, D.C. and the Amoco Building in Chicago  (www.ncgranite.com).

 

Geologic Map of Stone Mountain Area
Rock Unit Descriptions
Geologic Map of Stone Mountain and surrounding area.  Map and key modified from NCGS (1985) map

On the geological map of the state (see above map), the Stone Mountain pluton shows up as a pink blob in the midst of bands of different colors that run southwest to northeast.  The SW-NE bands represent older metamorphic rocks, mainly schist and gneiss, into which the magma was intruded.  These metamorphic rocks originated as layered sedimentary and volcanic rocks associated with the opening of the ancient Iapetus Ocean.  They are Late Precambrian in age (900 – 600 million years old) and belong to the Alligator Back formation.  The Alligator Back formation strikes about N50E, which explains the bands on the map.  Because the Stone Mt. Pluton intrudes the Alligator Back fm., it must be younger.  In fact, rocks of the pluton have been dated at 335.6 million years old (Miller and others, 2006).

 
Geological History and Tectonics
 

The evolution of the southern Appalachians was not a simple tectonic event.  In fact, geologic evidence suggests that several collisions between formerly separated continents, volcanic arcs, and perhaps other chunks of crust (terranes), occurring over a period of about 350 million years, were involved in the formation of these mountains.  The last and best preserved event was the collision of the proto-African continent with ancient North America during the Alleghenian orogeny, about 320 - 260 million years ago.  However, other tectonic events occurred during the Taconic and Acadian orogenies, from about 450 to 350 million years ago, and still older significant terrane interactions occurred between 600 and 500 million years ago.  Granitic magma is thought to have been generated by subduction or along strike-slip fault zones during these tectonic events.  Recent research by Hatcher and colleagues suggests that subduction of the Cat Square and the Tugaloo terranes during the Acadian Orogeny (around 360 million year ago) created conditions necessary to produce granitic intrusions (SAMS; Gatewood and others, 2006), including the Stone Mountain pluton.