Rock Types - Stone Mountain State Park

The dome of Stone Mountain is almost entirely composed of medium- to coarse-grained granite.   The mineralogy of the granite consists of quartz, alkali feldspar, plagioclase feldspar, and biotite and muscovite mica. According to McCarter and others (2001) the Stone Mountain pluton is composite in nature, having "at least twelve cross-cutting phases." Petrologically, "the marginal phases are fine- to medium-grained (~0.05-2 mm) muscovite-biotite tonalite to granodiorite, and the interior of the pluton is largely medium- to coarse-grained (~1 mm-1 cm) leucocratic muscovite-biotite quartz monzonite to granite" (McCarter and others, 2001).  The pluton is included with the northern group of the Spruce Pine plutonic suite by McSween and others (1991).

Alligator Back Formation

This Late Precambrian formation is composed of metamorphosed sedimentary and volcanic rocks and can be found in several places in the Blue Ridge Mountains.  It can be found in the valley of Stone Mountain State Park.  Also, the formation is present as xenoliths (see Features and Processes page) on the dome.  The picture to the left is such a xenolith located on the dome's north side of the trail..


Rock Types_Stone Mountain State Park
Photograph by Shawn Fitzmaurice.

Features and Processes - Stone Mountain State Park
Exfoliation Jointing

Mountain climbers frequent Stone Mountain to take advantage of the natural grips resulting from the geological process known as exfoliation jointing.  Sheets of granite parallel to the surface of the mountain appear to flake off, similar to how layers of onions do.  This process occurs as material, referred to as overburden by geologists, is removed over millions of years.  At Stone Mountain, the removal of overburden decreased the confining pressure exerted on the now hardened pluton.  The granitic rock was then allowed to expand since it no longer had so much material weighing it down.  Other processes accelerate the propagation of the joints (Martel, 2004).   Pressures exerted by water flowing through the joints and the expansion of water as it freezes also contribute to the highly jointed surface of the mountain.

exfoliation features at Stone Mountain State Park
Example of exfoliation weathering at Stone Mountain State Park.  Photograph by Shawn Fitzmaurice. 
Weathering Pits

Numerous depressions ranging from centimeters to meters in depth and diameter can be found on the higher altitude surfaces of the Mountain.  These depressions may originate from the erosive power of water, and sometimes appear to be aligned with cracks (joints) in the granite.  In places, these shallow pits may support the growth of primitive plants such as lichen.  In fact, there may be a symbiotic relationship between rock and plant, as the plant produces weak acids that aid in the chemical weathering of the granite, and by this process the feldspar in the granite transforms to clay minerals and releases ions.  The clay allows roots to attach and the ions provide nutrients for the plants.

weathering pits Stone Mountain State Park
Example of weathering pits at Stone Mountain State Park.  Photograph by Nathan Lyons. 

Fragments of rocks with composition differing from the granitic majority can be found protruding from the face of the Mountain.  The rock to the left, which belongs to the Alligator Back formation, was engulfed by the pluton during its formation - when it still was a liquid magma.  The fragment is known to be an older specimen than the granite surrounding it since the fragment would have had to exist prior to its inclusion in the pluton.  Finding older material enclosed within younger material is one of the basic ways that geologists determine the sequence of past geological events.

xenolith at Stone Mountain State Park
Example of xenolith at Stone Mountain State Park.  Photograph by Nathan Lyons.  Dime for scale. 

An observant park visitor may wonder about the origin of the rounded rocks at the bottom of the small pools in the streams.  These rocks are rounded during times of high water, as the water current swirls rapidly in the pothole.  In addition to these cobbles and pebbles being rounded by this process, the depression itself grows, and is in fact caused by, the impact of the sediment against the sides of the depression - almost like a hydraulic drill.

potholes Stone Mountain State Park
Example of pot holes at Stone Mountain State Park.  Photograph by Shawn Fitzmaurice.