Rock Type: Graphite
schist
Geologic terrane or major geologic element: Crabtree terrane
Age: Late Proterozoic
to Cambrian – approximately 620-520 million years old
Location: Google
Maps Link
USGS 7.5-minute Quadrangle:
Raleigh West
Site Access: From Lead Mine Road, turn onto Lakepark Dr.
and then right onto Rushingbrook Dr.
Park at the greenway trail entrance on the left. Follow the trail down, parallel to the creek. Walk past the volleyball court and then to
the right, near the greenway bridge.
This section of greenway trail is the Lake Park Connector Trail. It connects to the paved greenway trail that runs
around Shelley Lake.
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Technical Information: Blake and others, 2001, A Temporal
view of terranes and structures in the eastern North Carolina Piedmont, in Geological Society of America,
Southeastern Section, Field Trip Guide for 2001. See description for Stop 4 on p. 168-169.
Lumpkin, B. L., Stoddard, E. F.,
and Blake, D. E., 1994, The Raleigh graphite schist, in Carolina
Geological Society Field Trip Guide, 1994, p. 19-24. See also the description for Stop 3 on pages
91-92.
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Introduction
The Crabtree terrane contains a wide variety of
metamorphosed igneous and sedimentary rocks, of which the graphite schist is
undoubtedly the most distinctive.
Although it occurs only in Wake County, the graphite schist runs in
several narrow bands that may be traced from south of Lake Wheeler north to
Falls Lake. This very soft rock is sooty
black and easily marks paper (and skin).
Graphite is a mineral form of carbon (diamond is the other!) that occurs
in metamorphic rocks. Graphite may be
mined for use as a lubricant, for batteries, heat-resistant containers, and for
pencil “lead.” In Wake County, several
historic mines produced graphite from the mid-19th century to the
early 20th century. Lead Mine
Road is named for an old graphite mine, but there is no real lead (Pb), just
graphite (C). Today, most graphite used
in manufacturing must be very pure, and much of it is synthetic. Graphite also is an excellent conductor of
electricity.
Raleigh graphite schist
In addition to graphite, the schist contains muscovite mica,
garnet, a small amount of quartz and feldspar, and commonly one the metamorphic
minerals staurolite and/or kyanite. The
percentage of graphite may vary greatly (perhaps 5-50%) but is very high
compared with schist from elsewhere. If
you crush some of the schist, you should be able to find a few of these other
minerals, which are much harder than the graphite.
At this site, the large exposure of graphite schist is the
result of stream erosion along the outside of a bend, creating a cut bank
(Figure 1).
Around west Raleigh, construction activities frequently
reveal the presence of graphite schist beneath the soil. If you happen to see very dark gray to black
material in a fresh excavation, it very well could be this rock. An example is shown in Figure 2, which is a
photo taken in the 1980s at the site of the A. E. Finley YMCA on Baileywick
Road in north Raleigh.
Significance of the graphite schist
Graphite is a form of carbon, and analysis of the Raleigh
graphite shows that the carbon is most likely to be of organic origin. The age of the graphite schist is not certain
but based on its geological relationships with other rock units whose ages are
known, it must be older than 500 Ma (million years). This age is well before the age of woody
plants and coal deposits, which are younger than 400 Ma. The most likely organic source for the
Raleigh graphite then is algae. The high
concentration of carbon, and the fact that the graphite schist is interlayered
with metamorphic rocks that likely originated as sand and clay, suggests an
ancient shallow-water algal mat environment.
Such environments exist today along the coast of Western Australia.
