Rock Types: Gneiss and pegmatite
Geologic terrane or
major geologic element: Raleigh
terrane
Age: Late Proterozoic to Cambrian – approximately
620-520 million years old
USGS 7.5-minute
Quadrangle: Raleigh West
Site Access: Fallon Park is
located along a creek that parallels Oxford Road in a residential neighborhood
near downtown Raleigh. The creek is a
tributary of Crabtree Creek, which it enters about one km to the northeast.
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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 the description for Stop 1 on p. 163-164.
Stoddard, E. F.,
and Blake, D. E., 1994, Carolina
Geological Society Field Trip Guide, 1994.
See the description for Stop 12 on pages 107-108.
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Introduction
Most of the downtown area of
Raleigh lies within the Raleigh gneiss, a rock unit that makes up most of the
Raleigh terrane. Many creeks that flow
through or near downtown have created natural outcrops of Raleigh gneiss. Construction activities periodically provide exposures
of the gneiss as well. Along Capital
Blvd., Pigeon House Branch contains two excellent exposures. The city’s greenways also pass by several
good examples.
Raleigh terrane and Raleigh gneiss
The Raleigh terrane is situated
between the granitic Rolesville batholith, to the east, and the Crabtree
terrane to the west. The Raleigh gneiss
is the dominant unit within the terrane.
The gneiss represents the highest metamorphic grade within the county,
and thus its rocks were buried more deeply than the other metamorphic rocks,
reaching higher temperatures and pressures.
The gneiss is characterized by alternating layers of darker and lighter
color, reflecting the differing mineral content of the layers. Dark layers contain biotite (black mica) with
or without hornblende (black amphibole).
The light layers contain mostly quartz, feldspar, and sometimes
muscovite (white mica). The grain size
varies from fine to coarse. In addition
to hornblende gneiss, biotite gneiss, and amphibolite (hornblende-feldspar
rock), the Raleigh gneiss includes considerable weakly layered light-colored
granitic gneiss. The precursor of the
gneiss was most likely plutonic igneous rocks.
That is, bodies of igneous rock that crystallized from magma well below
the earth’s surface. These bodies
probably were of varied composition, ranging from granite (felsic) to diorite
(intermediate) to gabbro (mafic). These
plutons are believed to represent the roots of the ancient volcanic arc from
which the Carolina terrane originated. Their
age is not well known, but they are most likely to be between 625 and 550 Ma
(million years old). Around 550 Ma, well
before these rocks were metamorphosed, they were intruded by an unusual
granitic pluton, now represented by the Falls leucogneiss. Nearly all exposures of Raleigh gneiss also
include some younger (about 300 Ma) granite or pegmatite (a very coarse variety
of granite) related to the Rolesville batholith.
Rocks at Fallon Park
Begin at the upper end of the
park and follow the trail downstream.
There are several very good exposures of rock in the creek bed. One of the first things to notice is the
foliation or layering of the rock. The
foliation reflects layers of gneiss with different mineral content that have
slightly differing resistance to erosion; layers that are more easily eroded
are indented. Also notice that the
foliation is roughly parallel to the stream (Figure 1). This is not a coincidence; the structure of
bedrock commonly exerts an influence on the course of rivers and streams.
Figure 1. Creek-bed outcrop of Raleigh gneiss near the
upper end of Fallon Park. Note that the
stream follows the foliation (gneissic layering) in the gneiss.
As you walk downstream, you will
see more outcrops. Eventually you will
come to a footbridge near the brick remains of an old structure, likely a small
mill. Here you can clearly see that the
foliation, which runs about north-south, is also inclined quite steeply,
perhaps 70 degrees. See Figure 2. Strike is the
geological term used to refer to the direction of the trend of layers; dip
refers to the inclination of the layers, downward from horizontal.
You will also encounter
cross-cutting dikes of pegmatite (Figure 3).
Pegmatite is a coarse-grained variety of granite. This pegmatite is likely related to the
nearby Rolesville batholith. It consists
of feldspar, quartz and muscovite (white mica).
Note how the pegmatite dike cuts across the layering of the gneiss. This is a very important basic geological
observation that indicates the pegmatite is younger than the gneiss. From such observations, complex geological
histories are pieced together.
Figure 3. Pegmatite dike transecting Raleigh gneiss.
