Site number on county
map: 14
Rock Types: Gneiss and granite
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: Pigeon House
Branch runs along Capital Boulevard through downtown Raleigh. It is a tributary of Crabtree Creek, which it
enters about 1.5 km to the east of this site.
At the present, this outcrop may be accessed by scrambling down a
kudzu-covered embankment where the creek runs between the in-bound onramp from
Wake Forest Road to Capital, and an unpaved parking area for an abandoned
warehouse (formerly Raleigh Bonded Warehouse).
(It’s best to visit this outcrop in the winter.) A second exposure along the same creek may be
visited from the parking lot of Urban Ministries, located upstream and on the
other side of Capital Boulevard. Planned
construction along this corridor jeopardizes these outcrops and access to them.
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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. Erosion and the fresh rock exposures are
greatly enhanced due to the impervious manmade surfaces, and the increased
surface runoff that results. In
addition, construction activities periodically provide exposures of the gneiss,
and 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 in Pigeon House Branch
You will notice the dominant rock
type is grayish in color, with alternating layers or bands that differ slightly
in hue. This is Raleigh gneiss. You will also see numerous light-colored
streaks that cut through the gneiss.
These are granite dikes if they cut across the gneiss layers, or granite
sills if they are parallel to the layers.
Differing shades of gray in the gneiss likely reflect different parent material,
with a darker color reflecting a more mafic parent. This is a great rock exposure to observe an
important principle in geology:
cross-cutting relationships. If
one rock cuts across another rock, then it must be younger. See if you can find a spot where the white igneous
rock (granite) cuts across the gray metamorphic rock (gneiss). And recall that the gneiss is over 500
million years old, while the granite is a mere 300 million years old!
Figure 1. Outcrop of Raleigh gneiss in Pigeon House Branch.
Figure 2. Granite dike transecting Raleigh gneiss.