Tuesday, August 27, 2019

Raleigh Terrane Rocks, Pigeon House Branch - Raleigh, NC

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

Location:  Google Maps Link

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.
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.


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.

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