Graphite Schist of the Crabtree Terrane, Lake Park Connector Greenway Trail - Raleigh, NC

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-19^th century to the early 20^th 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).

Figure 1.  Cut bank exposure of Raleigh graphite schist along Lake Park Connector trail.  Photo taken from greenway trail bridge.  Loose pieces of schist may be found in the creek.

       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.

Figure 2.  Excavation for the A. E. Finley YMCA.  New construction commonly reveals the graphite schist.

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.

Ultramafic Rocks in the Falls Lake terrane – Upper Barton Creek at Falls Lake - Raleigh, NC

Rock Type:  Metamorphosed ultramafic rocks

Geologic terrane or major geologic element:  Falls Lake terrane

Age:  Late Proterozoic to Cambrian – approximately 620-520 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Bayleaf

Site Access:  Park in the lot for fishing access on the west side of Six Forks Road, just south of the Upper Barton Creek arm of Falls Lake, and south of the boat ramp (Figure 1).  Follow the trail west across a tiny creek and then head to the left onto a low ridge.

Figure 1.  Parking for fishing access on west side of Six Forks Road.

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Technical Information:  Stoddard, E. F., and Blake, D. E., 1994,  Carolina Geological Society Field Trip Guide, 1994).  See the description for Stop 6 on pages 96-97 and the section on the Falls Lake mélange on page 7.

Horton, J. W., Blake, D. E., Wylie, A. S., Jr., and Stoddard, E. F., 1986, Metamorphosed mélange terrane in the eastern Piedmont of North Carolina:  Geology, vol. 14, no. 7, p. 551-553.

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Introduction

       Although it is not an extensive rock unit, the Falls Lake terrane (formerly known as the Falls Lake mélange) is very distinctive.  Bounded on the west by the Carolina terrane and on the east by the Crabtree terrane, it pinches out south of Falls Lake and continues to the north into Granville County.  The terrane consists mainly of mica schist within which there are scattered hundreds of blocks and pods of other rock types.  These rocks include soapstone, serpentinite, chlorite schist, and amphibolite, and a variety of characteristic minerals including talc, chlorite, actinolite, hornblende, serpentine, magnetite, chromite, and Cr-rich corundum (ruby).  Before metamorphism and deformation, the blocks were ultramafic igneous rocks, mainly peridotite, with lesser amounts of the mafic igneous rocks gabbro and basalt.  Blocks and pods that have been studied in the Falls Lake terrane range from fist-sized to more than a kilometer in length (Figure 2).

Figure 2.  Portion of geological map of the Bayleaf Quadrangle, showing location of this site (red diamond).  Falls Lake schist is in gray; ultramafic pods in blue.

       On the ridge, you will encounter a large outcrop of massive, light green to dark green ultramafic rock (Figure 3).  This very hard rock consists of a dense aggregate of very fine-grained actinolite and/or serpentine.  If you look closely, you will see small metallic grains of magnetite, which will attract a sensitive magnet.  You will also see veinlets of tiny quartz crystals.  These were introduced very recently into the rock (geologically speaking); they have nothing to do with the rock’s origin and early history.

Figure 3.  Outcrop of metamorphosed ultramafic rocks on low ridge.

       If you explore this ridge, walking uphill toward the south, and downhill toward the west, you should be able to find examples of talc (slippery to the touch and softer than a fingernail), chlorite (green and platy, will peel into flakes), hornblende (jet-black and lustrous), and perhaps crystals of actinolite (dark green, lustrous, and breaks into long fragments with smooth crystal faces).  In this part of Falls Lake, the lakeshore has numerous exposures of similar ultramafic rocks.  To see more, you can follow one of these on-line Falls Lake geology guides.  One guide is for a trip by boat; the other is along a hiking trail.

Significance of the Falls Lake terrane

       There have been several different interpretations for the rocks of the Falls Lake terrane; each proposes a different mechanism to achieve the “block-in-matrix” nature of the unit.  The earliest studies interpreted the ultramafic rocks as intrusions into the schist.  In this scenario, extremely hot ultramafic magma was injected into the host “country rock.”  The magma cooled, and much later both rock types were metamorphosed.  In this case, we would expect to see cross-cutting contacts between the ultramafic rocks and the schist, as well as evidence of heating of the schist where it was in contact with the hot magma.  (We don’t see either.)

       In the early 1980s, these rocks were inferred to represent a characteristic assemblage of rocks that forms at the site of an ocean trench above a subduction zone, where an oceanic plate descends beneath another plate.  Such assemblages are called accretionary wedges, or mélanges.  They consist mostly of sediment that may be scraped off the down-going plate or derived from erosion of the upper plate.  In the subduction process, pieces of lower crustal and upper mantle material (ultramafic and mafic rock) may be broken off and incorporated in the sediment, producing a chaotic mixture (mélange in French).  In this scenario, the precursor of the schist would be sedimentary rocks such as siltstone, mudstone, and sandstone.  We might expect to be able to trace out specific layers of the original sedimentary rock within the schist.  (We can’t).  Because feldspar weathers quickly in the sedimentary environment, we would not expect to find much in the schist.  (In fact, the schist is unusually rich in feldspar.)  We would also expect that individual grains of highly resistant minerals like zircon would be rounded from sedimentary transport.  (They aren’t.  Instead they are well-formed crystals like those that crystallize in igneous rocks, directly from a magma).

       Tracing the Falls Lake terrane north into Granville County, the metamorphic intensity decreases.  Whereas the Wake County area was subjected to a medium to high grade of metamorphism, the Granville rocks only experienced low-grade metamorphism.  We therefore gain a more confident understanding of the original rocks because they have not been modified so much.  In fact, it seems the precursor of the Falls Lake schist was itself an igneous rock related to granite – the Gibbs Creek metagranodiorite.  The Falls Lake schist represents a large igneous pluton that is related to the volcanic rocks of the Carolina terrane.  It may represent the roots of the ancient volcanic arc.  The ultramafic and mafic blocks and pods are pieces of wall rock that were incorporated into the magma as it rose (xenoliths).  In this scenario, the evidence that counters the first two theories makes perfect sense.  Further testing of the third theory is underway.

       The evolving theory of the Falls Lake mélange/Falls Lake terrane is a very good case study of how science works, by suggesting hypotheses and then testing them.

Felsic Metavolcanic Rocks – Umstead State Park - Raleigh, NC

Rock Type:  Metamorphosed volcanic tuff

Geologic terrane or major geologic element:  Carolina terrane

Age:  Late Proterozoic to Cambrian – approximately 620-520 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Southeast Durham

Site Access:  From Glenwood Ave. (US 70) turn in to Umstead State Park.  Proceed past the Visitor’s Center and Maintenance Drive to the first parking lot on the left.  Park at the north end of the lot and follow the Oak Rock Trail markers.  The outcrop is near the northern end of the loop trail.  Other outcrops can be seen along the Potts Branch Trail.

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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 especially description for Stop 7 on p. 173-174.

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Introduction

       The Carolina terrane (formerly known as the Carolina slate belt) is a major geological region in central North Carolina.  It consists of lightly metamorphosed volcanic and related igneous and sedimentary rocks of Late Proterozoic to Early Paleozoic age, or approximately 620-520 million years old.  These rocks were formed as part of an ancient volcanic arc (Figure 1) on the far side of an ocean that no longer exists.  Later, when earth movements caused that ocean to close, the volcanic arc collided with ancient North America, helping to form the Appalachian mountain chain.  The eastern edge of the Carolina terrane runs through Cary; the western edge is near Greensboro.  Carolina terrane rocks are well exposed in the Uwharrie Mountains to the west, but In Wake County, Umstead State Park is a good place to see them.

Figure 1.  A volcanic island arc forms above a subduction zone.  Eventually, the ocean will close, causing plates to collide.  This is the primary way that mountain belts form.  The Carolina terrane is an ancient volcanic arc.

Big Lake – Raven Rock schist

       The Big Lake – Raven Rock schist is an extensive rock unit within the easternmost part of the Carolina terrane.  It is named for exposures near Big Lake in Umstead State Park and others at Raven Rock to the south in Harnett County.  This rock originated mainly as dacitic tuff, formed by explosive volcanic eruptions.  These eruptions involved varying amounts of crystals, rock fragments, and ash that were blown into the air and accumulated on the flanks of volcanoes.  Lithic tuff features rock fragments, crystal tuff features phenocrysts, or crystals, of quartz and feldspar; crystal-lithic tuff has both.

       These volcanic features have been somewhat obscured by later geological events, namely deformation and metamorphism that came as the result of the Appalachian mountain-building events later in the Paleozoic era.  This modified the volcanic rocks into metamorphic phyllite and schist with the growth and alignment of the platy minerals mica and chlorite.  The direction of the lined-up minerals is the foliation of the metamorphic rock.

       At Oak Rock (Figure 2), the foliation is steeply dipping, nearly vertical.  As you explore the park’s trails, you will quickly see that the foliation is not consistent.  The reason for the variation is that these rocks have been bent into folds by the collision between plates that formed the Appalachians.

       If you look closely, and ignore the lichen on the rock, you may see patches of the rock that have slightly differing color.  These are the flattened rock fragments of this metamorphosed lithic tuff.

Figure 2.  Oak Rock, near the north end of the Oak Rock Trail.

To the southeast of Oak Rock, the Potts Branch Trail crosses over a very nice exposure of crystal tuff (Figure 3), in which you can see bumpy white phenocrysts of quartz and feldspar.

Figure 3.  Exposure of metamorphosed crystal tuff on the Potts Branch Trail.  Light colored spots are crystals of quartz and feldspar.

Granite “Pavement” Exposure – Mitchell Mill State Natural Area - Rolesville, NC

Rock Type:  Granite

Geologic terrane or major geologic element:  Rolesville batholith

Age:  Late Paleozoic – approximately 300 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Rolesville

Site Access:  Heading south on Highway NC 96, there is a space to pull off the road on the right.  Park here and follow the path by the big blocks of rock.  You will see an extensive exposure of granite.  From this exposure, follow the trail from the back corner, near more blocks, and you will come to an even larger expanse of granite.  This site is Mitchell's Mill State Natural Area, part of the NC State Parks system.

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Technical Information:  Speer, J. A., 1994, Nature of the Rolesville batholith, North Carolina (pages 57-62 in Carolina Geological Society Field Trip Guide, 1994); See also description for Stop 11 on pages 105-107.

Speer, J.A., McSween, H.Y., and Gates. A.E., 1994, Generation, segregation, ascent and emplacement of Alleghanian granitoid plutons in the Southern Appalachians:  Journal of Geology, v. 102, p. 249-267.

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Introduction

       The Rolesville batholith is the largest body of granite in the southern Appalachian region.  It measures about 15 x 50 miles, and occupies the eastern third of Wake County, and about two-thirds of Franklin County.  At this site there is a huge flat exposure of granite that is a part of the batholith.

The rock body

       A pluton is a three-dimensional body of igneous rock.  Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which by definition covers an area greater than 100 square kilometers.
       Granite is an igneous rock that occurs in three-dimensional bodies called plutons.  Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which (by definition) covers an area greater than 100 square kilometers.  In three dimensions, plutons may be shaped like mushrooms or inverted teardrops, so they may extend to considerable depth beneath the surface. In the eastern Piedmont, almost all granitic plutons are between 280 and 320 million years old – Late Paleozoic in age (Figure 1). The largest such body is the Rolesville batholith, which is (in reality) ten or twenty separate plutons that intruded the same region over a period of geologic time, cutting across each other and coalescing to form the batholith (a mega-pluton). 

The rock itself

       Granite is composed of two types of feldspar – orthoclase and Na-plagioclase, plus quartz and biotite mica. Sometimes it may contain hornblende or muscovite mica or garnet. It always also contains some tiny minerals in small amounts. Granite is the most common type of rock that is quarried for crushed stone, used in making concrete and asphalt, among other things.  The granite here is a medium-grained biotite granite.

       You may notice certain features in the granite at Mitchell’s Millpond (Figure 2).  Cracks or fractures in the granite are of two types.  Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock.  Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses.  Below the surface, groundwater moves along these cracks.  You may also see thin bands of slightly different rock cutting across the granite (Figure 3).  These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite).  In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).

Origin

       The granite magmas that intruded this region during the late Paleozoic were generated in the lower crust of the earth, and moved upward until they stopped, cooled, and crystallized. The magmas formed as a result of the tremendous collision between continental plates that raised the Appalachian Mountains. Many of these magmas ascended along fault zones that were active at that time, such as the Nutbush Creek fault that runs through central Wake County. There is no evidence that any of these granitic magmas reached the surface – if they had, we would expect to see some remnants of volcanic rock of the same composition and age.
       You may notice certain features in the granite at Mitchell’s Millpond (Figure 2).  Cracks or fractures in the granite are of two types.  Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock.  Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses.  Below the ground surface, groundwater moves along joints; if there are numerous joints and they intersect, then wells can produce ample water.  In eastern Wake County, a majority of homeowners rely on wells drilled into the granite.  You may also see thin bands of slightly different rock cutting across the granite (Figure 3).  These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite).  In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).

Surface features

       This 93-acre site is a Registered Heritage Area.  It contains some of the best examples of native plant communities that grow in such a “granitic flatrock” environment.  You can see that erosion by running water proceeds very slowly, due to the resistance of this hard rock.  Note the numerous potholes (Figure 5).  You may notice that some of them line up along a fracture in the granite.  These potholes form over time as the result of small pebbles that get caught along a crack, and then drill away at the bedrock when the stream water is high and fast.

Figure 1.  Late Paleozoic granite plutons in the southern Appalachian Piedmont (RV=Rolesville).  From Speer and others (1994).

Figure 2.  Granite “flatrock” at Mitchell’s Millpond State Natural Area; large pothole in foreground.

Figure 3.  Dikes and veins cutting through the granite.

Figure 4.  Biotite schlieren in the granite.

Figure 5.  Pothole at Mitchell’s Millpond.  In the bottom of this hole, you would find rounded pebbles that act as a “drill bit” when they are swirled by fast-moving water.

Spheroidally Weathered Diabase Dike – Garner, NC

Rock Type:  Diabase

Geologic terrane, element, or event:  Opening of the Atlantic Ocean

Age:  Jurassic – 200 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Garner

Site Access:  This is an embankment next to an electrical substation.  Exercise caution, avoid the fenced-in enclosure and do not block access.

Introduction

Here, a ridge was partially excavated to make room for the substation.  The excavation exposed part of a large diabase dike.

Diabase dikes

A dike is a sheet-like body of igneous rock intruded as molten magma that cuts across the older rocks.  In Wake County and surrounding areas, most diabase dikes are sheets that dip very steeply and trend close to north-south (Figure 1).  Diabase is a dark-colored igneous rock that is a variety of basalt, the rock type that makes up all the ocean floors of the earth.  Whereas basalt is volcanic, diabase is intrusive, meaning that the molten magma cooled below the earth's surface.  The age of the diabase in our region has been determined quite precisely; it is almost exactly 200 Ma (million years old).  In fact, our diabase was formed in response to the stretching and eventual breakup of the supercontinent Pangea, and the opening of the Atlantic Ocean.  Its age helps to date that event.  The dike exposed here runs through downtown Garner, roughly along Creech Road, passing along the west edge of Southeast Raleigh High School, and through Southgate Park.  It continues north at least as far as Millbrook Road.

Figure 1.  Example of geological map of a portion of northeastern Wake County, showing several diabase dikes.  They are the red lines labelled Jd that run N-S or NNW-SSE.  The solid red lines are dikes that have been confirmed by fieldwork and the dotted red lines are dikes that are inferred from their magnetic signatures.

Spheroidal weathering

Weathering is the gradual breakdown of hard rock at or very near the earth’s surface.  Chemical weathering transforms hard minerals like feldspar and hornblende into soft clay and iron oxide; it is key to the formation of soil.  At this site, the chemical process known as spheroidal weathering is displayed in spectacular fashion.  Imagine a body of rock, beneath the ground, that has a network of parallel horizontal and vertical cracks, intersecting at right angles (like a Rubik's Cube).  The cracks break the rock into many cubes or rectangular blocks.  Groundwater seeps into the cracks and the rock begins slowly to chemically decompose.  The process attacks the corners of the "blocks" most intensely because there are three rock surfaces in contact with the water.  Over time, this chemical weathering modifies these cubic blocks into rounded spheroids.  The “Rubik’s Cube” becomes a bunch of hard rounded rocks separated by soft decomposed material.  Erosion then can remove the soft material and only the rounded rocks remain (Figures 2 and 3).  Spheroidal weathering typically occurs in igneous rocks, including diabase, granite, and gabbro.

Figure 2.  Approach to the site, showing the electrical substation (left) and the embankment with diabase spheroids (right).

Figure 3.  Close-up of several diabase spheroids.  Such weathering commonly involves peeling off of successive layers, sometimes called “onion-skin” weathering.

Triassic Conglomerates - Deep River Triassic Basin - Morrisville, NC

Rock Type: Conglomerate

Geologic terrane or major geologic element:  Deep River Triassic basin - Durham sub-basin

Age: Triassic - approximately 220 million years old

Location: Google Maps Link

Site Access: This is an active railroad line!  Great care must be taken while visiting!  Note:  This railroad interchange has been re-configured so that the rail line runs on an overpass above the highway.  Now, the sedimentary rocks are only visible on the west side of the tracks.  The exposure on the east side of the tracks, part of which is shown in Figure 4, is now completely covered by granitic rip-rap.

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Technical Information:  Carolina Geological Society Field Trip guide 1994 - Stop 2

http://carolinageologicalsociety.org/CGS/1990s_files/gb%201994.pdf

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Simplified Information:

The rocks exposed in the railroad cut help tell part of the geologic story of North Carolina.

When the Supercontinent Pangea (Figure 1) began to split apart approximately 245 million years ago, a system of rift basins (similar to the modern day East African Rift system) were formed all along the east coast of North America (Figure 2).  Called the Newark Rift System, the splitting apart of Pangea formed the Atlantic Ocean and several inland fault bounded rift valleys.

Figure 1: Sketch of the supercontinent Pangea.

Figure 2: Sketch of rift basins along the Atlantic Ocean.

Land to either side of the rift basin began to erode rapidly filling the fault bounded lowlands with boulders, cobbles, sand, silt and clay.  The deposits later turned into the red to maroon colored conglomerates, sandstones, siltstones and mudstones common in the basin (Figures 3 and 4). Excellent examples of these deposits are exposed in the railroad cut.

Figure 3: Photograph of outcrop at railroad cut showing Triassic-aged sedimentary rocks.

Figure 4: Photograph of details of outcrop at railroad cut showing alternating beds of Triassic-aged conglomerates.

The Morrisville area during the late Triassic was full of life.  North Carolina was located near the equator and had a semi-arid tropical climate. Situated between rugged mountains, the ancient area was dominated by lakes, swamps and meandering rivers and streams that would periodically dry-up.  Crocodile-like animals called phytosaurs, early dinosaur ancestors, and primitive mammals roamed the land of the Triassic basin; fish, clams and various crustaceans lived in the lakes and rivers; insects crawled on the ground and flew through the air; and abundant conifer trees and cycads grew.  Evidence of this abundant life is seen by the common occurrence of petrified wood in Triassic sediments, the occasional discovery of fossilized bones of reptiles and the footprints of early dinosaurs (Figure 5) in Triassic basin sediments throughout North Carolina.

Figure 5: Artist's rendition of flora and fauna of the Triassic basins – From News and Observer.

House Creek Trail

The House Creek Trail begins at Blue Ridge Road near the McDonald's restaurant behind Crabtree Valley Mall, where it links with the Crabtree Creek Trail.  It runs south for 2.8 miles, ending at the Reedy Creek Trail at the east side of the pedestrian bridge crossing over the I-440 Beltline just north of Wade Avenue.  The trail crosses over the Beltline on Glen Eden Drive, then runs between the Beltline and Ridge Road, following the course of House Creek.  At the south end of the trail, you can turn west on the Reedy Creek Trail and ride over the Beltline to the NC Museum of Art, continuing on along Reedy Creek Road all the way to Umstead State Park.  Alternatively, you can turn east on the Reedy Creek Trail, and pass along the edge of the campus of Meredith College to Faircloth Street.  From that point, you can continue eastward along the Rocky Branch Trail.

Some of the other greenway trails, such as the Crabtree Creek and Walnut Creek Trails, run east-west, and thus cut across the trend, or strike, of the rock units in the area.  In contrast, the House Creek Trail runs generally parallel to strike, passing through schist and gneiss of the Crabtree terrane.  These metamorphic rocks are thought to have originated as explosive volcanic layers and muddy sedimentary layers.  Perhaps the most distinctive rock type in this terrane is graphite schist, a soft black metamorphic rock that occurs in several thin layers, and was once mined for the graphite.  The graphite mines are the source of the name Lead Mine Road - it was pencil lead, not the metal lead.  Construction activity along Blue Ridge Road in 2011-2013 exposed abundant graphite schist, but it is has since been covered.

Graphite schist exposed during construction along

Blue Ridge Road, Summer 2013

Location is about 100 yards south of trail.

At its north end at Blue Ridge Road, the trail passes through Marshall Memorial Park, featuring a monument to a local World War II hero.  At the memorial, there are several huge blocks of a very interesting variety of banded gneiss, containing the minerals hornblende, biotite, and garnet, as well as feldspars.  There are also some zones of granite in some of these rocks. Unfortunately, these rocks are not locally derived; they are probably from the Blue Ridge Mountains.  (If anyone reading this has information, please send a comment!)

As you continue along the trail, passing behind the new condominiums, you may see sparse exposure of weathered, light-colored schist in the creek.  Continue up the hill and you soon will see tennis courts, picnic shelters, and playground of Glen Eden Pilot Park.  The trail continues along the Glen Eden Drive overpass across the Beltline, then does a U-turn and passes through a tunnel beneath Glen Eden.  The stretch from here to Lake Boone Trail passes through thick woods.  As you go south, you have noisy traffic of the Beltline on your right and bucolic House Creek on your left.  There is not much geology to see for most of the year.  In late fall and winter, you may get a glimpse of some very beautiful stretches of the creek, with some locally excellent rock outcrops.

Two layers of the graphite schist run roughly parallel to Ridge Road, off to your left.  On the geological map figure below, you may be able to see that the thickest of these layers runs through the neighborhoods between the creek and Ridge Road.  The locations of two former mines are shown along this layer, between Glen Eden Drive and Lake Boone Trail.

Excerpt from Geologic Map of the Raleigh West quadrangle

(Blake, D.E., 2008, NC Geol. Survey Geol. Map Series 15)

Graphite schist layers are gray bands in eastern half, old mine

locations are indicated by square symbols and red numbers.

In the early 1990's, a homeowner "rediscovered" one of these mines while digging in his back yard!

Exploring a 150-year-old graphite mine in Raleigh

The graphite layers continue south, running through the campus of Meredith College, under the Method Road greenhouses of NCSU, across Kaplan Drive, Avent Ferry Road, and the Walnut Creek Trail just downstream from Lake Dam Road.

Continue south and through the tunnel beneath Lake Boone Trail.  During construction of this portion of the trail, nearly horizontal layers of rock were exposed where the large retaining wall now stands.  If you continue just a bit further, to where the trail goes downhill and bends to the right, you can see a natural ledge outcrop of schist in a small creek on the left side of the trail.  If you take a minute to examine this exposure, you will see that the schist layers are nearly horizontal, or very gently dipping.  This is different from most of the metamorphic rocks in the Raleigh area, which dip steeply, even vertical in some cases.  The reason for the shallow dip here is that this location is right along the hinge-line of a large fold, the Raleigh antiform.  The antiform is basically a huge arch of rock layers, and we are right at the top of the arch here.  This antiform can also be studied at Shelley Lake.

Nearly horizontal layers of schist along the hinge of

the Raleigh antiform

Continue just a short distance ahead and you may see some large outcrops of light-colored gneiss in House Creek below you to the right.  These were probably volcanic in origin.

Metavolcanic rock outcrop in House Creek

just east of Horton Street

From this point, continue south along the trail, up one of the steepest hills in the greenway system, and at the top you connect with the Reedy Creek Trail.

If you turn left and take the Reedy Creek Trail, you can connect with the Rocky Branch Trail, and eventually the Walnut Creek Trail, and continue all the way to the Neuse River Trail if you so desire.