Raleigh gneiss of the Raleigh Terrane, Fallon Park - Raleigh, NC

Rock Types:  Gneiss and pegmatite

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:  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.

Figure 2.  Steeply dipping gneissic foliation near the remains of an old structure.  Photo by Blake Perdue from Google Maps.

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.

Falls Leucogneiss – Median of Centennial Parkway – Raleigh, NC

Rock Type:  Lineated granitic gneiss

Geologic terrane or major geologic element:  Raleigh terrane

Age:  Late Proterozoic  – approximately 550 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Raleigh West

Site Access:  Centennial Parkway runs in a pair of large arcs between Avent Ferry Road and Lake Wheeler Road.  It borders the Centennial Campus of NC State University and the State Farmers Market.  Traffic on Centennial Parkway may be heavy at times and moves quickly.  This site should be approached on foot and safely.  Parking may be found in the nearby Mission Valley Shopping Center.  Large fresh blocks of Falls leucogneiss, that were excavated during construction of the Parkway, have been placed in the median (Figure 1).

Figure 1.  Median of Centennial Parkway with blocks of leucogneiss.

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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 2 on p. 164-166.

Stoddard, E. F., and Blake, D. E., 1994, Carolina Geological Society Field Trip Guide, 1994.  See the descriptions for Stop 9 and 9A on pages 101-103.

Caslin, L. A., 2001, Age and significance of the Falls leucogneiss, Wake County, North Carolina:  M.S. thesis, NC State University, 39 p.

Farrar, S. S., and Owen, B. E., 2001, A north-south transect of the Goochland terrane and associated A-type granites – Virginia and North Carolina, in Geological Society of America, Southeastern Section, Field Trip Guide for 2001.

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Introduction

The Falls leucogneiss is a very distinctive and unusual rock unit that runs beneath downtown Raleigh.  It is important in the geological history of the region, and it has also exerted significant influence on the region’s human history.  Because it is unusually hard and resistant to weathering and erosion, there are many natural exposures of Falls leucogneiss in Wake County.

Nature of the rock

The prefix “leuco” means light-colored, as in leucocytes (white blood cells).  So this rock unit is a light-colored gneiss, meaning that it contains less than 10% dark minerals.  In fact, most samples of Falls leucogneiss have less than 5% dark minerals.  The remainder consists of quartz and two varieties of feldspar, and their relative percent classifies the igneous precursor of the leucogneiss as granite.  The sparse dark minerals are biotite (black mica) and magnetite.  All the mineral grains are small in size.  Gneiss is a metamorphic rock that is characterized by alternating darker and lighter layers.  The Falls leucogneiss then is a granitic gneiss.  However, the distinction between this rock and other varieties of granitic gneiss is its strong lineation.  In most gneisses, the layering is prominent, but in Falls leucogneiss it is difficult to discern.  Instead, this rock is characterized by very thin parallel lines of dark minerals that run through (Figure 2).

Figure 2.  Fresh block of Falls leucogneiss showing strong lineation.  Note how the thin dark lines are visible only on the side of the rock, not on the end.  When the leucogneiss decays by weathering, thin “pencils” of rock material are formed that accumulate on the ground.

Geometry and magnetic expression

The Falls leucogneiss runs in a narrow (typically about one km wide) band for about 80 km (50 miles) from near Lake Wheeler north to Henderson, NC.  The direction of its trace correlates with the trend of the lineation seen in outcrops of the leucogneiss.  Furthermore, the lineation is nearly horizontal in most exposures, or else it plunges gently toward the north or south.  Because of the lineated magnetite grains in the rock, the leucogneiss is relatively strongly magnetic.  (Most magnetic rocks contain mostly dark minerals; light-colored rocks are almost always non-magnetic.)  A sensitive magnet (for example, a small bar magnet tied to a string) will be attracted to a fresh piece of Falls leucogneiss.  The magnetism is strong enough that this rock unit shows up clearly on aeromagnetic maps of the region (Figure 3).

Figure 3.  Aeromagnetic map of the Wake County area.  The various patterns and trends are the effect of varying magnetism of the rocks.  The location of the Falls leucogneiss is indicated by the arrow.  Its NNE trend is clearly displayed.

Geological significance

Most of the Raleigh terrane is made up of Raleigh gneiss, which is typically dark gneiss or well-layered gneiss with some dark layers; most likely it was originally igneous rock.  The Falls leucogneiss is thought to represent granite that intruded into this igneous precursor rock of the Raleigh gneiss, then much later they were both deformed and metamorphosed, during the formation of the Appalachian Mountain belt.  One of the blocks of leucogneiss at the Centennial Parkway site contains some Raleigh gneiss (Figure 4).

Figure 4.  Block of Falls leucogneiss in the median of Centennial Parkway.  The dark material (hornblende gneiss) is thought to belong to another rock unit, Raleigh gneiss.  This is taken as evidence that the igneous precursor of the leucogneiss intruded the precursor of the Raleigh gneiss.

 There is a major fault zone that runs through the eastern Piedmont of North Carolina, parallel to and just west of the Falls leucogneiss.  It is called the Nutbush Creek fault.  Geologists know that the Nutbush Creek fault was active about 300 million years ago, and that it was a right-lateral strike slip fault.  In this type of fault, the two sides move sideways along the fault (not up and down), with the opposite side moving toward the right when viewed from across the fault.  (This is the same type of fault as the San Andreas fault in California.)  The Nutbush Creek fault (and other similar Piedmont faults) were formed during the collision of continental plates that created the Appalachians.  The colliding plates did not meet exactly head-on, but obliquely, forming right-lateral strike-slip faults like the Nutbush Creek.

Influence on streams and topography

Streams in the central and eastern Piedmont flow generally east and southeast, across the north-northeast trend of the Falls leucogneiss.  Because of its resistance to erosion, the leucogneiss presents a major barrier.  Consequently, streams may be diverted where they meet the rock unit (Figure 5), and where they do flow through it, the streambed is very rocky, with rapids and waterfalls prevalent.  In fact, this is how the old community of Falls, in northern Wake County, go its name (and of course how the leucogneiss got its name).  Naturally, the places where streams cross the leucogneiss made terrific locations for dams and mills.  Lake Wheeler, Lake Raleigh, and Falls Lake all have dams built across streams where the leucogneiss is located; Lassiter Mill, Yates Mill, and the old Falls Mill were also constructed there.  See Figure 6.

Figure 5.  Topographic map of a portion of Crabtree Creek, showing the right turn made by the creek as it encounters Falls leucogneiss (orange arrow), and the location of the Lassiter Mill dam (black arrow).

Figure 6.  Yates Mill is one of several historic Wake County mills that was built on Falls leucogneiss.

The leucogneiss also has produced north-northeast trending ridges in the local topography in a few places.  Two good examples are Lake Wheeler Road from Tryon Road south to Yates Mill, and Oberlin Road between Hillsborough Street and Glenwood Avenue.  Alas, neither Ridge Road nor Blue Ridge Road follow the leucogneiss.

Use as a building stone

Falls leucogneiss was a favored building stone during Raleigh’s history, and numerous small quarries produced blocks of stone that were used to construct many historic buildings, walls, and steps in the area.  The location of Glenwood Village Shopping Center, at the intersection of Oberlin Road and Glenwood Avenue, was one such quarry.  A small section of the wall of the former quarry may be seen behind the Harris Teeter grocery store there.  Broughton High School and several of the older churches in downtown Raleigh, as well as many older homes inside the beltline, and commercial buildings (for example, Mitch’s Tavern on Hillsborough Street) are all constructed from Falls leucogneiss.

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.

Blake, D. E., and others, 2016, Compiled geologic map of the Wilton 7.5-minute quadrangle, Granville, Vance, and Franklin Counties, North Carolina:  N. C. Geological Survey Open-File Report 2016-21, scale 1:24,000, in color.

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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 even tiny crystals of 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.  Soapstone is an especially characteristic rock in the Falls Lake terrane.  Soapstone is a massive rock consisting of very fine-grained talc.  It is so soft that it may be sawed with a wood saw or carved easily.  It was used by Native Americans and early European settlers to make bowls, pipes, and many other items.  There are outcrops of soapstone in Wake County that still show saw marks where rock was removed, and many old local cemeteries have soapstone grave markers.  To see more examples of the ultramafic rocks in the Falls Lake terrane, 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 630-520 million years old

Location:  GoogleMaps 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 630-520 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.

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
USGS 7.5-minute Quadrangle:  Cary
Site Access: This is an active railroad line.  Great care must be taken while visiting.
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Technical Information:  Carolina Geological Society Field Trip guide 1994 - Stop 2.  (Note that, since 1994, the railroad cut on the eastern side of the tracks has been removed during a construction project; only the western cut remains.)
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Introduction and geological history
The rocks exposed in this 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.

Rocks present
The sedimentary 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 sedimentary rocks 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.

Triassic life
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.

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.

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Technical Information:  Ragland, P. C., Hatcher, R. D., Jr., and Whittington, D., 1983, Juxtaposed Mesozoic diabase dike sets from the Carolinas:  A preliminary assessment:  Geology vol. 11, No. 7, p. 394-399.

Butler, J. R., 1986, Diabase dike near Lancaster, South Carolina:  The “Great Dyke of South Carolina:” Geological Society of America Centennial Field Guide, Southeastern Section, p. 245-246.

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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 (Figure 1).  In Wake County and surrounding areas, most diabase dikes are sheets that dip very steeply and trend close to north-south (Figure 2).  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.  Diabase dike exposed in the wall of a granite quarry in Nash County, NC.

Figure 2.  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.

Because diabase is a strongly magnetic rock, even when there is no diabase rock visible at the surface, the presence of a large dike can be determined from an aeromagnetic map.  Figure 3 is an aeromagnetic map showing several NNW-trending linear anomalies indicating the presence of diabase dikes in eastern Wake County.  In this area, the main bedrock, into which the diabase intruded, is magnetically "quiet" granite, so the dike anomalies are easily visible.  The western half and the southeastern corner of the map contain a variety of metamorphic rocks and have complicated magnetic patterns; magnetic anomalies from dikes in these places would difficult to discern.

Figure 3.  Aeromagnetic map of the Wake County area.  The NNW-trending features in the east half of this map indicate large diabase dikes that cut through the granite there.

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.  See Figure 4.  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 5 and 6).  Spheroidal weathering typically occurs in igneous rocks, including diabase, granite, and gabbro.

Figure 4.  The process of spheroidal weathering (Wikimedia Commons).

Figure 5.  Approach to the site, showing the electrical substation (left) and the embankment with diabase spheroids (right).

Figure 6.  Close-up of several diabase spheroids.  Such weathering commonly involves peeling off of successive layers, sometimes called “onion-skin” weathering.