Thursday, August 15, 2019

Granite of Rolesville batholith – Turnipseed Nature Preserve - Wendell, NC

Rock Type:  Granite

Geologic terrane, element, or event:  Rolesville batholith

Age:  Late Paleozoic – about 300 million years old

Location:  Google MapsLink

USGS 7.5-minute Quadrangles:  Knightdale and Clayton

Site Access:  Turnipseed Nature Preserve is part of the Wake County Department of Parks and Recreation.  There are two entrances with parking lots.  One is at 7100 Hunt Valley Tr., Wendell, NC  27591; the other is at 1525 Pleasants Rd., Wendell, NC  27591.
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.

       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, about two-thirds of Franklin County, and a portion of Johnston County as well.  This site is located in southeastern Wake County, near the Johnston County line.  Here, there are numerous “bouldery” exposures of granite and at least one flat “pavement” exposure of granite.  These outcrops are all part of the batholith.

The rock beneath the ground
       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, and 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 actually 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).

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

The rock itself
       Rocks are made up of minerals.  Granite is composed of two types of feldspar – orthoclase and Na-plagioclase, plus quartz and biotite mica.  Sometimes it contains hornblende, 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 at Turnipseed Nature Preserve is mostly a light-colored, medium-grained biotite granite; if feldspar crystals are larger than the other minerals in the rock it is said to have a porphyritic texture.  Some porphyritic granite occurs at Turnipseed Nature Preserve.  One of the most distinctive porphyritic varieties of Rolesville granite occurs in Johnston County.  It has crystals of orthoclase up to 1.5 inches and is termed a megacrystic granite.

Rocks at Turnipseed       
       As you explore the trails of the preserve, you will encounter numerous outcrops of granite.  Most common are large bouldery exposures (Figure 2).  Other outcrops are smooth rounded rock exposures (sometimes called “whalebacks”).  In fact, along the Boulder Trail, which extends south from the Pleasants Road entrance, there is a large whaleback outcrop that provides a vista of the wetland to the east (Figure 3a).  Here, you can see two varieties of granite (Figure 3b).
Figure 2.  Boulder outcrops of granite at Turnipseed Nature Preserve.

Figure 3a.  Large "whaleback" outcrop of granite overlooking a wetland along the Boulder Trail in Turnipseed Nature Preserve.
Figure 3b.  Close-up view of a portion of the rock in Figure 3a.  Here you can see the dominant medium-grained granite has been intruded by a dike of pink, coarse-grained granite called pegmatite.

Another type of granite outcrop is a flat pavement of bare rock; there is a nice example located in the area of the Lupine Loop trail (Figure 4).  In Wake County, there are several much larger pavements of granite to visit.

Figure 4.  Granite pavements exposure along the Lupine Loop trail at Turnipseed Nature Preserve.

Tuesday, August 13, 2019

Diabase Dike – Robertson Millpond Preserve - Wendell, NC

Rock Type:  Diabase

Geologic terrane, element, or event:  Continental rifting of Pangea; opening of the Atlantic Ocean

Age:  Jurassic – 200 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Knightdale

Site Access:  This preserve is a part of the Wake County Parks and Recreation Department.  The address is 6333 Robertson Pond Rd., Wendell, NC  27591.  Check website for hours and other information; during much of the year, it is open only on weekends.
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.

       The primary attractions for most visitors to Robertson Millpond Preserve are historical or ecological in nature (see the website and brochure).  However, the site has an interesting geological story to tell as well.  In this part of Wake County, Buffalo Creek runs in a south-southeast direction, parallel to and a few hundred feet east of a large diabase dike.  It is likely that the course of Buffalo Creek was somehow determined by the location of the dike.

Local geology
       This portion of Wake County is almost entirely underlain by granite of the Rolesville batholith.  Not far from Robertson Millpond, good exposures of the granite can be seen.  An example is Temple Flat Rock, just three miles northwest, off Watkins Road.  The granite is an igneous rock that is around 300 million years old.  However, as seen on a geological map, the granite is transected by thin bodies (dikes) of a younger igneous rock type.

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. 

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.

Diabase magnetism
       Geologists can often take advantage of a rock’s physical properties to determine its presence, even when it is not exposed at the earth’s surface.  Diabase is magnetic enough to stand out, especially when it is located among rocks that have low and consistent magnetism.  This is the exact situation when diabase dikes were intruded into granite, which is a “magnetically flat” rock.  After aerial magnetic surveys were conducted, a strong linear magnetic anomaly was seen along Buffalo Creek, and for that reason, a diabase dike running about north-south had been inferred in the Robertson Pond area (Figure 2).
Figure 2.  Aeromagnetic map of the vicinity of Buffalo Creek and Robertson Millpond.  Note the strong anomaly running along the creek; if this were a topographic map, it would be a sharp ridge.

Diabase rock occurrence

       In the woods along the west side of Robertson Millpond, abundant large boulders of diabase define the trace of the dike (Figures 3 and 4).  The best spot to see the diabase is near the northwest corner of the pond, and the best way to get there is by kayak or canoe.  Boulders may be seen near paddling trail markers 14-16.
       The trace of the dike along the ground appears to be offset to the west relative to the magnetic maximum.  Most likely, this is mainly because the dike is a slab that is tilted (dips) steeply toward the east.

Figure 3.  Topographic map of Robertson Millpond area showing location of diabase boulders (red dots).
Figure 4.  Example of diabase boulders at edge of Robertson Millpond.

Monday, August 5, 2019

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.
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.  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.  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 2).  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 2.  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.
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.

       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 stripe on the left side 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.

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).

       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.
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.

       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-19th century to the early 20th 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.
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.

       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.
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.

       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.