Mine Creek Trail

The Mine Creek Trail begins at the Crabtree Creek Trail greenway, and runs north along Mine Creek, crossing North Hills Drive, then passing beneath Millbrook Road, and then following the east side of the Shelley Lake Loop.  From the northwest corner of the Shelley Lake Loop, it continues north, through a narrow tunnel under North Hills Drive, and then under Lynn Road.  A short distance north of Lynn Road, the paved greenway bends to the northeast and becomes the East Fork Mine Creek Trail, while the Mine Creek Trail bends toward the northwest, continuing as an unpaved and poorly marked trail north to its termination at Sawmill Road.  A portion of the Mine Creek Trail south of Shelley Lake is referred to as the Ironwood Trail, owing to the presence of ironwood trees.

Geologically, the Mine Creek Trail lies within various rock units of the Crabtree terrane.  These metamorphic rocks are schists and gneisses that were originally mainly sedimentary and volcanic rocks.  The rock layers along the trail lie on the east flank of a large fold structure, the Raleigh antiform.  The layers run in a north-northeast direction, and generally dip toward the east at a moderate to steep angle, and locally vertically.  Among the more distinctive rock types in the Crabtree terrane are mica schist containing garnet and other metamorphic minerals, and schist that is rich in graphite, from which Lead Mine Road gets its name.  See the descriptions of the Shelley Lake Loop and Lakepark trails for more information.

At the beginning of the Mine Creek Trail, rock exposures can be seen along the east bank of Mine Creek.  A very short distance ahead, a bridge on the trail crosses over the creek.  Beneath the south end of the bridge, there is a large exposure of schist (photo).  A more accessible example of this rock can be seen above the trail, behind the apartment complex here, where huge blocks were left during construction.  The schist here contains garnet, as well as staurolite and kyanite, indicating that medium to high metamorphic temperatures and pressures were achieved by these rocks during formation of the Appalachian Mountains.

Continue northward, crossing North Hills Drive carefully.  In a short distance, a ridge extends from the west, almost reaching to the greenway (photo).

The rock here has been blasted during excavation for the sewer line; drill holes are visible in the outcrop (photo).

Garnets and small folds are also visible (photos).

         Mica schist containing garnets (small reddish spots).  

               The schist had tiny folds called crenulations, 

                 which resemble corrugations in cardboard.

Tight complex folds highlighted by quartz vein.

About 1/4 mile north, the trail bends to the left (west), following a sharp bend in the creek.  At this point, there is a huge rock exposure of schist on the outside of the bend (photo).

The creek's erosive power is concentrated on the outside of
 the bend, creating steep cut banks, while low point bars on the 
opposite side of the creek are sites where sediment is deposited.

A short distance ahead, there is an intersection with two connecting spur trails near milepost 1.  On the spur to the right (toward North Hills Drive), there is a very nice rock ledge at the base of the slope (photo).

This is a fine-grained biotite gneiss with abundant feldspar and 
quartz, making it resistant to weathering.  The distinctive banded 
appearance of the rock is the primary characteristic of gneiss.  
The banding strikes north 15 degrees east and dips about 60 degrees 
toward the east - recall we are on the east limb of the Raleigh antiform.

A very short distance ahead, long rod-shaped outcrops can be seen on both sides of Mine Creek (photo).

The long dimension of these rock exposures indicates 
the strike of the rock layers, like the grain of wood.

From here, the trail continues north and passes under Millbrook Road.  The lower parking lot for the Shelley Lake Loop trail is on the left; go to the right to continue on the Mine Creek Trail.  From here the trail follows the east side of Shelley Lake; points of interest are described in the Shelley Lake entry.  Toward the north end of the lake, the trail intersects with the Snelling Branch Trail greenway, which bends off to the right and uphill toward North Hills Drive.  Immediately after crossing the bridge over the creek, the Mine Creek Trail diverges from the Shelley Lake Loop, continuing north.

After passing under North Hills Drive through a narrow tunnel, the greenway takes a wide bend to the left, leaving the creek and following the hill slope.  A narrow unpaved footpath goes to the right to follow the creek here, rejoining the greenway in a short distance.  If you follow the footpath, you can see two large cut bank outcrops where the creek bends (photo).  Opposite these cut banks, there is a well developed sandy point bar.

One of two cut bank exposures between North Hills Drive and Lynn Road.

About 1/4 mile past Lynn Road, the Mine Creek Trail splits off to the left, continuing as an unpaved footpath along the creek.  The stretch of unpaved greenway trail continues north to its termination at Sawmill Road.

Sawmill Segment of Mine Creek Trail

After leaving the paved greenway, this narrow footpath squeezes between the southern end of a rocky ridge and Mine Creek, then follows the western flank of the ridge northward along the creek.  The ridge owes its existence to a rock type that is very hard and resistant to weathering.  It is a fine-grained, light-colored gneiss rich in quartz and feldspar (felsic gneiss) that has a strong lineation and other evidence of having been deformed and stretched by fault movements.  One of the first exposures encountered is an elongate rock outcrop that partially blocks the trail (photo).

Outcrop of strongly lineated felsic gneiss on the trail.
The lineation is parallel to the pocketknife.

In a very short distance, the slope to your right is strewn with chunks of the felsic gneiss (photo).  This is the southern end of a ridge that parallels the NNE strike of the rock structure.  The ridge was cut through over time by the erosive force of Mine Creek.   

Rocky southern end of ridge.

The creek and trail then take a sharp right bend and begin to run parallel to the strike of the rocks.  Some of the rocks along the slope here have features indicative of strong deformation, such as very thin bands and abundant quartz veins (photo).

Loose block of thin-banded felsic gneiss along slope.  This
rock shows evidence of having been deformed in a fault zone.

Along the slope, there are lots of large chunks of quartz and gneiss with quartz veins (photo).

Rock chunks littering the slope that parallels the rock structure.

You quickly encounter a very large flat "pavement" outcrop of felsic gneiss (photos).  It sits along the steep creek bank, just below the trail.  The rock foliation strikes N15^oE and dips 75^o toward the east.

Extensive pavement outcrop.  Trail is just above the steep bank;
view from across creek.  Note light-colored baseball cap for scale.

View from the trail of the same outcrop.  The nearly horizontal
lineation is visible.  Note this stretch of creek is roughly
parallel to the lineation.

The trail bends to the left, once again heading west across the strike of the rock structure.  Several sizable outcrops are visible in the creek and along the steep bank on the south side of the creek through this stretch (photo).

Outcrops of gneiss in and along creek.

From this point, the trail again takes a sharp bend and heads north-northeast, roughly paralleling the rock structure all the way to the trail end at Sawmill Road.  The terrain becomes flatter, and there are no more hard rocky stretches, because the trail has passed into a softer rock type - schist.

In about 1/4 mile, if the water is not high, you may be able to see a small elongate outcrop of weathered, moss-covered schist at a footbridge near some houses (photo).  The thin-layered, platy schist dips steeply toward the west.

View from footbridge, looking southwest, of schist outcrop.

In a couple hundred yards, there is a nice exposure of dark and light banded schist (photo).  Small folds are visible in this outcrop.

Outcrop of strongly banded schist; folds visible at right.

In a short distance, the trail runs along what appears to be an old dam.  Some of the rocks that were used to build the dam may be visible to the west.  The trail descends from the old dam and crosses a footbridge.  Here you may be able to see an example of the most distinctive rock type within the Crabtree terrane, graphite schist.  A sewer line crosses the creek just downstream from the bridge; a bit farther downstream, there is an excellent cut-bank exposure of vertically dipping white schist and black graphite schist (photo).

Eroded creek bank exposing black graphite schist just downstream
from sewer line (note pipe at left end of view).

If the water is not high, outcrops of steeply dipping schist are visible just downstream and upstream from the bridge (photos).

Outcrop of steeply dipping schist just downstream from bridge.

Outcrop of steeply dipping schist just upstream from the bridge.

Also, you may be able to explore the creek side just past (upstream from) bridge, and find loose pieces of garnet-bearing graphite schist (photo).  Graphite is soft enough to scratch with your fingernail, and it can mark paper - after all, it is pencil lead!.

Loose pieces of graphite-rich schist beside the creek, just
upstream from the footbridge.  Small dark spots are garnets.

In a short distance, the trail terminates at Sawmill Road.

Walnut Creek Trail

The Walnut Creek Trail extends from Lake Dam Road, just east of Lake Johnson, through the Centennial Campus of NCSU, then through the southern parts of Raleigh all the way to the Neuse River.  Geologically, it begins in rocks of the Crabtree terrane, then passes through the Raleigh terrane, and then into granite of the Rolesville batholith.

As you pass the first apartment complex on the right, you are traveling over a layer of graphite schist, a distinctive unit within the Crabtree terrane.  In the brush to the right there may still be evidence of it, but watch out for poison ivy.  From here the trail follows a low-lying area that is very susceptible to flooding.  After crossing Trailwood Drive, the trail continues on Avent Ferry Road and then back to the right, following a tributary of Walnut Creek.  Next the trail passes into the Centennial Campus of North Carolina State University, traversing the north side of Lake Raleigh.

When you approach the north end of the Lake Raleigh dam, there is a small outcrop next to the trail.  This rock is an example of Falls leucogneiss, a distinctive, strongly lineated rock unit within the Raleigh terrane (photo).  For a better look at this leucogneiss, examine the large blocks in the median of Centennial Parkway.

The lineation here trends a few degrees to the west of north.  A better and bigger outcrop can be seen just off the trail at this point (photo).

A little farther along, just before a tunnel, there are several small flat (pavement) outcrops of Falls leucogneiss just to the right of the trail.

From this point, the trail passes under Lake Wheeler Road, through an industrial area, under a railroad trestle, and under South Saunders Street.  Then it passes the old city water filtration plant, and on a sidewalk passes the Eliza Pool Park.  The large rock chunks around the park are from the underlying Raleigh gneiss, and include some banded gneiss as well as granite.  The trail then turns north along Fayetteville Street.  In a few blocks, the trail takes a sharp right turn, then follows an unpaved path down a steep slope and across a footbridge.  Turn right here to stay on the Walnut Creek Trail (to the left is the start of the Rocky Branch Trail).

Soon you come to a road (Peterson Street) and turn right paralleling it.  Across from you is Carnage Middle School.  In a very short distance the trail turns right again to re-enter the woods.  The Walnut Creek Wetland Center is located just ahead at this point on Peterson Street, and the Little Rock Trail greenway begins across the street.

In a short distance, the trail runs on a long boardwalk across a wetland; there is an interpretive sign.  About three tenths of a mile onward, you will see a large pile of dark colored rocks next to the trail (photo).

This is near the spot where Grantland Drive ends, and these rocks are diabase boulders, piled here during construction.  Note the rounded shapes of some, the result of the spheroidal weathering process.  Diabase is an igneous rock that occurs as steeply dipping thin bands, called dikes.  These dikes formed from hot magma that intruded into cracks in the older (500 - 600 million year old) rock of the Raleigh terrane.  Diabase is 200 million years old, and records the breakup of the supercontinent Pangaea and the birth of the Atlantic Ocean.

Soon, the trail leaves the woods and follows Little John Road and then continues by Worthdale Park.  On a hill in this stretch, you pass a small road cut of weathered granite (photo).

 This is the first evidence that you have passed out of Raleigh gneiss and have entered the Rolesville batholith, a huge body of granite that is about 300 million years old, much younger than the metamorphic rocks into which this igneous mass intruded.  You will be in this body of granite for the reminder of the Walnut Creek Trail.

After passing through the Walnut Creek Softball Complex, you cross under Interstate 440.  The Walnut Creek Amphitheater is off to the right.  Continue straight on the Walnut Creek Trail, and the pavement soon gives way to a gravel surface for about a mile.  The trail takes a sharp right bend.  At the top of the next hill, you may see a small exposure of granite bedrock on the left (uphill) side of the path (Photo; pocket knife for scale).

The tiny black specks in the rock are crystals of black mica (biotite).  You may also notice several irregular long clots or strings of biotite.  Such features, known as schlieren, are common in igneous rocks and are probably related to flow of the hot magma during its cooling and solidification.

A short distance ahead, the trail takes a sharp right and heads down a fairly steep slope.  Soon you pass beneath New Hope Road, and then beneath a power transmission line.  Next the trail passes through a large wetland area, and housing developments are seen on the left (north) side.  The blocks and boulders along this stretch are all granite belonging to the Rolesville batholith.  Many of them have been moved during construction activities.  Just beyond the 1 1/2 mile marker, in a curve, you will see a very rocky cut on the left side (Photo).

The rocks on this slope are part of the granite bedrock that underlies the ridge and is very well exposed at the surface.  Continue around the curve in the trail, and almost immediately you pass under another high power transmission line.  Here there is a flat pavement type outcrop of granite (Photo; hammer for scale.  Note poles for power line).

The trail continues through wetlands, with Walnut Creek to the right (south).  After a mile or so, you come to Barwell Road, where the trail crosses Walnut Creek on a trail bridge that parallels the highway bridge.  You may be able to see a large exposure of granite on the right side at the edge of the creek (Photo).

Then the trail follows a loop to pass under Barwell Road and continue along the south side of Walnut Creek.  If you have a clear view, you may notice two or three huge boulder outcrops down below you in the creek, beside a mobile home park.

In a short distance, the Walnut Creek ends at the Neuse River Trail.

Prairie Ridge Ecostation - The Geology

Prologue

The geologic story of the rock types and their geologic history was determined through many years of research by many investigators in the Piedmont of North Carolina and the immediate area surrounding Prairie Ridge.  This story combines many geologic investigations conducted within the Raleigh area and from other locations throughout the Southeast.  For a scientific review of the geologic interpretation of the various crystalline rock terranes in the Raleigh area, see Hibbard et al. (2002), Blake et al. (2001), and Stoddard and Blake (1994).  Descriptions of the rocks in the immediate vicinity of Prairie Ridge can be found as part of the detailed geologic map of the Raleigh area by Blake (2008).

The Rocks of Prairie Ridge and a Brief Review of their History

The various rock types and the soils that are derived from the rock form the foundation of Prairie Ridge.  But this foundation is rarely seen.  In the Raleigh area bedrock exposure is primarily restricted to creeks and streams, were the erosion power of water has stripped away feet of soil.  The rocks visible in the creek at Prairie Ridge can be generally separated into three rock types: 1) muscovite schist, 2) graphite-bearing schist, and 3) granitic gneiss (Figure 1).

Figure 1:  Map of Prairie Ridge Ecostation with major geologic features indicated.

Schist and gneiss are metamorphic rock types that formed from the metamorphism (changed through heat and pressure) of pre-existing rock types.  Typically schist is the result of the metamorphism of mudstones and siltstones.  Gneiss can be formed from the metamorphism of a wide variety of rock types – including sedimentary and igneous rock types.  Muscovite (a light-colored mica mineral) and graphite (a very soft mineral composed of carbon) are minerals that are prominent in the schists at Prairie Ridge.  The term “granitic” refers to the original rock having been granite (a plutonic igneous rock). 

To a geologist these rock types help tell the story of how a major portion of North Carolina formed.  The rock that underlie Prairie Ridge and the surrounding areas of Wake County record a long and sometimes violent geologic past of over a half billion years of Earth history.  Collectively the rocks of the Raleigh area record the formation of an ancient chain of volcanic islands (called Carolinia by geologists) that were active for millions of years with countless volcanic eruptions that formed piles of volcanic ash and lava flows.  During this same period of volcanic activity, erosion wore down the volcanic areas and deposited layers of sedimentary rocks in the form of mudstone, siltstone, and sandstone.  At Prairie Ridge, the schists are examples of the metamorphosed mudstones and siltstones that were deposited in this ancient volcanic area.  Primitive life, such as mats of algae, grew within and on the mud and silt layers.  At Prairie Ridge, the graphite-bearing schists are examples of the metamorphosed mudstones and siltstones that had abundant organic material (likely from algae) deposited in this ancient volcanic area.  Also at the same time, deep within the Earth below the volcanoes, magma (molten rock) -that did not make it to the surface to erupt as ash or lava – cooled very slowly and formed plutonic igneous rocks (like granite or other granitic rocks).  Over time, multiple generations of volcanoes and magma cut into older volcanic and their related sedimentary deposits.   At Prairie Ridge, the granitic gneiss is an example of magma that intruded the mudstones and siltstones after they were buried deeply. The magma cooled slowly and formed granite (a plutonic igneous rock).

    

Eventually, after many millions of years of volcanic activity, the volcanoes went extinct, and the layers of volcanic ash, lava, and associated sedimentary rocks were buried deep. Very generally, through plate tectonics and many millions of years, Carolinia – the chain of volcanic islands - inched across an ancient ocean and likely collided with another chain of islands that caused the first of several phases of folding and metamorphism.  Later, magma from a new period of volcanism started to erupt through the older rocks of the volcanic island arc.   This period of volcanism eventually ended, Carolinia continued to move slowly toward ancient North America and eventually after millions of years collided and “welded” itself to the North American continent.  This collision caused another period of metamorphism and folding. 

Millions of years later the ancient ocean that separated ancient North America from the ancient African continent closed and the continents collided forming the Supercontinent Pangea.  This collision again caused metamorphism, folding, and faulting and is last of the metamorphic events to affect the rocks at Prairie Ridge.  In the Raleigh area this collision formed a giant geologic structure called an antiform -  a type of fold.  (An antiform is similar to an anticline but geologists are unsure of the ages of the rocks so it is called an antiform.)  Geologic units are repeated from one side of the anticline/antiform to the other.  Geologists call this fold the Raleigh antiform (Figure 2).  

Figure 2: Block diagram sketch of the Raleigh antiform in a portion of the Raleigh West Geologic map.

Important marker units (rock layers) in the Raleigh antiform are schist layers that are composed almost entirely of graphite – called the graphite schist.  The graphite-bearing layers exposed in the creek at Prairie Ridge are part of the layers that are related to the graphite schist layers exposed on the opposite limb of the Raleigh antiform (Figure 2).  Geologists believe that the various layers of graphite-bearing and graphite schist were originally deposited as organic-rich mudstone and siltstone, as horizontal layers (like typical sedimentary layers), and through folding are now exposed as “thin strips” that are standing on their sides on both sides of the Raleigh antiform (Lumpkin et al., 1994).  

Essentially, the layers of graphite-bearing schist in the creek at Prairie Ridge are on their side (nearly vertical in places) because of the folding and they are part of the Raleigh antiform.

References:

Blake, D.E., Clark, T.W., and Heller, M.J., 2001, A temporal view of terranes and structures in the eastern North Carolina Piedmont, in Hoffman, C.W., ed. Field Trip Guidebook for the 50th Annual Meeting of the Southeastern Section, Geological Society of America, Raleigh, North Carolina, p. 149-180.

Blake, D.E., 2008, Geologic map of the Raleigh West 7.5-minute quadrangle, Wake County, North Carolina: North Carolina Geological Survey, Geologic Map Series – 15, scale 1:24,000.

Hibbard, J., Stoddard, E.F., Secor, D., Jr., and Dennis, A., 2002, The Carolina Zone: Overview of Neoproterozoic to early Paleozoic peri-Gondwanan terranes along the eastern flank of the southern Appalachians: Earth Science Reviews, v. 57, n. 3/4, p. 299-339.

Lumpkin, B.L., Stoddard, E.F., Blake, D.E., 1994, The Raleigh graphite schist, in  Stoddard, E.F. and Blake, D.E., eds., Geology and Field Trip Guide, Western Flank of the Raleigh Metamorphic Belt, North Carolina, Raleigh, North Carolina Geological Survey, Carolina Geological Society Guidebook for 1994, p. 19-24.

Stoddard, E.F. and Blake, D.E., eds., 1994, Geology and Field Trip Guide, Western Flank of the Raleigh metamorphic belt, North Carolina, Raleigh, North Carolina Geological Survey, Carolina Geological Society Guidebook for 1994, 110 p.

Shelley Lake Geology

 Shelley Lake Loop Trail

This geologic map shows the pattern made by layered rock units in the vicinity of Shelley Lake.  Each color represents a different rock unit.  Notice the crude symmetry of repeated rock units on either side of the Raleigh antiform.  The antiform is an arch-shaped fold that runs north and south through west Raleigh; its hinge-line lies along the heavy black line, and the short arrows pointing out identify it as an antiform.  Rock layers along the hinge-line should be nearly horizontal, while to each side the layers dip away from the hinge; in this case, they dip very steeply.  The Raleigh antiform is highlighted by the repetition of a unique graphite schist unit shown in dark gray.  Old graphite mines resulted in the name Mine Creek (now dammed to create Shelley Lake) and Lead Mine Road (graphite is pencil lead).

Google Earth KML File:
 https://docs.google.com/file/d/0B0fIscBEdGVzd2VpQWdsRUVfQjg/edit

At the east end of the Shelley Lake Dam, if the grass is not too high, in the field below you may be able to see bare patches in the soil that line up in a generally north-south direction (Photo).

These are actually the upturned edges of steeply dipping layers of rock.  The direction of these lines is called the strike of the rock layers.  In this case, the strike is N18°E, or 18 degrees east of north.

If you continue counterclockwise around the lake, you will soon encounter a large rock on the right side of the trail embedded in a steep bank.  This is felsic (light-colored, granitic) gneiss and is a block that was moved during trail construction.  You may notice a small cross-cutting lighter colored pegmatite dike in the rock.

About 100 yards ahead, you cross a small creek on a long bridge.  To your right, there is a small rock exposure in the creek, which may be visible in the winter (Photo).  The rock foliation strikes N15°E and dips to the east at about 75^o.

At the northern end of the lake, there is a large low-lying area that is underlain by sediment deposited by Mine Creek, which enters the lake from the north and deposits silt and sand.  This sediment constitutes a delta.  After crossing the bridge over Mine Creek and heading south, you can see a curve or meander in the creek.  On the outside of the curve, the creek runs faster and erodes the bank.  Note that large rocks, called rip-rap, have been placed here to combat erosion (Photo).

Also along this low stretch, you may notice that abundant sand and silt accumulates after heavy rains and flooding.  The city has to remove these deposits periodically.

Lake Park Connector Trail

This unpaved trail connects to the Shelley Lake loop trail at a point downhill from the Sertoma Arts Center parking area.  It runs northward through the Lake Park Swim Club and ends at Rushingbrook Drive in the Lake Park subdivision.  The trail follows a tributary creek to the west, then crosses over it on a bridge.  At the bridge you can see cascades in the creek, where the water flows over thin layers of gneiss that dip toward the west at a moderate angle, about 35^o (Photo).

Here you are on the western limb of the Raleigh antiform; the eastern limb is on the east side of Shelley Lake, and there the layers dip more steeply to the east or even vertically, such as the layers you can vaguely see from the east end of the dam described and pictured above.  The Raleigh antiform is an asymmetric fold, with a steeper east limb than west limb.

As you proceed uphill along the trail, you come to a spot where the creek bed is underlain by light-colored Crabtree Creek gneiss.  If you leave the trail you can see small waterfalls and cascades formed by this hard rock type (Photo).  Here the layers dip gently or moderately toward the west (Photos).

The rock is also strongly lineated, attesting to the squeezing forces that bent and stretched the rocks.

Continue along the trail, and soon cross a footbridge and arrive at the volleyball court of the Lake Park Swim Club.  If you walk a few steps to your left, you can see a spectacular cut bank exposure of soft, black graphite schist (Photo).

This rock is the source of the name Lead Mine Road, as this layer of graphite schist runs generally along that present-day road, and was the site of several graphite (carbon, not lead) mines.  Look carefully at this exposure and you will see that on the left (east) side, the graphite schist is in contact with light-colored, non-graphitic rock (Photo).

These layers dip moderately toward the west, indicating that we are still on the western limb of the Raleigh antiform.

Continue on the trail, cross another footbridge, and you will see a nice waterfall exposure of graphite schist dipping toward the west (Photo).