Monday, November 25, 2013

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 N15oE and dips 75toward 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.

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