Wednesday, October 16, 2013

Prairie Ridge Ecostation - The Geology


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.


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.

Wednesday, October 2, 2013

A Leisurely Geological Bike Ride along the Neuse River Greenway Trail

This 33 mile-long paved greenway follows the Neuse River, for the most part, from the Falls Lake Dam in northern Wake County south and southeast to Clayton in Johnston County.  The beginning point is at 12088 Falls of Neuse Road in Raleigh (27587; this is “Old” Falls of Neuse Road, just off New Falls of Neuse Road) and the ending point is at the Sam’s Branch Greenway Trailhead, located at 1358 N. O’Neil St. in Clayton (27528).  This geological greenway trail guide connects to a 60-mile geological hiking guide (Geological Guide to the Falls Lake Trail that begins at the Falls Lake Dam and heads west to Durham.

Most of the Neuse River Greenway Trail (NRGT) runs within the floodplain of the Neuse River.  For this reason, there are fewer exposures of bedrock than you will encounter on the Falls Lake hiking trail.  On the other hand, the more wide-open vistas and the asphalt surface make the NRGT perfect for a bicycle tour, and there is enough geological variety to provide for a few rest stops and see some interesting features.

The Falls Lake dam was constructed along an unusual rock unit called the Falls leucogneiss (colored beige on the geological map).  Prior to the dam, natural exposures of the leucogneiss in the Neuse River produced extensive rapids and small waterfalls ("Falls of Neuse"), for which the Falls community was named.  Very resistant to erosion, the Falls leucogneiss causes narrowing of stream valleys, and forms ridges along its trend.  The emergency spillway, located just north of the dam, was blasted through such a ridge.  Exposures of the leucogneiss are typically elongate narrow ledges, and some may be seen in the river between the old Falls of Neuse Road and the parking area for the NRGT trailhead.  (Photo 1; click on photos to see an enlargement)

As you proceed downstream (southeast) along the greenway, the leucogneiss quickly gives way to a new bedrock unit called Raleigh gneiss (blue on the map).  Gneiss (pronounced nice) is a banded metamorphic rock.  Raleigh gneiss is a heterogeneous rock unit that includes quite a bit of white or pink granite, but the gneiss commonly has a black and white banded appearance.  Outcrops along this stretch are quite rare, but beautiful examples of Raleigh gneiss can be seen in the quarried rip-rap blocks under the overpass for New Falls of Neuse Road. (Photo 2)
These rocks came from a quarry located just west of US Highway 1, between the highway and the river.  In fact, you may hear activity from the quarry as you proceed along the trail.

After crossing under a power line and over a small bridge, you may see a high bluff along the river with mountain laurel (Point of Interest A).  Here, at the edge of the river, there are huge blocks of gneiss and granite eroded from the bluff.  Such steep riverbanks, called cut banks, typically occur on the outside of a bend in the stream, where the water is deeper and flows faster.  At this location, the trail has been cut into the steep slope, and if you look carefully you may see a pinkish or creamy colored rock weathering out of the slope along the trail.  This is a feldspar-rich rock called pegmatite, a variety of granite with large crystals.  In former days, feldspar, quartz, and mica were mined from pegmatite within the Raleigh gneiss in Wake County.  Several old mines were located within a couple miles of this spot.

As you continue on the trail, the surface becomes flat and wide (Point of Interest B).  Here the Neuse River has a broad, well-developed floodplain, and for the most part the NRGT runs along it.  Occasionally within the floodplain, you may notice that the land actually slopes slightly uphill toward the river, indicating the existence of low ridges.  These ridges are natural levees, built up during flooding of the Neuse River, when sediment is deposited by rising floodwater that loses its carrying capacity when it escapes the riverbank.  (You can see this on a much larger scale in places like New Orleans, as well as human augmentation of the natural levees.)

The river and the trail take a sharp (90-degree) bend to the right.  Soon, you will see a dirt path toward the river that leads to a rope swing (Point of Interest C).  On the opposite side of the river, there is a very nice cutbank outcrop of granite.  This is part of a body of granite (called the Wyatt pluton) enclosed within Raleigh gneiss.  The rock quarry is very close to this point, up the hill beyond the outcrop.  You may be able to detect its depression on the geological map above.

As you continue along the trail, the floodplain widens again.  There is an interpretive trail marker describing floodplains, natural levees, and vegetated stream buffers (Point of Interest D).

After crossing under the railroad tracks and the old pumping station, you pass beneath U.S. Highway 1 (Capital Blvd.).  The next stretch is more extensive broad floodplain.  There is an interpretive marker describing the formation of an oxbow lake, which happens when a tight loop in a river is cut off during a flood, when the water takes a short-cut downstream (Point of Interest E).  Flat, broad floodplains encourage the development of sinuous river courses with such tight bends.  Horseshoe Farm Park, just west of U.S. Highway 401, is located within such a loop.  At Point E, there are picnic tables and a bench overlooking the river.  A major tributary, Smith Creek, enters the river on the far side (north side).

The NRGT takes a sharp left turn at the WRAL Soccer Complex on Perry Creek Road; you will see a paved spur trail coming from a large parking lot.  As you continue, the trail follows the outside of the curve of the "horseshoe" of Horseshoe Farm Park, located on the opposite side of the river.  By way of an elaborate boardwalk, the trail passes by several old farm buildings.  When you reach the top of elevated boardwalk, you can see several large outcrops of granitic rock on the steep slope below you, and in the river itself (Point of Interest F).  This steep north-facing slope also supports mountain laurel and holly.  In about a half mile, you encounter the spectacular trail suspension bridge that crosses to the north side of the river and leads to Horseshoe Farm Park.  Continue ahead and pass beneath U.S. Highway 401 (Louisburg Road).

At this point, we have left the Raleigh gneiss and have begun to traverse the most extensive rock unit in the eastern Piedmont of North Carolina.  This enormous body of granite, the Rolesville batholith, constitutes the bedrock of the eastern half of Wake County, as well as much of the neighboring counties.  If you continue along the NRGT, you will encounter sporadic exposures of it from here all the way to Clayton in Johnston County.  The granite is pink on the geological map.

At the 9 ¼ milepost, there is a weathered outcrop of granite (Photo 3).  You can easily see from its crumbly appearance that its mineral grains are slowly decaying into soil.

Between mile markers 10 and 10 ¼ there is a small flat exposure of granite (Photo 4). 
This type of outcrop is not crumbly but is hard; this is known as a pavement outcrop.  Farther along, just after crossing over to the other side of the river, there is another pavement (Photo 5).
This one occurs between the 11 ½ and 11 ¾ mile markers.

A short distance on, there is a bridge over a wetland.  Just before starting across the bridge, you may notice another nice pavement of granite on the left (east) side of the trail.  Then, just after crossing the wetland bridge, you will come upon an excellent exposure of fresh hard granite (Photo 6 – view from the bridge, note turtles).
This is one of the best outcrops on this greenway trail, and provides a good place to stop and examine the rock more closely.  The granite is composed of four major minerals:  two varieties of feldspar, quartz, and biotite (black mica) (Photo 7).
The colors of the feldspars and the size of the crystals vary throughout the Rolesville batholith, resulting in different varieties of granite.  Here you can also see how the granite is weathering into soil (Photo 8).

Along the next half mile or so (between the big rock outcrop and milepost 12.5), you pass through a cut that was excavated into the granite during construction of the sewer line, and modified in order to provide space for the trail surface.  If the embankment is not too overgrown, you may be able to see sporadic exposure of the bedrock.  If you look at the map, you can see that this stretch lies on the outside of a curve in the river's course (Point of Interest G).  Along a bend in a river, the water flows more rapidly and the water depth is greater, at the outside of the bend.  Conversely, it flows more slowly and is shallower on the inside.  This causes erosion to concentrate on the outside, and deposition on the inside.  The result is a steep cliff-like cut bank, commonly with rock exposures on one side, and a low-lying, flat and sandy point bar on the other.  If you look across the river along this stretch, you can see several homes on the lower point bar side.  Such home sites provide excellent river access, but also flooding potential.

The trail soon crosses back across the river on a scenic suspension bridge.  Just beyond the bridge is a large pavement outcrop of granite.  Soon, between mile markers 13 ½ and 13 ¾, you pass through a low road cut in weathered granite (Photo 9).
Then you may see another pavement outcrop of granite (Photo 10).

Between markers 15 and 15 ¼, turn left toward Loch Raven Parkway for a great view of the old Milburnie Dam (Photo 11).
The old powerhouse is visible to the left of the dam, and the large flat outcrop of granite below the dam is a favorite fishing spot.  There is an interpretive marker at the bridge.

If you take a short detour and ride across the bridge, you can examine some huge and spectacular boulders of Raleigh gneiss around the edge of the parking lot (Photos 11a and b).  They show intricate banding between darker, hornblende and biotite-rich layers and lighter feldspar-rich layers. These rocks were brought in from their outcrop belt to the west, during greenway construction activity.

Continuing south on the NRGT, you pass a small wetland, and then you will have a view of the river that features numerous large granite exposures.  Just before you reach the highway bridge ahead, you may see an old, long road cut in the granite, off to the right side of the trail (Photo 12).  It may be overgrown with kudzu during much of the year.
You may also see a pavement outcrop.

Next you pass beneath New Bern Avenue (U.S. 64 Business) and continue downstream to Anderson Point Park.  Along this stretch you will encounter well-developed floodplain with local natural levees, some more outcrops of granite, notably in the river, and wetlands.  There is also a bridge that crosses the river and connects with the Mingo Creek Greenway Trail, which runs east toward Knightdale.

Arriving at Anderson Point, you must go through the trail parking lot and over the bridge that spans busy U.S. 64/264 Bypass.  Anderson Point is the narrow triangle of land formed at the junction of the Neuse River and Crabtree Creek.

The map above is a generalized geological map showing the southern portion of the NRGT.  Anderson Point is at the upper left.

After you pass through the park itself, you encounter the trail bridge over Crabtree Creek (between markers 17 ¾ and 18).  There is a large outcrop of granite under the south end of this bridge (Photo 13).  From this bridge, Anderson Point and the confluence of Crabtree Creek with the Neuse River are visible to the east (left).
In another mile or so, after crossing under Poole Road, you cross a similar bridge over Walnut Creek.  If you stop on the bridge, and depending on the water level, you may be able to see the shallow mound of sediment deposited by Walnut Creek as it enters the Neuse (Photo 14).
This is a delta; if you imagine this on an enormous scale, it is the manner by which the Mississippi River formed Louisiana.

Between mile markers 19 ½ and 19 ¾, you will come upon a rounded outcrop of granite that has large crystals of beige or pinkish feldspar (Photo 15), and small intrusions (called dikes) of pegmatite (Photo 16).

From the bridge between 19 ¾ and 20, you may notice a large boulder outcrop of granite along a small creek (Photo 17).

From a small bridge between markers 21 ¼ and 21 ½, you can see a nice outcrop of granite below you (Photo 18).
Notice how the rock has lots of planar cracks in it.  These cracks are called joints by geologists.  In this part of North Carolina, joints in bedrock are where ground water occurs.  When you drill a well for water, you hope to intersect lots of joints that interconnect with one another and along which water can move.

Joints in hard rock tend to break the rock up into rough blocks.  Then the blocks slowly become rounded, not by erosion in a stream, but by the slow process of weathering.  Because weathering is more rapid at corners and edges of blocks that on the flat faces (because there is more surface area for chemical reactions to occur), they slowly but surely become more spherical.  This is a process known as spheroidal weathering.  In this area, when you see a round boulder on the ground (not in a streambed), this is probably how it formed.  Just after passing beneath Auburn Knightdale Road, there are some boulders that have formed in this manner along the access ramp (Photo 19).

Between mile markers 22 and 22 ¼, there is an impressive ledge of moss-covered granite about ten feet high (Photo 20).
Notice that it has flat surfaces (joints) running in several directions.  Typically there is one set of joints roughly horizontal (parallel to the ground) and at least two more that may be roughly vertical and commonly at right angles.  In this way the rock is divided into cubic or rectangular blocks, and then the spheroidal weathering process proceeds.

Soon the trail leaves the river and heads inland and uphill.  Just after crossing through the tunnel under Battle Bridge Road, you will encounter numerous rusty-weathering blocks and spheroids of a new rock type – diabase (Photo 21).

This igneous rock is much younger than the granite.  As a hot liquid, the diabase intruded into the granite by squeezing into long linear cracks, then cooling and solidifying.  Diabase dikes (steeply dipping sheets) formed this way are very common throughout the Piedmont.  They range from a few inches to over 100 feet thick, and some may be traced for miles.  On the geologic map figures, diabase dikes are represented by thin red lines.  Fresh diabase is a fine-grained black rock (Photo 22).  Whereas granite is felsic (light-colored), diabase is mafic.  Diabase is almost identical to the basalt that lies beneath all the ocean floors.  It is also almost always well jointed, and makes a good source for ground water, especially when it is surrounded by rocks that are poor sources, for example the sedimentary rocks of the Triassic basins of North Carolina.  Blocks of diabase may also be seen at the trail crossing on Brownfield Road (Point of Interest H).

The NRGT climbs up and around the Neuse River Wastewater Treatment Plant, where the city of Raleigh treats its wastewater before returning it to the river.  This is by far the longest steep stretch of the NRGT, and the only significant portion that departs from the river.  Referred to by some as "Wastewater Hill," it is also the highest elevation on the NRGT, at about 280 feet above sea level (Point of Interest I).  Interpretive markers along this stretch describe the treatment plant and associated fields that the trail passes.

Proceeding downhill along the trail, you cross a bridge to the other side of the river and soon pass into Johnston County.  Just past a bridge over a small creek, on the left (east) there is a large boulder of granite with conspicuous feldspar crystals (Photo 23).
This is known as porphyritic texture, when an igneous rock has one mineral whose crystals are larger and more prominent than those of the other minerals.  Nearby there is a small block of diabase, indicating that a dike must run through here somewhere.

A short distance ahead, if you look through the trees toward the river, you may catch a glimpse of some huge bouldery outcrops of granite (Photo 23A).

Between mile marker 28 and 28.5, if you stop and look back upstream (north), there is a very nice view of a bend in the river (Photo 24).
Here on the opposite side of the river you can see the inside of the bend, where the ground slopes gently and there is a large accumulation of sand (a point bar).  The NRGT trail is on the outside of the same bend, and on this side there is a steep bluff (cut bank), and a large outcrop of granite in the river.  Recall that this is how a river's course evolves, with erosion on the outside and deposition on the inside of the curve.  If you feel adventurous, you can climb down to inspect the rocks.

At marker 28.5, a bridge crosses a small creek.  Here, there is a nice pavement outcrop of granite in the creek (Photo 25).

A little farther along, there is a large pile of rocks at the edge of the woods (Photo 26).
The rocks are mostly granite, with some pegmatite.  If you look closely, you can see quartz, feldspar, and muscovite (white mica) in the pegmatite.  There are outcrops of similar rocks nearby.

Just before the trail passes through the Riverwood subdivision, you cross Marks Creek, a significant tributary of the Neuse.  Several conservation groups have been working toward preserving several thousand acres of land in the watershed of Marks Creek (see  As you ride through Riverwood, on both sides of the trail, there are several large spheroidal boulders of a very fine-grained variety of granite (Photo 27).

As you move on, just before mile marker 30.5, and just after passing the "Neuse Adventures" sign and river access, you pass through a cut in the slope that was made for the trail (Photo 28).

At first glance it may not look exciting, but look closer!  You will see bits of the bedrock here, and it is NOT granite!  This is mica schist, a new rock type (Photo 29).  It is made up mainly of muscovite (white mica), biotite (black mica), with some sparse garnets.  Here the schist has a corrugated appearance, owing to tiny folds in the rock (crenulations).  At mile 30.5, we have finally passed out of the Rolesville batholith, first entered at about mile 9.  The geological map depicts this rock unit in a gray color, and with the label gmbs.  If you look closely at the map, you may notice that it does not agree precisely with the observations we have made.  In particular, the map indicates that we should have encountered schist earlier, where we instead saw fine grained granite.  The map was made years before the greenway; if the authors of the map had had access to the rock exposures we have seen, they would have drawn the contacts between rock units slightly differently.


Just a short distance ahead, you cross under Covered Bridge Road.  If you look along the riverbank on the other side of the river beyond the bridge, and assuming the water is not too high, you can see a very nice exposure of layered metamorphic rocks at the water's edge (Photo 30).  The foliation (metamorphic layering) in these rocks dips moderately (maybe 35 degrees) toward the southeast (in the downstream direction).  The rock making up this exposure is called amphibolite, a dark-colored metamorphic rock composed primarily of hornblende and plagioclase feldspar.  This rock likely started out as a dark volcanic rock called basalt (similar to the rocks in Hawaii and Iceland), prior to being converted into a metamorphic rock,  This belongs to the unit labeled am and colored green on the geological map.

Continuing south, you cross back over the river.  Soon, the trail leaves the river and turns right, where it is part of the Sam’s Branch Greenway, operated by the town of Clayton.  You pass through a stretch featuring a display of artwork, and then a picnic area.  Soon, under the large power transmission line you will see a huge block of rock that was blasted out during construction (Photo 31).
Notice the drill hole in the rock.  This rock is a dark green banded metamorphic rock (greenstone, a relative of amphibolite) that shows evidence of being heated by the nearby granite.  Originally it may have been a lava flow from an ancient volcano.

A very short distance ahead, there is heavily weathered rock in the embankment on the right.  You may notice that the left end of the embankment is a slightly darker color than the right (Photo 32).
 This is because there is metamorphic rock on the right (phyllite), but it has been cut by a diabase dike on the left.  Look closely and you will see the different rock types.  Remember that diabase is a rusty-weathering black igneous rock that weathers into spheroids (Photo 33).

The NRGT ends at the Sam’s Branch Greenway trailhead parking area on O’Neil Street (Point of Interest J).