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.

Lake Johnson Geology

The Lake Johnson Greenway is divided in two by the causeway of Avent Ferry Road.  The western portion is a mostly unpaved hiking trail, while the eastern portion is paved and suitable for biking.  All rock exposures around Lake Johnson belong to various rock units within the Crabtree terrane.  The axis of a large fold, the Raleigh antiform, passes from north to south through the eastern half of the lake.

Lake Johnson West

From the parking lot at the north end of the causeway, proceed west along the northwestern edge of the lake.  The terrain is very flat here, but you should be able to see a more rugged shoreline on the other side to the south.  The western end of Lake Johnson is where Walnut Creek enters, and has deposited considerable sediment, forming a small delta that constitutes a nice wetland.  As you proceed along the unpaved path on the southwestern portion of the lake, you will encounter spots where the uphill side of the trail was cut and some low portions have been filled in, to make the trail more level.

If you look closely, you can see a small cut exposing weathered granitic gneiss, showing banding, under a pine tree.  A short distance ahead, you encounter a better outcrop of orange-colored granitic gneiss (Photo).

The banding, or foliation, of the gneiss dips moderately toward the west (in the uphill direction).

There is a short spur trail leading to a steep bluff where there is a bench and a nice overlook providing a lake vista.  Here there is an excellent outcrop of very light colored (leucocratic) gneiss (Photo).

The foliation strikes about north-south and dips about 20^o toward the east.  There is also a strong lineation, plunging about 13^o to the north.  This rock unit is called the Southwest Prong gneiss.

Ahead in a tributary creek, you will get a view of a big cut bank outcrop of fine-grained mica-rich gneiss or schist.

After reaching Avent Ferry Road, cross and walk north toward the causeway.  You will encounter a nice roadcut exposure at the south end of the causeway.  This rock is a mica-rich gneiss or schist that weathers into thin platy slabs (Photo).

The foliation strikes north-northwest and dips toward the southwest, and there is a lineation plunging south.

Lake Johnson East

Note:  From the parking lot along Avent Ferry Road south of the lake, this description follows the greenway trail counterclockwise.  Also, be forewarned that this section of trail has more steep hills than other trails and is usually very busy, especially if the weather is nice.

According to the geological map, the low point following the first steep hill is where the axial trace of the Raleigh antiform passes through.  This would mean that foliation (layering) in rock outcrops here should be nearly horizontal, whereas it dips away from this trace on either side.  Unfortunately, there are no outcrops visible from the trail at this point.  However, as you climb the second steep hill, you may be able to see a rock exposure in a small creek down to your left (Photo).

The foliation dips about 25 or 30 degrees toward the east.  Recall the outcrop near the south end of the causeway dips west, so we have indeed crossed the axis of the antiform (an arch-shaped fold).  A bit farther along the trail, just beyond the 2-mile marker, there is an outcrop to the right up on the hillslope and ridge. The rocks here also dip gently toward the east (Photo).

Continue along the trail until just before the dam.  There is a very large exposure of rock just below the spillway (Photo).

To see it, take the trail spur to the right toward Lake Dam Road, then walk up a small unmarked trail below the dam.  This outcrop consists of mica schist containing the metamorphic minerals garnet and staurolite. Elsewhere near Lake Johnson the mineral kyanite may also be found; together, these metamorphic minerals tell geologists that the rocks were buried to a depth of about 15 miles and reached temperatures of about 650^oC.  The foliation of the schist here dips eastward very steeply, around 80^o.  This rapid steepening on the east flank of the Raleigh antiform may also be seen at the greenways around Shelley Lake and along Crabtree Creek.  This indicates that the Raleigh antiform is asymmetric, more steeply dipping on the east than on the west.

You may choose to continue the Lake Johnson East Loop Trail, proceeding across the dam and then west toward the north shore of the lake.  Alternatively, the Walnut Creek Trail begins immediately across Lake Dam Road from the dam.