Trail Section 1 - Falls Lake Trail Geologic Guide

 

FLT section 1-3 map

Geologic information by Edward F. Stoddard (in Bold Text).

Mileage and Trail section descriptions from Mark Edelstein with updates from March – April 2011.

0.0 mi – Parking area of the Tailrace Fishing Area, just below the dam, off Falls of the Neuse Rd. Trail starts across from the restrooms, by three large boulders. Following white blazes, start on an open path.

1-A:     Most of these boulders are pieces of a distinctive rock unit called Falls leucogneiss (lew'-ko-nice).  They most likely were excavated during construction of the dam.  A couple boulders, notably the one on the extreme right, come from the next geological unit to the east, the Raleigh gneiss (Figure 1-A).

The Falls leucogneiss runs in a narrow band, typically one-half mile or less in width, from the city of Henderson in Vance County, southward through Franklin and Wake Counties, ending near Lake Wheeler.  This rock is geologically unusual and interesting, and it also has played a significant role in the history of the region.

The Falls leucogneiss (hereafter abbreviated Fln) is a very hard rock type, and is therefore more resistant to erosion than most other rocks of the region.  For this reason, wherever streams and rivers encounter the Fln, they have a difficult time cutting through.  Over time, this has resulted in the occurrence of natural rapids and waterfalls.  In addition, the stream valleys and floodplains tend to narrow at the Fln.  The Fln is therefore an example of how a Fall Line is created, with the stream too narrow, steep, and rocky, and the water flow too swift to allow boats to easily move upstream.  (In this part of the North Carolina Piedmont, we usually speak of a Fall Zone instead of a Fall Line, because the Fln is not the only hard rock type that provided an impediment to water travel, but it is an excellent example.)

Locations where sizeable streams cross the Fln typically provide outstanding sites for construction of dams because the valley is narrow, the stream gradient steep, and the bedrock hard.  Early settlers favored these locations for grist or saw mill sites; more recently, people have constructed dams for water supply, flood control, and recreation at these locations.  Such sites in Wake County, from south to north, include Lake Wheeler Dam on Swift Creek, Yates Millpond on Steephill Creek, Lake Raleigh on Walnut Creek, Lassiter Mill on Crabtree Creek, and of course Falls Lake Dam on the Neuse River.

Another topographic effect of the Fln is that it tends to express its location by creating a ridge, due to its resistance to weathering.  Because it runs, in large part, along a ridge running generally north-south, it was a logical place for trails and roads to develop.  Two roads in Wake County where you can see this effect are along Lake Wheeler Road south of Tryon Road, and on Oberlin Road between Clark Avenue and Fairview Road.

0.1 – By a trail post, turn L onto a more narrow path.

1-B:    The ridge above the trail to the right is an exposure of Fln (Figure 1-B).  Here the Fln appears to be less fine-grained than is typical.  There are also a few small (2" - 3" thick) dikes of pegmatite, a coarse-grained variety of granite, visible in the outcrop.

0.2 – Stay R, as an alternate blue-blazed trail goes off to the L.  (This trail goes 0.6 mi, by-passing the Dam Management Center, to rejoin the Falls Lake Trail).

0.3 – Cross a dam access road and turn R to walk on a wide, gravel path. Pay attention to trail signage through this section.

0.4 – Turn L off the gravel path and into woods.

0.5 – Reach another section of the access road and turn L, the quick R to enter an area of young pine growth (Restrooms, a public telephone and water fountain are within view).

0.6 – Cross a paved road near the Management Center and turn R onto a gravel footpath. In about 100 yd turn L off this path, onto new decking and stairs, to continue on the Falls Lake Trail.

1-C:    The long ridge to the left, parallel to the trail, is a spectacular but typical natural exposure of Falls leucogneiss.
Originally granite, an igneous rock type, when it was deep in the earth's crust, the Fln was modified into a metamorphic rock and was stretched out into a long thin band by a combination of heat, the pressure of overlying rock, and sideways (horizontal) movement along a major fault.  The fault, called the Nutbush Creek fault, is what geologists call a right-lateral strike slip fault.  A better-known fault of the same type is the San Andreas fault in California.  For both of these faults, rocks on the west side of the fault have moved north, and those on the east south.  Movement on the Nutbush Creek fault was active from about 300 to 250 million years ago, while the San Andreas is still moving today.

Evidence of the history of the Fln can be seen in a feature called lineation.  Examine closely a fresh piece of the rock.  You should be able to see that, although the rock is composed of about 95% light-colored minerals, the few dark grains are strung out into long thin lines.  This feature formed during the metamorphism and fault movement.  As it turns out, some of the dark minerals are magnetite, and so the Fln has a strong magnetic signature that shows up clearly on magnetic maps of the region.

The Falls leucogneiss was a favored building stone in the 19th and early 20th centuries in Wake County, and numerous quarries, all now inactive, exist along its outcrop length.  Many older stone buildings, homes, and rock walls, especially inside the Raleigh Beltline (I-440), are composed of this stone.  Examples include Broughton High School and Christ Episcopal Church.  An old quarry face can still be seen behind the Harris Teeter grocery store in Glenwood Village Shopping Center, at the intersection of Glenwood Avenue and Oberlin Road.

Along this stretch of the trail, there are several spots where evidence of quarrying activity may be seen along the rock face.  Some of the large blocks have holes where the rock was drilled for splitting from the outcrop; a rusty steel cable can also be seen wrapped around one block.  Whereas the rock surface of the natural outcrop will show signs of weathering, such as a slightly brownish or rusty color and a somewhat crumbly or gritty surface, the quarried blocks show the nature of the unweathered rock, harder and a light tan or pale pinkish color.

0.9 – Cross a paved service road by the lake.

1-C2:  Along this stretch of the trail, you may notice a number of small dark gray rocks with brown weathering in the trail.  Some of them have a fairly smooth surface.  These belong to a narrow band of igneous rock called diabase.  About 200 million years ago, molten rock (magma) of diabase squeezed into a crack in the much older leucogneiss and hardened into a structure called a dike.

1.0 – Reach a junction, L, with the alternate blue-blazed trail. The Falls Lake Trail stays R.

1.2 – Cross a small plank bridge over a feeder creek and turn L on the other side.

1-D:    In about 100 yards, the trail turns sharply to the right, near an outcrop of Falls leucogneiss in the creek.  Foliation is visible in this outcrop, dipping about 60° toward the east.  There are also several fractures in the rock.  The Fln here is coarser than is typical.

1.3 – Reach an open footpath and turn R, then quick L, to stay on the trail.

1-E:     The Nutbush Creek fault, mentioned above, runs approximately north-northeast, close to the trail portion we have been walking. Through this stretch, especially along a ridge, you may see numerous blocks of white quartz, or fine-grained, light-colored rocks with thin veins of quartz.  Though these rocks can be found just about anywhere in the Piedmont, they tend to occur in profusion along fault zones, along with two other rock types, mylonite and breccia.  Mylonite is a thinly banded rock produced at depth within ductile parts of a fault zone, while breccia is formed at shallow depths in a brittle manner, and is composed of jagged, broken bits of rock cemented together.  The Nutbush Creek fault has been traced from near Richmond, VA southward to the Cape Fear River near Lillington, and it may continue through South Carolina. In the Falls Lake area, this fault separates the Falls leucogneiss and other rocks on its east side, from metamorphic rocks belonging to the Crabtree terrane and the Falls Lake terrane to the west.

1.6 – Walk across a wide utility easement.

1-F:     Falls Lake serves as the water source for the city of Raleigh.  This easement is the location of the underground pipe that delivers water from the lake to the E. M. Johnson Water Treatment Plant, located at the intersection of Falls of the Neuse and Raven Ridge Roads.  A large diameter pipe here delivers from 47 to 86 million gallons of water each day depending on demand.  After use, Raleigh's wastewater is treated at the Neuse River Wastewater Treatment Plant in southeastern Wake County, near Clayton, and returned to the Neuse River.

2.1 – Cross a small creek on a footbridge.

1-G:   Outcrop of felsic (light-colored) gneiss beneath the bridge.  This feldspar and quartz-rich rock also contains both white mica (muscovite) and black mica (biotite).  The rock is part of the Crabtree terrane, indicating that we have passed out of the Fln and the Raleigh terrane.  Before being metamorphosed, this rock was originally either a sedimentary or volcanic in nature.  It shows a lineation similar to that of the Fln, though not as intense, indicating it has been affected by shearing associated with the Nutbush Creek fault.  In fact, the effects of the fault can be seen across a zone several miles wide.

2.4 – Use a footbridge to cross another small creek.

1-H:    In the creek to the right of the footbridge, there is an excellent outcrop of a very fine-grained felsic rock; you may have noticed it from the trail.  The rock contains tiny grains of feldspar and quartz, as well as biotite and magnetite.  As you continue uphill along the trail, you will see that this band of rock crosses the trail and extends uphill to the left.

Between the point where you cross the next low point and the next bridge, the trail runs along the boundary of the Annie Louise Wilkerson Nature Preserve Park, a new (2011) park maintained by the City of Raleigh.  Its road entrance is about 0.8 miles east from the end of this section of the trail at Raven Ridge Road, but a new spur hiking trail joins the Falls Lake Trail to Wilkerson Park.

2.6 – Use a footbridge to cross another small creek, at a low point passing around a cove.

2.8 – After a hillside descent, cross a creek on a footbridge and turn R to walk beside it. Eventually return to the lake.

1-I:      Along this long, straight stretch of creek, there is nearly continuous exposure of gneiss belonging to the Crabtree terrane.  Some of the gneiss is banded, with alternating darker and lighter layers.  The darker layers contain the black minerals biotite and hornblende.  Other portions of the gneiss are lighter colored.  You may also see that layers of the gneiss are bent into folds in some places.  Folds are the result of compression, or squeezing forces within the earth.

Here, the creek and the exposure of gneiss alongside illustrate one of the effects that bedrock may have on streams.  Notice that the layers of the gneiss run in precisely the same direction as the creek (Figure 1-I1).  This is known as a strike creek; the gneiss layers run about 20 degrees to the east of due north (N20°E) and the creek has created its channel along that strike, as a result of differences in the resistance to erosion of layers in the gneiss.  A close look at the layers in the outcrop of gneiss shows that they dip fairly steeply (about 65°) to the east.  Geologists describe the orientation of any non-horizontal rock layer or other planar feature in terms of its strike and dip.

In addition to the gneiss, another interesting rock you may see through here is pegmatite.  Pegmatite is a type of granite with very large mineral grains.  The pegmatite here is very light in color, and consists of feldspar, quartz, and muscovite, or white mica.  If you look closely, you may see evidence that the pegmatite is an igneous rock and is younger than the gneiss.  There are two ways to make this deduction.  First, the gneiss was subjected to a metamorphic event that caused it become foliated, or banded, but the pegmatite lacks the foliation, and so it was not in existence at the time the foliation formed.  Second, the pegmatite takes the form of dikes, small bodies that are not parallel to the foliation of the gneiss, but instead cut across it.  In fact, the pegmatite formed when hot magma forced its way into cracks in the gneiss, and then the minerals solidified as it cooled slowly.  Figure 1-I2 shows folded gneiss cut by a pegmatite dike.  In Wake County, pegmatite is much more common east of the Nutbush Creek fault, especially within the Raleigh gneiss, the geological unit to the east of the Falls leucogneiss.  Huge crystals of quartz, feldspar and mica can still be found, and at one time feldspar and mica were mined in the county.

This outcrop contains abundant evidence of the forces that affected the rocks here, about 300 million years ago.  The small folds that you see are asymmetric; if you look closely you can see that many of them form a series of Z’s (Figures 1-I2 and 1-I3).  Folds of this sort indicate that the rocks were deformed by right-lateral shear, meaning that the rocks on one side moved to the right relative to the other side.  A good way to visualize this is to imagine a deck of playing cards, set on its side so that the edges of the individual cards mimic the banding (foliation) in the gneiss.  Then imagine sliding the cards so that the cards farthest away from you slide to the right.  This sort of deforming force affected rocks over a wide zone in this region, as there are several faults having the same right-lateral shear.  Kinematic indicators are details preserved in rocks, such as these asymmetric folds, that can tell geologists the sense of motion in an ancient fault zone.

Another interesting feature in this outcrop is boudinage.  In a zone of layered rocks experiencing intense deformation, if one layer is more rigid than the others, it may be stretched and broken into a shape resembling a string of sausages (boudin is French for a type of sausage).  At this outcrop, you can see several good examples of boudinage (Figures 1-I4 and 1-I5).

 As you leave the long outcrop, you turn to head southwest and toward Raven Ridge Road.  Along this stretch of trail you will encounter two new rock types.  First you may see a small piece or two of a very dark rock called amphibolite (1-J).  It is a fine-grained rock consisting of hornblende and feldspar; in most cases if you look closely you can see that it is banded and it may have a black and white “salt and pepper” appearance.  Within a few hundred yards of the road, you will see a few pieces of quartz and quartz-mica rock (1-K).

1-L:     Here there is a small boulder of a very distinctive rock, a variety of a mica-rich rock called schist.  This rock is composed mainly of lots of mica, both white and black varieties, along with some harder light-colored feldspar and quartz.  In addition, it has two very distinctive minerals that tell geologists quite a lot about the origin of the rock.  These minerals, garnet and kyanite, form at great depth in the earth and indicate that the schist formed about 15 miles below the earth’s surface.  Such rocks form beneath crust that has been greatly thickened by the process of mountain building, a process that is happening today in the Himalayas, the result of India crashing into Asia.  So 15 miles beneath the summit of Mt. Everest, similar rocks are forming today.  Furthermore, we know that 15 miles of solid rock has been removed by erosion from this part of Wake County since the time of Appalachian mountain building, about 300 million years ago.

Garnet occurs as dark colored roundish crystals.  They may be the size of BBs up to a half-inch across.  When fresh, they are a deep red burgundy color and occur in crystals that have 12 or 24 sides; when weathered they are more roundish and may be rusty looking.  Kyanite forms in small thin rectangular crystals.  In Wake County the kyanite may be white, gray, or rarely blue.  Look for a “flash” from sunlight reflecting off the flat surface of the kyanite; this reflecting surface is called the mineral’s cleavage.  (When you see similar flashes from asphalt on a road surface, they are usually the result of the cleavage of embedded feldspar grains.)  An even closer look should reveal that the kyanite cleavage surface has minute parallel grooves on it.

1-M:    Because garnet and kyanite are very hard minerals, resistant to weathering, you may find grains on the ground or in the soil along this final stretch.  In addition, you may see flakes of white mica littering the trail.  When you reach the paved road, look in the embankment to the left for loose grains of these minerals.  Because the schist has lots of easily weathered mica and feldspar in it, the rock is weathered and eroded readily, transforming from rock to weathered rock (saprolite) and then to soil.  But the hard garnet and kyanite resist breakdown and may be left behind, along with red clay.

If you want to see hard unweathered garnet-kyanite schist, visit a site on Highway 98 about three miles north of here where a large roadcut was blasted when the highway was re-routed for Falls Lake in the early 80’s.  The site is on the north side of the highway, and just west of a major overhead power transmission line, and just west and downhill from the junction of Highway 98 with 98 Bypass (link to photographs of Highway 98 outcrop).

3.5 mi – Reach Raven Ridge Rd by a gravel pull-off. Turn R to cross the causeway (Honeycutt Creek) and continue on the trail.

End of Trail Section 1

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