Trail Section 6 - Falls Lake Trail Geologic Guide

 

Geologic Map Trail Section 4-7

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

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

Trail Section 6:

13.9 – By end of the guardrail, at a graveled pull-off, turn R. Note signboard welcoming you to Blue Jay Point, a Wake County Park. Reenter woods to continue on the Falls Lake Trail.

14.0– After crossing a small plank walkway, descend to cross a footbridge over a small creek.


6-A:  Just beneath the bridge is a small outcrop of chlorite schist.

6-B:  As you round the next point and the trail begins to go uphill, look for a few small pieces of light-colored talc schist.  Talc schist, like soapstone, is composed of talc and therefore soft enough to be scratched with a fingernail.  The difference between the two rock types is that the schist has a foliation, while soapstone is structureless.  If you look closely you may be able to detect tiny wavy folds that resemble the corrugations in cardboard.  Geologists call these crenulations; it is easy to see how a soft material might crumple in this manner.

14.5 – Cross several footbridges in an area with evidence of the ravages of hurricanes Fran and Floyd.

14.6 – Cross a wide footpath.

14.9 – Descend on some steps to cross a footbridge.

6-C:  Under this bridge is a small outcrop of mica schist, with a weathered exposure of similar material in the stream cut just downstream from the bridge.

This is a good spot to observe an important aspect of stream behavior.  There are steep, cliff-like cut banks on the outside of every bend along the stream, but on the inside of the bends, the ground is pretty flat.  Especially after a torrential rain, as water flows down the creek, it speeds up on the outside of the curve, and has more power to erode.  On the inside, the water slows, and instead of eroding, it drops what it is carrying, and the result is a flat deposit of sediment called a point bar.  Large rivers do the same thing, but on a quite different scale.
As you continue along the trail, you will encounter more talc schist, plus some other ultramafic rocks, locally containing the minerals chlorite and actinolite in addition to talc.

Soon you may notice that the slopes are littered with chunks of quartz.

15.2 - Cross a graveled, blue-blazed trail (R leads to park’s namesake, Blue Jay Point).

15.3 – Cross a small footbridge and turn R to stay on the trail (red-blazed trail turns L to ascend some steps and head uphill to a playground and restrooms).

6-D:  Just ahead there is a platform built over the creek bed.  Looking upstream from the platform, you will see a moss-covered outcrop of fine-grained chlorite schist.  It dips gently, about 20° to the west.

In a few hundred feet beside the trail, you may notice a chunk of quartz with peculiar cavities, having rectangular or parallelogram shapes.  These are molds where crystals of another mineral were present, but have weathered (perhaps dissolved) away.  In the Falls Lake terrane some of the ultramafic rocks also contain crystal molds.  When the mold shapes are particularly well preserved, they can give clues to the identity of the former mineral.  In some instances, it was most probably a highly soluble mineral called magnesite (magnesium carbonate).

15.4 – Stay straight at a junction with a red-blazed trail, L.

15.5 – Stay R at a junction with a yellow-blazed trail, L, as the trails run concurrently on a wide footpath.

15.6 – Turn R as the yellow-blazed trail turns off to L.

15.7 – Cross a narrow footbridge.

15.8 – Reach a wide blue-blazed path and turn L with this for about 100 yd, before turning L onto a narrower footpath.

15.9 – Reach a signboard by a parking area and turn R to cross a park road to continue on the Falls Lake Trail.

16.1– Cross a bridge over a creek bed. The lake is to the R, below you.

16.2 – After descent to the lake, cross 2 footbridges, with a bench between them.

6-E:  Just upstream from the first bridge, there is an excellent long ledge outcrop of fine-grained mica schist (both muscovite and biotite mica) with considerable feldspar and quartz as well.  These rocks dip very gently toward the west, and display a strong, nearly horizontal north-south lineation.  See Figure 6-E.

16.5 – Cross another footbridge, by the end of a cove.

6-F:  Here, along the creek, you will see a series of ledge exposures of flat-lying biotite-muscovite schist (Figure 6-F), creating several small waterfalls and pools.

As you proceed onward, you will begin to encounter pieces of various ultramafic rocks.  These begin with soapstone and talc schist, and as you continue downhill, more varieties are present, including actinolite-rich rocks with gnarly weathering black magnetite.  There is also some light green tremolite rock and serpentinite.

16.7 – Cross another footbridge.

6-G:  Here we are back in biotite-muscovite schist, gently west-dipping.  You may notice some gold-colored flakes in the weathered rock or soil; this is vermiculite, the product of weathering of the black biotite mica.

As you move onward, you pass a single quartzose boulder, then you will re-encounter soapstone for a while.  From here on, you will begin to see piles of a variety of ultramafic rocks.  Flaky dark green chlorite is present in some, as are green needles of actinolite.  Many of these rocks are quite rich in magnetite, and though most is the gnarly-looking form, some of the magnetite is in well-formed octahedral crystals.  An octahedron is a crystal shape having eight sides, each side an equilateral triangle.

Remember that in the Falls Lake terrane, the main rock type is the mica schist, such as we have seen in the gently west-dipping or flat outcrops.  Within a matrix of this material, there are hundreds of pods and blocks of the other rocks we have seen, mainly of ultramafic composition.

17.0 mi – Walk down some steps and reach Six Forks Rd. To continue on the trail, turn R to cross the causeway.  The boat ramp is in view here.

6-H:  This causeway spans Upper Barton Creek.  Up this creek to your left, the Falls Lake terrane contains more rocks similar to what you have hiked through, including a few locations where excellent specimens of actinolite may be found.  In addition, farther to the southwest near Pleasant Union Church Road, there are small areas of a different ultramafic rock type, chromitite.  This rock consists of the mineral chromite, which typically occurs as segregations in olivine-rich igneous rocks that form in the deep crust and upper mantle.  Here in the Falls Lake terrane, both the host rocks and the chromitite have been subjected to metamorphism, transforming the olivine into minerals such as talc and serpentine, and causing the growth of a number of minerals with the chromite.  These new minerals include black tourmaline (sometimes as rosettes), greenish kyanite, and even tiny rubies.  Chromium is a strategically important metal, and chromite is its primary source.  Consequently, the Falls Lake chromitite has been the subject of exploration studies, mainly in the 1940s and 50s, but was not found to be of commercial grade.

As you cross the causeway, if the water level is not too high, you will see a rock exposure next to the boat ramp.  This is another outcrop of ultramafic rock, including some nice chlorite.

Pause long enough to (safely) look down beneath the causeway.  Check out the large rocks (rip-rap) that have been piled here for erosion control.  They are quite different from the rocks we have seen on the trail.  They consist of pink and white granite, and some banded gneiss.  They come almost entirely from a rock quarry on US Highway 1 just south of Wake Forest.  The quarry lies within the Raleigh terrane to the east of the Falls leucogneiss and the Falls Dam.  The banded rock is called Raleigh gneiss; more of it occurs in the creeks along Capital Boulevard in downtown Raleigh.  The granite is much younger in age, and intruded into the gneiss about 300 million years ago.

End of Trail Section 6

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