Geologic Guide to the Hidden Rocks Trail
Annie Louise Wilkerson M.D. Nature Preserve
City of Raleigh Park
Draft - updated 9/21/2012
Falls leucogneiss (pronounced lou-ko nice): Resistant rock type that extends from Lake Wheeler to the north through Wake County to just south of Henderson. Rock is a distinctive pinkish to orange-tan colored metamorphosed granitic rock. Contains distinctive lineations of mineral magnetite.
Felsic gneiss: Typically a light-colored (gray) rock formed from the metamorphism of sediments that were likely volcanic in origin. Contains white-colored flakes of mica.
Horse Creek schist: Typically a silvery gray white and black mica-rich rock formed from the metamorphism of sediments (siltstones and mudstones). Contains the minerals garnet, kyanite, and minor staurolite.
Detailed geology from Horton et al. (1992) - Horton, J.W., Jr., Blake, D.E., Wylie, A.S., Jr., and Stoddard, E.F., 1992, Geologic map of the Falls Lake-Wake Forest area, northcentral North Carolina: U.S. Geological Survey Open File Report 92-269, scale 1:24,000.
Geologic Points of Interest Along the Hidden Rocks Trail
Text by Krista Brinchek – City of Raleigh Parks and Recreation Department
WP-1 Rock Boulders Near the Start of the Hidden Rocks Trail: The rock boulders you see near the start of the Hidden Rocks Trail and around the parking lot are not originally from this park. They were most likely transported from a quarry or construction site east of Raleigh. The igneous nature of the boulders is typical of what you would see from the Rolesville batholith, which formed approximately 300 million years ago. A batholith is a large mass of igneous rock that has intruded into the surrounding rock. The Rolesville batholith intruded the area of eastern Wake County. If you look closely at the different boulders you might see some pegmatite. Pegmatites are crystalline intrusive igneous rocks producing large mineral crystals such as mica, quartz etc..
WP-2 Quartz Cobbles: As you walk along the trail, keen observers will notice quartz cobbles scattered throughout the woods. Quartz is the second most abundant mineral in the earth’s crust behind feldspar and it is also very resistant to weathering. These two facts when combined allow us to see a plethora of quartz within the geological world. On some parts of the trail you might see piles of the cobbles, other times just a few scattered here and there. The cobble piles are an indicator of past land use, They might have been used to mark a property boundary or dug up during farm operations and placed in a pile. Quartz cobbles are also frequently seen because as the other minerals within the bedrock weather and erode away, the weather resistant quartz remains creating the cobbles of the hard mineral.
Fault Line: This location marks where geologists believe an ancient fault, similar in nature to the San Andreas fault, separates the rocks of the Crabtree and Raleigh terranes.
WP-3 Leucogneiss Outcrop: Be on the lookout for the first signs of bedrock. This location is the first bedrock outcrop on the trail and can be easily missed. Larger outcrops are present at WP-6 and will be discussed in more detail.
WP-4 City of Raleigh Water Line: The trail crosses over a grassy area that is the easement for the City of Raleigh water line. Water is being pumped from Falls Lake to the E.M Johnson Water Treatment Plant off of Falls of the Neuse Rd. Approximately 48 million gallons per day are pumped. Whoosh! Can you feel the flow under your feet?
WP-5 and WP-6 Falls Leucogneiss Outcrops: These monster rocks that you see might surprise you at first because typically here in Raleigh you don’t see large outcrops such as this. In the Northeastern US, outcrops are seen at the topographically high places such as ridges. But here in the Piedmont of North Carolina, geologists usually need to scour stream beds to find the existing bedrock. What makes this outcrop unique is the type of rock it is. The Falls leucogneiss is a metamorphic rock that contains high amounts of quartz, making it extremely resistant to weathering and erosion. If you look closely you might think that this rock is granite, and you wouldn’t be entirely wrong. When the rock cooled from magma approximately 545 million years ago it was a granitic rock! Approximately 300 million years ago, during the formation of the supercontinent Pangaea, the continental plates were shifting and a fault zone formed. The igneous rock was stretched like putty under extreme pressure, and transformed into the metamorphic leucogneiss. Another attribute of the Falls leucogneiss is its high abundance of magnetite. Magnetite is a ferrimagnetic mineral and the most magnetic of all the naturally occurring minerals in the world. If you look closely at the outcrop you will see the magnetite as either long black strands or small pen-like dots. If you had a sensitive magnet and held it close to the rock you would be able to feel the attraction to the magnetite grains.
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. Due to the rock’s resistance to weathering and erosion, streams and rivers have a difficult time cutting through the rock creating natural waterfalls and rapids. Locations where streams cross the Falls leucogneiss provide excellent sites for construction of dams, grist and saw mills. 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 Falls Lake Dam on the Neuse River.
As you explore the outcrops there are a couple of other things keen observers will notice. Do you see the potholes within some of the outcrops? These “holes” potentially formed from weathering and erosion of a less resistant mineral within the rock such as feldspar. As the mineral erodes away over time, the surrounding hard rock remains creating these pothole-like structures. Keep looking around the outcrops; if you look closely you might also observe some quartz veins within the outcrops. Quartz veins are crystallized minerals (quartz) that form within fractures of the rocks.
WP-7 Quartz Cobbles: As discussed earlier at WP-2 notice the abundance of quartz cobbles as you continue to hike through the woods. How do you think they ended up where they are? Look around for potential clues. Does this land look like it was ever farmed? How old are the trees? Do you see any other rock outcrops? Remember the quartz veins at WP-6!
WP-8 Pegmatite, Felsic Gneiss and Diabase: The trail crosses an intermittent stream here where large boulders of pegmatite, a small outcrop of felsic gneiss and a small boulder of diabase are present. Most obvious here are the boulders of pegmatite. Recall from WP-1 that pegmatites are igneous rocks with large-sized crystals. The pegmatite here consists of large pink potassium feldspars, white sodium feldspars, quartz, and mica flakes up to a few inches wide. This pegmatite most likely intruded the parent rock during the same time that the Roseville batholith intruded, approximately 300 million years ago. On the side of the stream you will notice what looks like rotting wood; however, this is a small outcrop of weathering felsic gneiss. The felsic gneiss has an abundance of white mica. Hidden within the ferns lies a small boulder of diabase. The diabase is an approximately 200 –million- year- old mafic igneous rock that is essentially like ocean floor basalt. You will be able to identify the diabase by the dark, fine grained crystals it is composed of. The diabase intruded the Piedmont when the Supercontinent Pangaea split apart.WP-9 Crabtree Terrane Felsic Gneiss: Visitors venturing down the Pond Loop trail can take a small side trip at the north end of the pond to see a good example of the felsic gneiss part of the Crabtree Terrane. We first encountered a visible outcrop of the felsic gneiss at WP-8 near the intermittent stream bed. Here at WP-9 you can see a less weathered version of the gneiss. The felsic gneiss of the Crabtree terrane is typically a light-colored rock formed from the metamorphism of sediments that were likely volcanic in origin. If you look closely, you will see it sparkle with the white-colored flakes of mica. The Nutbush Creek fault zone separates the Crabtree terrane from the Raleigh terrane. As you walked down the Hidden Rocks Trail you crossed over the fault zone, leaving the Falls leucogneiss (part of the Raleigh terrane) and entering into the Crabtree terrane.
End of Trail Section
iphone and ipad users!
Attention iphone and ipad users: Using the free app, Avenza PDF maps, you can view geopdfs on your iphone or ipad. Using this app and downloadable draft geopdf maps of the Hidden Rocks Trail, you can have real time location information while accessing the guide. Access Draft geologic geopdf maps here.