DRAFT
Geologic Guide to the Hidden Rocks Trail
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Annie Louise Wilkerson M.D. Nature Preserve
Falls Lake Trail
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Hidden Rocks
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RALEIGH TERRANE
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. This rock is a distinctive pinkish to orange-tan colored metamorphosed granitic rock. Contains distinctive lineations
of mineral magnetite.
CRABTREE TERRANE
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.
DATA SOURCES
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.
Horse Creek Schist
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 to 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. If you look closely at the different boulders you might see some pegmatite.
Pegmatites are crystalline intrusive igneous rocks producing large crystals.
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 pile. Quartz cobbles are also frequently seen because as the other minerals within the
bedrock weather and erode away; the quartz being resistant to w eathering remain creating the cobbles of the hard
mineral.
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Geologic Points of Interest Along the Hidden Rocks Trail
Text by Krista Brinchek – City of Raleigh Parks and Recreation Department
Felsic Gneiss
Falls Leucogneiss
Cartography by Steven Case NCGS Summer Volunteer 2012 Data Sources: North Carolina Geological Survey - June 2012
Fault Line: This location marks the location that geologists believe an ancient fault (that was similar in nature to the
San Andreas fault) separates the rocks of the Crabtree and Raleigh terranes.
WP-3 Leucogniess Outcrop: Be on the lookout for the first signs of bedrock. This location is 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 grassy area the trail crosses over 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 is 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 startle you at first because
typically here in Raleigh you don’t see large outcrops such as this. Unlike the Northeast US where outcrops can be
seen at the topographically high places such as ridges, geologists in the Piedmont of North Carolin usually need to
scour stream beds to find the existing bedrock. What makes this outcrop unique is the type of rock it is. It is Falls
leucogneiss. The Falls leucogneiss is a metamorphic rock consisting of high amounts of quartz making it extremely
resistant to w eathering and erosion. If you look closely you might think that this rock is a granite, and you w ouldn’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 the a fault zone formed taking the once igneous rock and stretching it like putty under extreme pressure
transforming it into the metamorphic leucogneiss. Another attribute that the Falls leucogneiss has is it’s 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 magnet and held it close to the rock you would be able to feel the attraction to the magnetite
minerals.
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 C ounties, ending near Lake Wheeler. Due to the rocks
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 pot holes
within some of the outcrop? 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 pot hole like structures. Keep looking around the outcrop; If you look closely you might also observe some quartz
veins within the outcrop. Quartz veins are crystallized minerals (quartz) that form within fractures of the rocks.
WP-7 City of Raleigh Water Line: The trail crosses the right of way for the City of Raleigh water supply line again.
WP-8 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-9 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 boulder of diabase are present. Most obvious are the boulders of
pegmatite. A pegmatite is an igneous rock 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. The pegmatite intruded
the rocks of the area probably when the Rolesville Batholith intruded into the area of eastern Wake County
approximately 300 million years ago. A small outcrop of felsic gneiss with abundant white mica is present in the
stream. Also, a boulder of diabase is present. Diabase is an approximately 200 million year old mafic igneous rock
that is essentially like ocean floor basalt. D iabase intruded the Piedmont when the Supercontinent Pangaea split
apart.
Wilkerson Park Boundary
Fault (Extinct)
Arrows indicate direction of fault movement
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Trail Start & End
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WP-1
Picnic Area
Wilkerson Park Geologic Points of Interest
Hidden Rocks Trail
Other Trails