Rock Type: Granite
Geologic terrane or major geologic element: Rolesville batholith
Age: Late Paleozoic –
approximately 300 million years old
USGS 7.5-minute Quadrangle:
Rolesville
Site Access: Heading south on Highway NC 96, there is a
space to pull off the road on the right.
Park here and follow the path by the big blocks of rock. You will see an extensive exposure of
granite. From this exposure, follow the
trail from the back corner, near more blocks, and you will come to an even
larger expanse of granite. This site is
Mitchell's Mill State Natural Area, part of the NC State Parks system.
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Technical Information: Speer, J. A., 1994, Nature of the Rolesville
batholith, North Carolina (pages 57-62 in Carolina
Geological Society Field Trip Guide, 1994); See also description for Stop
11 on pages 105-107.
Speer, J.A., McSween, H.Y., and
Gates. A.E., 1994, Generation, segregation, ascent and emplacement of
Alleghanian granitoid plutons in the Southern Appalachians: Journal of Geology, v. 102, p. 249-267.
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Introduction
The
Rolesville batholith is the largest body of granite in the southern Appalachian
region. It measures about 15 x 50 miles,
and occupies the eastern third of Wake County, and about two-thirds of Franklin
County. At this site there is a huge
flat exposure of granite that is a part of the batholith.
The rock body
A
pluton is a three-dimensional body of igneous rock. Granite plutons range in size from a
vein-like dike just a few inches wide up to the size of a batholith, which by
definition covers an area greater than 100 square kilometers.
Granite is an igneous rock that occurs in three-dimensional bodies called plutons. Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which (by definition) covers an area greater than 100 square kilometers. In three dimensions, plutons may be shaped like mushrooms or inverted teardrops, so they may extend to considerable depth beneath the surface. In the eastern Piedmont, almost all granitic plutons are between 280 and 320 million years old – Late Paleozoic in age (Figure 1). The largest such body is the Rolesville batholith, which is (in reality) ten or twenty separate plutons that intruded the same region over a period of geologic time, cutting across each other and coalescing to form the batholith (a mega-pluton).
Granite is an igneous rock that occurs in three-dimensional bodies called plutons. Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which (by definition) covers an area greater than 100 square kilometers. In three dimensions, plutons may be shaped like mushrooms or inverted teardrops, so they may extend to considerable depth beneath the surface. In the eastern Piedmont, almost all granitic plutons are between 280 and 320 million years old – Late Paleozoic in age (Figure 1). The largest such body is the Rolesville batholith, which is (in reality) ten or twenty separate plutons that intruded the same region over a period of geologic time, cutting across each other and coalescing to form the batholith (a mega-pluton).
The rock itself
Granite is composed of two types of
feldspar – orthoclase and Na-plagioclase, plus quartz and biotite mica. Sometimes it may contain hornblende or muscovite mica
or garnet. It always also contains some tiny minerals in small amounts. Granite
is the most common type of rock that is quarried for crushed stone, used in
making concrete and asphalt, among other things.
You may notice certain features in the granite at Mitchell’s Millpond (Figure 2). Cracks or fractures in the granite are of two types. Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock. Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses. Below the surface, groundwater moves along these cracks. You may also see thin bands of slightly different rock cutting across the granite (Figure 3). These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite). In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).
You may notice certain features in the granite at Mitchell’s Millpond (Figure 2). Cracks or fractures in the granite are of two types. Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock. Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses. Below the surface, groundwater moves along these cracks. You may also see thin bands of slightly different rock cutting across the granite (Figure 3). These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite). In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).
Origin
The
granite magmas that intruded this region during the late Paleozoic were
generated in the lower crust of the earth, and moved upward until they stopped,
cooled, and crystallized. The magmas
formed as a result of the tremendous collision between continental plates that
raised the Appalachian Mountains. Many of these magmas ascended along fault
zones that were active at that time, such as the Nutbush Creek fault that runs through central Wake County. There is no evidence
that any of these granitic magmas reached the surface – if they had, we would
expect to see some remnants of volcanic rock of the same composition and age.
You may notice certain features in the granite at Mitchell’s Millpond (Figure 2). Cracks or fractures in the granite are of two types. Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock. Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses. Below the ground surface, groundwater moves along joints; if there are numerous joints and they intersect, then wells can produce ample water. In eastern Wake County, a majority of homeowners rely on wells drilled into the granite. You may also see thin bands of slightly different rock cutting across the granite (Figure 3). These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite). In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).
You may notice certain features in the granite at Mitchell’s Millpond (Figure 2). Cracks or fractures in the granite are of two types. Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock. Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses. Below the ground surface, groundwater moves along joints; if there are numerous joints and they intersect, then wells can produce ample water. In eastern Wake County, a majority of homeowners rely on wells drilled into the granite. You may also see thin bands of slightly different rock cutting across the granite (Figure 3). These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite). In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).
Surface features
This 93-acre
site is a Registered Heritage Area. It
contains some of the best examples of native plant communities that grow in
such a “granitic flatrock” environment. You
can see that erosion by running water proceeds very slowly, due to the
resistance of this hard rock. Note the
numerous potholes (Figure 5). You may
notice that some of them line up along a fracture in the granite. These potholes form over time as the result
of small pebbles that get caught along a crack, and then drill away at the
bedrock when the stream water is high and fast.
Figure 1. Late
Paleozoic granite plutons in the southern Appalachian Piedmont
(RV=Rolesville). From Speer and others
(1994).
Figure 3. Dikes and
veins cutting through the granite.
Figure 4. Biotite
schlieren in the granite.
Figure 5. Pothole at
Mitchell’s Millpond. In the bottom of
this hole, you would find rounded pebbles that act as a “drill bit” when they
are swirled by fast-moving water.
