Rocks are cool to look at – of course, they can also be an
irritant if you’re digging in a garden. But rocks do far more than
please our eye or hamper our shovel – they tell us how the land
under our feet was formed and where it actually may have came from.
It’s a mystery story worthy of Sir Arthur Conan Doyle.
From NC-24/27 east of Albemarle, go north on Stony Mountain
Road. At the Y, take a right and go up and down to Mountain Ridge
Road, and take another right. [Note Well: Slow down, as
Mountain Ridge Road is execrable.] When you see the Stony
Mountain Winery entrance sign, continue on and find a place to park
off to the side of the road. Your best (and most interesting) bet
is to continue all the way to the top of the hill, turn around, and
come down to park opposite the exit road from the winery. The
“cache” is close by.
[The following text is largely adapted from Three Beautiful
Old-Timers (GC1KKKM), and it will help you understand the geologic
history of this general area, up until about 530 million years ago
(Ma)] Over 550 Ma, parts of what would become Africa,
South America, and other southern continents, were joined together
in a huge continent geologists call Gondwana.
It was not a good time to live on the northwest corner of that
supercontinent. Active volcanism was depositing masses of ash and
rock over (and under) a broad oceanside area. Sedimentary deposits
of mud, clay, and silt were building up on the sea bottoms. As the
pyrotechnics continued, massive intrusions of a hard, volcanic rock
were squeezing up into the mudstones of the ocean floor. These
volcanic rocks are often called "rhyolite", but have more iron and
magnesium in them than rhyolite -- but not as much as in a heavier
rock called "dacite". Splitting the difference, the correct term
for these hard, resistant rocks is "rhyodacite". These are the
rocks underlying what are today’s Uwharrie Mountains in Stanly and
Montgomery counties.
It is probable that the volcanic activity starting 550 Ma in
northwest Gondwana was due to the subduction of a piece of ocean
crust under the gondwanan continent. "Subduction" is a process
whereby the leading edge of a piece of ocean crust runs into a
continent, and being heavier than the rocks in the continent, it
dives under the continent. This process is always accompanied by
volcanic activity on the continent, as is the case along the
Pacific coast of South America today.
While subduction may have been responsible for the growth of the
once-major Uwharrie Mountains, it did not produce the Stony
Mountain Gabbro. Sometime later after ~550 Ma, subduction had
ended, and the beginnings of a process took place whereby a chunk
of the continent (referred to as "Carolinia" by geologists) began
to break off and began to recede from Gondwana. This eventually did
occur, but the date is hard to pin down. Current estimates vary
widely, but we will consider ~530 Ma as a probable date. This date
has been very recently proposed by Dr. Jeff Pollock in his 2007
doctoral dissertation.
Rifting is not a nice, neat separation of one part of a land
mass from the remainder. In nature, nothing is easy, and rifting is
no exception. Starting about 530 Ma, the Carolinia block of crust
was being pulled away from the main body of Gondwana -- the
continent was stretched apart. This stretching made the crust very
shallow, allowing hot mafic magma to well up from deep within the
lower crust and upper mantle. This magma was rich in iron and
magnesium, and it welled up with great force, intruding both
sedimentary and igneous bodies of rock that had come before it.
This magma has long, long ago cooled and solidifed, and is now
known as the Stony Mountain gabbro, and it is found throughout this
area of the Piedmont.
While the gabbro outcrops are beautiful to the eyes of anyone
who appreciates natural outcrops of beautiful rocks, they also tell
us something important about our neighbors to the north. Somewhat
north of Carolinia is another ex-island micro-continent, which
geologists call Avalonia. For years, it was assumed that
Avalonia and Carolinia shared similar backgrounds, but Avalonia has
nothing like the Stony Mountain gabbro. It is obvious that, dating
back some 530 Ma, Avalonia and Carolinia went their separate ways.
Who would have thought that a bunch of rocks running up Stony
Mountain would serve as genealogical indicators of bits of ex-South
America and Africa 530 million years in the past?
Logging Requirements
Send me an e-mail -- not part of your log -- responding to the
following:
1. Make the subject of the e-mail "GC1MW8J, Stony Mountain
Gabbro"
2. How many people are in your party?
3. This gabbro is referred to as a "mafic" rock. What does mafic
mean, and what does it tell us about the composition of this
gabbro?
4. Just south of you, immediately across NC-24/27, is a contact
point between sedimentary rocks on the bottom and a gabbro sill
above. What is a "sill"?
References:
Bradley, P. Piedmont Geologist, North Carolina Geological
Survey. Personal correspondence, 2009.
Hibbard, J., et al. Links between Carolinia, Avalonia, and
Ganderia in the Appalachian peri-Gondwanan Realm. Geological
Society Special Paper, 2007.
Pollock, J. The Neoproterozoic-Early Paleozoic Tectonic
Evolution of the Peri-Gondwanan Margin of the Appalachian Orogen:
An Integrated Geochronolocial, Geochemical and Isotopic Study from
North Carolina and Newfoundland. Unpublished PhD Thesis, North
Carolina State University, 2007.
Pollock, J. and Hibbard, J. Geochemistry of the Stony Mountain
Gabbro, North Carolina: Implications for the early Paleozoic
Evolution of Carolinia. In, Guide Book, Carolina Geological Society
Annual Field Trip, 2006.
Additional Site Information
The winery is open for tours and tasting, with the hours listed
on its sign. If it is open, drive into it and park. The view from
the deck of the tasting/event building will absolutely knock your
socks off. It is a gorgeous overview of the Pee Dee River valley
and the adjacent Uwharrie Mountains.
When the winery's building was constructed, huge chunks of
gabbro were removed, and are piled up near and around the
structure. At least a few of them have been split, and have a
beautiful, fresh surface. Hand samples are allowed, but in the
EarthCache area, not around the building.
The area surrounding the winery grounds is part of a leased
hunting preserve. Do not venture more than 10-15 feet into the
woods from Mountain Ridge Road.
Platinum EarthCache Master