NOTE:
Please respect parking laws.
Park only in LEGAL street parking spots, or in the public lot a few
blocks away (look for the library).
Please respect the landscaping in this
beautiful little city park.
Introduction
(Click the image for an
interactive geologic map of North Carolina. You'll need
it!)
You are looking in Historic downtown
Kernersville for boulders of highly differentiated and beautifully
banded metamorphic rock called gneiss (pronounced Nice).
Among boulders of various type, There are 2 excellent examples of
this rock.
Much of this state's geology is covered, and much of what lies
beneath your feet is seen in ditches, gravel and “decorative
boulders” like these. So, if you’re looking for mountains, set your
time machine back 400 million years. You’ll find them, however
you’ll be deep inside solid rock! The amazing thing about rocks in
this region is that you are looking at the deformed cores of
ancient mountains, peaks worn flat by ravages of gravity and
time.
Gneiss
Gneiss is a
metamorphic rock. At great depths a rock
experiences tremendous heat and pressure. As the rock “cooks”
minerals alter to reach equilibrium with their new state.
Chemically some minerals disappear and give their atoms up to newly
forming minerals. Somehow they migrate to form the gneissic banding
you are seeing now, called foliation. These are NOT
sedimentary deposit layers! They are layers of
mineral differentiation formed from a piece of homogeneous
rock.
The folding of the bands resulted from compression forces placed
on them during mountain building. Under enough heat and pressure,
rocks are considered ductile and act a lot like silly putty, thus
the mountain building events that created the Appalachians (see way
below) squeezed these rocks, creating the gnarled appearance you
see here. To do this without breaking the rock requires tremendous
heat and pressure.
The Milton Belt
The geologic terrane in the North Carolina Piedmont known as the
Milton Belt (or the Milton Terrane as the
Virginia portion is called) is known for its highly metamorphosed
rock units, such as the gneiss you are looking for. Debate has
raged (as much as this kind of debate rages) over its regional
associations. It was once thought to be a part of the
Charlotte Belt but the work of Hibbard (1993),
Wortman et al (1996) and Coler et al (2000) build a strong case
against this.
- Wortman (1996) concludes that the
Milton Belt must have evolved as a completely
separate chunk of crust before the
Charlotte Belt and the
Carolina Slate Belt attached themselves… all
those years ago.
- Hibbard (1993) argues that the Central Piedmont Suture, the
boundary separating the
Charlotte Belt from the
Inner Piedmont to the west, extends northward
along the eastern boundary of the
Milton Belt, delineating it from the
Carolina Slate Belt. (Phew! Just look at the
maps in the links!)
- Coler (2000) indicates that the
Milton Belt was built on native Laurentian
crust, and was the same "chunk of rock" as the Chopawamsic terrane
in Virginia. The implication is that if it is not part of the Inner
Piedmont (see below) then at the very least it had a very similar
geologic history. (Come back to this one. It will make sense
later!!!)
All in all, very little recent information about the Milton Belt
is published, however Wortman (1996) suggests that the rocks of the
region may have formed as a result of a mixture of ancient
“Grenville Age” (over 1 Billion years old) rock and younger ones of
various origins. The implication (and he admits it is not a
conclusion) is that you are looking at something formed one billion
years or more ago. It was later intruded by
igneous rocks and/or overlain by sediments. The
whole thing was mixed like ingredients in a cake by high-grade
metamorphism and/or full-on melting become a new homogenous rock
consisting of the current chemical composition. Later, metamorphism
during the building of the Appalachians changed them to what you
see today.
What a mess!
What the Milton Belt Probably Is
So just where did the Milton Belt Come From?
In order to give the tectonic history, I’ll go with this. I am
not going to name the geologists so that I don’t misquote, but… I
have it on good authority that a scientist prominent in NC geology
has interpreted the Milton Belt as a more highly metamorphosed
section of the VERY COOL
Inner Piedmont. (If you can provide official
documentation, please email me!)
Between 565 and 735 million years ago, the supercontinent of
Rodinia rifted apart in two stages, hence the nearly 200 million
year spread. The main units formed were Gondwana (early Africa and
South America) and Laurentia (early North America).
In addition, a slice of Laurentia, a.k.a. today’s Inner Piedmont
(IP), broke off and drifted “some” distance from its native shores.
When the tectonic forces reversed, the western IP ocean crust broke
off and subducted beneath the Inner Piedmont forming a
subduction related volcanic island arc
system like the one in this image.
Imagine, volcanoes! Right here in Historic Kernersville!
Kernersville (and the rest of the IP and
Milton Belt) collided back into Laurentia and
was followed by the volcanic island system called Carolinia (which
is now the
Carolina Slate Belt and has a few great
Earthcaches,
like this one!) and then Gondwana. The
successive collisions, spanning 330 to 480 Million Years Ago,
assembled the supercontinent Pangaea and built the Appalachian
Mountains which were once tall and mighty, perhaps reminiscent
of the Himalayas! In the process, the rocks you are looking at
were deep within the mountain's core undergoing metamorphism and
compressions. These actions created the characteristic foliation
and folding you see here.
Although the reasons for everything east of the
Blue Ridge Escarpment being much flatter and
lower than the mountains are a topic for another discussion, the
highly metamorphic rocks of the Inner Piedmont and the Milton Belt
are the core of a large part of the Appalachians that have eroded
away.
The Point
You are not looking at tall mountains. Instead you are looking deep
inside of them! Gneiss, eh?
Thought Questions
Though there are no "credit" requirements, the following questions are available to stimulate thought and increase your geologic knowledge.
1. WHAT is the difference between gneissic foliation and
sedimentary layering?
2. The foliation on this rock is generally divided into two
color bands, black and white. The black is mostly
biotote mica, while the white is
probably quartzand
plagioclase feldspar. Regionally rock folds can
range from millimeters to kilometers! In these boulders the
foliation has some very tight folds and some rather broad ones (see
the “K’Ville Boulder image above). For Question number 2, estimate
the width of the tightest fold you can find. In other words, trace
the line of one of the black OR white foliation bands and measure
or estimate the distance across to the same line as it folds back
roughly parallel with itself.
3. Gneiss is a metamorphic rock. If the process involved a more
intense heat and the rock had melted and re-cooled, what kind of
rock would you be looking at?
Extra Credit kudos if you can describe how these rocks
hypothetically may have gone through the
the Rock Cycle in the past, and/or how they may again be a part
of it in the future. There is no correct answer to this, and I
prefer it recorded in the public log for this cache. I think it
will be a fun experiment to see what different people come up
with.
Go on, you could learn loads!
References:
Coler, David G. , Wortman, Greg L., Samson, Scott D., Hibbard,
James P., and Stern, Richard. U-Pb Geochronologic, Nd Isotopic, and
Geochemical Evidence for the Correlation of the Chopawamsic and
Milton Terranes, Piedmont Zone, Southern Appalachian Orogen.The
Journal of Geology. Vol. 108, No. 4 (Jul., 2000), pp. 363-380
Wortman, Greg L., Samson, Scott D, and Hibbard, James P.
Discrimination of the Milton Belt and the Carolina Terrane in the
Southern Appalachians: A Nd Isotopic Approach. The Journal of
Geology, Vol. 104, No. 2 (Mar., 1996), pp. 239-247
Hibbard, James, 1993. The Milton Belt - Carolina Slate Belt
Boundary: The Northern Extension of the Central Piedmont Suture.
Geological Society Fieldtrip Guidebook, 1993, 85p.
And thanks to Dr. Skip Stoddard from the North Carolina
Geological Survey
(NCGS) for
his input, suggestions and education.
This is Geology Waymark number 001 from the
Paddy-Whacked
Radio™ Geology Project.