FTF Congratulations to Night-Ranger and G.O. Turtle!!!
15 August, 2009.
Introduction
(Click the image for an interactive geologic map of North Carolina. You'll need it!)
For this cache, 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?
Logging Requirements
To claim your Earthcache credit you must first submit the standard photo of you or a member of your party in front of one of these boulders, picking the most foliated and folded one you can without disturbing the landscaping. Up to you whether you show your GPS. Post it in the log.
And then, in an email and NOT in the log, you must answer my questions three!
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 Earthcache number 001 from the Paddy-Whacked Radio™ Geology Project.