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Gold Hill: Mudstone Magic

A cache by Frograil Send Message to Owner Message this owner
Hidden : 8/31/2009
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Geocache Description:

You are standing on the most valuable rocks in the Gold Hill area.

You are in the parking lot for the Russell-Rufty Memorial Shelter.

The Shelter, incidentally, houses the Veterans’ Memorial Room, conference room, office and meeting hall for the Gold Hill Mines Historic Park, as well as many photos and other informative items about the area.


In his doctoral dissertation, Jeffrey Pollock stated the following:  "The Floyd Church Formation comprises medium- to thick-bedded epiclastic mudstone."  This technical statement has important meaning for Gold Hill and Rowan County, so let's decipher it:   A "formation" is a rock unit that can be mapped on a geological map, and which has an upper and lower limit.  That means you can look at the rocks in the countryside occupied by the Floyd Church Formation and see the same type of rocks everywhere, until you see something quite different, signaling that you're now in another formation.  Early geologists noted a particular rock type in the vicinity of the Floyd Baptist Church on its namesake road near Lexington, NC, and called the rock unit the Floyd Church Formation.  If you go anywhere between Asheboro, Denton, Gold Hill and Albemarle, and find the same kind of rocks, you're looking at Floyd Church Formation rocks

So, just what are these rocks? They are epiclastic mudstone, according to Dr. Pollock's description.  The term "clast" refers to pieces of rock and mineral, while "epi" denotes the fact that these fragments originated over or beyond the place where they were laid down.  Mudstone is a fine-grained sedimentary rock composed of mostly bits of clay or mud.  The size of the tiny grains making up the "mud" in mudstone is technically important to geologists, as it determines what the rock will be called, as well as many of the rock’s characteristics.  Mud has a grain size of only up to 0.0025 inch (0.0625 mm), so the individual grains are much too small to be seen without a microscope.

"Bedded" refers to how smoothly these trillions of tiny grains were laid down.  Because the beds are described as "medium- to thick-bedded", the grains were deposited quite uniformly over a long period of time -- millions of years, in fact.  Such a deposit history means that the area was well under the sea, and not in an above ground or tidal area. The period of deposit was roughly 25 million years, and it was centered on approximately 540 million years ago.

Yes, I can hear you say "Big woop, a bunch of mud rocks.  So what?"  Bear with me, as those rocks are going to change – and here's where the magic begins to happen.

Over the millions of years that the grains were piling up on the ocean floor, they got thicker and thicker, and both heat and pressure began to increase in the lower parts of the deposits.  This began to harden and compress the grains and turn them into consolidated rock.  Remember, it was a constant process -- a lot can happen over millions of years!

A more abrupt and dramatic change took place during the time that the continents of what are now Africa and South America collided with what we now call North America.  This colossal process pushed up the Appalachians to be one of the great mountain ranges on the planet -- probably on a par with today's Himalaya Mountains. While impressive mountains were thrust up to the west, the fate of the mudstones we're concerned with wasn't nearly so dramatic.  In fact, they were buried, and subjected to more heat and pressure than anything they had experienced before.  This changed the rock slightly, as it made what had been seafloor, fairly soft rock, into a pretty hard rock -- more magic.

Eventually, the supercontinent created by the collision described above began to break up, and the Atlantic Ocean was born between them (much like today's Red Sea, but on a much larger scale).  So, where are those huge mountains?  The Appalachians today are simply the eroded remnants of those monstrous peaks, and that wasn't the only place erosion was taking place.  Everything above ground, all the way to the shore line, was eroding. As billions of tons of rock and mineral were eroded and carried away by the rivers and streams of the past, the land rose.  Continents actually "float" on denser material, and when weight is removed from them, they "bob" upward.

The rocks first described around the Floyd Baptist Church well over 150 years ago have risen from miles deep within the earth, and the rocks that were above them are now in the Atlantic Ocean, building new mudstones.  More magic, indeed.

All the above brings us to where we are standing.  There is an immense quarry operation on Old Beatty Ford Road, just to the south of where you are. Huge rocks are brought out from a very large open pit, and are then crushed, graded, and cleaned to meet customer specifications.  Six to twelve railroad cars are removed at least 2-3 times a week and replaced with empty cars to be loaded. Dozens, if not hundreds, of trucks a day take loads of that crushed, cleaned and graded rock out to building sites, asphalt plants, and other end-users -- including the Historic Gold Hill and Mines Foundation, Inc., on whose Floyd Church Formation crushed rock you are now standing.

It is ironic that lowly mud grains at the bottom of an ancient sea have become, by far, the most economically important rock ever to be mined or quarried at Gold Hill. This is a geological magic trick that has taken over a half a billion years to unfold.

Logging Questions:

Send me an e-mail – not part of your log – responding to the following:

    1. Make the subject of the e-mail “GC1Y1KE, Gold Hill: Mudstone Magic”

    2. How many people were in your party?

    3. Post a picture of you and your party, and try to include some of the stones in the picture.

    4. When Africa and South America collided with North America, the mudstones were slightly changed. Some geologists insist that the correct term for the rock should be "metamudstone". What's the difference between mudstone and metamudstone, and which term do you think should be applied to the rocks quarried in Gold Hill?

    5. Some uses of the crushed stone produced at Gold Hill were mentioned (asphalt and building site underlayment) in the text. What is at least one other use of crushed rock?

    6. Look closely at the stones, and try to describe them for me. Do they contain large, obvious minerals, or do they seem to be just a mass of no obvious pieces or parts? This is the nub of the questions: Why do you suppose you see what you see?

    7. Extra (fun) credit: Can you find any that are the typical bluish-grey, but have a layer of tan-colored stone in them? If you can, you are very lucky. Can you tell me what could have formed that tan-colored layer?


The author thanks Phil Bradley, Senior Geologist, North Carolina Geological Survey, for his valuable corrections, additions and suggestions relative to this write-up. Any mistakes herein, however, are solely the responsibility of the author. 2009

The author thanks Vivian Hopkins, Vice President, The Historic Gold Hill and Mines Foundation, Inc., and Chair of the Foundation's History Committee. She has been a tour guide, source of knowledge, and careful fact checker for the author.

Hibbard, J, et al. The Heart of Carolinia: Stratigraphic and Tectonic Studies in the Carolina Terrane of Central North Carolina. Field guide for a pre-meeting field trip of the Southeastern Section, Geological Society of America meeting, 2008.

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.

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