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Morrow Mountain's Special Rock

A cache by frograil Send Message to Owner Message this owner
Hidden : 4/10/2009
Difficulty:
1 out of 5
Terrain:
3 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

Morrow Mountain stands hundreds of feet above the surrounding countryside, and is composed primarily of a hard, dark gray rock. 11,000 years ago, it was the site of one of the largest quarries in the Piedmont of today’s North and South Carolina.

Just a little east of the small town of Badin, in northeastern Stanly County, is an archeological dig location known as the Hardaway Site; and it has been a treasure trove for scholars and enthusiasts for decades. Stone tools and weapon points by the thousands have been excavated, and represent one of the most extensive bodies of Native American stone work in existence. This EarthCache is not about the archeology of the Site, however; it's about the stone used to produce these objects. The Hardaway Site has been referred to as "The Remington Arms Factory of 10,000 BC", but the raw material for those weapon points and tools came primarily from one location: The top of Morrow Mountain, about 5 miles away.

To understand why that rock was so suited for knapping1, we have to understand more about what the rock is, and for that we have to go back further in time than thousands of years. In fact, we need to go back somewhere around 530-550 million years ago. What is now much of the eastern piedmont of North and South Carolina was a volcanic island arc then, much like Japan or Indonesia, and it was hundreds of miles southeast of today's location on the east coast of North America.

A volcano in an island arc is fed with molten and semi-molten rock (magma), from deep within the earth. As the magma rises, the temperature and pressure surrounding it changes, causing changes in the chemical makeup of the body. Finally, as the magma nears the surface, the pressure increases tremendously, as water and other "volatiles" are released from the magma, but the overhead rock keeps everything inside the mountain. Eventually, the pressure becomes so great that the top or side of the mountain explodes, and the magma is released. The chemical composition of the magma determines whether the release will be violent or subdued. Think of the difference between Mount Saint Helens in 1980, and the on-going burbling of Mauna Loa in Hawaii, respectively. The amount of silica in the magma is largely the determining factor.

Morrow Mountain's magma had a lot of silica in it, making the mix very viscous, or "sticky". The greater the viscosity, the greater the pressure build up, and the greater the resultant explosive eruption. The magma was just a little less sticky than pure rhyolite, so the explosions were probably good things to be far, far away from.

Rhyolite and granite are composed of the same mix of elements and minerals, but granite forms deep within the mountain after all eruptions have stopped. Over hundreds of thousands of years, the body cools, and crystals form from its minerals, creating the beautiful material for kitchen counter tops and bank building exteriors common throughout the country.

Rhyolite is usually an extrusive form of the same mineral mix as granite, but it doesn't look anything like it. [There are three photos of samples of rocks from Morrow Mountain at the bottom of this EarthCache listing.] It's a fairly uniform, dark gray to almost black rock, and you need strong magnification to the see any crystals. That's because, once very near the surface, or extruded over it, the magma (now called lava, because it's outside of the volcano) cools very rapidly. That rapid cooling doesn't allow time for large crystals to form. And all of this blather brings us to Morrow Mountain and those Native Americans.

But first, a technical note. There is less silica in the Morrow Mountain rhyolite, and more iron and magnesium, than found in "pure" rhyolite, but not so much that the rock would be the equivalent of dacite. Therefore, it is more proper to refer to the rock as rhyodacite. Further, during the period when the volcanic island arc carrying the rocks of Morrow Mountain collided with ancient North America some 450 million years ago, Morrow Mountain and the surrounding Uwharrie Mountains were buried under thousands of feet of rock and dirt, and the rocks within the long-extinct volcano were lightly changed, or metamorphosed2, which means that the rock should technically be called "metarhyodacite", which is more than a mouthful, so we'll stick to "rhyolite" for this EarthCache discussion.

The chemical and mineral composition of the rock, and the tiny crystals within it, created a very hard rock that, when knapped, yielded a conchoid fracture. That's a concave, circular depression that has very sharp, hard edges.

For thousands of years, Native Americans mined the rhyolite at the top of the mountain, and carried it to places such as Hardaway, to work it into tools and weapon points. Over 20 ancient "quarries" of rhyolite and other rocks have been discovered throughout the Uwharrie region and other regions further east. Very complicated geochemical analyses of the tools and points have enabled scientists to determine where some of the original rock came from. The vast majority of the work done at Hardaway used Morrow Mountain rhyolite, even though there were other quarries closer to the Site. Modern workers have demonstrated the superiority of Morrow Mountain's rock, compared to other quarry locations.

Now, as you walk around the crest of the mountain, you see no outcroppings of rhyolite. So where did all of that rock come from, anyway. You don't see outcroppings because they're all gone; the quarry was played out. It became easier to get rock from other locations than to dig into the earth and extricate rhyolite, even though that other rock might have less quality than Morrow Mountain's.

From the coordinates, go over the wall and down the trail. You'll shortly cross a fairly deep gully on a footbridge, and then come to a junction in the trail. Take the sharp right and walk onto another footbridge over the same gully. Look closely at the gully's walls, which are close to 10 feet deep.

Besides dirt, you'll see chips and shards of grayish-blue rock. Those humble rocks are Morrow Mountain rhyolite, and they are the residue of the mining operation. Walk carefully over the ground off the trail, and brush aside the leaves (be careful in this area, as it is very steep in places, and the leaves are slippery). There are shards of the rock everywhere. Indeed, it is estimated by Dr. Kevin Stewart at the University of North Carolina in Chapel Hill that millions of tons of such debris form the entire top of the mountain.

Morrow Mountain’s rhyolite is, indeed, a very special rock.

A reminder, Morrow Mountain is in Morrow Mountain State Park, and the taking of rock samples is prohibited.

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Footnotes:

1. Knap: To break or chip (stone) with sharp blows, as in shaping flint or obsidian into tools. (Free OnLine Dictionary)

2. Metamorphis: Literally, to change the shape. (FOD and Wikipedia)

 

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Logging Requirements:

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

1. Make the subject of the e-mail "GC1PT46. Morrow Mountain's Special Rock"

2. How many people are in your party?

3. Morrow Mountain, as well as the other mountains in the Uwharries, is called an "inselberg " or "monadnock".

a. What type of mountain do these terms describe?

b. With respect to Morrow Mountain, are the terms applicable? Why?

4. Post a picture of yourself on the lower footbridge over the gully. Try to include you, a bridge railing, and the gully.

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References:

Bradley, P. Piedmont Geologist, North Carolina Geological Survery. Personal communication, 2009.

Daniel Jr., I. R., and Butler, J. R. An Archaeological Survey and Petrographic Description of Rhyolite Sources in the Uwharrie Mountains, North Carolina, in Southern Indian Studies, Vol. 45. 1996.

Miller, B. V. Samarium-Neodymium Isotope and Trace-Element Analysis of Metavolcanic Rocks from the Vicinity of Fort Bragg, North Carolina: A Pilot Study of Artifact Source Quarry Discrimination. Isotope Geochemistry Laboratory, University of North Carolina - Chapel Hill. 2002

Stewart, K. G., and Roberson, M-R. Exploring the Geology of the Carolinas. University of North Carolina Press, 2007

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Find...

  • Beautiful Rhyolite SpecimensThese fine specimens are part of a mini-collection which is kept within the Morrow Mountain State Park office. Photo by Tony Hill
  • Rhyolite chips and shardsThe white "rind" is typical of the shards found at the top of the mountain. It formed on rock that was exposed to the air, and was often chipped off and left behind by the Indians. Photo by Tony Hill
  • Rhyolite chips and shardsAnother example of the types of rock pieces you'll see at the summit. The rind is very prominent on the piece on the left. Photo courtesy of the North Carolina Geological Survey

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