Science – Joshua R Benton

January 10, 2015August 29, 2019Earth Science, Geology

Geology of Columbia, SC – Peach Tree Rock

Recently while on vacation visiting family for Christmas I had the pleasure of visiting a geologically interesting part of SC called The Peach Tree Reserve. The Reserve is a protected, 466 acre plot of land in the midlands of South Carolina that’s managed jointly by SC’s Department of Natural Resources and a nonprofit organization called The Nature Conservancy. The preserve is part of the Sand Hills physiographic region which marks the upper limit of the Coastal Plain in South Carolina. Within the reserve there are fantastic exposures of the underlying bedrock (Eocene Barnwell Formation) in the form of pyramid-shaped sandstone outcrops, one of which is called Peach Tree Rock.

Peach Tree Rock. Photo Credit: Kristine Hart / The Nature Conservancy

Since the famous rock actually toppled over in December of 2013 I had to use an older photo from the Conservancy’s website for the above picture. Below is a Gigapan I took of Peach Tree Rock in Dec. 2014 to show how it looks now in comparison:

Click on the photo to link to the Gigapan site.

The reserve has other interesting precariously balancing sandstone outcrops such as Little Peach Tree Rock:

What you’re seeing whenever you walk around the reserve are bedrock exposures that have resisted weathering and now poke out above the soil. The bedrock here is a quartz rich sandstone with portions that are more clay rich than others giving the rock a color variation from white to brown. As a result of the bedrock being sandstone this part of the state is rich in silica sands. So much so that some Carolinians have even established profitable businesses (such as Columbia Silica Sand, inc) mining this sand and selling it to be used for various industrial purposes.

These sandstones mark the upper limit of the Coastal Plain before the state transitions into The Piedmont only a few miles north of the reserve in the capitol city Columbia. This transition from Coastal Plain to Piedmont is known as the Fall Line and it is the reason why Columbia is where it is today. If you were to travel upstream along one of South Carolina’s major rivers (such as the Congaree) the Fall Line is typically where you encounter the first turbulent water where the rivers “fall” down from the rocks of the Piedmont into the Coastal Plain. This was crucial to early European settlers who used rivers as their primary conduits to explore and colonize the East Coast. Most of the East Coast’s major cities all sit upon the Fall Line (New York, Baltimore, Washington D.C., Richmond, ect.).

So where did all this sand come from? This is the type of stuff you would expect to accumulate on a beach or in a desert yet there is neither of these in Columbia. As it turns out these sands are all shoreline or near shore deposits and represent where the coastline used to be millions of years ago during a geologic epoch called the Eocene. If you owned property in around Columbia during the Eocene you might have had beach front property.

You can find evidence to support this within Peach Tree Reserve. Upon a more detailed examination of Little Peach Tree Rock you can find trace fossils of burrowing organisms that would have lived within a coastal marine environment. Here is a picture of one such trace fossil called Ophiomorpha that is interpreted to be the burrow of a shrimp:

Ophiomorpha within Little Peach Tree Rock. Note the bumpy exterior.

A sedimentary structure known as cross-bedding can also be observed within Little Peach Tree Rock:

Cross-beds like this are typical among sandstones that formed in beach or shallow-marine environments and from these one can infer the direction of the water current that originally deposited the sand. Using clues such as fossils and sedimentary structures we can learn more about South Carolina’s past, and The Peach Tree Rock Preserve reveals only a small fraction of the Palmetto State’s long and fascinating natural history.

August 15, 2014Earth Science, Geology, Uncategorized

Grand Canyon – The Precambrian

The Earth is very very very old. Roughly 4.54 billion years old, or 4,540 million years old, or 4,540,000 thousand years old (however you would like to think of it). Prior to the development of radiometric dating to give us numbers and absolute dates we mostly used fossils and other principles to obtain relative dates of rocks. So all of our subdivisions of time are based upon the fossil record. The Precambrian refers to an immense span of time (4,540 million years ago to 542 million years ago) where the rocks contain very few fossils due to the fact that organisms never developed hard parts that could be fossilized until later in Earth’s history.

Zoroaster Granite and Vishnu Schist

At the bottom of the Grand Canyon we find rocks that formed during the Precambrian, two of which are the Zoroaster Granite and Vishnu Schist. In general a granite is defined as an igneous rock that forms from the crystallization of magma while a schist is a metamorphic rock that forms when shale is subjected to heat and pressure.

These rocks formed roughly 1,700 million years ago when a tectonic collision occurred between an older section of our continent that stretches from Southern California to Wyoming (dubbed Wyomingland) collided with a volcanic island arc. This collision of Earth’s plates provided the heat and pressure to morph the shale that formed the Vishnu Schist while producing the magma that formed the Zoroaster Granite. The Zoroaster literally intruded into the Vishnu as magma before cooling and solidifying to become a rock itself. We came across the first exposures of these rocks within the inner gorge of the canyon along side Bright Angel Creek:

This is a spectacular example where you can see huge sections of the Vishnu schist that broke off into the magma chamber before the magma cooled and solidified locking them into the place they are now.

By observing that there are inclusions of the Vishnu inside of the Zoroaster you can determine which rock is older than the other. The Vishnu must have been there first for the magma to intrude into it and break off pieces.

The Great Unconformity

Further upstream we got some impressive views of an interesting natural phenomena called an unconformity. In fact, this is known as the Great Unconformity which is represented here by an angular unconformity. The Great Unconformity was first identified in the 1800s by John Wesley Powell, a one-armed civil war vet who was the first person to lead an expedition down the Colorado River and later became the second director of the USGS. Here is a picture of the unconformity:

Can you spot it? Does it help that Kristi is pointing directly to it? Clearly there are beds of tilted rock dipping towards the right but are truncated at the top by horizontal bedding of a different rock. What in the world would cause this to happen? These are two different types of sedimentary rocks sitting right on top of each other at different angles. Would natural processes deposit the original sediments like this right on top of each other? The answer is no. The two rocks are not conformable. The “line” separating the two is an erosional surface which represents a gap in time that we call an unconformity. The rock unit on top is known as the Tapeats Sandstone that’s roughly 520 million years old while the unit on the bottom is known as the Dox Formation that’s roughly 1,120 million years old so there’s somewhere around 600 million years of missing time in between the two.

Here’s how this works:

Pretty cool right? Unconformities are found throughout the Grand Canyon and all throughout the world. Sometimes they are hard to identify but this one stands out as a prominent feature along the rim of the inner gorge.