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.
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:
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:
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.