South Carolina Geology
South Carolina has a geologic history extending back over one billion years. Check back to this page often for new stories on South Carolina's geologic past, and continue below for the first installment - The "Clemson Conglomerate". Click here to see a geologic map of South Carolina. You can also learn more through the SC Geolgical Survey.
The "Clemson Conglomerate"
If you're familiar with the upstate of South Carolina, you can't get away from the red clay that forms our topsoil. But what's underneath all this? Well, sometimes stream cuts, road cuts, and construction excavations reveal what is buried under the clay. One construction site on the west side of the city of Clemson, just on the Pickens County side of the Seneca River, reveals an interesting geological cross-section.
While excavating an area behind the foundation of lakefront condominiums, a seven foot thick section of sand, clay, cobbles, and unusual white rock was exposed. What was this material and how did it get there?
The mixture of sand, clay, pebbles and cobbles is a type of sedimentary material known as conglomerate, which is commonly formed through deposition of detritus (i.e. the sand and pebbles) by streams and rivers. In our case, the conglomerate was deposited by the Seneca River. Over many thousands of years the water flowing through the river channel slowly cut down into the underlying rock. As this happened, the rock was broken down into smaller fragments and carried away (a process called erosion). The rock fragments were further abraded and broken down as they travelled, resulting in the smooth pebbles and cobbles seen in the outcrop. When the river's current wasn't strong enough to carry the load of large rock fragments, they were deposited and formed the river's bed (sedimentation). As water flow slowed down even more, the finer sand grains were deposited.
The white material visible in the outcrop is a type of rock known as gneiss (seen in the sample to the left). This rock, which is over one billion years old, formed when igneous rocks located miles below the earth's surface were subjected to very high temperatures and pressure. The gneiss in our outcrop is actually a metamorphosed granite that is composed of feldspar (white), quartz (gray), and mica (black). As the rock was metamorphosed it became folded, and the minerals were aligned in distinctive planes, giving the rock a banded appearance. This is the rock that the Seneca River cut through, and most of the rounded rocks in the conglomerate are gneiss.
At the top of the photo to the right, the ancient river bed can be seen on top of the gneiss. The boundary between these two units is an erosional surface known as a nonconformity. Some interesting observations on the gneiss: Fresh, unweathered rock is very hard and difficult to break with a rock hammer. However, it can become so highly weathered that it will crumble in your hands (at this point the material is called saprolite). The felspar and mica that form the gneiss slowly break down into microscopic grains, forming the clay so common in the Upstate. The red color is the result of the oxidation ("rusting") of iron locked up in the minerals in the clay. Gneiss is an important rock in the Upstate because it is rather porous, making it easy for groundwater to move through it. Many people still rely on wells drilled into the bedrock for their water supply.