Adams D. - Report

Field Trip to Peachtree Rock

By Danny Adams

Figure 1: Peach Tree Rock

We arrived at Peachtree Rock Preserve early on a rather cold South Carolina morning. At the preserve's small parking lot, the landscape was very unspectacular and typical of the terrain in the midlands of South Carolina: sandy soils supporting a longleaf pine forest and stands of scrub oaks. As we descended the trail, not only did the temperature drop substantially, but also the types of flora began to change. The decrease in temperature combined with the increase in moisture led us to a special place. Near the waterfall, longleaf pines and scrub oaks gave way to mountain laurels, fiddle head ferns, and mosses, all of which are atypical of this area of South Carolina (The Nature Conservancy, 2000).

The first stop on our tour of the preserve was at Peachtree Rock itself (Figure 1, above). I have heard that the rock is so-named because it once had a peach tree growing atop it, but who knows if this is true. The rock is roughly conic in shape, is standing on its point, and is approximately 10 to 15 feet in height. At its top, the rock is composed of relatively durable sandstone with mud serving as a matrix (Kite, 1985). The lower two thirds of the rock is composed of unconsolidated sandstone that can easily be rubBeds away with the fingers. The sandstone is riddled with holes of various diameters and depths. Members of our group developed many theories concerning the origin of these holes: plant roots could have formed the holes or they could have been produced through bioturbation (burrowing animals). Perhaps the most credible theory concerning the many holes is that they were produced by wasps (modern wasps) and are not a part of the original rock. Similarly, the holes could have been created by human activity and, therefore, can provide no clues to the past depositional setting. Outcrops show complex cross-bedding which is characteristic of most outcrops on the preserve. Many thin layers of clay have been deposited at irregular intervals.

As we observed Peachtree Rock, Dr. Kendall asked the group to formulate a series of hypotheses to explain the mode of formation of the many unusual sandstone rock outcrops on the preserve. It was fairly obvious that these sediments were deposited in a marine environment, but the exact type of marine environment is questionable. Some members of the group surmised that the geologic features in the area indicate that the sediments were deposited in and around a tidal channel. Others suggested that the sediments were deposited on the continental shelf, on an offshore bar, or on the beach. Perhaps the deepest sediments were deposited on the continental shelf, higher sediments deposited on an offshore bar, and the highest deposited on the beach. Finally, it was suggested that the sediments could have been deposited at or near the headwaters of a drainage basin. Future studies of the area may reveal the origins of the sediments and the exact type of environment in which they were deposited.

The next step on our tour of Peachtree Rock Preserve took us to perhaps the most popular site in the preserve, the waterfall. This is the only naturally occurring waterfall on South Carolina's coastal plain. The sandstones at and near the waterfall are similar to those found at Peachtree Rock, however, the unconsolidated sand layer is markedly thinner which suggests that this rock outcrop was formed in a slightly different sedimentary environment (Figure 5, below). Examination of the cap rock with a hand lens revealed that it was primarily composed of angular to subangular quartz crystals. It was suggested that the angularity of the crystals could indicate that the quartz was transported to the site of deposition from only a short distance away; transportation over long distances would, most likely, have rounded the quartz crystals. However, the durability and stability of quartz could allow it to be transported over great distances without marked degradation of the crystals.

After inspecting the area near the waterfall, we began a rather intensive fossil hunt. Several fossils molds were found at the rock outcrop associated with the waterfall. I personally found, not fossils, but a layer of mollusk shell fragments in the creek bed upstream of the waterfall. I say that I found them, but having visited Peachtree Rock Preserve many times in the past, I knew where they were. The layer of shell fragments was approximately 2 to 3 inches thick and was higher than the cap rock forming the waterfall. This suggests that the shells or shell fragments were deposited more recently than the underlying sediments.

After this field trip, it became apparent that many geological processes can lead to the creation of unusual sedimentary rock outcrops and that many theories can explain their formation. As mentioned earlier, I have visited the preserve many times in the past. Being a former biology major, I mainly concerned myself with the present-day flora and fauna of this area: how it got there and why it remains there. Recently my interests shifted from biology to marine science, specifically marine geology. The sandstone rock outcrops at Peachtree Rock Preserve are a perfect example that it is not absolutely necessary to study modern coastal environments in order to understand past depositional environments.

References

________, _______. 2000. Peachtree Rock. The Nature Conservancy. [On-line]. Available:

Kite, Lucille E. 1985. stratigraphy of Peachtree Rock Preserve, Southern Lexington County, South Carolina. South Carolina Geology. 29: Pp. 1-7.

Thursday, February 21, 2013
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