Bartley H. - Report

The Peachtree Rock Field Trip Field Trip Report by Heather Bartley

The Peachtree Rock Preserve is owned by the South Carolina Nature Conservancy. The preserve is located in southern Lexington County, SC, in the upper coastal plain. Our class journeyed to Peachtree Rock Preserve to observe some good exposures of Middle Eocene sandstone and use our observations to infer something about the depositional setting of the sedimentary rocks there.

At the preserve, erosion of loose sand overlain by resistant sandstone has produced some unusual rock formations. The most distinctive rock formation at the preserve is shaped almost like a peach (see Figure 1). We observed that this formation was composed of quartz sandstone with a few calcareous layers (see Figure 2). The sandstone showed cross-Beds, down-cutting, and cut-and-fill in both directions. It was clearly a clastic sedimentary rock. There were many burrows in the rock, although it is unclear whether the burrows were recent or fossiliferous.

We saw layers with white flakes of clay in the rock that superficially looked like fragments of shell. Though calcareous layers in the sandstone have beeb reported to contain fossil shell fragments identified as marine shells we only saw molds of some bivalves. On the basis of these fossils, the sandstone's depositional environment must have been some sort of marine or near marine environment. Some possibilities could be a beach, a tidal channel, or a tidal flat. However, a sedimentary structure at the final outcrop we visited considerably narrowed down the choices. At the final outcrop, we observed herringbone cross-stratification (see Figure 4), a sedimentary structure nearly exclusive to tidal flat sediments. The herringbone pattern is formed when "sediments deposited during flood tide dip in the opposite direction to those deposited almost immediately afterward during ebb tide" [text, pg.372].

The fact that Peachtree Rock was once a marine environment indicates that sea level was at one time at that altitude, 500 feet above the present sea level. Sea level does change over time, mainly due to the formation and melting of glaciers. In the Middle Eocene when this sandstone formed, the Earth was probably experiencing a warm period so that there were no polar ice caps.

As a class, we examined one other outcrop. About one hundred yards from the peach-shaped outcrop was a small cliff. The sandstone layer in the cliff was thinner than at the first outcrop. Also, the sandstone in the cliff possessed coarser grains and more layers containing flakes of clay than at the first outcrop. These small differences between two outcrops one hundred yards apart helped the class understand the difficulty of analyzing the stratigraphy of an area.

At this same outcrop, the sandy clay near the top of the small cliff had been silicified, causing that layer to be nonporous. Groundwater in the layer above the nonporous stratus spilled out in the form of a waterfall (see Figure 5). It was very picturesque. The waterfall was also depositing calcium carbonate in the form of travertine. I thought that the geology behind the waterfall was fascinating.


Figure 1. The peach-shaped outcrop composed of sandstone.

Figure 2. A layer rich in flakes of clay in the sandstone.

Figure 3. The herringbone cross-stratification observed in the final outcrop.

Figure 4. A fossil shell posing next to my beautiful pen.

Figure 5. The waterfall spilling out from above the nonporous silicified layer and depositing travertine.

Thursday, February 21, 2013
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