Jones A. L. - Report

Peachtree Rock Reserve Report By Anthony Lamar Jones

"Peachtree Rock Reserve consists of just over three hundred acres situated in a valley at the headwaters of Hunt branch, which eventually feeds into Second creek and then the Congaree River. The upland areas are dissected abruptly by relatively steep sided valleys, which yield limited exposure of the underlying Cretaceous and Tertiary sediments. Peachtree Rock Reserve, owned by the The South Carolina Nature Conservancy and located in southern Lexington County, is noted for its unusual silicified sandstone beds, which are in part fossiliferous and offer a significant amount of stratigraphic exposure. Detailed work in the Peachtree Rock Preserve and the immediate vicinity indicates the local stratigraphy is compatible with the more recent interpretations of the stratigraphic framework for the Upper coastal plain of South Carolina. The silicified sandstone exposed in the Peachtree Rock area marks the top of the middle Eocene sediments in this vicinity. The abundaformnt fossil shells concentrations associated with the sandstone outcrops have as yet been overlooked for detailed study and description." (Colquhoun, Nystrom, & Burns).

One working hypothesis for Peachtree Rock's origins is that it was once a beach setting. The reasoning behind this hypothesis is that the cross beds are similar to the offshore bars of the present coastline of South Carolina. Secondly, the orientation of the cross beds is bidirectional, which are similar to beachfronts. In beachfronts, there is cutting, filling and scouring of the bedding planes, there is sharp contact between the bedding planes and units, and there are burrows, that varies in intensity as the exposed section increases in height. Other explanations that this could have been a beach environment fed by a fluvial system transporting and collecting sand or a river system is transporting and collecting shells out of the banks of a river and depositing them in the bank of the channel. With this data we can definitely work towards the concept that Peachtree Rock is the result of a depositional environment. (i.e. channel, beach, river or a offshore shove). If this were an offshore, one would expect the sediments to be worked over by burrowing organisms. See picture below.


Knowing this bit of data, one can now classify this environment to have been offshore; alternatively the environment could have been just within wave base where the waves are breaking offshore. If an observer were to look upwards and shoreward, they would find a different group of sediments. If they were looking at the far shoreline, we would might find clays. If you goes to the area at the top of break point bars you would find shells and flat laying inclined sands with a low inclination cross beds which is somewhat different from what is in Peachtree Rock.

Where the waterfall is flowing, there is something that you see as far as sedimentary structure. What you see is that grain in the rock starts to get courser and more shells are present. The fact that the grains are coarser suggests that Peachtree Rock was a beach environment. However, if you look closely at the rocks, the grains are angular to sub-angular which is strange. The reason it is strange is because the composition of the rock is mainly quartz. (See picture above) Usually, the way you end up with a composition of mainly quartz are to transport a material far enough that you wearing down the components in the sediment. (The other components could be mafic minerals, feldspars, or rock fragments) quartz is the mineral that lasts the longest in terms of sedimentary transport. So, if you combine the two observations together, regardless of whatever Sedimentary model you come up with, there is a problem predominately at the quartz composition and yet the angularity of the grain suggest that the rock has not been transported a great distance. This would have to work into whatever depositional model you would want to come up with for these sediments. So now the big question is: Where would you go to find predominately quartz dominated sediments in a fluvial system where would you find predominately have coarse sediment and a homogeneous grain size of median to fine grain? With this observation enforced, the possibility that Peachtree Rock was once a beach environment is very unlikely.


References Cited

Colquhoun , J. Donald., Huddlestun, F. Paul., Jr Nystrom, G. Paul and Burn, Jean 1985,

stratigraphy of Peachtree Rock Preserve, Southern Lexington Country, South Carolina. South Carolina

Geology (1985), V. 29, No. 1

Sunday, February 24, 2013
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