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 formations, 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 abundant silicified 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 environment. The reasoning for this hypothesis is the effect of the cross beds that is similar to the offshore bar on the coastline of South Carolina. Secondly, the orientation of the crossbeds is in both directions, which are similar to beachfronts. (In beachfronts, you have cutting, filling and scouring in the bedding planes) Below the bedding planes, there is sharp contact between the bedding planes and units, which are called barrows, that varies in intensity as the formation increases in height. Other explanations that this could have been a beach environment are that fluvial systems is 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 information, we can definitely work towards the idea that Peachtree Rock is the result of a depositional environment. (i.e. channel, beach, river or a offshore shove) If this were an offshore shove, one would expect the sediments to be worked over by burrowing organisms. See picture below.

Knowing this bit of information, one can now classify this environment to have been offshore. Or the environment could have been just within waved based where the waves are breaking offshore. If an observer were to look upwards, (s) he would find a different group of Sedimentary Structures if (s) he was looking at the source sequence. If (s) he was looking at the far shoreline, we would probably find clays. If you go to the area at the top of breakpoint bars you would find shells and flat laying inclined sand with a lowered inclination, 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