The Geology of Georgia - Part Four of a Very Occasional Series

The Potato Creek and Rock Hawk Effigy

The metamorphic rocks of the Georgia Piedmont consist of two enormous stacks of folded thrust sheets. The upper stack, referred to as the Little River thrust stack, occurs to the south of the lower thrust stack, the Georgiabama, and underlies much of the Piedmont near the Fall Line.  The Little River thrust stack has been divided into the Macon Complex, the Little River Complex, and the Northern Florida platform sequence. 

The Macon Complex is a tectonic mélange that underlies an area more than 300 miles long and 60 miles wide. As such, the Macon Complex represents the largest mélange known in the Appalachians and one of the largest Paleozoic mélanges known in the world, comparable in size and complexity with the enormous Franciscan mélange of coastal California and Oregon.

Much of the Macon Complex is a chaotic mixture of well rounded to angular clasts to slabs of contrasting rock types “floating” in a highly deformed matrix.  The complex was subsequently intruded by Devonian and Carboniferous granitic plutons, such as the Siloam and Elberton granites. Much of the remainder of the complex consists of metapelitic rocks and metagraywacke.

Mike Higgins identified three separate units within the Macon Complex: the Juliette, the Po Biddy, and the Falls Lake mélanges.  The Juliette mélange includes a body of rock Higgins called the Potato Creek facies.  The Potato Creek is characterized by an abundance of clastic rocks and the local presence of thin tuffaceous metacherts, and contains clasts ranging in size from small fragments seen in thin sections to mile-long slabs, and ranging in shape from well rounded to angular. The matrix of the Potato Creek is essentially an imbricate complex of the following:

1. Massive (but broken), coarse-grained, feldspathic, biotitic metagraywacke
2. Metamorphosed pebbly mudstones
3. Semi-schists with thin, broken metagraywacke beds
4. Thin tuffaceous metacherts interbedded with scaly schist

The Macon Complex apparently developed on the oceanward side of an ancient island arc off the coast of  the proto–North American continent. Higgins suggested that many of the rock bodies in the Macon Complex are clasts of oceanic crust and mantle off-scraped and incorporated into the mélange as proto–North America collided with a continent on the opposite side of the proto–Atlantic Ocean. The tuffaceous metacherts and scaly schists of the Potato Creek (number 4 above) were probably deep-sea sediments initially deposited onto the ocean floor and later off-scraped and incorporated into the mélange along with trench sediments. The metagraywackes (1 and 3 above) and associated metamorphosed pebbly mudstones (2 above) probably represent clastic continental sediments deposited on top of the mélange wedge and imbricated into it.

Geologic evidence indicates that many of the rocks in the Juliette mélange were emplaced during very latest Precambrian to early Late Cambrian time. Early Devonian metamorphism represents the time of collision and of thrusting of the rocks to their present positions on the Southern Piedmont.  The Siloam and Elberton granitic plutons subsequently intruded the rocks later in the Devonian and during the Carboniferous.

The Rock Hawk Effigy is an archaeological site in Putnam County, Georgia, within the area underlain by the Potato Creek.  The Rock Hawk Effigy consists of thousands of pieces of quartzite laid in the shape of a large bird. Although it is most often referred to as a hawk, scholars do not know exactly what type of bird the original builders intended to portray. Only two such effigy mounds have been found east of the Mississippi River. The other, known as Rock Eagle, is also located in Putnam County, approximately thirteen miles to the northwest. The Rock Hawk site is located along Wallace Dam Road, off State Route 16, several miles east of Eatonton, near the shores of Lake Oconee.

Current archaeology suggests that the site was built between 1,000 and 3,000 years ago by Woodland Indians. These Native Americans may have been part of the Adena or Hopewell cultures, although it is more likely that they represented a unique group.