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Throughout the world, geology built the stage on which archaeology and history have played out. In Georgia, there is a geologic story behind the locations of the largest cities (including Atlanta) and most of the important archaeological sites. The discovery of gold on Cherokee lands led to America’s first popular rush for gold, and to the notorious relocation of that people to Oklahoma. Several Civil War battles in Georgia owe their location and, it has been argued, their outcome, to geological factors. The location of mineral resources such as kaolin, as well as the fertility of soils based partly on rock type, both factor into differences in wealth that have shaped Georgia politics. Many Georgia buildings and monuments are built of (or decorated with) stone, and stone from Georgia has found its way into famous structures everywhere. The Roadside Geology of Georgia authors learned and wrote about these connections. We especially enjoyed using the New Georgia Encyclopedia and the Longstreet Highroad Guides as references. While most of the material will be found in the book, some details were omitted from the book as it was edited for length. As time permits we will publish some of that material at this web site as vignettes.
The Fall Line: Geology, Archaeology, History Geology literally splits Georgia in two along the Fall Line. Geologically, the Fall Line is the surface trace of an unconformity (not a fault, as it is sometimes misidentified). This is a place where younger strata have been deposited on rock that formed at a much earlier time. At the Fall Line, Coastal Plain sediments that accumulated near an Atlantic seashore (when sea level was much higher than today), from around 70 to 50 million years ago, rest on deeply eroded Piedmont metamorphic and igneous rocks that formed many miles below the surface, more than 200 million years earlier.
Streams coming from the Piedmont descend over a few miles of rapids and low waterfalls as they cross the last of the hard rock onto the loose Coastal Plain sediments. People navigating the rivers from the coast had to make portage at these rapids. This led to settlement by Indians (including the mound builders whose work is preserved in Ocmulgee National Monument at Macon). Later Columbus, Macon, Milledgeville, and Augusta were founded on the Chattahoochee, Ocmulgee, Oconee, and Savannah River portages, respectively, for the same reason. The availability of waterpower from the Fall Line helped these settlements grow into cities. Sometimes the Fall Line is referred to as the Fall Zone, because, as the map shows, the edge of Coastal Plain sediments wanders back and forth over a band about 20 miles wide. On any given river, though, the transition from hard rock to soft sediments is sharply defined, and the rapids owing to the transition extend over just a few miles. Another Important Boundary: Faults of the Blue Ridge front: Another major geologic boundary is the northwest edge of the Blue Ridge/Piedmont metamorphic rocks. For travelers along I-75 interested in archaeology, there is a mirror image here to the position of the Ocmulgee Mounds just outside the Blue Ridge-Piedmont at Macon: Etowah Mounds. This edge of the Blue Ridge/Piedmont is the surface trace of a long-dead fault, or a series of faults, which once carried Blue Ridge/Piedmont rocks more than 100 miles to the northwest over sedimentary rocks. Limestone and shale northwest of the fault are far less resistant to erosion, so a Great Valley runs along them. (Ridges and mountains to the northwest of the Great Valley are made of mostly of sandstone, a sedimentary rock which resists erosion well.) The Great Valley forms an easy path from Virginia to Alabama. At Cartersville (and nearby Etowah Mounds) is its closest approach to the Atlantic seaboard. Both I-75 and the 1840s’ Western and Atlantic railroad (now CSX) take advantage of this geography. Atlanta was founded where the Western and Atlantic intersected the Georgia Railroad, built westward from Augusta close to present I-20. Atlanta sits astride the Eastern Continental Divide (streams to the southeast flow to the Atlantic; those to the northwest flow to the Gulf of Mexico). Segments of the railroads followed that feature to minimize grade changes and stream crossings. Historical Vignettes Narratives connecting Georgia’s geology to history and archaeology, which had to be left out of Roadside Geology of Georgia for reasons of space, will be placed here as time permits.