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Georgia Geology

Introduction

Georgia consists of four distinct geologic regions, so it makes the most sense first to talk about those four regions separately, and then to see how they fit together. From northwest to southeast, those four regions are the Valley and Ridge, The Blue Ridge, the Piedmont, and the Coastal Plain. All of these geologic regions extend into the surrounding states, but Georgia is the only state south of Virginia to have all four regions.

The Valley and Ridge

The Valley and Ridge consists of Paleozoic sedimentary rocks that have been folded and faulted to cause long northeast-southwest-trending valleys and ridges that give the region its name. The faults are all thrust faults at which sheets of limestone, sandstone, and shale have been pushed northwestward on top of each other.

The strata of the Valley and Ridge include numerous carbonate units, such as the Cambro-Ordovician Knox Dolostone and the Ordovician Chickamauga Limestone, and thus caves and karst terrane exist across large parts of the region. In the far northwest corner of the state, Pennsylvanian siliciclastics underlie what some would consider the Cumberland Plateau, but the continuity of lithology and deformation allow one to include the entire region in the Valley and Ridge.

Geologic resources of the Valley and Ridge include construction-grade limestone, which is quarried by such companies as Vulcan Materials. Barite and ochre have been mined from the Lower Cambrian Shady Dolomite near Cartersville. Coal was once mined from Pennsylvanian strata in far northwest Georgia too. From a collector's perspective, the fossil-collecting localities of the Valley and Ridge are another geologic resource.

The Valley and Ridge extends southwest into Alabama to around Birmingham and northeast in Tennessee from Chattanooga to Knoxville and beyond. In Georgia, the eastern and southern boundary of the Valley and Ridge is the Cartersville-Great Valley fault system, which runs south from Chatsworth to a point southeast of Cartersville and then west to the Polk-Haralson County line. Across this major fault system, metamorphic grade increases into the Blue Ridge and/or Piedmont.

The Blue Ridge

The Blue Ridge is a region of low-to-high-grade metamorphic rocks. Many of the rocks of the Blue Ridge appear to be the metamorphosed equivalents of Proterozoic and/or Paleozoic sedimentary rocks. Others are metamorphosed igneous rocks, such as the Corbin Metagranite, the Fort Mountain Gneiss, various mafic and ultramafic rocks, and the metavolcanic rocks of the Gold Belt.

The Blue Ridge region forms the North Georgia Mountains or the southern Appalachians. Some of the highest points are balds like Brasstown Bald. The topography is not as patterned as that in the Valley and Ridge, and drainage systems are generally dendritic, whereas they are linear in the Valley and Ridge.

Geologic resources of the Blue Ridge presently include marble, much of which is mined by Georgia Marble Company. Talc has been mined in the western Blue Ridge just east of Chatsworth. Gold was mined at Dahlonega in the early 1800's, and in fact a U.S. mint produced gold coins there from 1830 to 1861. The North Georgia gold rush of the 1830's precipitated the eviction of the Cherokee and their forced migration on the Trail of Tears.

Georgia is the southwest end of the Blue Ridge, which extends northeast to Virginia through Great Smoky Mountain and Shenandoah National Parks. The southern boundary of the Blue Ridge in Georgia depends on one's perspective. A purely topographic approach would limit the Blue Ridge to just a few ridges extending southwestward from North Carolina, so that the Piedmont would extend all the way to the Georgia-Tennessee state line. Some geologists, in contrast, would extend the Blue Ridge region all the way to the Brevard Fault zone, which runs through northwest Atlanta and Gainesville. One of the most commonly accepted boundaries, which is based on changes in rock types, would run just southeast of Canton, Dawsonville, Dahlonega, and Helen.

The Piedmont

The Piedmont is a region of moderate-to-high-grade metamorphic rocks , such as schists, amphibolites, gneisses and migmatites, and igneous rocks like granite. Topographically, the Piedmont mostly consists of rolling hills, although faulting has produced the impressive ridge of Pine Mountain near Warm Springs. Isolated granitic plutons also rise above the Piedmont landscape to give prominent features like Stone Mountain.

One major feature cutting across the Piedmont (as defined here) is the Brevard Fault zone. The Brevard Fault Zone runs from northeast to southwest and passes through Centralhatchee in Heard County, northwest Atlanta, Duluth, Buford, and Gainesville before leaving Georgia at the westernmost point on the Tugaloo River in northernmost Stephens County. The Chattahoochee River follows the Brevard Zone too. However, the regional extent of the Brevard Zone is reflected by the fact that it is named after the town of Brevard, North Carolina. The Brevard Zone has been interpreted as a variety of different kinds of faults or discontinuities, and its true nature remains enigmatic.

Piedmont soils are commonly a red color for which Georgia is famous. Those soils consist of kaolinite and halloysite (1:1 aluminosilicate clay minerals) and of iron oxides. They result from the intense weathering of feldspar-rich igneous and metamorphic rocks. This intense weathering dissolves or alters nearly all minerals and leaves behind a residue of aluminum-bearing clays and iron-bearing iron oxides because of the low solubilities of aluminum and iron at earth-surface conditions. Those iron oxides give the red color to the clay-rich soil, yielding the red clay that has come to be almost synonymous with central Georgia, and the abundance of clay has contributed to a tradition of folk pottery in central and north Georgia.

Mineral resources of the Piedmont include hard crushed stone, which is quarried by such companies as Vulcan Materials. Granite has long been quarried for tombstones and other monuments in the eastern Piedmont near Elberton, and it was once quarried from the Stone Mountain granite at Stone Mountain Park. Soapstone was mined by Native Americans in southwestern Dekalb County at Soapstone Ridge. One well-known kyanite mine in the Piedmont was at Grave's Mountain. Groundwater in the Piedmont largely flows along faults and fractures, making it difficult to find but often locally abundant.

The granitic rocks of the Piedmont make radon a potential concern in the region. The USGS map of geologic radon potential shows the Piedmont, as well as the Blue Ridge, as a region of "moderate" radon potential, whereas that potential is "low" in the Valley and Ridge and Coastal Plain.

Athens and Atlanta are two cities in the Georgia Piedmont. The Piedmont extends a little bit westward into Alabama before it pinches out between the Valley and Ridge and the Coastal Plain. To the northeast, it cuts a broad swath across South Carolina, North Carolina, and Virginia. Spartanburg, South Carolina, and Greensboro and Winston-Salem, North Carolina, are Piedmont cities to the northeast of Georgia.

The Fall Line

The Fall Line is the boundary between the Piedmont and the Coastal Plain. Its name arises from the occurrence of waterfalls and rapids that are the inland barriers to navigation on Georgia's major rivers. Thus the cities of Columbus, Macon, Milledgeville, and Augusta developed where boats had to be unloaded on the Chatahoochee, Ocmulgee, Oconee, and Savannah Rivers, respectively. Those waterfalls and rapids occur where the rivers drop off the hard crystalline rocks of the Piedmont onto the more readily eroded sedimentary rocks of the Coastal Plain. The Georgia Department of Transportation intends to link Columbus, Macon, Milledgeville, and Augusta with the Fall Line Freeway (Georgia 540) someday. Other Fall Line cities include Tuscaloosa, Alabama, Columbia, South Carolina, and Richmond, Virginia.

The Fall Line is a boundary of bedrock geology, but it can also be recognized from stream geomorphology. Upstream from the Fall Line, rivers and streams typically have very small floodplains, if they have any at all, and they do not have well-developed meanders (tight bends in the river's course). Within a mile or so downstream from the Fall Line, rivers and streams typically have floodplains or marshes across which they flow, and within three or four miles they meander. The most pronounced example is in the Savannah River's course at Augusta, but the same change can be seen in Brier Creek, the Ogeechee River, Buffalo Creek, the Oconee River, the Ocmulgee River, Echeconnee Creek, the Flint River, Upatoi Creek, and the Chattahoochee River from east to west across Georgia.

The Coastal Plain

The Coastal Plain is a region of Cretaceous and Cenozoic sedimentary rocks and sediments. These strata dip toward the southeast, and so they are younger nearer the coast. At least near the Fall Line, they are ultimately underlain by igneous and metamorphic rocks like those of the Piedmont. The sedimentary rocks of the Coastal Plain partly consist of sediment eroded from the Piedmont over the last 100 million years or so, and partly of limestones generated by marine organisms and processes at sea. One could crudely generalize that buried Triassic rocks in the subsurface are various rift-basin siliciclastics, the Cretaceous strata are sandstones and shales, the Tertiary strata are limestones and shales, and that the Quaternary strata are sands and muds. Tektites, the glassy products of meteorite impacts, are a minute proportion but a historically quite significant component of the Tertiary strata of the Coastal Plain.

The lower Coastal Plain consists of a series of Quaternary beach complexes that parallel the modern coast and are younger nearer the coast. These beach complexes make subtle ridges like Trail Ridge, which is the crest of the Wicomio beach complex; the somewhat aptly named town of Mount Pleasant sits atop the Talbot beach complex. The Ogeechee and Altamaha Rivers break through these ridges and go directly to the sea, but the Saltilla River makes a 20-mile dogleg where it is trapped between the Penholoway and Talbot beach complexes before it breaks through the latter to the sea. The Saint Mary's River makes an even larger dogleg in the opposite direction. As a result, there is a large poorly drained area behind Trail Ridge in Charlton and Ware Counties. That area is the Okefenokee Swamp, home of the Okefenokee National Wildlife Refuge .

The modern beach consists largely of white quartz sand, but it also has dark-colored concentrations or placers of dense minerals (or so-called "heavy minerals"). The same is true of the older beach ridges inland, and those dense minerals include titanium-rich minerals like rutile, ilmenite, and sphene. As reported in the Atlanta Journal-Consititution (Jan. 1, 1998, p. A16), The Dupont Company owns or leases 38,000 acres along Trail Ridge on the east side of the Okefenokee Swamp, and Dupont wants to start a 50-year project to mine that acreage for titanium ore. This has raised considerable concern about the Okefenokee environment because the swamp is obviously sensitve to hydrology and because mining in the sands of Trail Ridge would be likely to disrupt the hydrologic setting.

The most economically significant mineral resource of the Coastal Plain is kaolin, a clay-rich rock that is mined in pits near the Fall Line. Kaolinite, the dominant mineral in kaolins, is used in a variety of industries from pharmaceuticals to paper. Kaolin is mined by such companies as the Imerys, J.M. Huber Corporation, and several other companies.

Limestone is quarried in southwest Georgia. However, its quality as aggregate is not as high as that of the limestone in the Valley and Ridge. The reasons lie largely in the greater porosity of the relatively Coastal Plain limestones, whereas the older limestones of the Valley and Ridge have lost nearly all of their fine-scale porosity.

Wells have been drilled for petroleum in the Coastal Plain, but a scarcity of petroleum-generating source rocks seems to have caused a dearth of oil and gas. Petroleum exploration nonetheless continues, if slowly, in the Georgia Coastal Plain. For example, in late October 1996, the Atlanta Journal-Constitution reported that two wells were being drilled in Dooly and Crawford Counties, and that permits had been issued for a 16,000 foot well in Turner County. The latter was to be drilled by Surface Exploration Resources of Dallas, Texas, and if it reached its permitted depth it would be the deepest well drilled in Georgia.

A major geologic resource in the Coastal Plain is groundwater. The less porous rocks of the other regions of Georgia provide less groundwater, but the aquifers of the Coastal Plain provide groundwater for domestic consumption, for industry, and for agricultural irrigation. The USGS actively monitors Ground-Water Conditions in Georgia.

Geologic hazards in the Coastal Plain include sinkholes and coastal erosion. Sinkholes can form in areas of limestone bedrock when subsurface dissolution of rock leads to collapse of the earth surface. Some examples exist in the region around Albany. On the Georgia coast, movement of sand southward along the barrier islands means that parts of the islands, especially their northern ends, are prone to erosion. On the other hand, transport of sand commonly allows the southern ends of islands to grow as the channels between the islands slowly migrate south.

One spectacular example of inland erosion of the Coastal Plain is at Providence Canyon State Park south of Columbus. Farming began in that region in about the 1820s, and by the late 1800s erosion of the poorly managed soil had turned gullys into ravines and then into canyons. The canyons are as much as 150 feet deep and are impressively steep-sided, so that hikers in Providence Canyon have just one way in and out.

The Coastal Plain is the southern or southeastern half of Georgia, and Americus, Albany, Valdosta, Waycross, and Savannah are all Coastal Plain cities. Montgomery, Alabama, Florence, South Carolina, and Greenville, North Carolina, are inland cities in the Coastal Plain, which extends all the way to New Jersey.

Beyond the Georgia coastline is the Continental Shelf, a gently sloping region of seafloor that is underlain by more sediments and sedimentary rocks. The shelf extends about 180 miles beyond the shoreline before depths exceed 200 meters. One feature on the shelf about 20 miles east of Sapelo Island is Gray's Reef, a sponge-coral reef that is now a National Marine Sanctuary.

Geologic History of Georgia

The position of these regions is best understood in terms of the plate-tectonic collision of continents that built the Appalachians about 300 million years ago. When North America and Africa collided then to make the Pangaean supercontinent, the compression where they met shoved sheets of sedimentary rock over each other to make the Valley and Ridge. Further southeast, in the heart of the collision, intense metamorphism and melting deeper in the earth generated the metamorphic and igneous rocks of the Piedmont. Less intense metamorphism combined with thrusting generated the Blue Ridge region. The rocks of the Piedmont and Blue Ridge were deeply buried in the collision, but erosion of the overlying mountain mass has subsequently exposed those rocks today.

About 220 million years ago, Pangaea broke apart as the Atlantic Ocean began to open. This rifting of the Pangaean supercontinent left a new continental margin - a new edge of a continent - in the Southeast. Subsequent accumulation of sediments on that new eastern margin of North America has generated the Coastal Plain. The upper Coastal Plain is now being eroded, and the presence of remnants of Coastal Plain strata on the southernmost Piedmont suggests that the Fall Line has migrated southward and will continue to do so.

The Four Geologic Regions and Their Highways

The regions about which we've been talking are defined by their geology, but they can almost be recognized from a highway map. Roads and highways in the Valley and Ridge largely run northeast-southwest in the valleys, with only occasional links over gaps in the ridges. That same pattern exists in Valley and Ridge all the way to Virginia. In the Blue Ridge, roads have a more random geometric pattern, but they often follow rivers and streams to achieve the lowest passage through the mountains. The opposite is true of at least minor roads in the Piedmont, where roads follow the flat ridgetops between stream valleys so that the number of bridges is minimized. In the Coastal Plain, and especially in the lower Coastal Plain, highways are often strikingly straight as they traverse the flat landscape. The roads radiating from Jesup, Waycross, Pearson, Valdosta, Thomasville, Bainbridge, and Colquitt provide fine examples.

Georgia Highway 17 is a good example of the above changes as it makes its way from Savannah to Hiawassee. In the Coastal Plain, it has a perfectly straight interval 12.8 miles long between Guyton and Oliver in nortwest Effingham County, another 8.5 miles long north of Wrens across the Jefferson-Warren county line, and other shorter straight intervals in between. In the Piedmont, Highway 17 follows ridges and skirts the headwaters of streams, and it does not cross a single permanent stream for about 45 miles from Falling Creek southeast of Elberton through Bowman, Royston, Lavonia, and Martin to Eastanolle Creek southeast of Toccoa. In the Blue Ridge, Highway 17 follows the Chattahoochee River and Spoilcane Creek north from Helen until it crosses the Tennessee Valley Divide, and then it comes down along the Hiawassee River on its way to the Tennessee state line. On the west side of the state, Georgia Highway 1 is another good example as it works its way southwest through valleys from Lafayette to Summerville and from Rome to Cedartown in the Valley and Ridge, as it follows flat ridges from Buchanan through Carrollton to Centralhatchee in the Piedmont, and with its first straightaway between Columbus and Cusseta as it enters the Coastal Plain.

Major Geologic Hazards

The lack of major geologic hazards like volcanoes and earthquakes in Georgia results from Georgia's location on a rifted passive margin of a continent, where there is a stable transition from the continental crust of North America to the ocean crust of the Atlantic. That's in contrast to the west coast of the North America and South America, where oceanic and continental crust move together to cause earthquakes and, in some places, to generate volcanoes. Georgia's main risk of earthquakes may be from distant epicenters, such as that of the Charleston earthquake of 1886 or of the New Madrid, Missouri, earthquakes of 1811. However, much smaller earthquakes are a regular occurrence in Georgia.

Additional Resources

Further Reading

Geological Surveys

Earth Science Departments

Geological Societies

Disclaimers

This page has links to some commercial pages. Links to those pages do not constitute an endorsement of the products of the companies sponsoring those pages.

No information in this document should be construed as advice for making economic or other decisions. None of the statements in this document should be construed as official opinions of the Department of Geology of the University of Georgia, of the University of Georgia, or of the State of Georgia. Consult a specialist in your area before using geological information to make decisions about investment, development, personal safety, or environmental safety.

Comments or suggestions? Please contact Bruce Railsback at rlsbk@gly.uga.edu.

Saturday, September 04, 2010 03:47 PM EDT