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 Dry Branch Kaolin Company, J.M. Huber Company, and a number of 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, AL, Florence, SC, and Greenville, NC, 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.
Why These Regions Exist
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.
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.