Dupont Corporation’s plan to mine titanium from heavy mineral sands in Trail Ridge, which borders the eastern boundary of the Okefenokee Swamp, sparked a crash course study of the geology and ecology of the Okefenokee Swamp. Scientists and environmentalists feared the impact of the mine could have seriously degraded the swamp. The potential environmental degradation was so feared, that even conservative politicians opposed the project, and ten years ago Dupont agreed to abandon the proposal and sell the land to the state and the Nature Conservancy. But the resulting research papers provided a wonderful source of knowledge and excellent fodder for my blog.
The Geological History of the Okefenokee Swamp Basin
The existence of the Okefenokee basin is due to many different geological factors. It’s located in a region known as the southeast Georgia embayment. This area was submerged under the Atlantic Ocean during the Cretaceous and for most of the Eocene. During the Eocene, sediment intermittently began to be deposited, but late in the Eocene, strong ocean currents are believed to have washed these away. U.S. Geological Survey bore holes near the Okefenokee Swamp reach late Miocene strata about 65-70 feet below the surface. This strata consists of limestone pitted with the impressions of fossil sea shells–evidence that as recently as 5 million years ago, the southeast Georgia embayment was still periodically inundated with sea water. Above this limestone layer is an impermeable clay of Pliocene age. Here we find two reasons for the Okefenokee basin’s existence: the basin sagged because rainwater caused the limestone bedrock to dissolve (albeit unevenly) in a process known as karstification, and the clay layer above this prevents water from draining, thus providing the optimum conditions for wetlands to develop within the basin.
Hardwood hammocks, the islands within the swamp, are elevated above the basin. These were formed during the late Miocene/early Pliocene when the area was still under sea water. Ocean currents and river sedimentation built these little islands. Later, when the basin emerged above sea level, the islands remained in place as dry land humps in the swamp.
The southeast Georgia embayment emerged above sea level during the Pliocene, and the resulting geological uplift caused the Suwanee River to backflow and reverse course, providing the source of all the fresh water accumulated there.
Map of Georgia’s Pliocene shoreline. The present day location of the Okefenokee Swamp was submerged under the Atlantic Ocean. The Suwannee River flowed from west to east. After the emergence of the Okefenokee Swamp basin above sea level, geological uplift caused the Suwanee River to back flow into the basin and reversed its course, so that it flows west instead of east. Note the islands which later became hardwood hammocks. Illustration from ‘The Geological and Natural History of the Okefenokee Swamp and Trail Ridge’ by Rich and Bishop.
Some scientists hypothesized that the early Okefenokee Swamp basin was a massive estuary and salt marsh, but there’s no evidence of this. Instead, evidence from the nearby Trail Ridge to the east of the swamp suggests the basin has been a freshwater swamp since early in its existence. Trail Ridge was originally a massive barrier island 130 miles long which is far longer than any current barrier island off the Atlantic Coast, the longest in Georgia being Cumberland Island which is a mere 16 miles long. Cape Hatteras off the North Carolina Coast is only 48 miles long. Trail Ridge runs from south Georgia to North Florida as the following map will show. This geological feature too contributes to the back flow of water into the swamp but is considered a minor influence compared to geological uplift, limestone sinkage, and the impermeable clay layer. Scientists have discovered fossil freshwater swamp vegetation buried under Trail Ridge sands–evidence that a swamp already existed in the area late in the Pliocene when the barrier island that is now Trail Ridge first formed during a high stand of the Atlantic Ocean.
Trail Ridge is the oldest terrace on Georgia’s coastal plain. Terraces run parallel to the Atlantic Ocean and were all formerly barrier islands that formed during warm interglacials or interstadials when sea level was higher than that of today’s. Logically, the oldest terraces are further west because subsequent high stands of water would have washed younger ones away.
Map of Pleistocene terraces on the Georgia coastal plain. Terraces were barrier islands formed during warm climate cycles when sea level was higher than that of today. From west to east they are aged from oldest to youngest. Scientists name these terraces. The following is a list of the terraces and their estimated ages. (Note: scientists don’t always agree with these age estimates. I’m using those from a college thesis that I list in my references.) The Wicomicico Shoreline (Trail Ridge)–1.8 million years; the Penholoway Shoreline–780,000 years; the Talbot shoreline–240,000 years; the Pamlico shoreline–130,000 years; the Princess Anne shoreline–80,000 years; and the Silver Bluff shoreline–40,000 years. The Talbot shoreline formed during the Yarmouthian interglacial. The Pamlico shoreline formed during the Sangamonian Interglacial. The Princess Anne shoreline formed during the early Wisconsin Ice Age when sea level was dropping but paused during an interstadial. The Silver Bluff shoreline also formed during a pause in sea level drop caused by an interstadial. Eventually, during glacial cycles, the shoreline could be located as much as 50 miles to the east of where it is today due to a lowering of sea level when much of the planet’s water was locked in ice. Pollen and fossil wood has been recovered from Grays Reef, 11 miles off Sapelo Island, showing that forest and prairie grew in areas deep under water today. It’s interesting to note that the Silver Bluff and Princess Anne shorelines formed when ice core data suggest temperatures were cooler than those of today. I think this shows that a certain threshold must be met before sea level fell drastically during glacial cycles. The sea level had been falling during the Wisconsin Ice Age but paused when the climate warmed up, even though it wasn’t as warm as that of today. It also shows that sea level lags behind climate change by thousands of years.
The Ecological History of the Okefenokee Swamp Basin
The Okefenokee Swamp apparently has existed since the late Pliocene, but it has been an intermittent existence. During the Pleistocene whenever there was a cycle of glacial expansion, the climate became cool and arid. Sea level retreated many miles to the east and the water table fell, causing the Okefenokee Swamp to become a relatively dry basin. Instead of hosting primarily wetland vegetation, oaks and pines and grasses grew in the basin. Wetland species such as cypress, tupelo, and water lily, still existed but were relegated to small scale marshes bordering the rivers and streams that still incised the basin.
Some time during a glacial cycle, wind and remaining water eroded 4 elliptically shaped Carolina Bays into the swamp basin. The bays, along with the remaining rivers, provided relic habitat that allowed wetland plants to rapidly recolonize the basin when climatic conditions once again became favorable to swamp development. Scientist have taken cores of the peat in the swamp and discovered that the peat in today’s Okefenokee has only been accumulating for 6600 years. It’s likely the Okefenokee basin was dryland habitat from about 36,000 years BP to about 7,000 years BP. The last glacial maximum, the coldest and dryest climate cycle of the late Pleistocene, lasted from 28,000 BP-15,000 BP. So it took thousands of years for the water table to rise to swamp level again, following the drying out of the swamp during the Ice Age.
Rich, Frederick, and Gale Bishop
Geology and Natural History of the Okefenokee Swamp and Trail Ridge, southeastern Georgia and northern Florida.
Georgia Geological Society Field Guides 1998