40 million years ago, the entire coastal plain of southeastern North America was below sea level. In Georgia sea shore occurred along a line that roughly corresponds with the latitudes of Columbus, Macon, and Augusta. Rich zones of zooplankton nourished near shore oyster beds populated by a species that grew up to 20 inches in length. Fossils of this extinct giant oyster ( Crassostrea gigantissima ) are exposed at many locations along the ancient shoreline wherever rivers or creeks erode into Eocene Age formations. Perhaps the best exposure can be found at Shell Bluff in Burke County, Georgia. This site is a 30 minute drive from my house, and I have long wanted to visit it, but alas it is private property not generally open to the public.
Location of Burke County, Georgia. Shellbluff is located on the eastern boundary by the river. A small local community is named after the site.
I couldn’t find a photo of Shellbluff that I could directly link to my blog, but the above linked Georgia Journal of Science article has a nice picture in the pdf file.
Old photo of the fossil oyster bed at Shell Bluff.
Look at the size of the extinct Crassostrea gigantissima. A chemical analysis of these giant oyster shells determined a cool shift in climate occurred during the late Eocene. Average winter sea surface temperatures were the same then as they are today, but summer sea surface temperatures were 3-12 degrees F cooler than those of today.
The bluff is 150 feet high and reportedly the giant oyster shell beds are 80-100 feet from the Savannah River. The soil near the bluff consists of limestone and sandy marl, and it is rich in calcium. Because of the unique microclimate and calcium-rich soil, there is a natural community quite different here from the surrounding fire-adapted longleaf pine/turkey oak sand hills. This natural community is known as a bluff forest with northern affinities or as some other botanists refer to it, a mesic slope forest. The steep slope and cooling river protect this forest from fire, and the north-northeast exposure helps keep temperatures cooler than in the surrounding terrain. Many of the plants growing here are disjunct populations of species more commonly found in the Appalachian Mountains or the Midwest. Species of northern affinities present at Shell Bluff include green violet, tall bellflower, wild ginger, black cohosh, ravine grass, and black walnut. The overstory consists of white oak, beech, pignut hickory, basswood, and black walnut. Dogwood, red buckeye, hop hornbeam, 2 species of pawpaw, beautyberry, Carolina buckthorn, and redbud comprise the midstory. 3 of these species–red buckeye, Carolina buckthorn, and red bud–are notable calciphiles (plants that prefer calcium rich soils). Some rare plants grow here too such as the Ocmulgeee skullcap.
Shellbluff is home to this rare mint–Ocmulgee Skullcap (Scutellaria ocmulgee).
William Bartram found mock orange (Philadelphus inodorous) growing at Shell Bluff in 1775.
John Bartram and his son, William, visited this site in 1765, and William returned 10 years later. They saw the forest before it was ever logged. The virgin timber consisted of white oaks, beech, and sweetgum with trunks that were 5 feet in diameter. Cypress trees were over 6 feet in diameter. There are probably few, if any, trees this large at the site today. Bartram included tupelo, tulip, and mulberry in his list of tree species here. I’m not sure, if these species still exist on the site since it has been logged. Other rare plants that Bartram cataloged may also be extirpated from the site including mock orange, leather wood, Carolina spice bush, and ginseng.
Bluff forests with northern affinities are relic habitats that represent natural communities formerly more widespread in the surrounding region. Oak and beech forests with cool climate associates likely formed a more continuous range throughout the mid to deep south during cool moist interstadials. (Though interstadials were warm phases of climate within Ice Ages, average temperatures were still cooler than those of the present day.) But these mesic forests also waned during arid cold stadials when grasslands and scrub habitat expanded. River bluffs have provided refuge for this type of forest during both hot and cold extreme shifts in climate, probably for millions of years.
“Shell Bluff–A Fossiliferous Ridge, The Site of the Extinct Oyster Crassostrea gigantissima and History of its Identification”
Georgia Journal of Science 74 (2) 2016