Shell Bluff, Burke County, Georgia

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.

http://digitalcommons.gaacademy.org/cgi/viewcontent.cgi?article=1012&context=gjs

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.

Reference:

Edwards, Elliott

“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

 

 

Advertisement

Beech-Magnolia Climax Forests–Another Relic Environment of the Pleistocene?

In the piedmont and coastal plain of Georgia and other southern states, bluff forests of northern affinities and bluff and slope forests both occur as relics in areas that are too steep to cultivate and too moist to burn.  The species composition of both  forest types would probably be more widespread, if not for the influence of man.  It is a safe assumption that both forest types were common across Pleistocene landscapes, especially during interglacials, interstadials, and even the weak stadials that preceded the Last Glacial Maximum, though during stadials, prairie and coniferous woods were more predominant.

I’m lumping bluff forests of northern affinities together with bluff and slope forests because they are so similar–the only distinction is that the former holds a greater variety of northern species of plants, including disjunct populations of some.  The dominant plants in a bluff forest consist of tulip, hickory, black walnut, beech, sweetgum, basswood, oaks (swamp chestnut, red, black, cherrybark), southern sugar maple, cottonwood, umbrella magnolia, red buckeye, mulberry, pawpaw, hornbeam, mayapple, maidenhair fern, Canadian ginger, and bluebell.

Bluebells (Canpanula americana) blooming in the forest undergrowth.  Disjunct populations of bluebells occur in bluff forests of northern affinities in the deep south.  The southern Pleistocene landscapes must have included scenes like this during phases of  equible climates.

Surprisingly, mountain laurel grows as far south as the piedmont in bluff forests with northern affinities.

These lush forests consisting of cool climate species such as beech and warm climate species such as magnolia were probably the most common climax forests during much of the Pleistocene, particularly when climate phases were more equible than they are today with much cooler summers, but only slightly cooler winters.  The only important missing floral component of this type of forest that was prevalent during the Pleistocene was the extinct Critchfield’s spruce.

Today, both types of bluff forests now only occur on land that slopes toward a river and faces north.  These conditions create a cool microclimate.  Moreover, feeder creeks and seepage springs protect the mature woods from frequent fire.  Before Indians began their annual burning of forests to improve conditions for game, beech-magnolia climax woods probably  occupied land that was eventually transformed into open pine savannah.  Pollen records show that early in the Holocene about 10,000 calender years BP, hardwoods were more prevalent than pine, even in south Georgia.  After the Last Glacial Maximum and into the early Holocene (~15,000 BP-~10,000 BP) beech was much more common in the south than it is today.  So for much of the Pleistocene prior to man, primeval forests on moist sites resembled the bluff forests that now can only be found in a few sites along the Savannah, Altamaha, Chattahoochee, and Appalachiacola Rivers.  Bluff forests at the latter river hold the southernmost population of American beech which grows alongside southern species such as magnolia, cypress, and palm trees.  Other examples of piedmont and coastal plain bluff forests are Magnolia Bluff in Camden County, the Alapaha Bluff, Springhill Plantation, William Bluff Preserve, Altamaha Bluff, and Shell Bluff in Burke County.

Beech-Magnolia climax forest at the Tall Timbers Research Station near Tallahassee, Florida.  Reportedly, this tract is adjacent to long-leaf pine savannahs.

Though the biomass of large mammal species in Beech-Magnolia climax forests of the Pleistocene wasn’t as great as that of Ice Age grasslands in the south, diversity was high.  Ice Age grasslands, which probably occurred adjacent to hardwood hammocks, held large herds of mammoths, bison, horses, llamas, and elk.  But during interglacials and interstadials  when primeval climax forests were more widespread than grasslands, gaps within them supported smaller herds of grazers as well as the beasts of the forest.  White-tailed deer, long-nosed peccary, tapirs, Jefferson’s ground sloth, mastodons, and bears all lived in these truly virgin forests where the mast from giant mature trees must have been exceedingly abundant.  The forests were the haunt of saber-tooth, jaguar, and dire wolf.  The birdlife of today’s world is likely the tiniest of remnants compared to that of the original unspoiled environments because every species living had taken millions of years adapting to it.  American birds have only had a few thousand years to evolve to survive in anthropogenic environments, and recent environmental change is occurring at an even more accelerated rate.

Common mammal species in South American forests.  A similar composition of animals probably occurred in Pleistocene beech-magnolia forests, though the species differed–long-nosed peccary instead of collared, white-tailed deer instead of brocket, Vero tapir instead of the extant South American species.  There were no pacas and agoutis but other unusual species such as Jefferson’s ground sloth and mastodons lived there.

The closest bluff forest near my house is Shell Bluff, named for the Eocene-age fossil oyster shells found there.  Access to Shell Bluff is either a long hike through private property or a boat ride down the river.  I didn’t think a trip to see it was feasible for me.  However, Dr. Charles Wharton wrote that nearby Griffin’s Landing is an even better example of a bluff forest.  I almost explored this area last summer, but it is next to a nuclear plant, and guards from Plant Vogtle close the gate to Griffin’s Landing at dark. I didn’t want to get stuck overnight behind the gate, but now that I know a bluff forest exists there along with fossil oysters, it may be worth a day trip later this year.