Most motorists driving over the Orangeburg Escarpment probably don’t even notice the gentle decline in elevation that marks the boundary between the upper and lower coastal plain. Most would be surprised to learn this gradual slope is the remnant of scenic sea cliffs that stretched from Jesup, Georgia to Fayetteville, North Carolina. Today, the elevation along the Orangeburg Escarpment lowers from 50-100 feet over a distance of 1-2 miles, but 3.5 million years ago it was a long chain of limestone cliffs, facing ocean waves. These cliffs ranged from 180-250 feet high with the higher cliffs on the northern end. Over time, the ocean has receded to the east and erosion has reduced the cliffs to a barely noticeable slope.
The lower figure shows where the Orangeburg Escarpment is located. They are named after a small town in South Carolina. South of this long chain of sea cliffs was a barrier island that was longer than any barrier island currently in existence on the Atlantic Coast of North America. It probably developed from sediment carried by longshore currents eroding away the sea cliffs.
Geologists are uncertain about the origin of the Orangeburg Escarpment, but they believe it must be the site of a deep fault where limestone and other rocks were thrusted upward well above the surrounding elevation. This occurred at sea level, and the Atlantic Ocean eroded into the uplifted rocks, creating the magnificent cliffs. An unusually long barrier island existed just to the south of these cliffs. Longshore currents carried eroded sediment from the cliffs, resulting in a barrier island that was far longer than any present day islands along the Atlantic coast. It was 100 miles long compared to the 48 mile long Cape Hatteras, North Carolina which is the longest present day barrier island on the Atlantic Coast of North America. (See: https://markgelbart.wordpress.com/2010/11/19/the-geological-and-ecological-history-of-the-okenfenokee-swamp-part-one/) Today, this former barrier island is known as Trail Ridge and is the geological structure that causes water to backflow into the Okefenokee Swamp.
The Bunda Sea Cliffs of Australia. Sea Cliffs similar to this, occurred from Jesup, Georgia to Fayetteville, North Carolina about 3.5 million years ago. Today, these cliffs are a gentle slope that separates the upper coastal plain and lower coastal plain. People driving over them on state highways barely notice the drop in elevation.
The Etretat Sea Cliffs in France.
Sea Cliffs on Moneghan Island, Maine. These are the closest sea cliffs to Georgia today. Hard to believe Georgia, South Carolina, and North Carolina once hosted sea cliffs at least as spectacular as these.
Ice Ages had begun occurring during the middle Pliocene when the Orangeburg Escarpment was still young. In between Ice Ages, sea level rose due to the melting of glaciers, and the ocean reached the escarpment. But gradually Ice Ages became longer and more intense and the ocean kept receding more and more to the east, leaving the cliffs high and dry until rainwater eventually eroded them into the flaccid slopes of the present. Subsequent sea level landforms east of the cliffs consisted of barrier island complexes.
The natural beauty of the cliffs and surrounding Pliocene-aged wilderness must have been quite impressive. The cliffs undoubtedly hosted dozens of species of nesting sea birds, eagles, and falcons; most of them probable ancestors of present day species. Avian diversity was higher then than it is today; later Ice Ages became too harsh for many species. The ocean in front of the cliffs was completely untouched by man and during most of the Pliocene, water temperatures were warmer than those of today. There were many more species of whales–a major marine mammal extinction event caused by the formation of the Central American landbridge had yet to occur. Carcharodon megalodon, the giant great white shark, still swam the seas. Inland, a cavalcade of large mammals inhabited the land, including 4-tusked mastodons, 3-toed horses, llamas, peccaries, an early species of deer, saber-tooths, the bone eating dog (Borophagus), hyenas, and bears.
Just imagine those spectacular sea cliffs located in a completely virgin pristine wilderness. I wish time travel vacations to places like this were possible.
This species of 3-toed horse, hyppohippus, became extinct about 9 million years ago, but similar species of 3-toed horses were likely the most common large mammals on the land behind the southeastern sea cliffs about 3.5 million years BP.