Phosphate, Fossils, and Geological Ghosts in the SC Lowcountry: the mysterious Edisto Formation
By Robert W. Boessenecker, Ph.D.
In early 1865 the US Civil War was winding down – Sherman’s march to the sea had cut the Confederacy in two, Robert E. Lee was encircled by Union troops in the Siege of Petersburg, and on February 15, Confederate general P.G.T. Beauregard ordered the evacuation of confederate troops from Charleston. Three days later, the flag of the Union was raised at Fort Sumter, where the war had started four years prior – symbolically ending the war in Charleston. The South Carolina lowcountry had a plantation economy that, up until this point, relied on slave labor for harvesting bountiful crops of rice, indigo, and sea island cotton – the finest cotton produced in the south. With enslaved Africans freed by Union troops enforcing the Emancipation Proclamation, and the passage of the Thirteenth Amendment later in 1865 – plantation owners no longer had a captive workforce. South Carolina slipped into poverty quickly after the war.
Francis Holmes, a former planter (a polite antebellum word for ‘slaveowner’), had amassed a spectacular fossil collection and in 1850 was appointed as curator of the first College of Charleston Museum. In 1868, the College reduced his salary and he resigned. Shortly thereafter, Holmes discovered the utility of phosphate rock in the Charleston lowcountry as a potential source of agricultural phosphate for fertilizer – and started the Ingleside Mining and Manufacturing company, the first of many phosphate mining companies during the postwar boom. Within a few years, phosphate mining boomed and over a couple dozen companies were involved with strip mining or dredging phosphate from our river bottoms. The mining region was called the Ashley Phosphate Mining District – more of an economic than a geologic label. Mining was focused in modern West Ashley (north of Highway 17, west of I-526, and west towards Bacons Bridge Road) and North Charleston just across the Ashley River. In this region, the sediments dating to the Pleistocene (last 2.5 million years – aka “the Ice Age”) are thin and only 5-10 feet below the current land surface. These Pleistocene deposits rested directly upon Oligocene aged limestone of the Ashley Formation (28-30 Myo), and occasionally, the slightly younger Chandler Bridge Formation (24-25 Myo). The gap between these Oligocene rocks and the Pleistocene layers is an erosional surface mantled with phosphate rock.
Two large blocks of white phosphate, collected in the vicinity of Bees Ferry Road in West Ashley, by Ashby Gale and given to me during the pandemic. The reworked blocks of white phosphate are the very "land phosphate" sought after by the phosphate miners after the U.S. Civil War. Photo by author.
Phosphate forms from a precursor sandstone, siltstone, claystone, or limestone rock during periods of low sea level and sediment starvation – in other words, during certain parts of sea level change cycles, sediment is trapped in rivers and the rate that marine sediment accumulates out on the continental shelf is quite slow. If this coincides with high primary productivity and upwelling, phosphorus-rich seawater can enrich the sediment and trigger the formation of the mineral Calcium Phosphate. This happens to be the same mineral that bones and teeth are made out of. Lastly, the mineral that most invertebrate shells (clams, oysters, sea snails, sand dollars, coral) is calcium carbonate – and this tends to dissolve during periods of ‘phosphogenesis’. As a result, bones and teeth are far more common than they normally would be – and invertebrates much more rare. Phosphate deposits tend to have lots of phosphate nodules, bones and teeth, and phosphatic molds (steinkerns) of mollusks. Much of the phosphate in Charleston is black and brown, with river phosphate including large black concretions – but the “land phosphate”, strip mined from the Ashley Phosphate Beds, consisted of large white to light tan blocks. A wide variety of marine and nonmarine vertebrate fossils, including sharks, fish, sea turtles, birds, whales, dolphins, sea cows, and land mammals – from Oligocene, Miocene, Pliocene, and Pleistocene species. This mixing is a result of the erosion and reworking of fossils common during phosphogenesis.
Phosphate was strip-mined by hand, with overburden stripped away by ploughs and oxen, and the blocks of phosphate hand-dug, transported by mine cart, washed, and chemically dissolved into liquid phosphate fertilizer. The strip mines were worked chiefly by former slaves and prisoners, the conditions described as “hellish” and perhaps worse than the working conditions on plantations. Plantation owners, bereft of their enslaved workforce, suddenly had to adapt to a completely different economic paradigm. Because most were unable to adapt to paying a large force to tend the fields, many plantations sold off land. But in the area of the Ashley River, where the largest plantations were – Drayton Hall, Magnolia, Middleton Place, Runnymede – they leased the land to phosphate mining companies.
LIDAR scanning of the land surface in West Ashley shows the scars left behind by late 19th century phosphate mining. Indeed, a drive down highway 61 in West Ashley in the vicinity of Drayton Hall and Magnolia Plantation reveals 2-3 meter tall ridges just a stone's throw off the road - piled up by ox and manpower 150 years ago and hardly disturbed since. Image by Patrick Lockamy, SCDNR, and augmented by B. Brussee and B. Lockett, from Post and Courier: https://www.postandcourier.com/phosphate-mines-locator/html_a35d7372-929b-11e9-b5ed-c7b713d9fceb.html
The exact source of the phosphate rock in the “Ashley Phosphate Beds” has been the focus of some disagreement in the geological literature. Only some of the rock layers underlying Charleston had been named prior to the Civil War (the Cooper Marl, named by the father of sedimentary geology, British geologist Charles Lyell, in the 1840s), and by the time geologic research matured and finally caught up in the 1890s – phosphate mining slowed down. More economically viable phosphates were discovered in Florida, leading to the shutdown of mining activities. This meant that the phosphate deposits were not properly studied throughout the duration of mining.
Closeup images of the 'white phosphate' samples from above collected from West Ashley. Left: broken section showing fresh white phosphate and shell molds. Center: some kind of holdfast on the surface of the hardground exposed on the seafloor, generally resembling gorgonian holdfasts (e.g. octocorals like sea whip, sea fans). Right: internal mold of solitary cup coral Balanophyllia.
In 1908, geologist Earle Sloane interpreted much of the phosphate rock from the Ashley Phosphate Beds as being derived from the “Edisto Phase”, later referred to as the Edisto Marl. This would later be named the Edisto Formation. This unit consisted of a quartz-rich limestone containing many of the marine invertebrates also present in the Ashley Formation. However, some invertebrates were identified as being Miocene in age – indicating an age of 23 million or slightly younger. Preeminent malacologist William Dall identified the Miocene snail Ecphora quadricostata from blocks of the Edisto Formation in the 1890s – a beautiful mollusk well-known to collectors along Calvert Cliffs, and the state fossil of Maryland. However, no figure of the specimen was provided, nor was any specimen number, so the observation cannot be confirmed. Ashby and I have found many internal molds of an older species of Ecphora infilled with Ashley Formation – and perhaps Dall was mistaken? Unlikely, but possible. Sloan also identified the oyster Ostrea haitiensis as indicating an early Miocene age. This observation has been repeated through the past century of papers mentioning the Edisto Formation, but published images of this species that could confirm future field discoveries of this oyster are not forthcoming.
The problem was, after the turn of the century, nobody had actually seen the Edisto Formation in person. Virtually all of the published observations through to the mid 20th century were based on out of place (ex situ) phosphate nodules and not on an actual in place (in situ) rock layer. Another geologist, C. Wythe Cooke, agreed in 1936 that the white “land phosphate” was derived from the Edisto Formation. Later, a different geologist, Harold E. Malde wrote in 1959 that the phosphate was probably from the Ashley Formation. Part of this I imagine was Malde’s inability to view outcrops of the Edisto Formation, and the natural skepticism associated with consideration of such a mystery unit. Indeed, being unable to examine the rocks in the field is a significant source of skepticism.
Stratigraphic column of rocks along the Ashley River at Givhan's Ferry State Park, including the Edisto Formation. From Ward and others (1979).
Molar of the extinct horse Anchippus texanus, derived from the Edisto Formation, from Leidy (1869).
The "chin" of the fearsome entelodont Dinohyus mento, reported by Allen (1926) from somewhere in Charleston. This specimen still has Edisto Formation matrix attached to it - and indicates an age corresponding to the late Arikareean land mammal age.
In recent years, a new formalized concept of the Edisto Formation was established, at first by Ward and others (1979). They identified an early Miocene age sandy limestone, somewhat sandier than the Ashley Formation, overlying the Ashley but stratigraphically below Pliocene rock layers like the Goose Creek Limestone and Raysor Formation. These authors established a new "type section" (lectotype) for the Edisto Formation at Givhan's Ferry State Park. They highlighted the initial report of fossil oysters of Miocene age, Ostrea haitiensis, originally reported from the Edisto. In 2016, Weems and others summarized new evidence for the age of the Edisto Formation. They collected further specimens of Ostrea haitiensis, and subjected the shells to Strontium isotope dating. This provided a date of 23.5 Myo, actually indicating a latest Oligocene rather than early Miocene age - though the formation almost certainly extends into the Miocene (in my opinion). Further tantalizing evidence comes in the form of two terrestrial mammal specimens: a tooth of the horse Anchippus texanus, and the "chin" end of the mandible of a large entelodont (aka "terminator pig"), Dinohyus mento. These two land mammals are only known from the late Arikareen land mammal age, indicating some land mammal-bearing rock unit dating to no older than about 24 Myo. Further, each specimen has matrix attached that is consistent with the Edisto Formation (but easily mistaken for the Ashley).
I used to dismiss the Edisto Formation as a source of fossils and phosphate, but recently we’ve been able to confirm a few of the observations supporting the existence of such a mystery unit.
Three specimens of Ecphora: left, Ecphora quadricostata; middle and right, Ecphora spp., possibly Ecphora wheeleri or Ecphora tampaensis. It is difficult to see, but the specimen on the left has four ridges and the other specimens have at least 5-7. These are internal molds (steinkerns) and are much more challenging to identify than the actual shell. Photo by author.
The first is marine invertebrate evidence. Very recently, we found our first good candidate for an internal mold of Ecphora quadricostata (frequently also referred to as E. gardnerae). This specimen could be as old as early Miocene, or possibly Pliocene in age. There is a Pliocene rock unit, the Goose Creek Limestone, that is encountered in the subsurface, and many large blocks of it have been found out on Folly Beach after the infamous 2014 renourishment – but we’ve never seen any good examples of that lithology at these dredge sites. Or, other typical Pliocene age mollusks such as Carolinapecten eboreus or Chesapecten septenarius. So, odds are, our specimen of Ecphora quadricostata likely represents the Edisto Formation.
Phosphate block size. We come across a large number of relatively large brick-like blocks of phosphate, packed full of marine invertebrate molds. These sorts of blocks are typically brown or nearly black externally, but are quite light in color when you break them open with a hammer – and have a lovely petroliferous smell, which I sort of like, but Ashby found repulsive. I wonder if these large phosphate blocks are from the Edisto Formation rather than the Ashley; a closer look is warranted, as the Edisto is supposed to be more quartz-rich than the Ashley. These may be the same large white phosphate blocks found during the Ashley Phosphate mining days, simply stained dark by river water.
Ear bones (periotics) of two dolphins (cf. Eoplatanista, left; Platanistoidea indet., right) that are not found in the well-sampled Oligocene Chandler Bridge Formation (~23-25 Myo) in Charleston, nor in the intensely sampled Pungo River Limestone of North Carolina (~18-15 Myo) or the lower parts of the Calvert Formation of Maryland and Virginia (~18-16 Myo). These periotics most likely derived from a rock unit from the intervening time - 23-18 Myo - during which time the Edisto Formation was deposited. Photo by author.
A tooth of true Squalodon - collected by CFA client Barney Hammond and donated for study. This is NOT a tooth of Ankylorhiza, the giant dolphin I named in 2020 from the Chandler Bridge Formation - this is bona fide, Miocene age Squalodon. Admittedly, this specimen could have also come from the much younger 18 Myo Marks Head Formation. It is probably assignable to Squalodon whitmorei based on its large size. Regardless, it indicates the presence of Miocene fossils under our lowcountry waterways - possibly the Edisto Formation. Photo by author.
Marine mammal fossils. This in my mind is perhaps the most slap-you-in-the-face obvious: we find tons of earbones out along the riverbanks, and they do not really match anything from the Ashley Formation. We get all sorts of more derived-looking periotics that match critters from the Miocene, such as eurhinodelphinids and even a tooth of Squalodon (NOT Ankylorhiza). Perhaps more critically, we’ve found a large number of periotics matching species from the earliest Miocene – distinctive periotics identifiable as Eoplatanista, Papahu, and an unnamed dolphin from the early Miocene of Europe. Critically, to my knowledge, none of these taxa have been collected from Calvert Cliffs and none are evident in collections from the Pungo River Limestone at the Lee Creek Mine. What these two localities have in common is Burdigalian to Langhian age – late early Miocene to middle Miocene. Eoplatanista is known from the earliest Miocene of Europe – Aquitanian stage – which just so happens to coincide with the younger side of age determinations for the Edisto Formation, thought to straddle the Oligocene-Miocene boundary.
What does this all mean? We have some evidence of fossils and phosphate from the Edisto Formation. We don’t know what proportion of phosphate – we see plenty of specimens likely dating to the Ashley Formation (early Oligocene), but perhaps a plurality of marine mammal specimens originating from younger strata. It’s unclear what this means in the context of phosphate source rocks – but in general, does support the idea that at least some phosphate originates from the Edisto Formation.
Problems and Future Directions. Being able to see the Edisto Formation in situ would be very useful – Weems et al. (2016) identified the section in the cliffs at Givhan’s Ferry State Park along the banks of the Edisto River as the reference section – the best we can do in the absence of a type locality or type section. This is the new type section, so to speak (equivalent with a neotype in zoological taxonomy). We’ve planned a trip to check out this locality and hope it’s not too overgrown. Such a visit will no doubt ease our concerns about the Edisto Formation and whether or not it’s a true ‘phantom’ rock layer.
The Edisto Formation is frequently identified in the subsurface from auger hole ‘cuttings’ – I can imagine that this is a bit challenging, as it is similar to ‘mudlogging’. No core sample that can be measured is pulled out – the disaggregated sediment comes out of the top of the auger drill and you need to do a bit of math (speed of the drilling, known depth of the drill) in order to estimate the depth and thickness of the rock layer. In many cases the Edisto Formation has been observed directly over the Ashley Formation without the Chandler Bridge Formation separating them. That’s fine – the Chandler Bridge Formation is notoriously patchy (though not as extremely so as the Edisto) – but the lithology of the Edisto Formation is so similar to the Ashley that it could simply be a misidentified upper part of the Ashley. It’s not like fossil data can be used to verify these observations – these are based chiefly on lithology, and perhaps overconfidently at that in the past.
So, stay tuned! We're hoping to find out much more about the Edisto Formation this winter.
References & Further Reading
Allen, G.M. 1926. Fossil mammals from South Carolina. Harvard University Museum of Comparative Zoology Bulletin 8:447-467. https://archive.org/details/biostor-97868
Boessenecker, R.W. 2018. The Ashley Phosphate Beds: the reconstruction era, vertebrate paleontology, fossil preservation, and stratigraphic confusion in Charleston, South Carolina. The Coastal Paleontologist: https://coastalpaleo.blogspot.com/2018/05/the-ashley-phosphate-beds.html
Cooke, C.W. 1936. Geology of the coastal plain of South Carolina. US Geological Survey Bulletin 867:1-196. https://pubs.usgs.gov/bul/0867/report.pdf
Leidy, J. 1869. On the extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America. Journal of the Academy of Natural Sciences of Philadelphia, 2nd series 7:1–472. https://www.biodiversitylibrary.org/bibliography/20910
Malde, H.E. 1959. Geology of the Charleston phosphate area, South Carolina. US Geological Survey Bulletin 1079:1-105. https://pubs.usgs.gov/bul/1079/report.pdf
Ward, L.W., Blackwelder, B.W., Gohn, G.S., and R.Z. Poore. 1979. Stratigraphic revision of Eocene, Oligocene, and lower Miocene formations of South Carolina. Geologic Notes 23:2-23.
Weems, R.E., Bybell, L.M., Edwards, L.E., Lewis, W.C., Self-Trail, J.M., Albright, L.B., III, Cicimurri, D.J., Harris, W.B., Osborne, J.E., and A.E. Sanders. 2016. Stratigraphic Revision of the Cooper Group and the Chandler Bridge and Edisto Formations in the coastal plain of South Carolina. South Carolina Geology 49:1-24.