Carolina bays
Carolina bays are elliptical depressions concentrated along the Atlantic seaboard within coastal Delaware, Maryland, New York, New Jersey, North Carolina, South Carolina, Virginia, Georgia, and north Florida. In Maryland, they are called Maryland basins. Within the Delmarva Peninsula, they and other coastal ponds are also called Delmarva bays.
They are named for the bay trees frequently found in them, not because of the frequent ponding of water.
Geographic extent
Carolina bays are present in the U.S. Atlantic Coastal Plain from New York to north Florida. In Maryland, they are called Maryland basins. Within the Delmarva Peninsula, they and other coastal ponds are also called Delmarva bays.Geomorphology
Carolina bays vary in size from one to several thousand acres. About 500,000 of them are present in the classic area of the Atlantic Coastal Plain, often in groups aligned in a northwest-southeast direction. Generally the southeastern end has a higher rim composed of white sand.Orientation
According to published papers and monographs, the orientation of the long axes of Carolina bays systematically rotate northward along the Atlantic Coastal Plain from northern Georgia to northern Virginia; the average trend of the long axes of Carolina Bays varies from N16°W in east-central Georgia to N22°W in southern South Carolina, N39°W in northern South Carolina, N49°W in North Carolina, and N64°W in Virginia. Within this part of the Atlantic Coastal Plain, the orientation of the long axes of Carolina bays varies by 10 to 15 degrees. If the long axes of these Carolina bays, as measured by Johnson, are projected westward, then they converge, neither in the Great Lakes nor Canada, but in the area of southeastern Indiana and southwestern Ohio.At the northern end of the distribution of Carolina bays within the Delmarva Peninsula and New Jersey, the average orientation of the long axes abruptly shifts by about 112 degrees to N48°E. Further north, the orientation of the long axes becomes, at best, distinctly bimodal, and exhibits two greatly divergent directions and, at worst, completely random and lacking any preferred direction. Plate 3 of Rasmussen and Slaughter, which is reproduced as Figure 51 of Kacrovowski, illustrates the disorganized nature of the orientations of the long axes of Carolina bays within the northernmost part of their distribution within Somerset, Wicomico, and Worcester counties, Maryland.
At the southern end of their distribution, the Carolina bays in southern Georgia and northern Florida are approximately circular in shape. In this area, they have a weak northerly orientation.
elevation image of of Carolina bays in Robeson County, N.C.
Stratigraphic setting
Most Carolina bays consist of a few meters of sand and mud that rest on a unconformity above a harder substrate that does not show signs of deformation or other disturbance. The composition and the age of this harder substrate varies from location to location.Furthermore, stratigraphic relations of some Carolina bays with fields of eolian dunes in river valleys suggest that Carolina bays formed episodically during different times at different places. For example:
Stratigraphy within the Carolina bays and sand ridges
Cores taken within several Carolina bays have revealed a stratigraphy of a few meters of sand and mud resting on a unconformity above a harder substrate. Carolina bays for which the stratigraphy has been described in some detail include Lake Mattamuskeet, Wilson's Bay, Herndon Bay, Big Bay, Flamingo Bay, and Duke's Pond.Lake Mattamuskeet : Cores from within this Carolina bay revealed a 0.3-1.2 m thick unit of sand and silty sand that rests on an unconformity above an undisturbed unit of gray clay and sandy clay of Pleistocene age. Cores from the adjacent sand rims revealed a 2.6-2.9 m thick unit of silt, sand silt, and silty sand that rests on an unconformity above an undisturbed unit of gray clay and sandy clay of Pleistocene age. Charcoal and wood from a western sand rim yielded radiocarbon ages of ~5,760 and 1,270 years before present. Organic sediment and charcoal from an eastern sand rim yielded radiocarbon ages ranging from ~7,750 to 2,780 years BP.
Wilson's Bay : Cores and augers from within this Carolina bay revealed a 1.5-3.2 m thick unit of sand, sandy silt, and silty sand that rests on an unconformity above an undisturbed unit of saprolite. These lacustrine deposits yielded a radiocarbon age of ~21,920 years BP. Cores and augers from the adjacent sand rims revealed a 1.5-4.0 m thick unit of muddy sand, sand, and gravel that rests on an unconformity above an undisturbed unit of saprolite/weathered felsic gneiss. Organic material within the bay yielded an age of ~21,920 radiocarbon years BP.
Herndon Bay : Cores drilled into four different sand ridges associated with this Carolina bay revealed that the sand ridges are composed of 2.5-4.5 m thick accumulations of fine to coarse sand that rest on an unconformity above an undisturbed unit of black mud of Cretaceous age. Sediment samples from sand rims associated with this Carolina bay have yielded three optically stimulated luminescence ages of ~36,700 years ago; ~29,600 years ago; and ~27,200 years ago.
Big Bay : A core drilled within this Carolina bay went through the following units: Drilling depth 0 to 4.5 m = eolian sand sheet that overlies the Carolina bay; Drilling depth 4.5 to 9.0 m = silty sand and sandy mud with abundant organic material; and Drilling depth 9.0 to 10.6 m = sandy clay of Pliocene age. Sediment samples from sand rims associated with this Carolina bay have yielded four optically stimulated luminescence ages of ~35,700 years ago; ~25,200 years ago; ~11,200 years ago; and ~2,100 years ago.
Within cores of undisturbed sediments recovered from Big Bay, North Carolina, Brook and others documented well-defined pollen zones consisting of distinct pollen assemblages. They found a stratigraphically consistent series of pollen zones, which increased in age consistently with depth from Holocene Stage to the Wisconsin Stage, back into marine isotope stage 5
Flamingo Bay : A core taken within this Carolina bay revealed an 0.94 m thick unit of quartz sand that rests on an unconformity above an undisturbed unit of sandy silt and clay of Eocene age. Charcoal samples within the 0.94 m thick unit of quartz sand yielded radiocarbon ages of ~4,500 to 2,500 years BP. A core taken from adjacent sand rim revealed a 1.85 m thick unit of Quaternary sand that rests on an unconformity above an undisturbed unit of sandy silt and clay of Eocene age. Moore et al. reported that sediment samples from sand ridges associated with this Carolina bay have yielded five OSL ages of ~15,000 years ago; ~13,100 years ago; ~11,500 years ago; ~9,200 years ago; and ~5,000 years ago. Brooks et al. reported that sediment samples from sand ridges associated with this Carolina Bay yielded OSL ages of ~108,700 years ago; and ~40,300 years ago.
Duke's Pond : A sediment sample from a sand rim at the margin of this Carolina has yielded an OSL age of ~23,600 years ago. Basal peat bog sediment within this Carolina bay yielded an age of ~8,600 radiocarbon years ago.
Additional notes on stratigraphy
In a study of several Carolina bays in North Carolina, Gamble et al. stated that drilling and coring indicated that the bedding and sediments underlying Carolina bays are undisturbed. Studies by Frey, Watts, and Whitehead have also documented that the sediments filling Carolina bays are generally undisturbed. Several cores have found that the sediments that fill Carolina bays have distinct and conformably layers or beds.The dating of the sand rims of a number of Carolina bays by optically stimulated luminescence techniques has yielded ages ranging from ~109,000 to ~2,000 years ago, but most ages from the sand rims range from ~40,000 to ~11,000 years ago.
Radiocarbon dates have been obtained from organic matter collected from the undisturbed sediments filling Carolina bays by Bliley and Burney, Mixon and Pilkey, Thom, and Kaczorowski. Some radiocarbon dates obtained from organic matter within undisturbed sediments are greater than 14,000 BP radiocarbon in age. The radiocarbon dates range from 27,700 ±2,600 to 440 ± 50 radiocarbon years BP. Some cores have contained organic matter that was too old for dating by radiocarbon methods, resulting in "greater than" dates. For example, samples from some Carolina bays have been dated at greater than 38,000 to 49,550 radiocarbon years BP. In cases where multiple radiocarbon dates have been determined from a single core, most radiocarbon dates are typically consistent in terms of their stratigraphic position within a core, and accumulation rates calculated from them only are rarely anomalous. Given the nature of radiocarbon dating, discordant dates occasionally occur even in undisturbed deposits, when multiple samples were dated. The occasional discordant dates by themselves are meaningless as an indicator of disturbance. The intact internal stratigraphy of the Carolina bay sediments, as indicated by paleosols and pollen zones refutes such arguments.
As discussed by Gaiser and others, radiocarbon dates reported from any Carolina bay are minimum dates for their formation. The radiocarbon dates only represent times during which organic matter accumulated and was preserved in Carolina bays. At other times, datable organic matter either might not have been preserved as sediment accumulated within them, or older organic matter might have been destroyed when the bays dried out. During times when the water table was below the bottom of a Carolina bay, organic matter could have been destroyed by oxidization and weathering. Also, during such times, eolian processes could have eroded any existing sediments at the bottom of Carolina bays. There are some who suggest that the oldest radiocarbon date from a Carolina bay only indicates the time when the water table rose high enough for a permanent lake or swamp to exist within it. This interpretation, however, may depend upon the nature of the overlying sediment. For example, eolian processes can bury and preserve organic matter, and thus the preservation of organic matter can occur independently of water table behavior.
Ecological significance and biodiversity
The bays have many different vegetative structures, based on the depression depth, size, hydrology, and subsurface. Many are marshy; a few of the larger ones are lakes; Lake Waccamaw is an undrained example. Some bays are predominantly open water with large scattered pond cypress, while others are composed of thick, shrubby areas, with vegetation growing on floating peat mats.The bays are especially rich in biodiversity, including some rare and/or endangered species. Species that thrive in the bays' habitats include birds, such as wood storks, herons, egrets, and other migratory waterfowl, mammals such as deer, black bears, raccoons, skunks, and opossums. Other residents include dragonflies, green anoles and green tree frogs.
The bays contain trees such as black gum, bald cypress, pond cypress, sweet bay, loblolly bay, red bay, sweet gum, maple, magnolia, pond pine, and shrubs such as fetterbush, clethra, sumac, button bush, zenobia, and gallberry. Plants common in Carolina bays are water lilies, sedges and various grasses. Several carnivorous plants inhabit Carolina bays, including bladderwort, butterwort, pitcher plant, and sundew.
Some of the bays have been greatly modified within human history, under pressure from farming, highway building, housing developments and golf courses. Carvers Bay, a large one in Georgetown County was used as a bombing practice range during World War II. It has been drained and is mostly used for tree farming today. Others are used for vegetable or field crops with drainage. A study of bays located on the Delmarva peninsula estimated that 70% had been partially or fully converted to agriculture.
In South Carolina, Woods Bay, on the Sumter-Florence county line near Olanta has been designated a state park to preserve it as much as possible in its natural state. Also in Clarendon County, another bay, Bennett's Bay, is a Heritage Preserve.
Another bay in Bamberg County, South Carolina is owned by the South Carolina Native Plant Society, which has been developing a preserve called the Lisa Matthews Memorial Bay, which is trying to preserve and increase the federally endangered wildflower Oxypolis canbyi in the bay. The uplands area surrounding the bay is being restored from a loblolly pine plantation to the original longleaf pine. Included in the longleaf restoration is the restoration of wiregrass as a key understory plant. Its flammability aids in periodic burning, which is necessary for Canby's Dropwort and many of the other species unique to the environment.
Theories of origin
Most geologists today interpret the Carolina bays as relict geomorphological features that developed via various eolian and lacustrine processes. Furthermore, multiple lines of evidence, e.g. radiocarbon dating, optically stimulated luminescence dating, and palynology, indicate that the Carolina bays predate the start of the Holocene. The range of dates can be interpreted that they were either created episodically over the last tens of thousands of years or were created at time over a hundred thousand years ago and have since been episodically modified.Relict thermokarst lakes
Recent work by the U.S. Geological Survey has interpreted Carolina bays as relict thermokarst lakes that have been modified by eolian and lacustrine processes. Thermokarst lakes develop by thawing of frozen ground, with subsequent modification by wind and waves. Thus, this interpretation implies that frozen ground once extended as far south as the Carolina bays. This interpretation is consistent with the optically stimulated luminescence dates, which suggest that the Carolina bays are relict features that formed when the climate was colder, drier, and windier.Quaternary geologists and geomorphologists argue that the peculiar features of the Carolina bays can be readily explained by known terrestrial processes and repeated modification by eolian and lacustrine processes. Also, Quaternary geologists and geomorphologists believe to have found a correspondence in time between when the active modification of the rims of Carolina bays most commonly occurred and when adjacent sand dunes were active during the Wisconsin glaciation between 15,000 and 40,000 years and 70,000 to 80,000 years BP. In addition, Quaternary geologists and geomorphologists have repeatedly found that the orientations of the Carolina bays are consistent with the wind patterns that existed during the Wisconsin glaciation, as reconstructed from the orientations of parabolic dunes in river valleys.
Fossil pollen recovered from cores of undisturbed sediment taken from various Carolina bays in North Carolina by Frey, Watts, and Whitehead document the presence of full glacial pollen zones within the sediments filling some Carolina bays.
Within the Atlantic Coast Plain, the measured orientation of the long axes of Carolina bays and the inferred direction of movement of adjacent sand dunes, where present, are generally oblique to each other. In southern Georgia and northern Florida, the northerly orientation is matched by an inferred westerly direction of movement of Pleistocene sand dunes. Northward from northern Georgia to Virginia, the average orientation of direction of movement of Pleistocene parabolic sand dunes systematically shifts along with the average orientation of the long axes of Carolina bays as to lie oblique to them. In the Delmarva Peninsula, the 112 degree shift in the average trend of the long axes is also accompanied by a corresponding shift in the average direction of movement of Pleistocene parabolic sand dunes such that their direction of movement is also oblique to the long axes, as is the case in the rest of the Atlantic Coastal Plain.
Alternative interpretations
Alternative interpretations of Carolina bays that are no longer viewed favorably by most geologists include:- the action of sea currents when the area was under the ocean;
- the upwelling of ground water at a later time;
- the formation of siliciclastic karst by solution of subsurface material during glacial sealevel lowstands;
- extraterrestrial impact hypotheses:
A extraterrestrial impact origin of Carolina bays was proposed again in association with the Younger Dryas impact hypothesis.
However, the idea that the Carolina bays were created by a low density comet exploding above or impacting on the Laurentide Ice Sheet about 12,900 years ago has been discredited by OSL dating of the rims of the Carolina bays, paleoenvironmental records obtained from cores of Carolina bay sediments, and other research related to the Laurentide Ice Sheet.. Fine-scale scrutiny of radiocarbon data does not support synchroneity of the Younger Dryas impact hypothesis.
Similar landforms in the Gulf of Mexico coastal plain
Other landform depressions, not widely accepted as Carolina bays, are present within the northern Gulf of Mexico coastal plain in southern Mississippi and Alabama, where they are known as either Grady ponds or Citronelle ponds. They are also known by a variety of names such as pocks, pock marks, bagols, lacs ronds, and natural ponds. These features in southern Mississippi and Alabama are elliptical to roughly circular in shape. The measurement of the long axes of 200 elliptical Grady / Citronelle ponds in southwestern Baldwin County, Alabama found a very distinct orientation tightly clustered about N25°W.Undrained depressions, circular-to-oval in shape and exhibiting a wide range of area and depth, are also a feature of the Gulf of Mexico coastal plain in Texas and southwest Louisiana. These depressions vary in size from in diameter. Within Harris County, Texas, raised rims, which are about high, partially enclose these depressions.