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Carolina bays are elliptical depressions concentrated along the Atlantic seaboard within coastal Delaware, Maryland, New Jersey, North Carolina, South Carolina, Virginia, Georgia, and northcentral Florida (Prouty 1952, Kaczorowski 1977). In Maryland, they are called Maryland Basins (Rasmussen and Slaughter 1955). Within the Delmarva Peninsula, they and other coastal ponds are also called Delmarva bays (Coleman nd).
Other landform depressions, not widely accepted as Carolina bays, are found within the northern Gulf of Mexico coastal plain in southeast Mississippi and Alabama, where they are known as either Grady ponds or Citronelle ponds (Otvos 1976, Folkerts 1997). 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, with each bay invariably aligned in a northwest-southeast direction. 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 (or were before drainage) lakes; 36 square km (14 square mile) Lake Waccamaw is an undrained one. Some bays are predominantly open water with large scattered pond cypress, while others are composed of thick, shrubby areas (pocosins), with vegetation growing on floating peat mats. Generally the southeastern end has a higher rim composed of white sand. They are named for the bay trees frequently found in them, not because of the frequent ponding of water (Sharitz 2003).
Undrained, often circular to oval, depressions exhibiting a wide range of area and depth are also a very common feature of the Gulf of Mexico coastal plain within Texas and southwest Louisiana. These depressions vary in size from 0.4 to 3.6 km (0.25 to 2 miles) in diameter. Within Harris County, Texas, raised rims, which are about 0.65 m (2 ft) high, partially enclosed these depressions. In the scientific literature, they are known by a variety of names, including “pocks”, “pock marks”, “bagols”, “lacs ronds”, and “natural ponds” (Aronow nda, ndb).
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.
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.
In South Carolina, Woods Bay, on the Sumter-Clarendon County line near Turbeville has been designated a state park to preserve it as much as possible in its natural state. Also in Clarendon County (near Manning), 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 52-acre (210,000 m2) preserve called the Lisa Matthews Memorial Bay, which is trying to preserve and increase the federally endangered wildflower Oxypolis canbyi (Canby's Dropwort) 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 (Aristida beyrichiana) 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.
As measured by Johnson (1942) and Kacrovowski (1977), 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 eastcentral 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 (Johnson 1942, Kacrovowski, 1977, Carver and Brooks 1989). If the long axes of these Carolina bays, as measured by Johnson (1942), are projected westward, 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, 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 (Kacrovowski, 1977). Plate 3 of Rasmussen and Slaughter (1955), which is reproduced as Figure 51 of Kacrovowski (1977), 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 (Kacrovowski, 1977). The Carolina bays in southern Mississippi and Alabama are elliptical to roughly circular in shape. The measurement of the long axes of 200 elliptical Grady / Citronelle ponds found a very distinct orientation tightly clustered about N25°W in southwestern Baldwin County, Alabama (Otvos 1976).
Within the Atlantic Coast Plain, the measured orientation of the long axes of Carolina bays and the Pleistocene direction of movement of adjacent sand dunes, where present, are generally perpendicular to each other. In southern Georgia and northern Florida, the northerly orientation is matched by a westerly orientation of the direction of Pleistocene movement of sand dunes (Markewich and Markewich 1994). Northward from northern Georgia to Virginia, the average orientation of direction of Pleistocene movement of parabolic sand dunes systematically shifts along with the average orientation of the long axes of Carolina bays as to always lie approximately perpendicular to them. In The Delmarva Peninsula, the 112 degrees shift in the average trend of the long axes is also accompanied by a corresponding shift in the average direction of Pleistocene movement of parabolic sand dunes such that their direction of movement is also perpendicular to the long axes, as is the case in the rest of the Atlantic Coastal Plain (Carver and Brooks 1989).
The age of the Carolina bays is constrained by a variety of dating techniques, as predating the end of the Pleistocene by ten of thousands to over a hundred thousand years. The techniques, which demonstrate a preterminal Pleistocene age for the Carolina bays, are radiocarbon dating, optically stimulated luminescence (OSL) dating, and palynology.
As illustrated in Figure 3 of Heinrich (2005), numerous radiocarbon dates have been collected from the sediments, which fill the basins of Carolina bays. The majority of these samples, from which these dates were obtained, were collected from cores of undisturbed sediments that filled Carolina bays in North and South Carolina. These cores were collected to reconstruct region paleoenvironmental records using pollen, diatoms, and other fossils found in the distinctly and conformably layered sediments that fill the Carolina bays (Brooks et al. 2001, Frey, 1953, Frey 1955, Gaiser et al. 2001, Watts 1980, Whitehead 1981). Additional radiocarbon dates have been obtained from organic matter collected from the undisturbed sediments filling Carolina bays by Blilet and Burney (1957). Kaczorowski (1977), Mixon and Pilkey (1976), and Thom (1970). Many radiocarbon dates, which were obtained from organic matter preserved within undisturbed sediments are greater than 14,000 BP radiocarbon in age. The finite radiocarbon dates range in age from 440 ± 50 to 27,700 ±2,600 BP radiocarbon in age (Whitehead 1981, Gaiser et al. 2001). Some samples are so old, they contained insufficient radiocarbon for dating, which results in "greater than" dates. For example, samples from sediments filling Carolina bays have been dated at greater than 38,000 to 49,550 BP radiocarbon years (Frey 1955, Brooks et al. 2001).
In cases where multiple radiocarbon dates have been determined from a single core, radiocarbon dates are consistent in terms of their stratigraphic position within a core and accumulation rates calculated from them with only the occasional exception. Given the nature of radiocarbon dating, such 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, contrary to the arguments of Firestone (2006). The intact internal stratigraphy of the bay sediments, their paleosols, and their pollen zones, i.e. as observed by Brook et al. (2001) in case of Big Bay, refutes such arguments.
As discussed by Gaiser et al. (2001), radiocarbon dates reported from any Carolina bay are all minimum dates for their formation. Because only organic matter can be dated by radiocarbon dating, the reported radiocarbon dates only represents times during which organic matter of some type accumulated in Carolina bays and was later preserved. At other times, datable organic matter would either not have been preserved as sediment accumulated within them, or older organic matter destroyed when they dried out completely. During glacial periods when sea level was 130 meters (400 ft) below present, the water table would have been below the bottom of the vast majority of the bays. At such times, any organic matter would have been destroyed by oxidization and weathering of the lake bottom. Also, at that time eolian processes would have eroded any existing sediments filling the bottom of many bays removing any older lake sediments and the pollen, and datable organic matter. As a result, it is highly unlikely that organic matter dating to the exact age of any Carolina Bay would have been preserved except in the deepest of them. Thus, the oldest radiocarbon date from a Carolina Bay only indicates when the water table rose high enough for a permanent lake or swamp to exist within it (Gaiser et al. 2001).
Over the last several years, Ivester et al. (2002, 2003, 2004a, 2004b, 2007) have dated the sand rims of numerous Carolina bays using optically stimulated luminescence (OSL). They found sand rims of many Carolina bays to be as old as 80,000 to 100,000 BP. For example, Ivester et al. (2002) wrote about Flamingo Bay, a Carolina bay:
In the upper Coastal Plain, dates from Flamingo Bay indicate the rim was active at 108.7 ± 10.9 ka BP and again at 40.3 ± 4.0 ka BP. The nearby Bay-40 had an actively forming sand rim at 77.9 ± 7.6 ka BP. Near the confluence of the Wateree and Congaree Rivers in the middle Coastal Plain, an eolian sand sheet was dated to 74.3 ± 7.1 ka BP.
About Carolina bays in general, Ivester et al. (2004a) concluded:
Luminescence and radiocarbon dating of inland dunes and Carolina bay rims indicate activity during multiple phases over the past 100,000 years. Some bays have evolved through phases of activity and inactivity over tens of thousands of years, as evidenced both by multiple rims along a single bay and by multiple ages within single rims.
Both dunes and bays were active during the Wisconsin glaciation, with ages tending to fall between 15,000 and 40,000 years BP, and near the isotope stage 5/stage 4 boundary 70,000 to 80,000 years BP.
Based on OSL dating, Brooks et al. (1996, 2001), Grant et al. (1998), and Ivester et al. (2002, 2003, 2004b) argue that lacustrine and eolian processes have periodically modified the shape and size of the Carolina bays during the last 100,000 to 120,000 years. For example, Ivester et al. (2003), using OSL dating, dated the nested, concentric sand rims, which are found in Big Bay, a Carolina bay in South Carolina. From the outside to the inside the age of the rim becomes progressively younger, i.e. 35,660±2600; 25,210±1900; 11,160±900; and 2,150±300 years BP. These dates demonstrate that Big Bay has shrunk over the last 36,000 years by 1.6 miles (2.6 km). Ivester et al. (2004a) also found these rims to be composed, not of impact ejecta, but rather "are composed of both shoreface and eolian deposits". Concerning the Carolina bays, they concluded:
The optical dating results indicate that present-day bay morphology is not the result of a single event, catastrophic formation, but rather they have evolved through multiple phases of activity and inactivity over tens of thousands of years. This is evidenced both by multiple rims of differing ages along the same bay, and by multiple ages within single rims.
On the basis of 45 OSL dates from and sedimentological analyses of rims of Carolina bays in Georgia and South Carolina, Ivester et al. (2007) concluded that a single Carolina bay was actively modified between 12,000 to 50,000 BP; 60,000 to 80,000 BP; and 120,000 to 140,000 BP. His conclusions is collaborated by the OSL dating done by Brooks et al. (1996, 2001), Grant et al. (1998), and Ivester et al. (2002, 2003, 2004b) on other Carolina Bays and the fact not all Carolina Bays are as perfectly aligned as they are claimed to be. In any one location, the orientation of their long axes varies by 10 to 15 degrees as discussed in Johnson (1942), Kacrovowski (1977), and Carver and Brooks (1989). Plate 3 of Kacrovowski (1977) also shows the long axes of Carolina bays becomes, at best, distinctly bimodal and exhibits two greatly divergent directions and, at worst, completely random and lacking any preferred direction within the northernmost part of their distribution, i.e. Somerset, Wicomico and Worcester counties, Maryland.
The physical relationship of Pleistocene sand dunes to Big Bay, North Carolina, and adjacent Carolina bays further demonstrates that they are tens of thousands of years old (Brooks et al. 2001). In this case, during the Pleistocene, sand dunes have migrated from the valley wall of the Wateree River into, and partially filled, Big Bay and an adjacent Carolina bay. According to basic principles of cross-cutting relationships and superposition, both Carolina bays were first created and the sand dunes later migrated into them. Thus, the sand dunes must be younger than Carolina bays. Since the sand dunes have been dated by OSL dating at 29,600 ± 2,400 to 33,200 ± 2,800 BP, both Carolina bays must be older than these dates.
The sequences of pollen zones recovered from cores taken from various Carolina bays by Frey (1953, 1955), Watt (1980), and Whitehead (1964, 1981) document the presence of full glacial pollen zones within the sediments filling Carolina bays. The thick sediments, which were recovered in these cores and contain pollen characteristic of full glacial conditions could have accumulated within these Carolina bays only if they had existed prior to end of the last glacial epoch. The radiocarbon dates reported by Frey (1953, 1955), Watts (1980), and Whitehead (1964, 1981) from these Carolina bay cores fully corroborate both the glacial age of the pollen and the undisturbed nature of the sediments filling these Carolina bays as indicated by the reported layering of the sediments filling them and increasing age with depth of the pollen they contain.
Within cores of undisturbed sediments recovered from Big Bay, North Carolina, Brook et al. (2001) 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 interglacial epoch to the Wisconsinan glacial epoch, back into oxygen isotope stage 5, 75,000 to 134,000 years BP. These pollen zones corroborate the dating of Big Bay by OSL and radiocarbon dating.
Theories of the origin of the Carolina bays fall into two major categories: that these features were created by forces within the Earth, or that they were gouged by an astronomical event or set of events.
Various geomorphological hypotheses have been proposed to account for the bays, including action of sea currents when the area was under the ocean or the upwelling of ground water at a later time. One major theory within the earth sciences academic community is that a combination of processes created the shapes and orientations of these ancient landforms, including climate change, the formation of siliciclastic karst by solution of subsurface material during glacial sealevel lowstands and later modification of these depressions by periodic eolian and lacustrine processes.
Quaternary geologists and geomorphologists argue that the peculiar features of Carolina bays can be readily explained by known terrestrial processes and repeated modification by eolian and lacustrine processes of them over the past 70,000 to 100,000 years. 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 which existed during the Wisconsin glaciation as reconstructed from Pleistocene parabolic dunes, a time when the shape of the Carolina bays was being modified.
The cometary impact theory of the origin of the bays was popular among earth scientists of the 1940s and 50s. After considerable debate and research, geologists determined the depressions were too shallow to be and lacked evidence of impact features. Reports of magnetic anomalies turned out not to show consistency across the sites. There were no meteorite fragments, shatter cones or planar deformation features. None of the necessary evidence for hyperspeed impacts was found. The conclusion was to reject the theory that the Carolina bays were created by impacts of asteroids or comets (Rajmon 2009).
A new type of extraterrestrial impact hypothesis was proposed as the result of interest by both popular writers and professional geologists in the possibility of a terminal Pleistocene extraterrestrial impacts, including the Younger Dryas Impact hypothesis. It said 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. However, this theory 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 that shows that many of them are as old as, or older than, 60,000 to 140,000 BP (Brooks et al.1996, 2001, Grant et al. 1998, and Ivester et al. 2002, 2003, 2004b).