Carolina bays

300px|right|thumb|[[LIDAR elevation image of of Carolina bays in Robeson County, North Carolina]]

Carolina bays, also regionally known as Maryland basins and Delmarva bays, are elliptical to circular depressions concentrated along the East Coast of the United States from New York to north Florida.

Geographic extent

Carolina bays are present in the U.S. Atlantic Coastal Plain within coastal New York, New Jersey, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, and north Florida. In Maryland, they are called Maryland basins.

Origin of name

The name "Carolina bay" is sometimes attributed to the writings of the English explorer John Lawson, who explored North Carolina, South Carolina, and Georgia during the early 18th century. This attribution, however, is not correct. Lawson described visiting a swamp that contained bay trees, but there is no indication that he wanted to name the swamp with the word "bay". Furthermore, Lawson said that this swamp had steep margins and that he could see mountains to the west from the vicinity of the swamp. Thus, it seems more likely that this swamp was an inter-dune depression among the Carolina Sandhills, rather than a Carolina bay. Nevertheless, bay trees are present in some Carolina bays.

The earliest scientific description of Carolina bays is by L. C. Glenn (1895), who used the term "bay" (which he described as "lake-like expanses") to refer to these features near the town of Darlington, South Carolina. Glenn put quotation marks around the word "bay" but did not use the phrase "Carolina bay". A subsequent publication by F.A.Melton and William Schriever (1933) used the phrase "The Carolina Bays" (with quotation marks around the word "Bays"). Later, G. R. MacCarthy (1937) published a paper titled "The Carolina Bays", using this phrase throughout the publication (without quotation marks, and with a capital "B" for the word "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, many in groups aligned in a northwest–southeast direction. Generally, the southeastern end has a higher rim composed of white sand. 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. which is reproduced as Figure 51 of Kacrovowski, These eolian dunes have yielded an optically stimulated luminescence date of ~23,600 years, and thus this Carolina Bay must be younger than this OSL date. Another example is Bear Swamp, which is a Carolina bay that is inset into eolian dunes in the valley of the Great Pee Dee River (Marion County, South Carolina). Cores from within this Carolina bay revealed a 0.3–1.2 m thick unit of sand and silty sand (lacustrine deposits and paleosols) that rests on an unconformity above an undisturbed unit of gray clay and sandy clay (with marine shells and burrows) 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 (interpreted as paleosols, shoreline, loess, and eolian deposits) that rests on an unconformity above an undisturbed unit of gray clay and sandy clay (with marine shells and burrows) of Pleistocene age (the same unit that was encountered in cores from within the Carolina bay). Charcoal and wood from a western sand rim (closer to the bay) yielded radiocarbon ages of ~5,760 and 1,270 years before present (BP). Organic sediment and charcoal from an eastern sand rim (farther from the bay) yielded radiocarbon ages ranging from ~7,750 to 2,780 years BP.

Wilson's Bay (Johnston County, North Carolina): 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 (Black Creek Formation). Sediment samples from sand rims associated with this Carolina bay have yielded three optically stimulated luminescence (OSL) ages of ~36,700 years ago; ~29,600 years ago; and ~27,200 years ago.

Big Bay (Sumter County, South Carolina): 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. (2010) Studies by Frey,

The dating of the sand rims of a number of Carolina bays by optically stimulated luminescence (OSL) 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. Several carnivorous plants inhabit Carolina bays, including bladderwort, butterwort, pitcher plant, and sundew.

Some bays have been greatly modified by human activities including farming, highway building, and construction of housing developments and golf courses. For example, Carvers Bay, a large bay in Georgetown County, South Carolina, 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, was designated a state park to preserve it as much as possible in its natural state. Also, 66 Bennett's Bay, near Manning, in Clarendon County, South Carolina, is a designated 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 (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.

Interpretations (theories of origin)

thumb|More than a dozen bays are shown in this photo in southeastern [[North Carolina. Several are cleared and drained for farming.]]

Most geologists today interpret the Carolina bays as relict geomorphological features that developed via various eolian and lacustrine processes. 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. 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. The range of dates can be interpreted that Carolina bays 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 Also, Quaternary geologists and geomorphologists have found a correspondence in time between when 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 (Late Wisconsin) and 70,000 to 80,000 years BP (Early Wisconsin). Within the Atlantic Coast Plain, the 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 orientation is matched by an inferred west to east direction of movement of Pleistocene sand dunes. Northward from northern Georgia to Virginia, the average inferred 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 also corresponds with a shift in the average inferred 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.

Some of the disfavored alternative hypotheses involve extraterrestrial events of some type. These hypotheses include meteorite impacts; secondary impacts of glacial ice boulders ejected from a Younger Dryas impact in the Laurentide Ice Sheet; and speculative "cavitation" processes in a unproven, regional sandy, Mid-Pleistocene impact ejecta. These hypotheses are dismissed for a number of reasons including the wholesale lack of primary evidence for the extensive contemporary impact ejecta, impact processes, impactors, and suitable source impact crater for either sand or ice boulder ejecta.

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, , 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.

Geographic extent

Origin of name

Geomorphology

Interpretations (theories of origin)

Relict thermokarst lakes

Similar landforms in the Gulf of Mexico coastal plain

See also

References

Further reading