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. There is no single definitive or universally agreed-upon origin for them, though wind-and-water processes remain the leading scientific consensus.
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.Kaczorowski, R. T. (1977) The Carolina Bays: a Comparison with Modern Oriented Lakes Technical Report no. 13-CRD, Coastal research Division, Department of Geology, University of South Carolina, Columbia. In Maryland, they are called Maryland basins. Within the Delmarva Peninsula, they and other coastal ponds are also called Delmarva bays.Coleman, D. (2001) Delmarva Bays: Natural Enigmas. Maryland Department of Natural Resources , Annapolis. Maryland.
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".Lawson, J., A New Voyage to Carolina 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.
Orientation According to published papers and monographs, 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 (1942), are projected westward, then they generally converge 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. Farther north, the orientation of the long axes becomes, at best, distinctly bimodal, exhibiting two greatly divergent directions, and, at worst, completely random, lacking any preferred direction. Plate 3 of Rasmussen and Slaughter,Rasmussen, W. C., and T. H. Slaughter (1955) "The ground water resources, in The water resources of Somerset, Wicomico, and Worcester Counties". Bulletin no. 16, Maryland Geological Survey, Baltimore, Maryland. which is reproduced as Figure 51 of Kacrovowski, illustrates the disorganized nature of the orientations of the long axes of Carolina bays in 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.
Stratigraphic setting Most Carolina bays consist of a few meters of sand and/or mud that rest on an 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.
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:
In some places, Carolina bays are inset into fields of eolian dunes in river valleys, and thus these Carolina bays must be younger than the underlying eolian dunes. One such example is Dukes Pond, which is a Carolina bay that is inset into eolian dunes in the valley of the Ohoopee River (Tattnall County, Georgia). 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).
In other places, Carolina bays are overlain by eolian dunes that are now vegetated, and thus these Carolina bays must be older than the overlying eolian dunes. One such example is Big Bay, which is a Carolina bay that is overlain by eolian dunes in the valley of the Wateree River (Sumter County, South Carolina). These eolian sand dunes at Big Bay have been dated by optically stimulated luminescence techniques at 29,600 ± 2,400 to 33,200 ± 2,800 BP, and thus this Carolina bay must be older than these dates.
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/or mud resting on a unconformity above a harder substrate. Carolina bays for which the stratigraphy has been described in some detail include Lake Mattamuskeet (Hyde County, North Carolina), Wilson's Bay (Johnston County, North Carolina), Herndon Bay (Robeson County, North Carolina), Big Bay (Sumter County, South Carolina), Flamingo Bay (Aiken County, South Carolina), and Duke's Pond (Tattnall County, Georgia).
Lake Mattamuskeet (Hyde County, North 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 and augers from within this Carolina bay revealed a 1.5–3.2 m thick unit of sand, sandy silt, and silty sand (lacustrine deposits) that rests on an unconformity above an undisturbed unit of saprolite (weathered felsic gneiss). 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 (the same unit that was encountered in cores from within the Carolina bay). Organic material within the bay yielded an age of ~21,920 radiocarbon years BP.
Herndon Bay (Robeson 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): A core (drill hole D1/2) drilled within this Carolina bay went through the following units: (1) Drilling depth 0 to 4.5 m = eolian sand sheet that overlies the Carolina bay; (2) Drilling depth 4.5 to 9.0 m = silty sand and sandy mud with abundant organic material; and (3) Drilling depth 9.0 to 10.6 m = sandy clay of Pliocene age (Duplin Formation). Sediment samples from sand rims associated with this Carolina bay have yielded four optically stimulated luminescence (OSL) 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 (Aiken County, South Carolina): A core (C1) taken within this Carolina bay revealed a 0.94 m thick unit of quartz sand that rests on an unconformity (paleosol) 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 (P25) taken from adjacent sand rim revealed a 1.85 m thick unit of Quaternary sand that rests on an unconformity (paleosol) above an undisturbed unit of sandy silt and clay of Eocene age (the same unit that was encountered in core C1 from within the Carolina bay). Moore et al. (2012) 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) 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 (Tattnall County, Georgia): 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. (1977) stated that drilling and coring indicated that the bedding and sediments underlying Carolina bays are undisturbed.Gamble EE, Daniels RB, Wheeler WH (1977), "Primary and secondary rims of Carolina Bays": Southeastern Geology, v. 18, p. 199–212. 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.