How Many Carolina Bays?

Carolina bays are elliptical or oval geomorphologic depressions (up to several meters deep), often characterized by the presence of shallow ponds or swamps (wetlands), that occur in the Atlantic Coastal Plain of United States from Delaware to Florida (Johnson 1942; Prouty 1952; Gamble et al. 1977; Bennett and Nelson 1991; Richardson and Gibbons 1993; Sharitz 2003). Glenn (1895) and later Bennett and Nelson (1991) listed the following nine characteristics common to each bay and even if not always present, they seem to define the general physical structure of the bays: oval shape, northwest to southeast orientation, parallelism of their axes, sand rims raised above the general level, interior surface below the general level, the soil inside the rims is different from the external ones, relatively shallow depth, flat sandy bottoms beneath the interior fill and their apparent independence of inflowing and outflowing streams. Explanations of their origin include meteor impacts, subsidence from subsurface karst, and thermokarst or thaw lakes. Quantification of the number of Carolina bays, which may help in forming theoretical explanations for their origin, is also elusive ranging from 10,000 to 500,000. For example, Bennett and Nelson (1991) counted only 2651 in South Carolina and Prouty (1952) estimated some 500,000 exist within the U.S. Prouty’s 500,000 estimate is the most commonly cited reliable estimate. This research demonstrates the large variation in estimates of Carolina bays is a function of mapping scale, the most important factor in enumerating topographic features. The systematic skills from analysts and cartographers in the USGS who mapped Carolina bays on two different scale map series in the National Mapping Program was exploited in this research as a data source. The methodology in this work included 1) an evaluation of the criteria and symbology used for mapping bay and bay-like wetlands on USGS topographic map series (Figure 1) and 2) an enumeration and scaling of observed bay frequency for other mapping scales and geographic regions. The location and characteristics of all Carolina bays in Orangeburg County, South Carolina, were digitized, measured, and enumerated on USGS topographic map series at the scales of 1:24,000 and 1:100,000 (Figure 2). The total number of Carolina bays in South Carolina using the same mapping unit as previous works (e.g. Bennett and Nelson 1991) was derived. Finally, an empirical approach for estimating and modeling Carolina bays at any scale/MMU is presented and, based on previous Carolina bay regional extents, the number of Carolina bays in the Eastern United States was modeled. The results indicate the number of Carolina bays is five times greater when enumerated at a 1:24,000 scale as compared to a 1:100,000 scale. Using a fitted function to observed frequencies at different size classes, an empirical model of the frequency of Carolina bays was developed and applied to estimate bay frequency at any mapping unit. A more reliable estimate of the number of Carolina bays in the U.S. using an MMU of 0.1 ha (0.25 acres) is between 35,000 and 50,000.