PUTATIVE STRUCTURES AND SALINITY GRADIENTS AT THE SEWEE SHELL RING, SOUTH CAROLINA: EVIDENCE FOR ANCIENT CONTROL OF FRESHWATER RESOURCES

摘要

The Sewee shell ring is located on the lower coastal plain of South Carolina. This structure was built approximately 4,100 conventional yrs before present (BP) when eustatic sea-level was 0.8 to 3.0 m lower than today. An elevation benchmark, based on high tide heights, was established at 1.86 m above mean lower low water (MLLW) at the study site. The shell ring and adjacent features including a brackish-to-freshwater Slough and Stream were mapped. Individual putative features designated as the Upper, Middle and Lower Weirs, and a Slough Shell Structure were also mapped. Two baselines, R-R' and S-S', extending from the intertidal zone to the supratidal zone, bisected these putative shell ring features. Surface and sub-surface water heights and salinities were monitored along each baseline on low and high tides to determine if changes in these variables were associated with land elevations, shell ring structures, tidal stages or prior week rainfall. There was a positive correlation between surface and subsurface water heights and land elevations. Conversely, salinities decreased with increasing land elevations. Increases in unconfined groundwater heights and decreases in salinity within the confines of the Sewee shell ring were correlated with prior week rainfall amounts. Statistically significant step-wise increases in water heights and decreases in salinity were associated with shell ring structural features located along baselines R-R' and S-S'. The locations of these structural features and a freshwater stream suggest that the Sewee shell ring could have been constructed to control the directional flow and storage of unconfined coastal freshwater resources.