STORM-DEPOSITED BOULDER RIDGES ON SAN SALVADOR ISLAND, BAHAMAS: CHARACTERISTICS AND IMPLICATIONS FOR COASTAL DEVELOPMENT

Coastal ridges comprised primarily of limestone boulders (clasts >25 cm in diameter) are common but little studied features along rocky shorelines on islands of the Bahama Archipelago. Prominent ridges of two coastal areas on San Salvador were studied in detail, one along the reef- and lagoon-protected northern coast west of Singer Bar Point (SBP, length ~790 m), and the other on the high-energy southern coast of the island west of The Gulf (TG, length ~460 m). After field reconnaissance that included GPS data-point recording and measurements for evenly spaced transects (4 transects for SBP and 3 for TG), both boulder ridges were mapped using ArcMap, and transects were plotted. The SBP ridge is wide (up to 14 m) and has a low crest (~1.5 m above mean sea level), whereas the TG ridge is generally narrower, has a sharp crest, and is located on a cliffed bench 3-5 m above mean sea level. For transects of both areas, the largest boulders were measured (length, width, thickness), characterized by composition (subtidal calcarenite, coral rubblestone, eolianite, paleosol), shape (tabular or irregular), and degree of roundness. The largest boulders from each site were photographed and located with GPS coordinates for future reference. Largest boulders at SBP are generally smaller (15 boulders; ~150-4000 kg; with most <1500 kg) and more rounded than those at TG (12 boulders; ~700-4500 kg; with all but one >1000 kg). Source for boulders at both sites was the seaward rocky coast, with boulders having been peeled away from bedrock, transported and deposited by high-energy storm waves. Boulders and smaller clasts with noticeable rounding are in much higher percentage at SBP than TG, indicating multiple events of milling in the surf prior to final deposition along this low-profile coast as compared to the cliffed, higher profile coast at TG. In contrast, the larger overall size of boulders and presence of fossil coral rubblestone boulders at TG indicates that much stronger storm waves were required to form these deposits. Boulder ridges provide direct evidence of past storm activity and represent natural coastal zone hazard markers. Their continued mapping throughout the Bahamas is warranted, and the presence of such ridges should be considered by coastal managers in site planning for future development projects.