Potholes and Resident Boulders on a Contemporary Limestone Shore (Sal Island, Cabo Verde Archipelago, Northeast Atlantic Ocean)

Johnson, M.E.; Ramalho, R.S.; Uchman, A.; Martín-González, E.; Martins, G.M.; Hipólito, A.; Marques, S.; Ávila, G.; Madeira, P., and Ávila, S.P., 2025. Potholes and resident boulders on a contemporary limestone shore (Sal Island, Cabo Verde Archipelago, Northeast Atlantic Ocean). Journal of Coastal Research, 41(4), 657–665. Charlotte (North Carolina), ISSN 0749-0208. The north coast of Sal Island in the Cabo Verde Archipelago is affected by wave swell generated by persistent northeast trade winds, seasonal winter swell from the western Atlantic, and rare tropical storms. An unroofed sea cave between Ponta Norte and Ponta Palhona exposes a natural amphitheater enclosed by the 6-m cliff face of a Pleistocene marine terrace. Covering approximately 250 m2, its floor sits like a bench with a seaward edge another 8 m above sea level. Multiple semicircular potholes with a maximum depth of 1 m are bored into the floor. All contain sea salt, the largest of which retain liquid brine. An oval-shaped pothole that measures 2.8 m × 1.6 m includes a boulder 0.70 m in diameter, estimated to weight 40 kg based on a medium value for limestone density at 2.25 g/cm3. The potholes result from mortar-and-pestle style action by limestone grinders within limestone depressions. Salt indicates replenishment of seawater during intervals with highest surf or rare gales, as swell generated by the pervasive trade winds is insufficient to break over the bench. Winter brings stronger swell from the west, known by locals as lavarias (i.e. washing-out time). These provide sufficient wave shock to loosen limestone blocks from the sea cliffs surrounding the amphitheater. The surface is flushed by high seas, whereas lesser disturbances are expected to leave all but the largest boulders in place. The resident limestone boulder within a large pot hole is too heavy to be removed by normal sea swell and may have been eroded during a rare gale. The wave height necessary to dislodge such a block from the cliff face can be estimated. Repeated wave action is expected to reduce such blocks to spherical shapes as they rotate within potholes that increase in size over time.