Very Large Boulders on the Coast of Grand Cayman: The Effects of Giant Waves on Rocky Coastlines

Abstract

Two stretches of rocky coastline on Grand Cayman are characterized by boulders that are up to 5.5 m long, 3.4 m. wide, and 2 3 m. high (estimated to weigh as much as 40 tonnes). Clusters of boulders, which are Irregularly distributed through the area, occur up to 100 m inland from the present day shoreline. All of the boulders were derived from the microcrystalline dolostone of the Bluff Formation that forms the coastal terraces of these areas. The occurrence of sponge borings, Lithophaga borings, encrusting vermetid gastropods, and encrustmg Homotrema on some boulders, however, shows that some must have been submerged in seawater prior to transportation to their present position. One boulder is also covered with Astrangia solitaria. Radiocarbon dating of this coral yielded an age of 1662 AD (1625 to 1688 AD at 68.3% confidence limits); an age that probably reflects the time when the coral was removed from its marine environment. Analysis of the boulders suggests that they were transported to their present position by a giant waves that swept across Grand Cayman approximately 330 years ago. Such waves moved some blocks, estimated to weigh 10 tonnes up to 18 m vertically and 50 to 60 m horizontally. These boulders may have been moved by hurricane-generated waves or a tsunami that was triggered by an earthquake or slumping on submarine slopes. No wave of comparable power has affected the island since 1662.