Dolomitization of Pleistocene carbonates on San Andres Island has occurred as a result of brine seepage in the spray zone. Brines have formed by supratidal evaporation of surf spray and sea water trapped in splash pools on a rocky coast line where the solutions percolated downward through metastable algal packstone. The formation of evaporites and dolomite is terminated when brines are diluted by meteoric water. The dolomitized algal limestone grades laterally into unaltered algal limestone exhibiting typical cementation by sparry calcite. The algal limestone is accreted to the weathered surface of a cliff-forming limestone of Miocene age.