Peritidal beachrock ledges along the south shore of Bardawil Lagoon, a body of water located in the eastern Mediterranean off the coast of the Sinai Peninsula, consist of gypsum or gyprock and change along strike to a thoroughly cemented, solid, tough dolostone. Microprobe analysis shows that this dolostone is a non-stoichiometric dolomite with mol % CaCO 3 of 65-70%. Except for fossils and included terrigenous particles, this modern dolostone is solid rock composed entirely of dolomite. Under the microscope, the rock is a peloidal micrite (dolomicrite) in which abundant fragments of solid particulate organic matter are dispersed. In comparison with other kinds of beachrock, this dolostone is tight. The age of the dolostone is < 5700 YBP. The delta 13 C PDB of dolostone samples is -39.3 per thousand and -31.2 per thousand . These strongly negative delta 13 C values suggest that the carbon of the dolomite came from methane. Bacterial reactions form methane when the bacteria have reduced all available sulfate. I infer that bacteria were responsible for the origin of this dolomite through the methane pathway. High desert temperatures may have facilitated such a dolomitization process.