delta 18 O PDB and delta 13 C PDB values have been measured on 107 calcite cement samples from a laterally extensive (> 3 km) and continuous calcite-cemented layer 0.5 m thick in the coastal exposures of the Lower Jurassic shallow-marine Bridport Sands in Dorset, southern England. The samples were taken from a two-dimensional grid with 10-cm horizontal and vertical spacing between samples and along individual vertical lines across the calcite-cemented layer. delta 18 O PDB values vary between -4.8 per thousand and -9.2 per thousand and decrease radially outwards from points with lateral spacings on the order of 0.5-1 m in the middle of the calcite-cemented layer. The delta 18 O PDB values therefore indicate that the calcite-cemented layer was formed by merging of concretions. All delta 13 C PDB values measured are in the narrow range -2.2 per thousand to -0.5 per thousand , which suggests that the dominant source of calcite cement in the layer was biogenic carbonate.