A wide range of total carbonate (2%-80%) are recognized; the highest, containing more than 40% carbonate, occurs on the western shorelines of the north and south Cornish coasts. Two low carbonate sections (<20%) exist on the eastern beaches of north and south Devon coasts. Intermediate areas on both northern and southern coastlines (20%-40%) complete the pattern of carbonate distribution. On a 100% carbonate basis, mineralogy of these molluscan sands consists of 43% aragonite, 45% low-Mg calcite and 12% high-Mg calcite. Average concentration of Sr in the carbonate fraction is approximately 2,000 p.p.m. and Mg is approximately 12,500 p.p.m. Carbonate levels of the beach systems are negatively correlated with input of terrigenous clastic sediment. Highest concentrations occur where local cliff lithologies are resistant to weathering; rocky intertidal zones permit large accumulations of skeletal invertebrates; and unprotected beaches allow their destruction by storm/wave action. Comparison with modern beach sands from South Africa and eastern Australia demonstrates that terrigenous sediment dilution can be important in controlling carbonate accumulation at all latitudes.--Modified journal abstract.