Limestone is a non-clastic sedimentary rock composed largely of calcium carbonate (CaCO3), usually formed at the bottom of shallow seas. The solubility of limestone makes it easily eroded; caves and sink-holes are typical features of limestone terrains. Limestone can vary substantially in hardness, the softer forms being very easy to cut, the harder ones often as hard as marble. It also varies in colour, from brown to grey, yellow to white. Limestones of various types were widely used in the Roman world for building and sculpture, especially in those area lacking in fine marble sources. In north-western Europe, North Africa and the Balkans limestone was commonly used for most projects.