oolitic limestone

During the Jurassic Period (195 to 135 million years ago) the region that is now Bath was under a shallow sea. Layers of Marine sediment built and individual spherical grains were coated with lime as they rolled around the sea bed forming the Bathonian Series of rocks. Under the microscope, these grains or ooliths (egg stone) are sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg.

Bath Stone is an oolitic limestone comprising granular fragments of calcium carbonate. Originally obtained from the Combe Down and Bathampton Down Mines under Combe Down, Somerset, England, its warm, honey colouring gives the World Heritage City of Bath its distinctive appearance. An important feature of Bath Stone is that it is a freestone, that is one that can be sawn or 'squared up' in any direction, unlike other rocks such as slate, which forms distinct layers.