Combe Down stone
Combe Down stone is a Bath Stone and an oolitic (from the Greek òoion meaning egg and líthos meaning stone) limestone. It is formed from grains of calcium carbonate deposited during the Jurassic Period (195 to 135 million years ago) when the region was under a shallow sea. The stone occurs within the Chalfield Oolite Formation, of which the Bath Oolite and the Combe Down Oolite are members, within the Great Oolite Group.
Bath Stone is a shallow-marine limestone, consisting of ooids with variable amounts of broken up skeletal debris from marine organisms such as bivalves, brachiopods and corals. The ooids are spherical, sand-sized grains of calcite precipitated from seawater, probably with the help of microbes, around a nucleus, most commonly a shell fragment. Ooids are mostly formed in shallow-water areas of moderate to high energy, less than 2 metres deep, where waves and currents are moving the oolitic sand around in dunes, sand waves and ripples. Hence, cross-bedding and lamination are frequently seen in Bath Stone. Environments where ooids are forming today occur in the Bahamas and, along the Trucial Coast of Abu Dhabi in the Arabian Gulf.
The Bath Stone is part of the Bathonian Series of rocks. The Bathonian is one of the stages of the Middle Jurassic. It lasted from approximately 168 to around 166 million years ago. Under the microscope, the ooids can be observed as spherical grains with a diameter 0.25 – 2 mm. They commonly have concentric layers developed around a nucleus. The grains are cemented by calcite, but the rock still has a high porosity (20%).
The Combe Down Oolite, which contains the freestone worked in the mines, is the lowest member of the Chalfield Oolite Formation. Above it are the Twerton Beds which form the plateau of Combe Down.
The top 1.5 to 2 m of the Combe Down Oolite consist of a buff, thinly to medium bedded, fine-grained, slightly oolitic limestone with abundant shell debris. This stone is stronger, more thinly bedded and has a different fracture pattern than the worked freestone below. It is probably for these reasons that the old miners referred to it as the ‘Bastard Stone’ and it was left by them to form the roof beds over much of the quarries. Above this stone there occurs a unit with many corals, referred to as the Combe Down coral reef.
The worked stone of the Combe Down Oolite is up to 9 m in thickness and consists of a buff, medium to thickly bedded oolitic limestone. Although some cross-bedding is present in Springfield Quarry, the medium to thickly bedded strata in the Firs and Byfield Mines and is almost entirely effected by cross-bedding, although, light weathering of the stone must occur for it to be seen in the Combe Down Oolite.
Beneath the worked stone of the Combe Down Oolite is 2 to 3 m of pale buff/grey, thinly bedded, crystalline, shelly, oolitic limestone of the Lower Ragstone (the lowest unit of the Great Oolite). Due to the greater strength and variable character of this limestone it was not extracted by the old miners. It was used for paving stones and walls, however.