Mining 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 laid down during the Jurassic Period (195 to 135 million years ago) when the region was under a shallow sea. It is classified as part of the Great Oolite Group of the Chalfield Oolite Formation of which the Bath Oolite and the Combe Down Oolite are members.
It was formed when layers of marine sediment (rock and soil particles, remains of marine organisms, products of submarine volcanism, chemical precipitates from seawater) were deposited. The tiny individual spherical grains, or ooliths, were coated with lime as they rolled around the sea bed and, later, formed sedimentary rocks.
These are the Bathonian Series of rocks. Bathonian is a stage of the Middle Jurassic. It lasted from approximately 168 to around 166 million years ago. Under the microscope, the ooliths are formed from ooids, with a diameter 0.25 – 2 mm, in concentric layers, that look like fish eggs.
The stratigraphic unit (or layer) below the Twinhoe Beds of the Great Oolite which form the uppermost part of the Combe Down plateau is the Combe Down Oolite which contained the mined stone.
The top 1.5 to 2m of the Combe Down Oolite consists 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. 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.
The worked stone of the Combe Down Oolite is up to 9m in thickness and consists of a buff, medium or thickly bedded oolitic limestone. Although some cross bedding is known to exist in Springfield Quarry, the medium to thickly bedded strata in the Firs and Byfield Mines do not show this phenomenon. Beneath the worked stone of the Combe Down Oolite is 2 to 3m 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.