1911 Encyclopædia Britannica/Bradford Clay
BRADFORD CLAY, in geology, a thin, rather inconstant bed of clay or marl situated in England at the base of the Forest Marble, the two together constituting the Bradfordian group in the Bathonian series of Jurassic rocks. The term “Bradford Clay” appears to have been first used by J. de. C. Sowerby in 1823 (Mineral Conchology, vol. v.) as an alternative for W. Smith's “Clay on Upper Oolite.” The clay came into notice late in the 18th century on account of the local abundance of the crinoid Apiocrinus Parkinsoni. It takes its name from Bradford-on-Avon in Wiltshire, whence it is traceable southward to the Dorset coast and northward towards Cirencester. It may be regarded as a local phase of the basement beds of the Forest Marble, from which it cannot be separated upon either stratigraphical or palaeontological grounds. It is seldom more than 10 ft. thick, and it contains as a rule a few irregular layers of limestone and calcareous sandstone. The lowest layer is often highly fossiliferous; some of the common forms being Arca minuta, Ostrea gregaria, Waldheimia digona, Terebratula coarctata, Cidaris bradfordensis, &c.