OXFORDIAN, in geology, the name given to a series of strata in the middle Oolites which occur between the Corallian beds and the Cornbrash; the division is now taken to include the Oxford Clay with the underlying Callovian stage (q.v.). The argillaceous beds were called "Clunch Clay and Shale" by William Smith (1815-1816); in 1818 W. Buckland described them under the unwieldy title "Oxford, Forest or Fen Clay." The term Oxfordian was introduced by d'Orbigny in 1844. The name is derived from the English county of Oxford, where the beds are well developed, but they crop out almost continuously from Dorsetshire to the coast of Yorkshire, generally forming low, broad valleys. They are well exposed at Weymouth, Oxford, Bedford, Peterborough, and in the cliffs at Scarborough, Red Cliff and Gristhorpe Bay. Rocks of this age are found also in Uig and Skye.
The Oxford Clay is usually bluish or greenish-grey in colour, weathering brown or yellow; in the lower portions it is somewhat more shaly. The beds frequently tend to be calcareous and bituminous, while in places there is a considerable amount of lignite. Septaria of large size are common, they have been cut and polished at Radipole and Melbury Osmund in Dorsetshire, where they are known as Melbury marble or "turtle-stones"; they were used to form table-tops, &c. In Yorkshire the Oxford Clay is usually a grey sandy shale. In the central and southern English counties the Oxford Clay is divisible as follows: Upper zone of S Clays with septaria and ironstone nodules. Clays with Cardioceras cordatum pyritized fossils (subzone of Quenstedtoceras Lamberti). Lower zone of I Shales with pyritized fossils (subzone of Cosmoceras Jason). Cosmoceras ornatum The upper zone contains also Gryphaea dilatata (large forms), Serpula vertebralis, Belemnites hastatus, Aspidoceras perarmatum, Cardioceras vertebrale. The lower zone yields Reineckia anceps, Peltoceras athleta, Quenstedtoceras Mariae, Cosmoceras Jason, Cerithium muricatum, and a small form of Gryphaea dilatata. The remains of fishes and saurian reptiles have been found. The Oxford Clay is dug for brick-making at Weymouth, Trowbridge, Chippenham, Oxford, Bedford, Peterborough and Fletton.
The "Oxfordian" of the continent of Europe is divided according to A. de Lapparent into an upper (Argovian) and a lower (Neuvizyen) substage. In the former he includes part of the English Coralline Oolite and in the latter the lower Calcareous Grit, while a portion of the lower Oxford Clay is placed in the Divesian or upper substage of the Callovian. In north-west Germany the Oxford Clay is represented by the Hersumer beds. Most of the European formations on this horizon are clays and marls with occasional limestone and ironstone beds.
See JURASSIC, CALLOVIAN, CORALLIAN. (J. A. H.)
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