The Jurassic Series in England

THE JURASSIC SERIES IN ENGLAND The sequence of Jurassic rocks as developed in England is full, varied and highly fossiliferous, and may be taken as typical. Although there were no orogenic movements during the period, small earth-movements were probably going on continuously, so that the sediments deposited vary in lithology both in area and in time, and there are many slight unconformities. The fauna to a certain extent varies with the lithology; thus in the clays, belem nites are common and corals rare, while in the limestones, belem nites are rare and corals abundant. The ammonites, on which cor relation depends, are found in both types of deposit, and serve to demonstrate that a lithological unit may be of different ages in different places.

Lower Lias.

The name Lias is supposed to be a corruption of the word "layers," used by the quarrymen in the west of England in reference to the stratified clays and limestones of the lower beds. There is a gradual passage from the Trias through the Rhaetic beds into the Lias.

The Lower Lias, the thickest division, is usually divided into the Hettangian (from Hettange, Lorraine) below, the Sinemurian (from Sinemurum = Semur, Cote-d'Or) in the middle, and the Charmouthian (from Charmouth, Dorset) above. It consists al most exclusively of clay, with thin layers of argillaceous lime stone. It covers a considerable area in England from the coast of Dorset to north Yorkshire, forming much of the central plain of England. 58o ft. thick in Dorset, it thins out almost completely against the Mendip Hills, thickening again to from 44o ft. to 700 ft. in the Midlands, and 700 ft. in Yorkshire.

See S. S. Buckman, Jurassic Chronology, I. Lias, Q.J.G.S., vol.

(1917), p. 257, and vol. lxxvi. (192o), p. 62 ; W. D. Lang, and others, in various papers on the Lower Lias of the Dorset Coast, Q.J.G.S., vol. lxxix. , P• 47; vol. lxxxii. (1926), p. 144; vol. lxxxiv. (1928), p. 179 ; and Proc. Geol. Assoc., vol. xxxv. (1924), p. 169 ; A. E. Trueman, on the Lower Lias of the Cardiff District, Proc. Geol. Assoc.,

vol. xxxi. (192o), p. 93 and of Glamorganshire, ibid. vol. xxxiv. (1923), p. 246, of Somerset (with J. W. Tutcher), Q.J.G.S., vol. lxxxi. (1925), P. 595.

Middle Lias

or Domerian consists principally of sands in the lower part and ferruginous limestone (marlstone) in the upper part. In Dorset, it consists of sands and sandy clays, about 35o ft.

thick, with a seam of marlstone (r ft.) at the top; it is attenuated against the Mendips. Towards the Cotswolds it thickens, the sands to 150 ft., the overlying marlstone to 15 ft. In Oxfordshire, Leicestershire and south Lincolnshire, the marlstone is extensively worked as an iron ore; in Yorkshire, the Middle Lias consists of sandy clays with the valuable Cleveland Ironstone in the upper part.

Upper Lias

or Toarcian (from Toarcium = Tours) is sub divided into a Lower series or Whitbian (from Whitby, Yorkshire) which consists principally of clays, and an Upper series or Yeo vilian (from Yeovil, Somerset) which is almost entirely arena ceous. The thickness of clays deposited during the Whitbian is greatest in Yorkshire (28o ft.), where they have yielded to corn merce alum and jet. In Northamptonshire the clays shrink to r6o ft., and at Fawler, north of Oxford, all the zones are repre sented in a remanie bed a few inches thick. In the Cotswolds, the Whitbian attains its greatest thickness (320 ft.), of which the highest 190 ft.—the Cotswold Sands—are arenaceous. In Dorset, the Whitbian is represented by ro ft. of clays, resting on a cephalo pod bed, 5 ft. thick.

The most important development of the Yeovilian is in Northamptonshire, where, together with the topmost zone of the Whitbian, it contains the Northampton Iron Ore, extensively worked all along the outcrop. The Yeovilian is represented in Yorkshire by sands (50 ft.) ; near Oxford, by To ft. of unfossil iferous clay; in the Cotswold, by a cephalopod bed, some 3 ft. thick; and in Dorset, by the Bridport and Yeovil Sands, 13o ft. thick.