Cornbrash Formation

The Cornbrash Formation is a Middle Jurassic geological formation in England. It ranges in age from Bathonian to Callovian, the uppermost part of the Middle Jurassic. Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus. The name Cornbrash is an old English agricultural name applied in Wiltshire to a variety of loose rubble or brash which, in that part of the country, forms a good soil for growing corn. The name was adopted by William Smith for a thin band of shelly limestone which, in the south of England, breaks up in the manner indicated. Although only a thin group of rocks (10–25 feet c. 3–7 m), it is remarkably persistent; it may be traced from Weymouth to the Yorkshire coast, but in north Lincolnshire it is very thin, and probably dies out in the neighborhood of the Humber. It appears again, however, as a thin bed in Gristhorpe Bay, Cayton Bay, Wheatcroft, Newton Dale and Langdale. In the inland exposures in Yorkshire it is difficult to follow on account of its thinness, and the fact that it passes up into dark shales in many places the so-called clays of the Cornbrash, with Avicula echinata. The Cornbrash is of little value for building or road-making, although it is used locally; in the south of England it is not oolitic, but in Yorkshire it is a rubbly, marly, frequently ironshot oolitic limestone. In Bedfordshire it has been termed the Bedford limestone.

Cornbrash Formation
Stratigraphic range: Bathonian-Callovian
Cornbrash Formation at Shorncote Quarry in Gloucestershire
Type	Geological formation
Unit of	Great Oolite Group
Underlies	Kellaways Formation, Cayton Clay Formation
Overlies	Forest Marble Formation, Blisworth Clay Formation, Scalby Formation
Thickness	0-10.5 m
Lithology
Primary	Limestone
Location
Region	England
Country	UK
Extent	Dorset coast to Yorkshire coast

Fossils

The Cornbrash is a very fossiliferous formation; the fauna indicates a transition from the Lower to the Middle Oolites, though it is probably more nearly related to that of the beds above than to those below. Good localities for fossils are Radipole near Weymouth, Closworth, Wincanton, Trowbridge, Cirencester, Witney, Peterborough and Sudbrook Park near Lincoln. A few of the important fossils are: Waldheimia lagenalis, Pecten levis, Avicula echinata, Ostrea fiabelloides, Mycicites decurtatus, Echinobrissus clunicularis. Macrocephalites macrocephalus is abundant in the midland counties but rarer in the south; belemnites are not known. Indeterminate stegosaurian dinosaur material (sometimes known under the nomen nudum "Eoplophysis") have been discovered in this formation. The teleosaurid crocodyliformes Yvridiosuchus, Seldsienean, Clovesuurdameredeor and Deslongchampsina are known from the formation.

See also

• List of dinosaur-bearing rock formations
  • List of stratigraphic units with indeterminate dinosaur fossils

References

• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.
• This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Cornbrash". Encyclopædia Britannica. Vol. 7 (11th ed.). Cambridge University Press. p. 163.