In geology, Cornbrash was the name applied to the uppermost member of the Bathonian stage of the Jurassic formation in England. It 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 (1025 feet c. 300 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 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. The remains of saurians (Steneosaurus) are occasionally found. 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.