Page:VCH Norfolk 1.djvu/33

 GEOLOGY but it is not known what older formation occurs below. Paljeozoic rocks, possibly even Coal Measures, might be found, as may be inferred from the fact that at Harwich old slaty rocks, presumably Silurian, were reached at a depth of little over i,ooo feet ; and at Stutton, in Suffolk, similar rocks were found at a little less than i,ooo feet — in both cases immediately below the Gault. In a well-boring at Holkham, clay, presumed to be Kimeridge Clay, was touched at a depth of 743 feet from the surface beneath the Lower Greensand ; but the evidence was not satisfactory, as the clay may belong to the Lower Greensand. We know nothing eastwards of this locality of any beds beneath the Gault in Norfolk. Hence in any trial-borings for Coal Measures it would be necessary to estimate in the western part of the county for a considerable thickness of Jurassic clay beneath a reduced thickness of the Chalk, etc. ; while in eastern Norfolk, although there would be a much greater thickness of Chalk, yet the Palaeozoic floor might be reached immediately beneath the Gault.* OXFORD AND CORALLIAN CLAYS The oldest formation which has been proved in Norfolk is the Oxford Clay, a marine mud well developed in the neighbourhood of Oxford, and which forms vales of meadow land across the country from Dorset to Lincolnshire. A boring made in 1827 at Lynn to a depth of 630 feet, was carried through about 50 feet of alluvium and 580 feet of Jurassic clays. These clays include portions of the Oxford Clay with Gryphaa dilatata ; of overlying Corallian Clay with Ammonites decipiens^ Bekmnites abbreviatus, and Ostrea bullata ; and of Kimeridge Clay with Ostrea deltoidea. Corallian clay may also occur beneath the alluvium at Denver Sluice, south-west of Downham Market, where Ammonites decipiens has been obtained.* KIMERIDGE CLAY The oldest formation which comes to the surface in Norfolk is the Kimeridge Clay, so named from its development at Kimeridge, on the Dorset coast. It consists of dark blue, almost black, clay and shale, with occasional cement-stones, or septaria, and bands of limestone ; and it has been dug for the manufacture of bricks and tiles at Downham Market, Watlington, and West Winch. As a formation it enters very little into the structure of the county ; it forms part of the islet in the Fens on which Southery is situated, and it borders the eastern side of the Fenland from Downham Market to a little north of Lynn, concealed in places by intervening portions of marshland. Its soil, naturally a stiff clay suitable (References are there given to observations by C. B. Rose and Fitton.) 3
 * See H. B. Woodward, * Geology of Norwich' {Geol. Survey), 1881, pp. 6, 169.
 * See H. B. Woodward, 'Jurassic Rocks of Britain' {Geol. Survey), vol. v., 1895, pp. 61, 147.