Fig. 9. Lias clay being worked for the manufacture of hand-made sand-faced roofing tiles. (By courtesy of Messrs Webb Bros. Ltd., Cheltenham.)
The 'Lias clays'—the lowest of the Jurassic formation—are chiefly dark, bituminous shales, including the 'alum shales,' and are often seriously contaminated with pyrites and ironstone. When carefully selected they may be used to advantage in the production of most red articles such as bricks, tiles, chimney pots, etc. They shrink less in the kiln than do most clays, and are easily fusible on account of the lime they contain, but on the whole this formation is of great value for the manufacture of the articles just mentioned.
Agriculturally, the Lias clays are laid down for grass, but the lighter soils are useful for arable purposes.
The 'Oolitic clays,' which are also Jurassic, usually contain limestone in the form of nodules, but are nevertheless important. They form a broad belt above the Lias from Dorset to Yorkshire, and include the blue clays of the Purbeck beds, stiff blue bituminous Kimeridge clays, the irregular, sandy Coral Rag clays, the famous Oxford clay (from which the Peterborough and Fletton bricks are made), the Kellaways blue clay, and the Fuller's Earth deposits.
The 'Kimeridge clays' are dark, stiff laminated clays, closely resembling gault, and are much used in the West and Midlands for brickmaking. A well-known deposit of this character has long been used at Pickering in Yorkshire, but the most typical deposits are in Huntingdonshire. The Kimeridge clays contain a bituminous shale, or Sapropelic Coal, which evolves a characteristic odour on burning.
Agriculturally, the Kimeridge clays resemble gault and are difficult to work as arable land, though they form first-rate pasturage.
Fig. 10. Oxford clay near Peterborough. (By courtesy of Messrs Ruston, Proctor & Co. Ltd.)