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 GEOLOGY between the two estuarine series; probably it does, but absence of sediment rather than loss of it by later denudation accounts for the missing beds/ The clays are dug for brick-making, and have been used for fire- clay and terra-cotta manufacture ; the ironstone yields a good quality of iron, but does not pay to work. Agriculturally these beds are probably the worst in the county, producing cold wet lands, and the heartily disliked oyster-bed soils, locally known as pen-earth or penny-earth. The Upper Estuarine series represents in time the Fuller's Earth of Gloucestershire and Somersetshire, indeed the limestone, 5^, may be a deposit contemporary with the Fuller's Earth rock. The upper part of 5a, together with the lower part of the limestones above, probably corresponds in time with the Stonesfield slate. The Great Oolite Limestone This set of beds consists of yellowish or white limestone in various courses, much jointed, sometimes compact and blue-hearted, and mostly bluish when deep-seated. The partings between the courses of stone may consist either of sand, marl, dirty clay, oyster-beds, or com- minuted shell. The limestone is seldom oolitic, and only occasionally can be worked as a freestone like its contemporary the Bath oolite, nevertheless it has been most extensively used in the county for building, both of churches and houses, and for walls, often without mortar. Some of the hard, blue, shelly and subcrystalline limestone will take a good polish ; around Castor and Alwalton such stone was formerly quarried and used under the name of Alwalton marble, but it appears to be lacking in durability. The limestone is much quarried for the production of lime, but to a still larger extent as a flux for local ironstone. The limestone division of the Great Oolite series retains consider- able uniformity in character and thickness throughout the county ; this is the more notable since not far from Banbury, in a south-westerly direction, it gets very sandy, and has been mistaken for the Northampton Sand ; and in Lincolnshire it almost disappears. It is distinctly a marine formation, as shown by the abundant fauna, yet the frequent alternation of oyster-beds, the common occurrence of plant remains, the change to sandy conditions to the south-west and dying out to the north-east, as well as its interposition between beds of an estuarine character, point to shallow water conditions and nearness to land. The Great Oolite Clay The clay named Great Oolite Clay by Prof Judd is the same as the Blisworth Clay of Mr. Sharp,^ and no doubt represents, in time, the vol. xvi. p. 226 (Nov. 1899). (Aug. 1870), p. 354. 19
 * Beeby Thompson, ' Excursion to Weldon, Dene, and Gretton,' Proc. Gcol. Assoc,
 * Samuel Sharp, 'The Oolites of Northamptonshire,' pt. i., Quart. Journ. Gcol. Soc.