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 



* Beeby Thompson, ' Excursion to Weldon, Dene, and Gretton,' Proc. Gcol. Assoc, 

 vol. xvi. p. 226 (Nov. 1899). 



* Samuel Sharp, 'The Oolites of Northamptonshire,' pt. i., Quart. Journ. Gcol. Soc. 

 (Aug. 1870), p. 354. 



19 



