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Encyclopedia Britannica - Main :: GRA-GUI |
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GREENSAND , in geology, the name that has been applied to no fewer than three distinct members of the Cretaceous System, viz. the Upper Greensand (see GAULT), the Lower Greensand and the so-called Cambridge Greensand, a local phase of the base of the Chalk (q.v.). The term was introduced by the early English geologists for certain sandy rocks which frequentlyexhibited a greenish colour on account of the presence of minute grains of the green mineral
The Lower Greensand was first examined in detail by W. H. Fitton (Q.J.G.S. iii., 1847), who, in 1845, had proposed the name " Vectine " for the formation. The name was revived under the form " Vectian " in 1885 by A. J. Jukes-Browne, because, although sands and sandstones prevail, the green colour has often changed by oxidation of the iron to various shades of red and brown, and other lithological types, clays and limestones represent this horizon in certain areas. The Lower Greensand is typically developed in the Wealden
district
east
Wealden
series is 800 ft. thick at Atherfield in the Isle of Wight, but it thins rapidly westward. It is usually clearly marked off from the overlying Gault.In the Wealden area the Lower Greensand has been sub-divided as follows, although the several members are not every-where recognizable: Isle of Wight. Folkestone Beds (7o-too ft.) . Carstone and Sand rock series . Sandgate Beds (75-too ft.) . Ferruginous Sands (Shanklin sands). Hythe Beds (80-30o ft.) . Ferruginous Sands (Walpen sands). Atherfield Clay (2o-90 ft.). . Atherfield Clay.The Atherfield Clay is usually a sandy clay, fossiliferous. The basal portion, 5-6 ft., is known as the " Perna bed " from the abundance of Perna Mulleti; other fossils are Hoplites Deshayesii, Exogyra sinuata, Ancyloceras Mathesonianum. The Hythe beds are interstratified thin limestones and sandstones; the former are bluish-grey in colour, compact and hard, with a certain amount of quartz and glauconite. The limestone is known locally as " rag "; the Kentish Rag has been largely employed as a building stone and roadstone; it frequently contains layers of chert (known as Sevenoaks stone near that town). The sandy portions are very variable; the stone is often clayey and calcareous and rarely hard enough to make a good building stone; locally it is called " hassock " (or Calkstone). The two stones are well exposed in the Iguanodon Quarry near Maidstone (so called from the discovery of the bones of that reptile). South-west of Dorking sandstone and grit become more prevalent, and it is known there as " Bargate stone," much used around Godalming. Pulborough stone is another local sandstone of the Hythe beds. Fuller's earth occurs in parts of this formation in Surrey. The Sandgate beds, mainly dark, argillaceous sand and clay, are well developed in east
The Lower Greensand usually forms picturesque, healthy country, as about Leith Hill, Hindhead, Midhurst, Petworth, at Woburn, or at Shanklin and Sandown in the Isle of Wight. Outside the southern area the Lower Greensand is represented by the Faringdon sponge-bearing beds in Berkshire, the Sandy and Potton beds in Bedfordshire, the Shotover iron sands of Oxford-shire, the sands and fuller's earth of Woburn, the Leighton Buzzard
Aylesford
Aylesbury
This formation is continuous across the channel into France, where it is well developed in Boulonnais. According to the continental classification the Atherfield Clay is equivalent to the Urgonian or Barremian; the Sandgate and Hythe beds belong to the Aptian (q.v.); while the upper part of the Folkestone beds would fall within the lower Albian (q.v.).See the Memoirs of the Geological Survey, " Geology of the Weald " (1875), " Geology of the Isle of Wight " (2nd ed., 1889), " Geology of the Isle of Purbeck " (1898); and the Record of Excursions, Geologists' Association (London, 1891). (J. A. H.) End of Article: GREENSAND If you wish, you can link directly to this article.
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