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A roadcut near the Llano Uplift exposing Cambrian greensand in the lower unit. Notice the normal fault cutting through the formation.

Greensand is an olive-green coloured sandstone rock which is commonly found in narrow bands, particularly associated with bands of chalk and clay worldwide; it has been deposited in marine environments at various times during Earth history, such as during the Jurassic and Cretaceous periods.

Contents

Formation and locations

Greensand forms in anoxic marine environments that are rich in organic detritus and low in sedimentary input.[1] Greensands are occasionally fossil-rich, such as in the late Cretaceous deposits of New Jersey, USA.[2] Important exposures are known from both northern and western Europe and North America.

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Great Britain

In Great Britain, greensand usually refers to specific rock strata of Early Cretaceous age. A distinction is made between the Upper Greensand and Lower Greensand; the term greensand was originally applied by William Smith to glauconitic sandstones in the west of England and subsequently used for the similar deposits of the Weald, before it was appreciated that the latter are actually two distinct formations separated by the Gault Clay.[3]

Both Upper and Lower Greensand outcrops appear in the scarp slopes surrounding the London Basin and the Weald. Prominent seams are to be found in the Vale of White Horse, in Bedfordshire, in Kent, Surrey, the South Downs National Park[4] and Hampshire and the Jurassic Coast in Dorset.

The soil of the greensand is quite varied, ranging from fertile to fairly sterile. On the fertile soils chestnut and stands of hazel and oak are common, while Scots Pine and Birch colonise the poorer soils.[5] These Greensand Ridges are popular long distance walking routes, for instance the Greensand Way in Kent.

A sample of Cretaceous greensand from near Swanage, Dorset.

Lower Greensand

The Lower Greensand (known as the Woburn Sand north of the London Basin) is of Aptian age. In the Weald the Lower Greensand consists of four deposits which are partly diachronous: the Atherfield Clay 15–50 ft thick, the Folkestone Beds 60–250 ft thick; the Hythe beds 60–350 ft thick and the Sandgate Beds 5–120 ft thick.[6] Although it appears both north and south of the London Basin it is not present everywhere beneath the Chalk which underlies the basin; the Gault lies directly on eroded Jurassic or Devonian rocks under much of the area.

Upper Greensand

The Upper Greensand is of Albian age. It represents a sandy lithofacies deposited in areas of stronger currents than the Gault Clay. Like the Lower Greensand it is not present beneath the whole of the London Basin, apparently passing laterally into Gault clay east of a line between Dunstable and Tatsfield and of uncertain extent to the east of London.[7][8]

Outcrops of the Upper Greensand occur in the south-west of England including the Blackdown Hills and East Devon Plateau and the Haldon Hills, remnants of a once much wider extent.[9]

Properties and uses

The green colour of greensand is due to variable amounts of the mineral glauconite, an iron potassium silicate with very low weathering resistance; as a result, greensand tends to be weak and friable. It is a common ingredient as a source of potassium in garden fertilisers, such as in organic gardening and organic farming. Due to its chemical exchange properties, the glauconite of greensand is used as a water softener. Greensand coated with manganese oxide, known as manganese greensand, is used in well water treatment systems to remove insoluble ferric (oxidized) iron and manganese. It is also used as a type of rock for stone walls in areas where greensand is common.

In Roman times coarse grits derived from the lower greensand were used to line the inner surface of mortaria (grinding bowls) produced in the Oxfordshire pottery kilns.[10]

See also

References

  1. ^ Odin, G.S. (ed., 1988). Green marine clays. Development in sedimentology, 45. Elsevier, Amsterdam.
  2. ^ Dale A. Russell, An Odyssey in Time (Toronto: University of Toronto Press, 1989), pp. 137-9.
  3. ^ Gallois R.W. & Edmunds M.A. (4th Ed 1965), The Wealden District, British Regional Geology series, British Geological Survey, ISBN 011884078-9
  4. ^ South Downs Integrated Landscape Character Assessment
  5. ^ Greensand Way in Kent, 1992, Kent Count Council, ISBN 1-873010-23-0
  6. ^ Britain's Structure and Scenery, L.Dudley Stamp, Pub Sept 1946, Collins New Naturalist Series.
  7. ^ Sumbler M.G. (1996), London and the Thames Valley, British Regional Geology series, British Geological Survey, ISBN 0-11-884522-5
  8. ^ Ellison R.A. et al. 2004, Geology of London: Special Memoir for 1:50,000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales), British Geological Survey, Keyworth, ISBN 0-85272478-0
  9. ^ Edmonds E.A.,McKeown M.C. & Williams M. (1975), South-West England, British Regional Geology series, British Geological Survey, ISBN 0118807137
  10. ^ Henig M. & Booth P. (2000), Roman Oxfordshire, pg.166

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

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. The term was introduced by the early English geologists for certain sandy rocks which frequently exhibited a greenish colour on account of the presence of minute grains of the green mineral glauconite. Until the fossils of these rocks came to be carefully studied there was much confusion between what is now known as the Upper Greensand (Selbornian) and the Lower Greensand. Here we shall confine our attention to the latter.

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, in the Isle of Wight, in Dorsetshire about Swanage, and it appears again beneath the northern outcrop of the Chalk in Berkshire, Oxfordshire and Bedfordshire, and thence it is traceable through Norfolk and Lincolnshire into east Yorkshire. It rests conformably upon the Wealden formation in the south of England, but it is clearly separable from the beds beneath by the occurrence of marine fossils, and by the fact that there is a marked overlap of the Lower Greensand on the Weald in Wiltshire, and derived pebbles are found in the basal beds. The whole series is Boo 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 subdivided as follows, although the several members are not everywhere recognizable: - Isle of Wight.

Folkestone Beds (70-100 ft.). Carstone and Sand rock series. Sandgate Beds (75-100 ft.). Ferruginous Sands (Shanklin sands). Hythe Beds (80-300 ft.).. Ferruginous Sands (Walpen sands). Atherfield Clay (20-90 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). Southwest 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 Kent, and about Midhurst, Pulborough and Petworth. At Nutfield the celebrated fuller's earth deposits occur on this horizon; it is also found near Maidstone, at Bletchingley and Red Hill. The Folkestone beds are light-coloured, rather coarse sands, enclosing layers of siliceous limestone (Folkestone stone) and chert; a phosphatic bed is found near the top. These beds are well seen in the cliffs at Folkestone and near Reigate. At Ightham there is a fine, hard, white sandstone along with a green, quartzitic variety (Ightham stone). In Sussex the limestone and chert are usually lacking, but a ferruginous grit, "carstone," occurs in lenticular masses and layers, which is used for road metal at Pulborough, Fittleworth, &c.

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 Oxfordshire, the sands and fuller's earth of Woburn, the Leighton Buzzard sands, the brick clays of Snettisham, and perhaps the Sandringham sands of Norfolk, and the carstone of that county and Lincolnshire. The upper ironstone, limestone and clay of the Lincolnshire Tealby beds appear to belong to this horizon along with the upper part of the Speeton beds of Yorkshire. The sands of the Lower Greensand are largely employed for the manufacture of glass, for which purpose they are dug at Aylesford, Godstone, near Reigate, Hartshill, near Aylesbury and other places; the ferruginous sand is worked as an iron ore at Seend.

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; while the upper part of the Folkestone beds would fall within the lower Albian.

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.)


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