Greensand

Greensand or green sand is a sand or sandstone which has a greenish color. This term is specifically applied to shallow marine sediment, that contains noticeable quantities of rounded greenish grains. These grains are called glauconies and consist of a mixture of mixed-layer clay minerals, such as smectite and glauconite mica. Greensand is also loosely applied to any glauconitic sediment.[1][2][3]

Greensand
A roadcut within the Llano Uplift on Texas Highway 1431 about 18 km (11 mi) northwest Marble Falls, Texas that exposes greensand of the Lion Mountain Sandstone (Cambrian) in the lower unit. Notice the normal fault cutting through the formation.
Glauconitic Siltstone
Occurrence of glauconitic siltstone in the Serra da Saudade ridge, in the Alto Paranaíba region, Minas Gerais, Brazil.

Formation

Greensand forms in anoxic marine environments that are rich in organic detritus and low in sedimentary input.[1] Having accumulated in marine environments, greensands can be fossil-rich, such as in the late-Cretaceous deposits of New Jersey.[4]

Occurrence

Important exposures are known from both northern and western Europe, North America, southeastern Brazil and north Africa. Well known and important greensands are the Upper and Lower Greensands of England and occur within Eocene and Cretaceous sedimentary strata underlying the coastal plains of New Jersey and Delaware. Although greensand has been found throughout Phanerozoic and Late Precambrian sedimentary deposits, it appears to be most common in Eocene, Cambrian, and Cretaceous sedimentary deposits.[1][2]

Brazil

In Brazil, greensand refers to a fertilizer produced from glauconitic siltstone unit belonging to the Serra da Saudade Formation, Bambuí Group, of Neoproterozoic/Ediacaran age. The outcrops occur in the Serra da Saudade ridge, in Alto Paranaíba region, Minas Gerais.[5] It is a silt-clay sedimentary rock, laminated, bluish-green, composed of glauconite (40–80%), potassium feldspar (10–15%), quartz (10–60%), muscovite (5%) and minor quantities of biotite (2%), goethite (<1%), titanium and manganese oxides (<1%), barium phosphate and rare-earth elements phosphates (<1%).

Enriched levels of potash have K2O grades between 8% and 12%, thickness up to 50 m (160 ft) and are associated to the glauconitic levels, dark green in color. Glauconite is authigenic and highly mature. The high concentration of this mineral is related to a depositional environment with a low sedimentation rate. The glauconitic siltstone has resulted from a high level flooding event in the Bambuí Basin. The sedimentary provenance is from supracrustal feldsic elements on a continental margin environment with an acidic magmatic arc (foreland basin).

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.[6] The Upper Greensand was also once known as either the "Malm" or "Malm Rock Of Western Sussex"[7]

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,[8] elsewhere in Hampshire, the Isle of Wight, 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.[9] These Greensand Ridges are popular long distance walking routes, for instance the Greensand Way in Kent.

GreensandSample
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 5–15 m (15–50 ft) thick, the Folkestone Beds 20–80 m (60–250 ft) thick; the Hythe beds 20–110 m (60–350 ft) thick and the Sandgate Beds 2–37 m (5–120 ft) thick.[10] Although it appears both north and south of the London Basin it is not present everywhere beneath the Chalk Group 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.[11][12]

Outcrops of the Upper Greensand occur in the southwest of England including the Blackdown Hills and East Devon Plateau and the Haldon Hills, remnants of a once much wider extent.[13]

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 organic gardening and farming fertilisers. Greensand glauconite is used as a water softener for its chemical-exchange properties. Greensand coated with manganese oxide (called manganese greensand) is used in well water treatment systems to remove dissolved (reduced) iron and manganese with the addition of an oxidant, usually potassium permanganate, under controlled pH conditions.[14] It is also used as a type of rock for stone walls in areas where greensand is common.

In Roman times in Britain, coarse grits derived from the lower greensand were used to line the inner surface of mortars (grinding bowls) produced in Oxfordshire pottery kilns.[15]

Recently, glauconitic greensand has become a popular organic soil amendment. The porous properties of glauconite greensand allows for the absorption of water and minerals, making irrigation and nutrient delivery much more efficient (see soil conditioner). Greensand can be used to absorb excess water in clay-rich soils and to prevent water loss in sandy soils.

See also

References

  1. ^ a b c Odin, G. S., ed. (1988). Green Marine Clays. Developments in Sedimentology. 45. Amsterdam: Elsevier. ISBN 978-0-444-87120-6.
  2. ^ a b Pettijohn, F. J.; Potter, P.; Siever, R. (1987). Sand and Sandstone. New York, NY: Springer-Verlag.
  3. ^ Neuendorf, K. K. E.; Mehl, J. P., Jr; Jackson, J. A., eds. (2005). Glossary of Geology (5th ed.). Alexandria, VA: American Geological Institute. ISBN 0-922152-76-4.
  4. ^ Russell, Dale A. (1989). An Odyssey in Time. Toronto, ON: University of Toronto Press. p. 137–139.
  5. ^ Moreira, Débora (2016). "Estratigrafia, petrografia e mineralização de potássio em siltitos verdes do grupo Bambuí na região de São Gotardo, Minas Gerais" (PDF). Revista Geociências. São Paulo: UNESP. 35: 157–171.
  6. ^ Gallois, R. W.; Edmunds, M. A. (1965). The Wealden District. British Regional Geology (4th ed.). British Geological Survey. ISBN 0-11-884078-9.
  7. ^ "Upper Greensand Formation". BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 22 May 2013.
  8. ^ "South Downs Integrated Landscape Character Assessment" (PDF). Archived from the original (PDF) on 2007-09-28.
  9. ^ Greensand Way in Kent. Kent County Council. 1992. ISBN 1-873010-23-0.
  10. ^ Stamp, L. Dudley (1946). Britain's Structure and Scenery. New Naturalist Series. Collins.
  11. ^ Sumbler, M. G. (1996). London and the Thames Valley. British Regional Geology. British Geological Survey. ISBN 0-11-884522-5.
  12. ^ 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). Keyworth, Nottinghamshire: British Geological Survey. ISBN 0-85272-478-0.
  13. ^ Edmonds, E. A.; McKeown, M. C.; Williams, M. (1975). South-West England. British Regional Geology. British Geological Survey. ISBN 0-11-880713-7.
  14. ^ MWH (2005). Crittenden, J.; et al. (eds.). Water Treatment: Principles and Design (2nd ed.). John Wiley & Sons. p. 1587–1588. ISBN 0-471-11018-3.
  15. ^ Henig, M; Booth, P. (2000). Roman Oxfordshire. p. 166.

External links

Black Down and Sampford Commons

Black Down and Sampford Commons (grid reference ST118161) is a 155.2-hectare (384-acre) biological Site of Special Scientific Interest in Devon and Somerset, notified in 1952.

The Little Breach reserve, which forms part of the SSSI is an area of heathy grassland on Greensand, with some blackthorn and birch, noted for its butterflies and moths.Blackdown and Sampford Commons have the finest and most extensive surviving examples of the heathland, carr woodland and marshy grassland habitats that have developed on the acidic soils overlying the Greensand and Keuper Marls of the Blackdown Hills. The heathland supports a typical invertebrate fauna, including a wide variety of butterfly species, and with spiders notably abundant. The site is regionally important for birds which favour heathland habitats.

Cambridge Greensand

The Cambridge Greensand is a geological formation in England whose strata date back to the Cenomanian stage of the Late Cretaceous. It forms the transitional bed between the Gault Formation and the Chalk Group in the vicinity of Cambridgeshire, and technically forms the lowest member bed of the West Melbury Marly Chalk Formation. The lithology is made out of glauconitic marl, with a concentration of phosphatic nodules and bones at the base. Dinosaur remains are among the fossils that have been recovered from the formation.

Dinodocus

Dinodocus (meaning "terrible beam") is the name of a genus of sauropod dinosaur, named by Richard Owen in 1884. The name is now usually considered a nomen dubium. The only species, D. mackesoni, a name given to some fossil limb bones from the Lower Greensand (Lower Cretaceous) of Kent, England, was formerly placed in the genus Pelorosaurus (Mantell, 1850), but review by Upchurch et al. (2004) concludes that Dinodocus is a nomen dubium.

Gault

The Gault Formation is a geological formation of stiff blue clay deposited in a calm, fairly deep-water marine environment during the Lower Cretaceous Period (Upper and Middle Albian). It is well exposed in the coastal cliffs at Copt Point in Folkestone, Kent, England, where it overlays the Lower Greensand formation, and underlies the Upper Greensand Formation. These represent different facies, with the sandier parts probably being deposited close to the shore and the clay in quieter water further from the source of sediment; both are believed to be shallow-water deposits.The etymology of the name is uncertain and probably of local origin.

Geography of Kent

Kent is the south-easternmost county in England. It is bounded on the north by the River Thames and the North Sea, and on the south by the Straits of Dover and the English Channel. The continent of Europe is a mere 21 miles across the Strait.The major geographical features of the county are determined by a series of ridges running from west to east across the county. These ridges are the remains of the Wealden dome, a denuded anticline across Kent, Surrey and Sussex, which was the result of uplifting caused by the Alpine movements between 10-20 million years ago. The dome was formed of an upper layer of Chalk above subsequent layers of Upper Greensand, Gault, Lower Greensand, Weald Clay and the Hastings Beds. The top of the dome eventually eroded away through weathering and ridges and valleys resulted across Kent and Sussex due to the exposed clay eroding at a faster rate than the exposed chalk, greensand and red sandstone and normal sandstone. The following ridges and the valleys have formed across Kent, listed from north to south:

the low lying London Clay marshlands along the Thames/Medway estuaries and along the North Kent coast;

the chalk North Downs, containing the highest point of the county, Betsom's Hill, at 251m/823 ft.

the Vale of Holmesdale formed from Gault Clay overlaid in the north with the upper layer greensand;

the Greensand Ridge, formed from the lower layer of greensand, containing the source of the River Medway and its tributaries;

the Low Weald, a Weald Clay valley

the sandstone High Weald.The chalk comes in three layers: the upper layer, about 500 feet thick, is a pure white limestone bedded and jointed with localised masses of flint (ideal for cement); the middle layer, about 170 feet thick, is a compact white chalk occasionally hard enough for building; the lower layer, about 170 feet thick, is a greyish marly chalk.Dartford, Gravesend, The Medway Towns, Sittingbourne, Faversham, Canterbury, Deal and Dover are built on chalk.The eastern part of the Wealden dome was eroded away by the sea. The White cliffs of Dover occur where the North Downs meets the coast. From there to Westerham is now the Kent Downs Area of Outstanding Natural Beauty. The chalk displays all its characteristic features such as steep sided dry valleys, and sunken roads.

Greensand is a calcareous sandstone containing an uneven distribution of the mineral glauconite, giving the sandstone a greenish tinge. On exposure to the air this oxidises into a yellow stain. Sevenoaks, Maidstone, Ashford, and Folkestone are built on the greensand. Greensand comes in four layers: the Folkestone Beds 60–250 ft thick; The Sandgate Beds 5–120 ft thick; the Hythe beds 60–350 ft thick and Atherfield Clays 15–50 ft thick. The soil of the greensand is quite varied, ranging from fertile to fairly sterile. On the fertile soils we see chestnut and stands of hazel and oak, while Scots Pine and Birch colonise the poorer soils.The Hastings Beds, which are resistant to weathering, leading to outcrops, such as High Rocks Tunbridge Wells, and sterile soil only suited to heathland and forests of Scots Pine. The Hastings Beds are divided into three formations: Tunbridge Wells Sand Formation 130–400 ft; Wadhurst Clay Formation 100–230 ft, shales with bands of sandstone and iron ore; and the Ashdown Formation 160–700 ft; sandstones. The Fairlight Clays form the upper part of the Ashdown Formation; grey and varigated shales. Tonbridge and Tunbridge Wells are built on the Hastings Beds.The Weald derives its ancient name from the Germanic word wald meaning simply woodland. Much of the area remains today densely wooded; where there are also heavy clays the tracks through are nearly impassable for much of the year.

The Wealden dome is a Mesozoic structure lying on a Palaeozoic foundation, which usually creates the right conditions for coal formation. This is found in East Kent, roughly between Deal, Canterbury, and Dover. The coal measures within the Westphalian Sandstone are deep (below 244m - 396m) and subject to flooding. They occur in two major troughs, which extend under the English Channel where similar coalfields are sited.Seismic activity has occasionally been recorded in Kent, though the epicentre is offshore. In 1382 and 1580 there were two earthquakes exceeding 6.0 on the Richter Scale. In 1776, 1950, and 28 April 2007 there were earthquakes of around 4.3. The 2007 earthquake caused physical damage in Folkestone.The coastline of Kent is continually changing, due to uplift, sedimentation, and marine erosion. The Isle of Thanet was till recently (AD 960) an island, formed around a deposit of chalk. The channels silted up with alluvium. Similarly Romney Marsh and Dungeness have been formed by accumulation of alluvium.Kent's principal river, the River Medway, rises near Edenbridge and flows some 25 miles (40 km) eastwards to a point near Maidstone, when it turns north. Here it breaks through the North Downs at Rochester before joining the River Thames as its final tributary near Sheerness. The river is tidal as far as Allington lock, but in earlier times cargo-carrying vessels reached as far upstream as Tonbridge. The Medway has captured the head waters of other rivers such as the River Darent. There are other rivers in Kent, most notably the River Stour in the east.

Glauconite

Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance.

It crystallizes with a monoclinic geometry. Its name is derived from the Greek glaucos (γλαυκος) meaning 'blue', referring to the common blue-green color of the mineral; its sheen (mica glimmer) and blue-green color presumably relating to the sea's surface. Its color ranges from olive green, black green to bluish green, and yellowish on exposed surfaces due to oxidation. In the Mohs scale it has hardness of 2. The relative specific gravity range is 2.4 - 2.95. It is normally found in dark green rounded brittle pellets, and with the dimension of a sand grain size. It can be confused with chlorite (also of green color) or with a clay mineral. Glauconite has the chemical formula – (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2.

Glauconite particles are one of the main components of greensand, glauconitic silstone and glauconitic sandstone. Glauconite has been called a marl in an old and broad sense of that word. Thus references to "greensand marl" sometimes refer specifically to glauconite. The Glauconitic Marl formation is named after it, and there is a Glauconitic Sandstone formation in the Mannville Group of Western Canada.

Greensand, New Jersey

Greensand is an unincorporated community located within Edison Township in Middlesex County, New Jersey, United States.

Greensand Ridge

The Greensand Ridge is an extensive, prominent, often wooded, mixed greensand/sandstone escarpment in south-east England. It runs to and from the East Sussex coast, around the Weald, a former dense forest in Sussex, Surrey and Kent. It reaches its highest elevation, 294 metres (965 ft), at Leith Hill in Surrey—the second highest point in south-east England, while another hill in its range, Blackdown, is the highest point in Sussex at 280 metres (919 ft). The eastern end of the ridge forms the northern boundary of Romney Marsh.

Greensand Way

The Greensand Way is a long distance path of 108 miles (174 km) in southeast England, from Haslemere in Surrey to Hamstreet in Kent. It follows the Greensand Ridge along the Surrey Hills and Chart Hills. The route is mostly rural, passing through woods, and alongside fruit orchards and hop farms in Kent and links with the Stour Valley Walk near Pluckley in Kent. The trail was opened on 15 June 1980 and is jointly managed by Surrey and Kent Councils who fully updated it in 2012 (route, waymarking, online guide).

Much of the land is designated an Area of Outstanding Natural Beauty (AONB). The stretch from Hindhead to Leith Hill has formed part of the Surrey Hills AONB since 1958. The Sevenoaks Ridge, from the Surrey–Kent border to Borough Green, is included in the Kent Downs AONB.

The waymarks alone are not sufficient to follow the trail – an OS map, or the online guide with maps, is required. An updated guide with maps and walk directions is available online from the Kent and Surrey Council websites. Public transport information, OS maps and GPS data is available online from the Saturday Walkers website. The original guidebook is out of print. On Ordnance Survey Explorer map 145, a spur – also labelled Greensand Way – is shown from Thursley to Farnham; this is not part of the path.

Holmbury Hill

Holmbury Hill is a wooded area of 261 metres (856 ft) above sea level and the site of an Iron Age hillfort. The Old Saxon word "holm" can be translated as hill and "bury" means fortified place. It sits along the undulating Greensand Ridge its summit being 805 feet (245 m) from the elevated and tightly clustered small village of Holmbury St. Mary in Surrey, England which was traditionally part of Shere, 8 kilometres (5.0 mi) away.

Lambert's Castle

Lambert's Castle is an Iron Age hill fort in the county of Dorset in southwest England. Since 1981 it has been designated as a Site of Special Scientific Interest (SSSI) on account of its geology, archaeology and ecology.

The hillfort is situated on a broad northerly spur at the summit of Lambert's Castle Hill, which rises to a height of 256 metres (840 ft). There are steep natural slopes on three sides of the fort, and linear ramparts across the flat southern approaches. The site is owned by the National Trust. A car park is accessible from the B3165 road. There are two other hill-forts near Lambert's Castle: Coney's Castle is about 1.5 kilometres (0.9 mi) to its south, and Pilsdon Pen is about 5 km (3.1 mi) to its north-east.

Lower Greensand Group

The Lower Greensand Group is a geological unit, which forms part of the underlying geological structure of southeast England. South of London in the counties of West Sussex, East Sussex, Surrey and Kent, which together form the wider Weald, the Lower Greensand can usually be subdivided to formational levels with varying properties into the Atherfield Clay Formation, the Hythe Formation, the Sandgate Formation, and the Folkestone Formation. In areas north and west of London, including Cambridgeshire, Bedfordshire and Buckinghamshire the Lower Greensand is referred to as the Woburn Sands Formation.

North Downs

The North Downs are a ridge of chalk hills in south east England that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. Westerham Heights, at the northern edge of the North Downs, near Bromley, South London, is the highest point in London at an elevation of 245 m (804 ft). The North Downs lie within two Areas of Outstanding Natural Beauty (AONBs), the Surrey Hills and the Kent Downs. The North Downs Way National Trail runs along the North Downs from Farnham to Dover.

Oxmoor Copse

Oxmoor Copse is just south of the village of Abinger Hammer and to the west of the village of Abinger Common, in Surrey. It is 1.23 hectares (3.0 acres) and is in an AONB lying within the Surrey Hills. The previous owners gave the copse to the Woodland Trust as a gift.

The copse lies on Greensand and the soil is acidic. The main trees are mature oaks and hazel; there are also bluebells. A considerable number of trees were blown down in the 1987 storm. Replanting took place in 1991. Arable and pastoral land surrounds the copse. On the fringes of the copse there is a considerable amount of Common Cudweed but generally the copse is species poor.The grid reference is TQ 090 467

River Sid

The River Sid is a minor river in East Devon. It flows for 6.5 miles (10.5 km) southwards from a source in Crowpits Covert (OSGB36 Grid reference SY138963) at a height of 206 metres above sea level. The source is at the head of a goyle or small ravine.

The underlying geology is impermeable silty mudstones and sandstones of the Triassic Keuper marl, overlain with permeable Greensand and clay-with-flints. The junction between the Greensand and Keuper Marl forms a spring line.

The river flows through Sidbury and Sidford to Sidmouth and is fed by springs flowing from East Hill and water from the Roncombe Stream, the Snod Brook and the Woolbrook. In Sidmouth the river outflows at the Ham through a shingle bar.The Sid Vale Association, the first Civic Society in Britain (founded in 1846) is based in the Sid Vale.

Surrey Hills AONB

Surrey Hills is a 422 km2 (163 sq mi) Area of Outstanding Natural Beauty (AONB) which covers one quarter of the county of Surrey, England. The AONB was designated in 1958 and adjoins the Kent Downs AONB to the east and the South Downs National Park in the south west.

Sutton Waldron

Sutton Waldron is a village and civil parish in north Dorset, England, situated on the A350 road between Iwerne Minster and Fontmell Magna, in the Blackmore Vale under the scarp of Cranborne Chase, 8 miles (13 km) north of Blandford Forum and 5 miles (8.0 km) south of Shaftesbury. In the 2011 census the parish had 93 dwellings, 87 households and a population of 200.The parish covers about 1,300 acres (530 ha) in a strip of land that, from west to east, is composed of Kimmeridge clay, Lower Greensand, Gault Clay, Upper Greensand and chalk.In 1086 in the Domesday Book Sutton Waldron was recorded as Sudtone; it had 24 households, one mill, six ploughlands, 6 acres (2.4 ha) of meadow and 40 acres (16 ha) of woodland. It was in the hundred of Gillingham and the lord and tenant-in-chief was Waleran the hunter.The parish church dates from 1847 and is constructed in the Decorated Gothic style.

Winterfold Forest

Winterfold Forest is a wooded area of the broadest plateau of the western Greensand Ridge in Surrey, England. It blends seamlessly into the Hurt Wood or Hurtwood.

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