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[[Image:culver_bill1.jpg|250px|thumb|[[Culver Down]], [[Isle of Wight]].]] |
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A '''downland''' is an area of open [[chalk]] upland. This term is especially used to describe the [[chalk]] countryside in southern [[England]]. Areas of downland are often referred to as '''Downs'''. |
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==Formation of downland== |
==Formation of downland== |
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Revision as of 12:48, 3 March 2005
A downland is an area of open chalk upland. This term is especially used to describe the chalk countryside in southern England. Areas of downland are often referred to as Downs.
Formation of downland
Downland is formed when Chalk Formations are forced above the sea level. Over centuries the chalk is slowly eroded to form sloping hills. Chalk deposits are very porous so the height of the water table in chalk hills rises in winter and falls in summer. Where the downs are being eroded quickly, steep white chalk cliffs are formed such as the White cliffs of Dover.
Downland soil
The soil profile of chalk downland in England is a thin soil overlaying the parent chalk. Weathering of the chalk has created a characteristic soil known as rendzina. Unlike many soils in which there are easily distinguished layers or horizons, a chalk rendzina soil consists of only a shallow dark humus rich surface layer which grades through a lighter brown hillwash containing small pellets of chalk, to the white of the chalk itself. This is largely because of the purity of the chalk which is here about 98% calcium carbonate and the consequent absence of soil-building clay minerals which are abundant, for example, in valley floors.
Steep slopes on chalk downland develop a ribbed pattern of grass covered horizontal steps a foot or two high. Although subsequently emphasised by cattle and sheep walking along them, these terracettes (commonly known as sheep tracks) were formed by the movement of soil downhill, a process known as soil creep.
Downland habitat
In temperate regions chalk downland is typically calcareous grassland, a habitat formed by grazing farm animals. Prior to farming, these areas would probably have been covered with forest. Chalk downland is often unspoilt because it is often unsuitable for modern arable farming, horticulture and housing, due to the poor shallow soil and difficult slopes. However, equally this shallow soil structure makes downland ecosystems extremely fragile and easy to destroy. The UK cover of lowland calcareous grassland has suffered a sharp decline in extent since the middle of the 20th century. There are no comprehensive figures, but a sample of chalk sites in England surveyed in 1966 and 1980 showed a 20% loss in that period and an assessment of chalk grassland in Dorset found that over 50% had been lost between the mid-1950s and the early 1990s. Much remaining chalk downland has been protected against future development, due to its unique biodiversity.
Examples of downland
The Southern England Chalk Formation
- Berkshire Downs
- Chiltern Hills (Dunstable Downs)
- Cranborne Chase
- Dorset Downs (Hambledon Hill, Hodd Hill, Maiden Castle, Cerne Abbas Giant)
- Isle of Wight (The Needles, Culver Down)
- North Downs (White cliffs of Dover)
- Purbeck Hills (Ballard Down, Lulworth Cove, Old Harry Rocks, White Nothe)
- Salisbury Plain
- South Downs (Beachy Head, Eastbourne, Seven Sisters, Watership Down, Hampshire)
United States
- Austin Chalk formation, which underlies much of central Texas