Downland

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This article is about the geographical feature. For the ship anchorage, see The Downs (ship anchorage).
Devil's Dyke, a dry valley in the South Downs

Downland, chalkland, chalk downs or just downs are areas of open chalk hills, such as the North Downs. This term is used to describe the characteristic landscape in southern England where chalk is exposed at the surface.[1] The name "downs" is derived from the Old English word dun, meaning "hill".[2]

Distribution

Downland areas in southern England

The largest area of downland in southern England is formed by

the Weald in Surrey, Kent and part of Greater London, forming the North Downs. To the southeast the downlands continue into West Sussex and East Sussex as the South Downs.[1] Similar chalk hills are also found further north in Lincolnshire and Yorkshire where they are known as the Wolds
.

Geology

Main article: Chalk Group

The Chalk Group is a sequence of

Gault Clay or permeable Upper Greensand Formation
above the Gault Clay.

Since its deposition, the chalk in southern England has been uplifted,

periglacial effects included significant amounts of dissolution of the chalk and the modification of existing valleys due to a combination of frozen ground and snowmelt.[6]

Formation

Beachy Head cliffs and Belle Tout Lighthouse

Downland develops when chalk rock becomes exposed at the surface. The chalk slowly erodes to form characteristic rolling hills and valleys. As the Cretaceous chalk layer in southern England is typically tilted, chalk downland hills often have a marked scarp slope on one side, which is very steep, and a much gentler dip slope on the other. Where the downs meet the sea, characteristic white chalk cliffs form, such as the White Cliffs of Dover and Beachy Head.[1]

Hydrology

Chalk deposits are generally very permeable, so the height of the water table in chalk hills rises in winter and falls in summer.[4] This leads to characteristic chalk downland features such as dry valleys or coombes, and seasonally-flowing streams or winterbournes. The practice of extracting water from this aquifer, in order to satisfy the increasing demand for water, may be putting some of these streams under stress.

In the valleys below the downs at the base of the chalk layer, greensand or gault clay comes to the surface and at the interface at the top of the gault a

dewponds.[1]

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.[7] Unlike many soils in which there are easily distinguished layers or soil 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 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

soil creep
.

Habitat

Galium verum (L.) Lady's Bedstraw, a typical English chalk downland plant

The dominant habitat in chalk downland is typically

scrub or other less rare habitat, essentially destroying the delicate calcareous grassland. The UK cover of lowland calcareous grassland has suffered a sharp decline in extent since the middle of the twentieth 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 to preserve its unique biodiversity.[1]

See also

References

  1. ^
    ISBN 9780198606192
    .
  2. ^ "down". Online Etymology Dictionary. Retrieved 9 October 2020.
  3. ISBN 9780632017331
    .
  4. ^
    S2CID 140713528
    .
  5. doi:10.1016/j.pgeola.2011.09.007
    .
  6. ISBN 9780521310161
    .
  7. ^ South Downs National Park Authority (2020). "Down to earth – the unique soil of the South Downs National Park". Retrieved 10 October 2020.

External links

Wikimedia Commons has media related to Downland.
Look up downland in Wiktionary, the free dictionary.
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