Downland

Last updated November 30, 2024

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 Celtic word "dun", meaning "fort" or "fastness" (and by extension "fortified settlement", from which it entered English as "town", similar to Germanic "burg"/"burough"), though the original meaning would have been "hill", as early forts were commonly hillforts - compare Germanic "burg" (fort) and "berg" (mountain).^[2]

Distribution

The largest area of downland in southern England is formed by Salisbury Plain, mainly in Wiltshire. To the southwest, downlands continue via Cranborne Chase into Dorset as the Dorset Downs and southwards through Hampshire as the Hampshire Downs onto the Isle of Wight. To the northeast, downlands continue along the Berkshire Downs and Chiltern Hills through parts of Berkshire, Oxfordshire, Buckinghamshire, Hertfordshire, Bedfordshire and into Cambridgeshire. To the east downlands are found north of 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

The Chalk Group is a sequence of Upper Cretaceous limestones. The dominant lithology is relatively soft porous white chalk with only poorly-defined bedding. The chalk is classified as a biomicrite,^[3] with microscopic coccoliths and other fine-grained fossil debris in a matrix of micrite mud. Small amounts of silica were also deposited, mainly from sponge spicules, which moved during diagenesis and accumulated to form flints. The Chalk Group either directly overlies the impermeable uppermost Lower Cretaceous Gault Clay or permeable Upper Greensand Formation above the Gault Clay.

Since its deposition, the chalk in southern England has been uplifted, faulted, fractured and folded by the distant effects of the Alpine Orogeny. The fracturing has greatly increased the chalk's permeability, such that it is a major aquifer.^[4] Sedimentary basins formed by rifting during the Triassic to Early Cretaceous were inverted during the Late Paleogene to Miocene leading to the formation of structures such as the Wealden Anticline and the Portland-Wight Monocline.^[5] Later erosion has produced the characteristic ridges of the downland landscape. The landscape was further modified during the Quaternary period by the area's proximity to the southern edge of the ice sheets formed during the last ice age. These 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

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 springline can occur where water emerges from the porous chalk or the underlying greensand. Along this line, settlements and farms were often built, as on the higher land no water was available. This is demonstrated very clearly beneath the scarp of the White Horse Hills, above the Vale of White Horse. In many chalk downland areas there is no surface water at all other than artificially created 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 terracettes (commonly known as sheep tracks) were formed by the movement of soil downhill, a process known as soil creep.

Habitat

The dominant habitat in chalk downland is typically calcareous grassland, formed by grazing from both livestock and wild animals. Chalk downland is often unsuitable for intensive agriculture, horticulture, or development because of the nutrient-poor, shallow soil and difficult slopes. For this reason downland often survived uncultivated when other, more easily worked land was ploughed or reseeded. This shallow soil structure makes downland ecosystems extremely fragile and easy to destroy. With modern machinery and fertilising techniques, it has become possible to use some previously uncultivated downland for farming, and the decline of extensive grazing has meant that many areas of downland, neither cultivated nor grazed, revert to 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]

Related Research Articles

Salisbury Plain is a chalk plateau in southern England covering 300 square miles (780 km²). It is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the Chalk Group and largely lies within the county of Wiltshire, but stretches into Hampshire.

The Chiltern Hills or the Chilterns are a chalk escarpment in southern England, northwest of London, covering 660 square miles (1,700 km²) across Oxfordshire, Buckinghamshire, Hertfordshire, and Bedfordshire, stretching 45 miles (72 km) from Goring-on-Thames in the southwest to Hitchin in the northeast. The hills are 12 miles (19 km) at their widest.

The South Downs are a range of chalk hills in the south-eastern coastal counties of England that extends for about 260 sq mi (670 km²) across the south-eastern coastal counties of England from the Itchen valley of Hampshire in the west to Beachy Head, in the Eastbourne Downland Estate, East Sussex, in the east. The Downs are bounded on the northern side by a steep escarpment, from whose crest there are extensive views northwards across the Weald. The South Downs National Park forms a much larger area than the chalk range of the South Downs, and includes large parts of the Weald.

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. Much of the North Downs comprises 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.

The South Downs National Park is England's newest national park, designated on 31 March 2010. The park, covering an area of 1,627 square kilometres (628 sq mi) in southern England, stretches for 140 kilometres (87 mi) from Winchester in the west to Eastbourne in the east through the counties of Hampshire, West Sussex and East Sussex. The national park covers the chalk hills of the South Downs and a substantial part of a separate physiographic region, the western Weald, with its heavily wooded sandstone and clay hills and vales. The South Downs Way spans the entire length of the park and is the only National Trail that lies wholly within a national park.

The Chalk Group is the lithostratigraphic unit which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest European chalk 'province'. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment.

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. Greensand is also loosely applied to any glauconitic sediment.

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. 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 Dorset Downs are an area of chalk downland in the centre of the county Dorset in south west England. The downs are the most western part of a larger chalk formation which also includes Cranborne Chase, Salisbury Plain, Hampshire Downs, Chiltern Hills, North Downs and South Downs.

Dorset is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology, which includes an almost unbroken sequence of rocks from 200 to 40 million years ago (Mya) and superficial deposits from 2 Mya to the present. In general, the oldest rocks appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.

The visible geology of Hampshire in southern England broadly comprises a folded succession of sedimentary rocks dating from the Cretaceous and Palaeogene periods – mostly gentle folding in the north, more complex folding along the south coast. The lower (early) Cretaceous rocks are sandstones and mudstones whilst those of the upper (late) Cretaceous are the various formations that comprise the Chalk Group and give rise to the county's downlands. Overlying these rocks are the less consolidated Palaeogene clays, sands, gravels and silts of the Lambeth, Thames and Bracklesham Groups which characterise the Hampshire Basin.

Rendzina is a soil type recognized in various soil classification systems, including those of Britain and Germany as well as some obsolete systems. They are humus-rich shallow soils that are usually formed from carbonate- or occasionally sulfate-rich parent material. Rendzina soils are often found in karst and mountainous regions.

The Greensand Ridge, also known as the Wealden Greensand, is an extensive, prominent, often wooded, mixed greensand/sandstone escarpment in south-east England. Forming part of the Weald, a former dense forest in Sussex, Surrey and Kent, it runs to and from the East Sussex coast, wrapping around the High Weald and Low Weald. 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.

<span class="mw-page-title-main">Geography of Kent</span>

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 21 miles across the straits.

Cranborne Chase and West Wiltshire Downs, marketed as the Cranborne Chase National Landscape, is an Area of Outstanding Natural Beauty (AONB) covering 379 square miles (980 km²) of Dorset, Hampshire, Somerset and Wiltshire. It is the sixth largest AONB in England.

The western Weald is an area of undulating countryside in Hampshire and West Sussex containing a mixture of woodland and heathland areas.

The geology of Kent in southeast England largely consists of a succession of northward dipping late Mesozoic and Cenozoic sedimentary rocks overlain by a suite of unconsolidated deposits of more recent origin.

The geology of Surrey is dominated by sedimentary strata from the Cretaceous, overlaid by clay and superficial deposits from the Cenozoic.

Beeding Hill to Newtimber Hill is a 321 ha (790-acre) biological and geological Downland Site of Special Scientific Interest (SSSI) north of Shoreham-by-Sea in West Sussex. It is a Nature Conservation Review site, Grade 2, and it includes Devil's Dyke Geological Conservation Review site.

The geology of the South Downs National Park in South East England comprises a gently folded succession of sedimentary rocks from the Cretaceous and early Palaeogene periods overlain in places by a range of superficial deposits from the last 2.6 million years. Whereas the South Downs are formed from the Late Cretaceous age chalk, the South Downs National Park extends into the Weald to the north of the range and thereby includes older rock strata dating from the Early Cretaceous including sandstones and mudstones. The youngest solid rocks are found on the southern fringes of the National Park in the eastern extension of the Hampshire Basin and include sand, silt and clay deposited during the Palaeocene and Eocene epochs.

References

1 2 3 4 5 Bettey, J. (2002). "1. Downlands". In Thirsk, J. (ed.). Rural England: An Illustrated History of the Landscape. Oxford University Press. pp. 27–49. ISBN 9780198606192.
↑ "down". Online Etymology Dictionary. Retrieved 9 October 2020.
↑ Clayton, C.R.I.; Matthews, M.C. (1987). "Deformation, diagenesis and the mechanical behaviour of chalk". In Jones, M.E.; Prescott, R.M.F.; Prescott, R.M.E. (eds.). Deformation of Sediments and Sedimentary Rocks. Geological Society, Special Publications. Vol. 29. p. 55–62. ISBN 9780632017331.
1 2 Butler, A.P.; Hughes, A.G.; Jackson, C.R.; Ireson, A.M.; Parker, S.J.; Wheatear, H.S.; Peach, D.W. (2012). "Advances in modelling groundwater behaviour in Chalk catchments". In Shepley, M.G.; Whiteman, M.I.; Hulme, P.J.; Grout, M.W. (eds.). Groundwater Resources Modelling: A Case Study from the UK. Geological Society, Special Publications. Vol. 364. doi:10.1144/SP364.9. ISBN 9781862393448. S2CID 140713528.
↑ Hopson P. (2011). "The geological history of the Isle of Wight: an overview of the 'diamond in Britain's geological crown'" (PDF). Proceedings of the Geologists' Association. 122 (5): 745–763. doi:10.1016/j.pgeola.2011.09.007.
↑ Ballantyne, C.K.; Harris, C. (1994). The Periglaciation of Great Britain. Cambridge University Press. pp. 153–154. ISBN 9780521310161.
↑ South Downs National Park Authority (2020). "Down to earth – the unique soil of the South Downs National Park" . Retrieved 10 October 2020.

UK Lowland calcareous grassland habitat action plan

External links

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[Bettey_2002-1] 1 2 3 4 5 Bettey, J. (2002). "1. Downlands". In Thirsk, J. (ed.). Rural England: An Illustrated History of the Landscape. Oxford University Press. pp. 27–49. ISBN 9780198606192.

[2] "down". Online Etymology Dictionary. Retrieved 9 October 2020.

[Clayton_&_Matthews_1987-3] Clayton, C.R.I.; Matthews, M.C. (1987). "Deformation, diagenesis and the mechanical behaviour of chalk". In Jones, M.E.; Prescott, R.M.F.; Prescott, R.M.E. (eds.). Deformation of Sediments and Sedimentary Rocks. Geological Society, Special Publications. Vol. 29. p. 55–62. ISBN 9780632017331.

[Butler_etal_2012-4] 1 2 Butler, A.P.; Hughes, A.G.; Jackson, C.R.; Ireson, A.M.; Parker, S.J.; Wheatear, H.S.; Peach, D.W. (2012). "Advances in modelling groundwater behaviour in Chalk catchments". In Shepley, M.G.; Whiteman, M.I.; Hulme, P.J.; Grout, M.W. (eds.). Groundwater Resources Modelling: A Case Study from the UK. Geological Society, Special Publications. Vol. 364. doi:10.1144/SP364.9. ISBN 9781862393448. S2CID 140713528.

[Hopson-5] Hopson P. (2011). "The geological history of the Isle of Wight: an overview of the 'diamond in Britain's geological crown'" (PDF). Proceedings of the Geologists' Association. 122 (5): 745–763. doi:10.1016/j.pgeola.2011.09.007.

[Ballantyne_&_Harris_1994-6] Ballantyne, C.K.; Harris, C. (1994). The Periglaciation of Great Britain. Cambridge University Press. pp. 153–154. ISBN 9780521310161.

[SDNPA-7] South Downs National Park Authority (2020). "Down to earth – the unique soil of the South Downs National Park" . Retrieved 10 October 2020.

[1]

[2]

[3]

[4]

[5]

[6]

[7]