Downland,chalkland,chalk downs or justdowns are areas of openchalkhills, such as theNorth Downs. This term is used to describe the characteristic landscape in southernEngland 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 commonlyhillforts - compare Germanic "burg" (fort) and "berg" (mountain).[2]
The largest area of downland in southern England is formed bySalisbury Plain, mainly inWiltshire. To the southwest, downlands continue viaCranborne Chase intoDorset as theDorset Downs and southwards throughHampshire as theHampshire Downs onto theIsle of Wight. To the northeast, downlands continue along theBerkshire Downs andChiltern Hills through parts ofBerkshire,Oxfordshire,Buckinghamshire,Hertfordshire,Bedfordshire and intoCambridgeshire. To the east downlands are found north ofthe Weald inSurrey,Kent and part ofGreater London, forming theNorth Downs. To the southeast the downlands continue intoWest Sussex andEast Sussex as theSouth Downs.[1] Similar chalk hills are also found further north inLincolnshire andYorkshire where they are known asthe Wolds.
The Chalk Group is a sequence ofUpper Cretaceouslimestones. The dominantlithology is relatively soft porous white chalk with only poorly-defined bedding. The chalk is classified as abiomicrite,[3] with microscopiccoccoliths and other fine-grained fossil debris in a matrix ofmicrite mud. Small amounts of silica were also deposited, mainly fromsponge spicules, which moved duringdiagenesis and accumulated to formflints. The Chalk Group either directly overlies the impermeable uppermostLower CretaceousGault Clay or permeableUpper Greensand Formation above the Gault Clay.
Since its deposition, the chalk in southern England has been uplifted,faulted,fractured andfolded by the distant effects of theAlpine Orogeny. The fracturing has greatly increased the chalk's permeability, such that it is a majoraquifer.[4]Sedimentary basins formed byrifting during the Triassic to Early Cretaceous wereinverted during the LatePaleogene toMiocene leading to the formation of structures such as theWealden Anticline and thePortland-Wight Monocline.[5] Later erosion has produced the characteristic ridges of the downland landscape. The landscape was further modified during theQuaternary period by the area's proximity to the southern edge of the ice sheets formed during thelast ice age. Theseperiglacial effects included significant amounts of dissolution of the chalk and the modification of existing valleys due to a combination of frozen ground andsnowmelt.[6]
Downland develops when chalk rock becomes exposed at the surface. The chalk slowlyerodes to form characteristic rolling hills and valleys. As theCretaceous chalk layer in southern England is typically tilted, chalk downland hills often have a markedscarp slope on one side, which is very steep, and a much gentlerdip slope on the other. Where the downs meet the sea, characteristic white chalkcliffs form, such as theWhite Cliffs of Dover andBeachy Head.[1]
Chalk deposits are generally very permeable, so the height of thewater table in chalk hills rises in winter and falls in summer.[4] This leads to characteristic chalk downland features such as dry valleys orcoombes, and seasonally-flowing streams orwinterbournes. 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 aspringline 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 theWhite Horse Hills, above theVale of White Horse. In many chalk downland areas there is no surface water at all other than artificially createddewponds.[1]
Thesoil profile of chalk downland in England is a thin soil overlaying theparentchalk. Weathering of the chalk has created a characteristic soil known asrendzina.[7] Unlike many soils in which there are easily distinguished layers orsoil horizons, a chalk rendzina soil consists of only a shallow darkhumus 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-buildingclay 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, theseterracettes (commonly known as sheep tracks) were formed by the movement of soil downhill, a process known assoil creep.
The dominant habitat in chalk downland is typicallycalcareous grassland, formed by grazing from bothlivestock and wild animals. Chalk downland is often unsuitable for intensiveagriculture,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 toscrub 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 inDorset 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 uniquebiodiversity.[1]
