Downland, chalkland, chalk downs or just downs are areas of open chalk
hill
A hill is a landform that extends above the surrounding terrain. It often has a distinct Summit (topography), summit.
Terminology
The distinction between a hill and a mountain is unclear and largely subjective, but a hill is universally con ...
s, such as the
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. Much of the North Downs comprises two Areas of Outstanding Natural Beauty (AONBs): the Surrey Hills and ...
. This term is used to describe the characteristic landscape in southern England where
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...
is exposed at the surface.
The name "downs" is derived from the Old English word dun, meaning "hill".
Distribution
The largest area of downland in southern England is formed by
Salisbury Plain
Salisbury Plain is a chalk plateau in the south western part of central southern England covering . 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 wi ...
, mainly in
Wiltshire
Wiltshire (; abbreviated Wilts) is a historic and ceremonial county in South West England with an area of . It is landlocked and borders the counties of Dorset to the southwest, Somerset to the west, Hampshire to the southeast, Gloucestershire ...
. To the southwest, downlands continue via
Cranborne Chase
Cranborne Chase () is an area of central southern England, straddling the counties Dorset, Hampshire and Wiltshire. It is part of the Cranborne Chase and West Wiltshire Downs Area of Outstanding Natural Beauty (AONB).
The area is dominated by, ...
into
Dorset
Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset (unitary authority), Dors ...
as the
Dorset Downs
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 (from west to east) Cranborne Chase, Salisbury Plain, Ham ...
and southwards through
Hampshire
Hampshire (, ; abbreviated to Hants) is a ceremonial county, ceremonial and non-metropolitan county, non-metropolitan counties of England, county in western South East England on the coast of the English Channel. Home to two major English citi ...
as the
Hampshire Downs
The Hampshire Downs form a large area of downland in central southern England, mainly in the county of Hampshire but with parts in Berkshire and Wiltshire. They are part of a belt of chalk downland that extends from the South Downs in the southe ...
onto the
Isle of Wight
The Isle of Wight ( ) is a county in the English Channel, off the coast of Hampshire, from which it is separated by the Solent. It is the largest and second-most populous island of England. Referred to as 'The Island' by residents, the Isle of ...
. To the northeast, downlands continue along the
Berkshire Downs
The Berkshire Downs are a range of chalk downland hills in South east England split between the counties of Berkshire and Oxfordshire. They are part of the North Wessex Downs Area of Outstanding Natural Beauty. The western parts of the downs ...
and
Chiltern Hills
The Chiltern Hills is a chalk escarpment in England.
The area, northwest of London, covers stretching from Goring-on-Thames in the southwest to Hitchin in the northeast - across Oxfordshire, Buckinghamshire, Hertfordshire, and Bedfordshire. ...
through parts of
Berkshire
Berkshire ( ; in the 17th century sometimes spelt phonetically as Barkeshire; abbreviated Berks.) is a historic county in South East England. One of the home counties, Berkshire was recognised by Queen Elizabeth II as the Royal County of Berk ...
,
Oxfordshire
Oxfordshire is a ceremonial and non-metropolitan county in the north west of South East England. It is a mainly rural county, with its largest settlement being the city of Oxford. The county is a centre of research and development, primarily ...
,
Buckinghamshire
Buckinghamshire (), abbreviated Bucks, is a ceremonial county in South East England that borders Greater London to the south-east, Berkshire to the south, Oxfordshire to the west, Northamptonshire to the north, Bedfordshire to the north-ea ...
,
Hertfordshire
Hertfordshire ( or ; often abbreviated Herts) is one of the home counties in southern England. It borders Bedfordshire and Cambridgeshire to the north, Essex to the east, Greater London to the south, and Buckinghamshire to the west. For govern ...
,
Bedfordshire
Bedfordshire (; abbreviated Beds) is a ceremonial county in the East of England. The county has been administered by three unitary authorities, Borough of Bedford, Central Bedfordshire and Borough of Luton, since Bedfordshire County Council wa ...
and into
Cambridgeshire
Cambridgeshire (abbreviated Cambs.) is a Counties of England, county in the East of England, bordering Lincolnshire to the north, Norfolk to the north-east, Suffolk to the east, Essex and Hertfordshire to the south, and Bedfordshire and North ...
. To the east downlands are found north of
the Weald
The Weald () is an area of South East England between the parallel chalk escarpments of the North and the South Downs. It crosses the counties of Hampshire, Surrey, Sussex and Kent. It has three separate parts: the sandstone "High Weald" in the ...
in
Surrey
Surrey () is a ceremonial and non-metropolitan county in South East England, bordering Greater London to the south west. Surrey has a large rural area, and several significant urban areas which form part of the Greater London Built-up Area. ...
,
Kent
Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...
and part of
Greater London
Greater may refer to:
*Greatness, the state of being great
*Greater than, in inequality (mathematics), inequality
*Greater (film), ''Greater'' (film), a 2016 American film
*Greater (flamingo), the oldest flamingo on record
*Greater (song), "Greate ...
, forming the
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. Much of the North Downs comprises two Areas of Outstanding Natural Beauty (AONBs): the Surrey Hills and ...
. To the southeast the downlands continue into
West Sussex
West Sussex is a county in South East England on the English Channel coast. The ceremonial county comprises the shire districts of Adur, Arun, Chichester, Horsham, and Mid Sussex, and the boroughs of Crawley and Worthing. Covering an ar ...
and
East Sussex
East Sussex is a ceremonial and non-metropolitan county in South East England on the English Channel coast. It is bordered by Kent to the north and east, West Sussex to the west, and Surrey to the north-west. The largest settlement in East Su ...
as the
South Downs
The South Downs are a range of chalk hills that extends for about 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. ...
.
Similar chalk hills are also found further north in
Lincolnshire
Lincolnshire (abbreviated Lincs.) is a county in the East Midlands of England, with a long coastline on the North Sea to the east. It borders Norfolk to the south-east, Cambridgeshire to the south, Rutland to the south-west, Leicestershire ...
and
Yorkshire
Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a Historic counties of England, historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other Eng ...
where they are known as
the Wolds
The Wolds is a term used in England to describe a range of hills which consists of open country overlying a base of limestone or chalk.
Geography
The Wolds comprise a series of low hills and steep valleys that are in the main underlain by calcare ...
.
Geology
The Chalk Group is a sequence of
Upper Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
s. The dominant
lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
is relatively soft porous white chalk with only poorly-defined bedding. The chalk is classified as a
biomicrite,
with microscopic
coccolith
Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere''. ...
s and other fine-grained fossil debris in a matrix of
micrite
Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud. Flügel, Erik, ''Microfacies of Carbonate Rocks: Analysis, Interpretation and Application,'' Springe ...
mud. Small amounts of silica were also deposited, mainly from
sponge spicule
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Sponge spicules are made of calcium carbon ...
s, which moved during
diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
and accumulated to form
flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and start fir ...
s. The Chalk Group either directly overlies the impermeable uppermost
Lower Cretaceous
Lower may refer to:
*Lower (surname)
*Lower Township, New Jersey
*Lower Receiver (firearms)
*Lower Wick
Lower Wick is a small hamlet located in the county of Gloucestershire, England. It is situated about five miles south west of Dursley, eight ...
Gault Clay
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 ...
or permeable
Upper Greensand Formation
The Upper Greensand Formation is a Cretaceous formation of Albian to Cenomanian in age, found within the Wessex Basin and parts of the Weald Basin in southern England. It overlies the Gault Clay and underlies the Chalk Group. It varies in thick ...
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
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
.
Sedimentary basin
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
s formed by
rift
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
ing during the Triassic to Early Cretaceous were
inverted during the Late
Paleogene
The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
to
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
leading to the formation of structures such as the
Wealden Anticline and the
Portland-Wight Monocline.
Later erosion has produced the characteristic ridges of the downland landscape. The landscape was further modified during the
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
period by the area's proximity to the southern edge of the ice sheets formed during the
last ice age. These
periglacial
Periglaciation (adjective: "periglacial", also referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and ot ...
effects included significant amounts of dissolution of the chalk and the modification of existing valleys due to a combination of frozen ground and
snowmelt
In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many part ...
.
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
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
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
cliff
In geography and geology, a cliff is an area of rock which has a general angle defined by the vertical, or nearly vertical. Cliffs are formed by the processes of weathering and erosion, with the effect of gravity. Cliffs are common on co ...
s form, such as the
White Cliffs of Dover
The White Cliffs of Dover is the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, deposi ...
and
Beachy Head
Beachy Head is a chalk headland in East Sussex, England. It is situated close to Eastbourne, immediately east of the Seven Sisters.
Beachy Head is located within the administrative area of Eastbourne Borough Council which owns the land, formin ...
.
Hydrology
Chalk deposits are generally very permeable, so the height of the
water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
in chalk hills rises in winter and falls in summer.
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
The Berkshire Downs are a range of chalk downland hills in South east England split between the counties of Berkshire and Oxfordshire. They are part of the North Wessex Downs Area of Outstanding Natural Beauty. The western parts of the downs ...
, above the
Vale of White Horse
The Vale of White Horse is a local government district of Oxfordshire in England. It was historically a north-west projection of Berkshire. The area is commonly referred to as the 'Vale of ''the'' White Horse'. It is crossed by the Ridgeway Nat ...
. In many chalk downland areas there is no surface water at all other than artificially created
dewponds.
Soil
The
soil profile
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
of chalk downland in England is a thin soil overlaying the
parent
A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male t ...
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...
. Weathering of the chalk has created a characteristic soil known as
rendzina
Rendzina (or ''rendsina'') 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 occ ...
.
Unlike many soils in which there are easily distinguished layers or
soil horizon
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
s, a chalk rendzina soil consists of only a shallow dark
humus
In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...
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
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
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
A terracette is a landform consisting of a hillside ridge arranged as part of sub-parallel "step-like sequences" of such ridges. Terracette occur on step hillsides and are regularly spaced. Various causes have been suggested to explain their origi ...
(commonly known as sheep tracks) were formed by the movement of soil downhill, a process known as
soil creep
Downhill creep, also known as soil creep or commonly just creep, is a type of creep characterized by the slow, downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of p ...
.
Habitat
The dominant habitat in chalk downland is typically
calcareous grassland
Calcareous grassland (or alkaline grassland) is an ecosystem associated with thin basic soil, such as that on chalk and limestone downland. Plants on calcareous grassland are typically short and hardy, and include grasses and herbs such as clover ...
, formed by grazing from both
livestock
Livestock are the domesticated animals raised in an agricultural setting to provide labor and produce diversified products for consumption such as meat, eggs, milk, fur, leather, and wool. The term is sometimes used to refer solely to animals ...
and wild animals. Chalk downland is often unsuitable for intensive
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
,
horticulture
Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...
, 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
Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset (unitary authority), Dors ...
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
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
.
See also
*
References
UK Lowland calcareous grassland habitat action plan
External links
{{wiktionary, downland
South Downs Virtual Information CentreSouth Downs information gatewayNorth Wessex Downs AONBChilterns AONBSurrey Hills AONBIsle of Wight AONBDorset AONBKent Downs AONBSalisbury Plain & West Wiltshire Downs AONB
Hills
Plateaus
Prairies