Formation of downland

Downland is formed when chalk formations are raised above the surrounding rocks. The chalk slowly erodes to form characteristic rolling hills and valleys. As the chalk layer is generally tilted, chalk downland formations typically have a marked scarp slope on one side, which is very steep, and a dip slope on the other, which is much shallower. Where the downs meet the sea, characteristic white chalk cliffs can be formed, such as the White cliffs of Dover and on the Isle of Wight.

Hydrology

Chalk deposits are very porous, so the height of the water table 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 modern practice of extracting water from this 'reservoir', in order to satisfy demand for water, may be putting some of these streams under extreme stress.

In the valleys below the downs there is typically a clay soil, and at the interface between the two a springline can occur where water emerges from the porous chalk. 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.

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

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Examples of downland

===The Southern England Chalk Formation===

Berkshire Downs and White Horse Hills

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, Box Hill, Langdon Bay)

Purbeck Hills (Ballard Down, Lulworth Cove, Old Harry Rocks, White Nothe)

Salisbury Plain