Wiltshire Geology

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Two-thirds of Wiltshire, a mostly rural county, lies on chalk, a kind of soft, white, porous limestone that is resistant to erosion, giving it a high chalk downland landscape. This chalk is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the Chalk Group and stretching from the Dorset Downs in the west to Dover in the east. The largest area of chalk in Wiltshire is Salisbury Plain, which is used mainly for arable agriculture and by the British Army as training ranges. The highest point in the county is the Tan HillMilk Hill ridge in the Pewsey Vale, just to the north of Salisbury Plain, at 295 m (968 ft) above sea level.

The chalk uplands run northeast into West Berkshire in the Marlborough Downs ridge, and southwest into Dorset as Cranborne Chase. Cranborne Chase, which straddles the border, has, like Salisbury Plain, yielded much Stone Age and Bronze Age archaeology. The Marlborough Downs are part of the North Wessex Downs AONB (Area of Outstanding Natural Beauty), a 1,730 km2 (670-square-mile) conservation area.

In the northwest of the county, on the border with South Gloucestershire and Bath and North East Somerset, the underlying rock is the resistant oolitelimestone of the Cotswolds. Part of the Cotswolds AONB is also in Wiltshire, in the county's northwestern corner.

Between the areas of chalk and limestone downland are clay valleys and vales. The largest of these vales is the Avon Vale. The Avon cuts diagonally through the north of the county, flowing through Bradford on Avon and into Bathand Bristol. The Vale of Pewsey has been cut through the chalk into Greensand and Oxford Clay in the centre of the county. In the south west of the county is the Vale of Wardour. The southeast of the county lies on the sandy soils of the northernmost area of the New Forest.

Chalk is a porous rock, so the chalk hills have little surface water. The main settlements in the county are therefore situated at wet points. Notably, Salisbury is situa


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