Chalk is a particularly soft form of limestone, which weathers to form smooth rounded hills known as "downs" or "wolds", often dissected by flat bottomed valleys associated with prominent, rounded spurs. The rock formation was laid down in shallow seas some sixty million years ago. Chalk rarely outcrops and only forms cliffs at the coast (e.g. the "White Cliffs of Dover"). English downland is broadly divisible into four main regions:-
- the North Downs of Kent and Surrey;
- the broader South Downs of Sussex, Hampshire and the Isle of Wight;
- the Chilterns;
- the Wessex downs of Wiltshire, Dorset, Hampshire and Berkshire, in the centre of which lies Salisbury Plain.
Until the middle of the last century these downlands were devoted almost exclusively to sheep-rearing. In many places on the chalk, woodland occurs, and in some areas such as the Chilterns, there are substantial beech forests. Ungrazed chalk grassland will soon be invaded by bushes, and eventually by woodland trees. These facts are the starting point for the conservation of downland because is indicates that grassland of to-day is stable only because of the grazing of animals— domestic sheep, and wild rabbits. It has been inferred, from the concentration of artifacts of late Neolithic culture on the Wessex downs, and, to a less extent, on the South Downs, that the original forest covering these chalk hills must have been at least partly removed by 2500 B.C.; and it may well be that in these areas a good deal of the chalk grassland turf is about 4,000 years old. But in other areas, e.g. the North Downs, there was no such widespread early culture, and the origin of the open downland must be a good deal more recent. Woodland of today many not be all that ancient; for in several places lynchets,' or old Celtic ploughing terraces, can be detected in present day mature beech woods.
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