The Dorset Downs are bounded on the north, along the steep scarp face, by the Blackmore Vale, a large clay and limestonevalley. On the east, the Downs were once, thousands of years ago, continuous with Cranborne Chase, but the River Stour now cuts a valley between them,[dubious – discuss][citation needed] which is the location of Blandford Forum and the eastern boundary of the downs. From the northern scarp face, the hills dip gently southwards before the chalk disappears beneath the Bagshot Beds which form the heathlands of the county, between Dorchester and Wareham. South of the River Frome, the chalk reappears in a narrower strip, forming coastal cliffs east of Weymouth and, further east, the steep ridge of the Purbeck Hills. This southerly strip of the visible chalk (sometimes referred to as the South Dorset Downs or South Dorset Ridgeway) continues westwards behind Weymouth, and rejoins the main body of the downs at their western extremity at Eggardon Hill. In the west the chalk dips down under marl.
The main land uses on the downs are arableagriculture, woodland and calcareous grassland, a habitat which is growing as farmers are encouraged to set aside land with subsidies. Chalk is a rough rock and the Dorset Downs hold a large water table which acts as a reservoir of drinking water for much of the county. This property of chalk also means there are many seasonal rivers, called winterbournes, that flow depending upon the level of the water table.
Hills
Bulbarrow Hill (274 m), site of Rawlsbury camp and the highest hill in the Downs.