The Purbeck Hills are a ridge of chalk downs that extend from the Dorset Downs south of Dorchester, through the Isle of Purbeck to Old Harry Rocks where the ridge meets the sea. The hills are part of the southern England Chalk Formation which also includes Salisbury Plain and the South Downs, and would once have been continuous with the Isle of Wight to the east. For most of their length the chalk is protected from coastal erosion by the a band of resistant Portland limestone, where this band ends, at Durlston Head , the clay and chalk behind has been eroded, creating Poole bay and the Solent.

The height of the chalk ridge and proximity to Poole Harbour and the south coast have made the hills of strategic importance. There are a number of iron age, Roman and Saxon archaeological sites, such as Nine Barrow Down . At Corfe Castle the hills are broken twice leaving a steep round hill between the ridges on which stood a medieval castle, guarding the only easy route through the hills, until the English Civil War of the 17th century, when it was ruined.

Some of the western end of the ridge, around the village of Tyneham, near Lulworth, has been closed to the public for use by the army as a firing range. This has protected them from damage from farming and development, and these areas are now natural reserves. At the eastern end Ballard Down is a National Trust nature reserve which is managed for its calcareous grassland habitat.

See also

• Isle of Purbeck, Geology of Dorset