Dorset ( /ˈdɔrsɨt/) (or archaically, Dorsetshire) is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology which includes an almost unbroken sequence of rocks from 200 Ma to 40 Ma and superficial deposits from 2 Ma to the present. In general the oldest rocks (Early Jurassic) appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; and although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.
Dorset's coastline is one of the most visited and studied coastlines in the world because it shows, along the course of 95 miles (153 km) (including some of east Devon), rocks from the beginning of Triassic, through the Jurassic and up to the end of the Cretaceous, documenting the entire Mesozoic era with well-preserved fossils. Throughout Dorset there are a number of limestone ridges. The largest and most notable is the band of Cretaceous chalk that runs from the south-west to the north-east of the county and forms part of the Chalk Group that underlies much of the south of England, including Salisbury Plain, the Isle of Wight and the South Downs. Between the bands of limestone and chalk are wide clay vales with flood plains.
South-east Dorset, around Poole, Bournemouth and the New Forest, lies on younger and less resistant beds: Eocene clays (mainly London Clay), sands and gravels. These rocks produce thin soils that historically have supported a heathland habitat. The chalk and limestone hills of the Purbecks lie atop Britain's largest onshore oil field. The field, operated from Wytch Farm, produces a high-quality oil and has the world's oldest continuously pumping well at Kimmeridge, which has been in use since the early 1960s. The source of this oil is the organic-rich shales found in the lower lias. Landslides along the coast have been known to ignite these shales causing cliff fires, the most recent of which occurred in 2000.