a building material and for restoring some of Britain's most famous landmarks. Almost every type of rock known from the Early Jurassic to the Eocene epochs can be found in the county. Dorset has a number of limestone ridges which are mostly covered in either arable fields or calcareous grassland supporting sheep. These limestone areas include a wide band of Cretaceous chalk which crosses the county as a range of hills from north-east to south-west, incorporating Cranborne Chase and the Dorset Downs, and a narrow band running from south-west to south-east, incorporating the Purbeck Hills. Between the chalk hills are large, wide vales and wide flood plains. These vales are dotted with small villages, farms and coppices, and include the Blackmore Vale (Stour valley) and Frome valley. The Blackmore Vale is composed of older Jurassic deposits, largely clays interspersed with limestones, and has traditionally been a centre for dairy agriculture. South-east Dorset, including the lower Frome valley and around Poole and Bournemouth, comprises younger Eocene deposits, mainly sands and clays of poor agricultural quality. The soils created from these deposits support a heathland habitat which sustains all six native British reptile species. Most of the Dorset heathland has Site of Special Scientific Interest (SSSI) status with three areas designated as internationally important Ramsar sites. In the far west of the county and along the coast there are frequent changes in rock strata, which appear in a less obviously sequential way compared to the landscapes of the chalk and the heath. In the west this results in a hilly landscape of diverse character that resembles that of neighbouring county Devon. Marshwood Vale, a valley of Lower Lias clay at the western tip of the county, lies to the south of the two highest points in Dorset: Lewesdon Hill at and Pilsdon Pen at . A former river valley flooded by rising sea levels 6,000 years ago, Poole Harbour is one of the largest natural harbours in the world. The harbour is very shallow in places and contains a number of islands, notably Brownsea Island, the birthplace of the Scouting movement and one of the few remaining sanctuaries for indigenous red squirrels in England. The harbour, and the chalk and limestone hills of the Isle of Purbeck to the south, lie atop Western Europe's largest onshore oil field. The field, operated by Perenco from Wytch Farm, has the world's oldest continuously pumping