Dorset - Physical Geography

Physical Geography

Dorset covers an area of 2,653 square kilometres (1,024 sq mi) and contains considerable variety in its underlying geology, which is partly responsible for the diversity of landscape. A large percentage (66%) of the county comprises either chalk, clay or mixed sand and gravels. The remainder is less straightforward and includes Portland and Purbeck stone, other limestones, calcareous clays and shales. Portland and Purbeck stone are of national importance as 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 279 metres (915 ft) and Pilsdon Pen at 277 metres (909 ft).

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 well at Kimmeridge which has been producing oil since the early 1960s.

Dorset's diverse geography ensures it has an assortment of rivers, although a moderate annual rainfall coupled with rolling hills, means most are typically lowland in nature. Much of the county drains into three rivers, the Frome, Piddle and Stour which all flow to the sea in a south-easterly direction. The Frome and Piddle are chalk streams but the Stour, which rises in Wiltshire to the north, has its origins in clay soil. The River Avon, which flows mainly through Wiltshire and Hampshire, enters Dorset towards the end of its journey at Christchurch Harbour. The rivers Axe and Yeo, which principally drain the counties of Devon and Somerset respectively, have their sources in the north-west of the county. In the south-west, a number of small rivers run into the sea along the Dorset coastline; most notable of these are the Char, Brit, Bride and Wey.

Most of Dorset's coastline is part of the Jurassic Coast, a World Heritage Site, which stretches for 153 kilometres (95 mi) between Studland and Exmouth in Devon. This coast documents the entire Mesozoic era, from Triassic to Cretaceous and is noted for its geological landforms. The Dorset section has yielded important fossils, including Jurassic trees and the first complete Ichthyosaur, discovered near Lyme Regis in 1811 by Mary Anning. The county features some notable coastal landforms, including examples of a cove (Lulworth Cove), a natural arch (Durdle Door) and chalk stacks (Old Harry Rocks). Jutting out into the English Channel at roughly the midpoint of the Dorset coastline is the Isle of Portland, a limestone island that is connected to the mainland by Chesil Beach, a 27-kilometre (17 mi) long shingle barrier beach protecting Britain's largest tidal lagoon.

The county has one of the highest proportions of conservation areas in England—including two Areas of Outstanding Natural Beauty which together cover 53% of the county. It has two Heritage Coasts totalling 92 kilometres (57 mi) and Sites of Special Scientific Interest covering 199 km2 (77 sq mi). The South West Coast Path, a National Trail, runs along the Dorset coast from the Devon boundary to South Haven Point near Poole.