Geology of south Dorset and south-east Devon and its World Heritage Coast: special memoir for 1:50,000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot

In recognition of its outstanding geology, the coast between Orcombe Rocks in south-east Devon and Old Harry Rocks in south Dorset was granted World Heritage status in December 2001. The geology of this coast is described, together with the recently mapped 1:50 000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth and 342/343 Swanage. The diverse geology ranges from the Late Permian to Quaternary, representing more than 200 million years of geological time and many different ancient environments that included arid desert, subtropical seas and cold periglacial conditions. The stratigraphy is described in detail and incorporates revisions of the Triassic, Jurassic, Cretaceous and Palaeogene successions. The region is justly renowned for its rich variety of fossils, found especially in the Lower Jurassic rocks around Lyme Regis and Charmouth. Many scientifically important specimens have come from these crumbling mudstone cliffs, including marine reptiles first made famous by the work of the 19th century Lyme Regis fossil collector Mary Anning. During the Quaternary, the district lay beyond the influence of the glaciers and outwash that covered much of northern Britain. Head and the residual claywith-flints are the main deposits preserved from that period together with river alluvium and terrace deposits. The structural geology is described in the context of basin evolution. This area was part of the Wessex–Channel Basin that developed throughout the Mesozoic. Numerous borehole records provide the basis for a detailed assessment of thickness variation and this is related to structural development. Mesozoic structures continued to influence later structural development. Periods of earth movement associated with the uplift of the Alps in southern Europe caused parts of the succession to be spectacularly folded, as seen for example at Durdle Door (Cover photograph). Many past geomorphological studies have emphasised the close relationship between geology and the development of coastal landscape in the region, and there are a number of spectacular natural arches and sea stacks. Landslips, both active and dormant, are described and occur in a variety of forms unrivalled in the UK. Inland too, the influence of geology on the landscape is very strong, from the high rolling chalk downland north of Dorchester to the lower lying sandy heaths formed of Cenozoic (Palaeogene) sediments farther east around Wareham. In the Isle of Purbeck, picturesque Corfe Castle sits astride the sharply defined east–west ridge of the Purbeck Hills, formed by steeply dipping beds of chalk, part of a huge monoclinal fold that extends eastwards to the Isle of Wight. Geology has also played a part in the development of the region, summarised in the Economic geology chapter. This region supplied the Portland Stone and Purbeck Marble that has been used in many great cathedrals and civic buildings. Palaeogene clays are an important source of raw material for ceramics, and the Wytch Farm Oilfield, near Bournemouth, is the largest onshore oilfield in the UK. The text is fully referenced, and additional information includes a borehole and a fossil inventory. The geological succession tabulated opposite summarises the age, stratigraphical classification, lithology and thickness of the rock types seen in south Dorset and on the World Heritage Coast.