Дорсет: Геология и ландшафт южной Англии.

none

Dorset '/d//ɔːr//s//ᵻ//t/ (or archaically, Dorsetshire) is a county in South West England on the English Channel coast. Covering an area of ; 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 to 40 million years ago (Mya) and superficial deposits from 2 Mya 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; 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 mile (153 km) (including some of east Devon), rocks from the beginning of Triassic, through the Jurassic, and 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 Purbeck lie atop Britain's largest onshore oil field. The field, operated from Wytch Farm, produces a high quality oil. Nearby is 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.

About 380 Mya, the landmass that was later to form Southern Britain was some 19° south of the equator and lay on the northern shore of an ocean basin that separated the continents of Laurasia and Gondwanaland. When these continents collided to form the single supercontinent of Pangaea, the sediments on the ocean floor were pushed up and over, while the molten rock below the surface was forced out. Today, these igneous intrusions and the red sandstones, visible in the neighbouring county of Devon, slope away from west to east, and are deeply buried beneath younger deposits in Dorset.

Through most of this period, southern Britain experienced continental conditions, initially with deposition by large river systems forming the Sherwood Sandstone Group. This was succeeded by mudstones with evaporites (including locally thick deposits of halite) deposited in a desert environment, forming the Mercia Mudstone Group. A major change to marine conditions is shown by the mudstones with limestones and sandstones of the overlying Rhaetian age Penarth Group. No rocks from this period crop out in Dorset, but they are known to be present in the subsurface from hydrocarbon exploration drilling.

Around 204 Mya, Dorset, now 30°N, was under water, and the first ammonites (Psiloceras planorbis) appear among the shales and limestones that make up the lower Lias.

Formed during the Late Rhaetian to Pliensbachian, in what was then a shallow marine environment, the lower Lias is composed of Blue Lias, Black Ven Marls, and Green Ammonite Beds (Charmouth Mudstone Formation). Mostly covered by lush vegetation, it forms the floor of Marshwood Vale in the west of Dorset and can be seen in stream beds, where the land has been excavated, and along the coast to the west of Seatown. The sides of the vale are mainly made from the clays and sands of the upper and lower Lias, while younger strata from the Cretaceous Period (145–66 Mya), crown the higher points. The Blue Lias is the lowest of the Liassic strata and where it is visible on the coast near Lyme Regis is layered with hard limestone and oil rich shale. The iron pyrites in the clay can heat up when exposed to the air and occasionally ignite the shales. Around Lyme Regis, during the 18th and 19th centuries, the collection and sale of fossils became a popular occupation. Landslides and the excavation of the clays, used in cement production, exposed not only an abundance of ammonites of varying size, but also much larger specimens such as ichthyosaurs and plesiosaurs. Although these rocks were formed under water, the discovery of fossil wood, land dwelling animals, and a pterosaur suggest that dry land was close by.

Laid down during the Pliensbachian to Early Toarcian in a shallower marine environment that was subjected to much more turbulent weather conditions, the sediments of the middle Lias are not as muddy as those in the lower Lias, and are much nearer to silt and sand. These sands are thought to have come from the islands that now form part of Cornwall and South Wales. Ammonites found in this layer had apparently been attacked by larger crustaceans. The fossilised remains of large numbers of brittle stars, found towards the end of this deposition, indicate that they were rapidly covered. Theories put forward suggest a great storm or tidal wave was the cause, and indeed many of the remains appear to have been swept along the sediment, some losing limbs on the way. This particular section of the lias is appropriately known as the "starfish bed". The succeeding layers of Downcliffe and Thorncombe sands also appear to have been deposited in a periodically stormy environment where silts and sands dominate. Pebbles and cobbles are found here, encrusted with fossilised animals, which suggest that some of the sediments were already lithified; also, some evidence exists of localised fault activity. The last layer in the middle Lias is the Marlstone Rock bed.

The lowest part of the upper Lias, the Beacon Limestone Formation, formed during the Pliensbachian to Toarcian, around 182 Mya. It is known as the junction bed and despite representing 3 million years of deposition, is only 0.6 to 1.5 m thick, where it can be seen at Eype Mouth. In Yorkshire, the equivalent stratum is around 100 m thick, suggesting that Dorset remained under water much longer and was robbed of supplies of sand and clay. This bed is composed of fine grained limestones from white to pale pink in colour. The sands form a band that runs inland to Yeovil. In the east of the county, these sands are some 900 m below the surface and form the Wytch Farm oilfield's middle reservoir. It contains many fossils, including bryozoans, brachiopods, ammonites, belemnites, gastropods, bivalves, and echinoids, and like the previous middle and upper Lias, shows signs of fault controlled deposition. In the upper section of the Inferior Oolite, two beds are packed with numerous types of sponges, which can be seen around Shipton Gorge and Burton Bradstock. Despite being formed in a marine environment, the oolite, like the lower Lias, contains some fossilised remains of land dwelling creatures, including two species of Megalosaurus that were found near Sherborne. A limestone known as Fuller's Earth rock is found in the north west of the county, between the clays south and east of Sherborne. It is rich in ammonites and bivalves. Previously referred to as the Upper Fuller's Earth Clay, the succeeding Frome Clay can be seen to the west of West Bay, where it forms the major part of the West Cliff. It contains many brachiopods, particularly in the Weymouth Anticline where the oyster beds are 5 m thick. Outcrops of Forest Marble are concentrated around the Weymouth Anticline, the coastal escarpment between Burton Bradstock and Abbotsbury, and inland as far as Bothenhampton, where they disappear below younger Cretaceous deposits. Surfacing once more at Rampisham, they turn east and then north into Somerset. The limestone does not form one continuous bed as previously thought, but rather forms pockets in a formation that is predominately clay. Severe tropical storms likely swept up large quantities of shells and other invertebrates and deposited them in this way. The shallow water in which the Cornbrash limestones were deposited was followed by marginally deeper water in which the Kellaway Beds were laid down. The Kellaway Beds are only visible in the banks of streams and rivers, and the brick pits at Chickerell and Rampisham, where the succeeding Oxford Clays were quarried. The Oxford Clays were deposited in much deeper water and cover the valley floors between Weymouth and the Blackmore Vale. They can also be seen in the cliffs at Tidmoor Point, Jordan Hill, and the previously mentioned brick pits at Chickerell. Poor exposure, coupled with a decline in the brick industry, has made detailed mapping of the clays difficult, and now relies heavily on trenches dug for services. These excavations have unearthed some impressive examples of Septarian nodules. The Corallian strata can be seen on the Weymouth anticline, between Abbotsbury and Osmington. It then disappears to emerge as a low escarpment between Mappowder and Cucklington on the Somerset border. Near the top of this formation is the Ringstead Waxy Clay, in which large delta shaped oysters are found and at the top of the Corallian is the Ringstead Coral Bed. Benecliff grit, a soft sandstone containing large doggers, is well exposed at the headland, Bran Point. Once a reservoir, oil sometimes seeps from the cliff here, and an oily residue is present in the sands nearby. Deposited in an oxygen deficient environment, the Kimmeridge beds make up the major part of the Purbeck Monocline and the periphery of the Weymouth Anticline. They form a low lying band running north to south through the Blackmore Vale, and underpin the strip of land between the isle of Portland and the mainland. The unusual iron rich sediment near Abbotsbury was, in a departure from the norm, deposited in shallow iron rich water and the pale bands throughout the formation were caused by coccoliths following periodic blooms of algae. Many of the fossilised remains are squashed flat, indicating that the Kimmeridge beds were heavily compacted, perhaps to an eighth of their original thickness. This was not sufficient to produce oil reservoirs like those beneath the North Sea, yet the oil rich shales have, in the past, been economically important as a local fuel.

Whether the formations created between the Upper Jurassic and Lower Cretaceous were deposited across the whole of Dorset like the preceding beds, and subsequently eroded, or only formed in the areas in which they are now found is unclear. The Portland sands were created in a deep marine environment, whereas the Portland stone was a near shore deposit. The marine algae, which formed large reefs, are responsible for the occasional stacked and arched bands in the formation and are best viewed in the Roach, an extremely fossiliferous limestone confined to the north of Isle of Portland. To the north of the oolite is a fine grained limestone, which was deposited in calm, shallow water. It contains flint like rocks known as chert, were it not for the presence of very large ammonites, might be mistaken for an early chalk. Previously thought to be around the middle of the Purbeck Strata, modern research suggests that the boundary between the Jurassic and Cretaceous periods lies within the basal few metres of the overlying Lower Purbeck.

The Purbeck Limestone Group is a succession of limestones, shales, and clays named after the Isle of Purbeck, laid down in the latest Tithonian to late Berriasian mainly in very shallow lagoons and freshwater lakes, with only occasional marine influence at the lower levels where halite, gypsum, and other evaporites are found. The muddled sediments around Lulworth are thought to be caused by a thick layer of these evaporites dissolving. Evidence also exists of uplift and erosion of earlier formations. The climate around this time had changed from a tropical to a subtropical one, with pronounced wet and dry seasons as indicated by the growth rings in the fossilised trees also found in the lower levels of the Purbeck strata. These trees grew around the low coastal areas and were, from time to time, subjected to drowning from the sea. The fallen trees and their stumps became covered by stromatolites which trapped sediments. The limestones at this level, which vary greatly in colour, are composed of densely packed shells of the freshwater snail Viviparus. Their ability to take a very high polish has led them to become known as Purbeck Marble. Like the older Forest Marble, it is a sedimentary marble rather than a true metamorphic one. The Purbeck Limestone Group has yielded a diverse selection of fossilised remains, including rare reptiles, amphibians, and mammals, for which the formation has become internationally important.

The Wealden Beds are confined to the south and increase in thickness from 107 to 716 m, between Upwey and Swanage. These rivers drained the upland areas to the north and west, and were responsible for depositing much of the succeeding sediment across southern Dorset. The Wealden Beds contain a great deal of plant matter and charcoal, indicating periodic fires in the region. At Mupe Bay, some of the sands have been bound together by oil and formed into sandstone boulders, suggesting that oil generation had already begun when these sands were deposited. The earliest deposit, the Lower Greensand, covers the preceding Wealden Beds in the south, and the Oxford and Kimmeridge clays in the north. In the west, overlying the eroded Lias, is a thin layer of Gault Clay of Albian age, which decreases in thickness from 27 to 7 m the further west it stretches, and can be located by the spring lines around the chalk ridges and escarpments. The water that sits between these two beds is responsible for the many landslips that have occurred throughout the county's history. Because geologists are certain all the formations from the Lias to the Kimmeridge Clay were deposited over the whole of the county, the Gault clay reveals the scale of uplift and erosion that must have taken place before Dorset was flooded. The overlying Gault is revealed in the spring lines around the chalk ridges and escarpments. Above the Gault is the Upper Greensand, which varies from the golden sands that crown the West Dorset heights, such as Pilsdon Pen and Golden Cap, to the rubbly sandstones found in the centre of the county. The first chalk deposits contained sands and grits thought to have originated in Cornubia. This gritty chalk was deposited across central Dorset, and can clearly be seen in the sides of Eggardon Hill. As the water deepened, plankton, coccoliths, and foraminifera became the main source for the calcareous soup, which would later form the Dorset chalk. Generally, the Dorset chalk is too soft to be used as a building stone except along the Ridgeway, where faulting and folding produced a particularly hard variety popularly used by the Romans in their mosaics. The chalk often contains larger fossils, including ammonites, belemnites, and brachiopods. Flint occurs throughout in both nodule and tabular layers. The sediments resting on the Dorset chalk are, for the same reason, distinctly different from those in other parts of the world, and the remains of the plants and animals within are much more recent.

The expansion of the North Atlantic, which first occurred during the Triassic period, now reached a point where it was affecting the Dorset landscape. Areas that were previously under water were exposed to erosion as the chalk was folded and forced upwards. The early deposits from this period, found across South East England, do not occur in Dorset. The small amounts found in Devon, however, coupled with the many solution holes found across the Dorset chalk, suggest that erosion may have removed them. Some of the larger solution holes, known as dolines, are found on the heaths between Dorchester and Bere Regis. Near Briantspuddle is Culpepper's Dish, 86 m wide and over 21 m deep. The first deposits found from this period in Dorset are the Reading Formation clays. A subdivision of the Lambeth Group, they are visible above the chalk at Studland. Mottled, brightly coloured, and generally lacking fossils, the clays are thought to be deposited beneath lagoons. The Bracklesham Group, found across much of the Dorset portion of the Hampshire Basin, were mainly deposited by rivers, and form the foundations of the Dorset heathland. The frequent and heavy rains that occurred during this period transported large amounts of material from northern uplands across the Purbeck and Weymouth areas. Blocks of quartzite can be found across the downlands of southern Dorset, and these sarsens have been used in some of the local prehistoric monuments. To the north and east of the county, much finer sands and gravels are found, including ball clay, which was widely mined on the heaths around Wareham. The fine particles that make up the clay are believed to have originated in the west, where rocks were being broken down by the alternating dry, humid conditions and torrential rain, which in turn formed rivers and carried the material to its current position. Fossilised palms taken from the cliffs at Bournemouth confirm that Dorset had a tropical climate at the time.

During this period, Dorset drifted a further 9° north, the climate cooled rapidly, and by the late Eocene was temperate. Mainly terrestrial with fresh and brackish water environments, the county, despite its low lying nature, remained largely without marine influences except for some minor incursions at the end of the ice ages. The late Palaeogene deposits that filled the Hampshire Basin as the sea levels fell are not found in Dorset, nor are the Neogene, which are absent from virtually all of the British Isles. Further, but less dramatic folding and uplift, along the county's subsurface faults, coupled with continuing erosion of Jurassic and Cretaceous sediments, created Dorset's modern landscape and drainage patterns.

During the last 2,600,000 years, Britain was subjected to a number of extreme fluctuations in temperature, which included three ice ages. Dorset was not affected by glaciation, but did experience very low temperatures with permafrost on the high ground. Sudden changes in sea level, and successive warming and cooling, significantly modified the landscape. The two raised beaches at Portland created 210,000 and 125,000 years ago are a dramatic testament to the fluctuations in sea levels. The causes of these two fluctuations were twofold the growth of the ice sheets, which locked away large quantities of water, and the weight of the ice in the north that caused the landmass to tilt. During the summers, partial melting of the ice created large rivers that washed enormous amounts of sand and gravel across the county, forming huge terraces in both the upland areas and valleys. These rivers cut down into the landscape as they made their way to the lower sea levels. This action caused many large scale landslips, most notably around Shaftesbury and Abbotsbury. Flint and chert tools from early human visitors have been retrieved from the river terraces around the Dorset and Devon border, although these may have been transported here with the other river deposits. These extensional faults were themselves reactivated during further rifting in Jurassic to early Cretaceous times. The area experienced the effects of the early stages of Alpine collision during the Late Cretaceous to Palaeogene, with reverse reactivation of many of the extensional faults, creating the most visible of the structures in the Dorset area, such as the Purbeck Monocline.

The Litton Cheney Fault is a west–to east trending, south dipping normal fault. The fault was active during deposition of the Lias Group and again during the late Jurassic to early Cretaceous. Some evidence exists of minor reversal on the fault during the late Cretaceous to Palaeogene inversion.

This major west–to east trending, south dipping normal fault was mainly active during the late Jurassic to early Cretaceous, as shown by the local preservation of the Wealden in its hanging wall. It was reactivated as a reverse fault during the late Cretaceous to Palaeogene, and now juxtaposes Portland and Purbeck beds in the hanging wall against chalk in the footwall. The fault is not directly linked to faults beneath the Triassic salt layer.

This monoclinal structure forms part of a semicontinuous structure extending eastwards through the Isle of Wight as the Portland Wight Monocline (or Disturbance). This was formed by reverse reactivation of the Purbeck–Isle of Wight fault zone.