Dorset is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth and Poole and Dorset. Covering an area of 2,653 square kilometres, Dorset borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, Hampshire to the east; the county town is Dorchester, in the south. After the reorganisation of local government in 1974 the county's border was extended eastward to incorporate the Hampshire towns of Bournemouth and Christchurch. Around half of the population lives in the South East Dorset conurbation, while the rest of the county is rural with a low population density; the county has a long history of human settlement stretching back to the Neolithic era. The Romans conquered Dorset's indigenous Celtic tribe, during the early Middle Ages, the Saxons settled the area and made Dorset a shire in the 7th century; the first recorded Viking raid on the British Isles occurred in Dorset during the eighth century, the Black Death entered England at Melcombe Regis in 1348.
Dorset has seen much civil unrest: in the English Civil War, an uprising of vigilantes was crushed by Oliver Cromwell's forces in a pitched battle near Shaftesbury. During the Second World War, Dorset was involved in the preparations for the invasion of Normandy, the large harbours of Portland and Poole were two of the main embarkation points; the former was the sailing venue in the 2012 Summer Olympics, both have clubs or hire venues for sailing, Cornish pilot gig rowing, sea kayaking and powerboating. Dorset has a varied landscape featuring broad elevated chalk downs, steep limestone ridges and low-lying clay valleys. Over half the county is designated as an Area of Outstanding Natural Beauty. Three-quarters of its coastline is part of the Jurassic Coast Natural World Heritage Site due to its geological and palaeontologic significance, it features notable landforms such as Lulworth Cove, the Isle of Portland, Chesil Beach and Durdle Door. Agriculture was traditionally the major industry of Dorset but is now in decline and tourism has become important to the economy.
There are no motorways in Dorset but a network of A roads cross the county and two railway main lines connect to London. Dorset has ports at Poole and Portland, an international airport; the county has a variety of museums and festivals, is host to the Great Dorset Steam Fair, one of the biggest events of its kind in Europe. It is the birthplace of Thomas Hardy, who used the county as the principal setting of his novels, William Barnes, whose poetry celebrates the ancient Dorset dialect. Dorset derives its name from the county town of Dorchester; the Romans established the settlement in the 1st century and named it Durnovaria, a Latinised version of a Common Brittonic word meaning "place with fist-sized pebbles". The Saxons named the town Dornwaraceaster and Dornsæte came into use as the name for the inhabitants of the area from "Dorn"—a reduced form of Dornwaraceaster—and the Old English word "sæte" meaning people, it is first mentioned in the Anglo-Saxon Chronicle in AD 845 and in the 10th century the county's archaic name, "Dorseteschyre", was first recorded.
The first human visitors to Dorset were Mesolithic hunters, from around 8000 BC. The first permanent Neolithic settlers appeared around 3000 BC and were responsible for the creation of the Dorset Cursus, a 10.5-kilometre monument for ritual or ceremonial purposes. From 2800 BC onwards Bronze Age farmers cleared Dorset's woodlands for agricultural use and Dorset's high chalk hills provided a location for numerous round barrows. During the Iron Age, the British tribe known as the Durotriges established a series of hill forts across the county—most notably Maiden Castle, one of the largest in Europe; the Romans arrived in Dorset during their conquest of Britain in AD 43. Maiden Castle was captured by a Roman legion under the command of Vespasian, the Roman settlement of Durnovaria was established nearby. Bokerley Dyke, a large defensive ditch built by the county's post-Roman inhabitants near the border with modern-day Hampshire, delayed the advance of the Saxons into Dorset for 150 years. However, by the end of the 7th century Dorset had fallen under Saxon control and been incorporated into the Kingdom of Wessex.
The Saxons established a diocese at Sherborne and Dorset was made a shire—an administrative district of Wessex and predecessor to the English county system—with borders that have changed little since. In 789 the first recorded Viking attack on the British Isles took place in Dorset on the Portland coast, they continued to raid into the county for the next two centuries. After the Norman Conquest in 1066, feudal rule was established in Dorset and the bulk of the land was divided between the Crown and ecclesiastical institutions; the Normans consolidated their control over the area by constructing castles at Corfe and Dorchester in the early part of the 12th century. Over the next 200 years Dorset's population grew and additional land was enclosed for farming to provide the extra food required; the wool trade, the quarrying of Purbeck Marble and the busy ports of Weymouth, Melcombe Regis, Lyme Regis and Bridport brought prosperity to the county. However, Dorset was devastated by the bubonic plague in 1348 which arrived in Melcombe Regis on a ship from Gascony.
The disease, more known as the Black Death, created an epidemic that spread a
Greensand or green sand is a sand or sandstone which has a greenish color. This term is applied to shallow marine sediment, that contains noticeable quantities of rounded greenish grains; these grains are called glauconies and consist of a mixture of mixed-layer clay minerals, such as smectite and glauconite mica. Greensand is loosely applied to any glauconitic sediment. Greensand forms in anoxic marine environments that are rich in organic detritus and low in sedimentary input. Having accumulated in marine environments, greensands can be fossil-rich, such as in the late-Cretaceous deposits of New Jersey. Important exposures are known from both northern and western Europe, North America, southeastern Brazil and north Africa. Well known and important greensands are the Upper and Lower Greensands of England and occur within Eocene and Cretaceous sedimentary strata underlying the coastal plains of New Jersey and Delaware. Although greensand has been found throughout Phanerozoic and Late Precambrian sedimentary deposits, it appears to be most common in Eocene and Cretaceous sedimentary deposits.
In Brazil, greensand refers to a fertilizer produced from glauconitic siltstone unit belonging to the Serra da Saudade Formation, Bambuí Group, of Neoproterozoic/Ediacaran age. The outcrops occur in Alto Paranaíba region, Minas Gerais, it is a silt-clay sedimentary rock, bluish-green, composed of glauconite, potassium feldspar, quartz and minor quantities of biotite, goethite and manganese oxides, barium phosphate and rare-earth elements phosphates. Enriched levels of potash have K2O grades between 8% and 12%, thickness up to 50 m and are associated to the glauconitic levels, dark green in color. Glauconite is authigenic and mature; the high concentration of this mineral is related to a depositional environment with a low sedimentation rate. The glauconitic siltstone has resulted from a high level flooding event in the Bambuí Basin; the sedimentary provenance is from supracrustal feldsic elements on a continental margin environment with an acidic magmatic arc. In Great Britain, greensand refers to specific rock strata of Early Cretaceous age.
A distinction is made between the Upper Lower Greensand. The term greensand was applied by William Smith to glauconitic sandstones in the west of England and subsequently used for the similar deposits of the Weald, before it was appreciated that the latter are two distinct formations separated by the Gault Clay; the Upper Greensand was once known as either the "Malm" or "Malm Rock Of Western Sussex"Both Upper and Lower Greensand outcrops appear in the scarp slopes surrounding the London Basin and the Weald. Prominent seams are to be found in the Vale of White Horse, in Bedfordshire, in Kent, the South Downs National Park, elsewhere in Hampshire, the Isle of Wight, the Jurassic Coast in Dorset; the soil of the greensand is quite varied, ranging from fertile to sterile. On the fertile soils chestnut and stands of hazel and oak are common, while Scots pine and birch colonise the poorer soils; these Greensand Ridges are popular long distance walking routes, for instance the Greensand Way in Kent.
The Lower Greensand is of Aptian age. In the Weald the Lower Greensand consists of four deposits which are diachronous: the Atherfield Clay 5–15 m thick, the Folkestone Beds 20–80 m thick. Although it appears both north and south of the London Basin it is not present everywhere beneath the Chalk Group which underlies the basin; the Upper Greensand is of Albian age. It represents. Like the Lower Greensand it is not present beneath the whole of the London Basin passing laterally into Gault clay east of a line between Dunstable and Tatsfield and of uncertain extent to the east of London. Outcrops of the Upper Greensand occur in the southwest of England including the Blackdown Hills and East Devon Plateau and the Haldon Hills, remnants of a once much wider extent; the green colour of greensand is due to variable amounts of the mineral glauconite, an iron potassium silicate with low weathering resistance. It is a common ingredient as a source of potassium in organic farming fertilisers. Greensand glauconite is used as a water softener for its chemical-exchange properties.
Greensand coated with manganese oxide is used in well water treatment systems to remove dissolved iron and manganese with the addition of an oxidant potassium permanganate, under controlled pH conditions. It is used as a type of rock for stone walls in areas where greensand is common. In Roman times in Britain, coarse grits derived from the lower greensand were used to line the inner surface of mortars produced in Oxfordshire pottery kilns. Glauconitic greensand has become a popular organic soil amendment; the porous properties of glauconite greensand allows for the absorption of water and minerals, making irrigation and nutrient delivery much more efficient. Greensand can be used to absorb excess water in clay-rich soils and to prevent water loss in sandy soils. Glauconite The dictionary definition of greensand at Wiktionary Super Greensand website Arkansas Geological Commission, "Greensand". Howe, John Allen. "Greensand". Encyclopædia Britannica
Chalk is a soft, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3, it forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells shed from micro-organisms called coccolithophores. Flint is common as bands parallel to the bedding or as nodules embedded in chalk, it is derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is deposited around larger fossils such as Echinoidea which may be silicified. Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones; this indicates stable conditions over tens of millions of years. Chalk has greater resistance to weathering and slumping than the clays with which it is associated, thus forming tall, steep cliffs where chalk ridges meet the sea.
Chalk hills, known as chalk downland form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water through dry seasons. Chalk is mined from chalk deposits both above underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks; some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty. The Chalk Group is a European stratigraphic unit, it forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is underlain by chalk deposits, which contain artificial caves used for wine storage; some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark – both once formed a single island.
Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Chalk was one of the earliest rocks made up of microscopic particles to be studied under the microscope, when it was found to be composed entirely of coccoliths, their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer built up over millions of years and, through the weight of overlying sediments became consolidated into rock. Earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level; the chemical composition of chalk is calcium carbonate, with minor amounts of clay. It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, made of mineral chalk, since it crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be erased.
Blackboard chalk manufacture now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum. While gypsum-based blackboard chalk is the lowest cost to produce, thus used in the developing world, calcium-based chalk can be made where the crumbling particles are larger and thus produce less dust, is marketed as "dustless chalk". Colored chalks, pastel chalks, sidewalk chalk, used to draw on sidewalks and driveways, are made of gypsum. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In southeast England, deneholes are a notable example of ancient chalk pits; such bell pits may mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but the most famous is the extensive complex at Grimes Graves in Norfolk. Woodworking joints may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface.
Chalk transferring to cover the complete surface indicates a good fit. Builder's putty mainly contains chalk as a filler in linseed oil. Chalk may be used for its properties as a base. In agriculture, chalk is used for raising pH in soils with high acidity; the most common forms are CaCO3 and CaO. Small doses of chalk can be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for polishing. For example, toothpaste contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a controlled grain size, for fine polishing of metals. Chalk can be used as fingerprint powder. Several traditional uses of chalk have been replaced by other substances, although the word "chalk" is still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth by tailors, it is now made of talc. Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court.
If a ball hits the line, a cloud of chalk or p
The Jurassic Coast is a World Heritage Site on the English Channel coast of southern England. It stretches from Exmouth in East Devon to Studland Bay in Dorset, a distance of about 96 miles, was inscribed on the World Heritage List in mid-December 2001; the site spans 185 million years of geological history, coastal erosion having exposed an continuous sequence of rock formation covering the Triassic and Cretaceous periods. At different times, this area has been desert, shallow tropical sea and marsh, the fossilised remains of the various creatures that lived here have been preserved in the rocks. Natural features seen on this stretch of coast include arches and stack rocks. In some places the sea has broken through resistant rocks to produce coves with restricted entrances, in one place, the Isle of Portland is connected to the land by a narrow spit. In some parts of the coast, landslides are common; these have exposed a wide range of fossils, the different rock types each having its own typical fauna and flora, thus providing evidence of how animals and plants evolved in this region.
The area around Lulworth Cove contains a fossil forest, 71 different rock strata have been identified at Lyme Regis, each with its own species of ammonite. The fossil collector Mary Anning lived here and her major discoveries of marine reptiles and other fossils were made at a time when the study of palaeontology was just starting to develop; the Charmouth Heritage Coast Centre provides information on the heritage coast, the whole length of the site can be visited via the South West Coast Path. The Jurassic Coast stretches from Orcombe Point near Exmouth in East Devon to Old Harry Rocks near Swanage in East Dorset, a distance of 96 miles. Inscribed on the World Heritage List in 2001, the Jurassic Coast was the first wholly natural World Heritage Site to be designated in the United Kingdom. At Orcombe Point, the "Geoneedle", an acute pyramidal sculpture, marks the western end of the heritage site; the cliffs on this part of the coast are being eroded as sections crumble away and landslides occur.
These processes reveal successive layers of sedimentary rock, uncovering the geological history at the modern coastline over a period of 185 million years, disclosing an continuous sequence of rock formations covering the Triassic and Cretaceous periods. The fossils found in the area and the coastal geomorphologic features of this dynamic coast, have advanced the study of earth sciences for more than two hundred years; the area covered by the designation comprises the land between the mean low water mark and the top of the cliffs or the back of the beach. The fossils found in abundance along this coastline provide evidence of how animals and plants evolved in this region. During the Triassic this area was a desert, while in the Jurassic it was part of a tropical sea, in the Cretaceous it was covered by swamps; the fossilised remains of the animals and plants that lived in those periods are well preserved, providing a wealth of information on their body shapes, the way they died and the fossilised remains of their last meals.
Fossil groups found here include crustaceans, molluscs, fish, reptiles and a few mammals. At Lulworth Cove there is a fossil forest of tree-ferns and cycads; the Jurassic Coast consists of Triassic and Cretaceous cliffs, spanning the Mesozoic, documenting 185 million years of geological history. The site can be best viewed from the sea, when the dipping nature of the rock strata becomes apparent. In East Devon, the coastal cliffs consist of steep cliffs of red sandstone from the Triassic, at Budleigh Salterton, the gravel cliffs contain red quartzite pebbles which accumulate on the beach below as Budleigh pebbles, locally protected. Further east at Ladram Bay, more sandstone cliffs give rise to spectacular red sandstone stacks. Around Lyme Regis and Charmouth the cliffs are of Jurassic clays and shale, landslips are common. Chesil Beach is a good example of a barrier beach and stretches for 18 miles from Burton Bradstock to the Isle of Portland; the beach encloses an intertidal lagoon, an internationally important Ramsar Convention site known for its biodiversity.
At Lulworth Cove, the waves have cut their way through the resistant Portland stone and created a horseshoe-shaped cove by eroding the softer sands and clays behind. Another feature of this part of the coast is a natural arch. Sea stacks and pinnacles, such as Old Harry Rocks at Handfast Point, have been formed by erosion of the chalk cliffs; the highest point on the Jurassic Coast, on the entire south coast of Britain, is Golden Cap at 627 ft between Bridport and Charmouth. This coast shows excellent examples of landforms, including the natural arch at Durdle Door, the cove and limestone folding at Lulworth Cove and a tied island, the Isle of Portland. Chesil Beach is a fine example of a storm beach; the site has stretches of discordant coastlines. Due to the quality of the varied geology, the site is the subject of international field studies; the many sedimentary layers on this coastline are rich with fossils, the remains of the animals and plants present in the area whose tissues became immersed in deposits of mud which hardened into rock.
At Lyme Regis, for example, geologists have identified 71 layers of rock, each one containing fossils of a different species of ammonite. At the end of the 18th century Georges Cuvier showed th
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
Geology of Dorset
Dorset is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres; the great variation in its landscape owes much to the underlying geology which includes an unbroken sequence of rocks from 200 Ma to 40 Ma and superficial deposits from 2 Ma to the present. In general the oldest rocks appear in the far west of the county, with the most recent in the far east. Jurassic rocks underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south 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, 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 and the New Forest, lies on younger and less resistant beds: Eocene clays and gravels; these rocks produce thin soils that 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, 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. 380 million years ago the landmass, to form Southern Britain was some 19 degrees 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 super-continent 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 buried beneath younger deposits in Dorset. Around 204 Ma Dorset, now 30 degrees north, was under water and the first ammonites appear among the shales and limestones that make up the lower Lias. Formed somewhere between 185 Ma and 204 Ma, in what was a shallow marine environment, the Lower Lias Lias is composed of Blue Lias, Black Ven Marls and Green Ammonite Beds. 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, along the coast to the west of Seatown; the sides of the vale are made from the clays and sands of the upper and lower Lias while younger strata from the Cretaceous Period, crown the higher points. The Blue Lias is the lowest of the Liassic stratum 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 ignite the shales. Around Lyme Regis, during the 18th and 19th centuries, the collecting 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 much larger specimens such as ichthyosaurs and plesiosaurs. Although these rocks were formed underwater, the discovery of fossil wood, land-dwelling animals and a pterosaur suggest that dry land was close by. Laid down between 185 Ma and 182 Ma years ago in a shallower marine environment, subjected to much more turbulent weather conditions, the sediments of the Middle Lias are not as muddy as those in the lower Lias, 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. Again, inland exposure is poor, although the middle Lias is visible along the spring line of the hills that surround the Marshwood Vale.
The cliffs either side of Eype and the coastal stretch between Thorncombe and Watton Cliff provide the best view. The base of the middle Lias is composed of three thick layers of calcareous sandstone beds separated by marls; these resistant bands form massive buttresses along the sea-cliffs and, where eroded, boulder aprons on the foreshore. Above these three layers is the Eype clay, deposited in deeper, calmer waters. Ammonites found in this layer have 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 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 appear to have been deposited in a periodically stormy environment where silts and sands
Isle of Purbeck
The Isle of Purbeck is a peninsula in Dorset, England. It is bordered by water on three sides: the English Channel to the south and east, where steep cliffs fall to the sea, its western boundary is less well defined, with some medieval sources placing it at Flower's Barrow above Worbarrow Bay. According to writer and broadcaster Ralph Wightman, Purbeck "is only an island if you accept the barren heaths between Arish Mell and Wareham as cutting off this corner of Dorset as as the sea." The most southerly point is St Alban's Head. Its coastline is suffering from erosion; the whole of the Isle of Purbeck lies within the local government district of Purbeck, named after it. However the district extends further north and west than the traditional boundary of the Isle of Purbeck along the River Frome. In terms of natural landscape areas, the southern part of the Isle of Purbeck and the coastal strip as far as Ringstead Bay in the west, have been designated as National Character Area 136 - South Purbeck by Natural England.
To the north are the Dorset Heaths and to the west, the Weymouth Lowlands. The geology of the Isle is complex, it has a discordant coastline along the concordant coastline along the south. The northern part is Eocene clay, including significant deposits of Purbeck Ball Clay. Where the land rises to the sea there are several parallel strata of Jurassic rocks, including Portland limestone and the Purbeck beds; the latter include Purbeck Marble, a hard limestone that can be polished. A ridge of Cretaceous chalk runs along the peninsula creating the Purbeck Hills, part of the Southern England Chalk Formation that includes Salisbury Plain, the Dorset Downs and the Isle of Wight; the cliffs here are some of the most spectacular in England, of great geological interest, both for the rock types and variety of landforms, notably Lulworth Cove and Durdle Door, the coast is part of the Jurassic Coast World Heritage Site because of the unique geology. In the past quarrying of limestone was concentrated around the western side of Swanage, the villages of Worth Matravers and Langton Matravers, the cliffs along the coast between Swanage and St. Aldhelm's Head.
The "caves" at Tilly Whim are former quarries, Dancing Ledge and Winspit are other cliff-edge quarries. Stone was removed from the cliff quarries either by sea, or using horse carts to transport large blocks to Swanage. Many of England's most famous cathedrals are adorned with Purbeck marble, much of London was rebuilt in Portland and Purbeck stone after the Great Fire of London. By contrast, the principal ball clay workings were in the area between Corfe Wareham; the clay was taken by pack horse to wharves on the River Frome and the south side of Poole Harbour. However, in the first half of the 19th century the pack horses were replaced by horse-drawn tramways. With the coming of the railway from Wareham to Swanage, most ball clay was dispatched by rail to the Potteries district of Staffordshire. Quarrying still takes place on Purbeck, with both Purbeck Ball Clay and limestones being transported from the area by road. There are now no functioning quarries of Purbeck Marble; the Purbeck Mineral and Mining Museum displays an exhibition about ball clays and the associated narrow gauge railways.
The isle has the highest number of species of native and anciently introduced wild flowers of any area of comparable size in Britain. This is due to the varied geology; the species most sought is Early Spider Orchid, which in Britain, is most common on Purbeck. Nearly 50,000 flowering spikes were counted in 2009. Late April is the best time, the largest population is in the field to the west of Dancing Ledge. Smaller numbers can be seen on a shorter walk in Durlston Country Park; this orchid is the logo of the Dorset Wildlife Trust. Cowslip meadows are at their best shortly afterwards and Durlston Country Park has several large ones. In early May several woods have carpets of Wild Garlic. King's Wood and Studland Wood, both owned by the National Trust, are good examples. At around the same time and some Downs have carpets of yellow Horseshoe Vetch and blue Chalk Milkwort. In late May the field near Old Harry Rocks has a carpet of yellow Kidney Vetch. Blue and white flowers of Sheep's bit and pink and flowers of Sea Bindweed lend colour to Studland dunes in June.
Both Heath Spotted Orchid and Southern Marsh Orchid are frequent on Corfe Common that month, Harebells and Purple Betony flowers add colour to the Common in July. Dorset Heath, the county flower, can be found in July and August in large numbers on and around Hartland Moor, in damper parts of the heathland. Bog Asphodel gives displays of yellow flowers there in early July. Marsh Gentian is found less in similar areas from mid August to mid September. A number of Romano-British sites have been discovered and studied on the Isle of Purbeck, including a villa at Bucknowle Farm near Corfe Castle, excavated between 1976 and 1991; the Kimmeridge shale of the isle was worked extensively during the Roman period, into jewellery, decorative panels and furniture. At the extreme southern tip of Purbeck is St Aldhelm's Chapel, Norman work but built on a Pre-Conquest Christian site marked with a circu