Dorset (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.Chaffey (pp.5–6) 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.Chaffey (p.6)
Dorset's coastline is one of the most visited and studied coastlines in the world because it shows, along the course of (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.House (1993) pp.1–4 Throughout Dorset there are a number of limestone ridges.Chaffey pp.5–7 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.Cullingford (p.122) 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.Ensom (1998) pp.22–23
Devonian to Carboniferous About 380 Mya, the landmass that later formed Southern Britain was some 19° south of the equator. It lay on the northern shore of an ocean basin separating 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.Ensom (p.13) 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.Ensom (p.14)
Triassic 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.
Latest Triassic to Jurassic Around 204 Mya, Dorset, now 30°N, was underwater, and the first ammonites (Psiloceras planorbis) appear among the shales and limestones that make up the lower Lias.Ensom (p.19)
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.Ensom (p.21) 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.Ensom (p.22) The iron pyrites in the clay can heat up when exposed to the air and occasionally ignite the shales.Ensom (p.23)
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.Ensom (p.23) Although these rocks were formed under water, the discovery of fossil wood, land-dwelling animals, and a pterosaur suggests that dry land was close by.Ensom (p.24)
Middle Lias 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.Ensom (p.25) 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, which was probably deposited in deeper, calmer waters.Ensom (p.26) 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".Ensom (P.26-27) 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.Ensom (p.28) The last layer in the Middle Lias is the Marlstone Rock bed.
Upper Lias
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.Ensom (p.29) In Yorkshire, the equivalent stratum is around 100 m thick, suggesting that Dorset remained underwater 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 clearest evidence of fault activity exists in a section of cliff between West Bay and Eype Mouth, where the junction bed thickens in a section of steps, and fractures in the rock contain sediment from the ocean bed, which most likely was sucked in as the cracks opened.
The remaining part of the upper Lias is the bright yellow Bridport sandstone of Toarcian age, which gives the cliffs between West Bay and Burton Bradstock their distinctive colour.Ensom (p.31) 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.
Inferior Oolite The succeeding Inferior Oolite was deposited in a shallow, tropical marine environment, 35° north of the equator, during the Aalenian to early Bathonian. It is only 2 m thick in the south, but increases to 20 m in the north, near Sherborne.Ensom (p.32) 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 some places, the sea floor appears to have been breached, and sediments and fossils were trapped like those in the junction bed between West Bay and Eype. Most of Dorset was starved of sediment, and condensed limestones collected. These limestone beds are often iron-rich, lending them a rusty colour. One particular bed, exposed at Burton Cliff, contains large numbers of orange, discus-shaped concretions, which on closer inspection reveal themselves to be pieces of shell from a species of mussel, coated with a thin layer of iron-rich sediment.Ensom (p.33)
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.Ensom (p.34) 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.
Great Oolite Fuller's Earth Formation clays were deposited across much of southern Dorset when the continental shelf subsided, forcing the region deeper underwater during the early Bathonian. The lower clays are visible at Burton Bradstock and contain large numbers of Bositra buchi. These bivalves may have been able to swim freely, thus able to avoid the soupy mud on the sea floor.Ensom (pp.35–36) 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.Ensom (p.36) 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. At the junction between the Frome Clay and the overlying Forest Marble is the Boueti Bed, so called because of the large numbers of the brachiopod Goniorhynchia boueti found there. It is best viewed on the Herbury peninsula south of Langton Herring. The limestone known as Forest Marble is not a true metamorphic marble, but it takes a high polish and has been used as a building material and marble substitute for many years.Ensom (pp.37–38) 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.Ensom (p.38) The limestone does not form one continuous bed as previously thought, but rather forms pockets in a formation that is predominantly clay. Severe tropical storms likely swept up large quantities of shells and other invertebrates and deposited them in this way. The formation of the Forest Marble suggests a shallowing of the sea, and the remains of pieces of tree and bits of land-dwelling animals are found among the turtles, frogs, and salamanders within.