Natural England - Dorset (including Poole and Bournemouth)

Dorset (including Poole and Bournemouth)

Dorset displays some of the best known and studied rocks in the World. Its varied geology, spans the last 200 million years, and is dominated by sedimentary rocks deposited in a range of marine, freshwater and terrestrial environments.

The Jurassic and Cretaceous rocks of the Dorset coast are now part of a World Heritage Site.

In general terms, rocks of Jurassic age occupy the western side of the County, sloping gently to the east, so that the oldest outcrop is in the Lyme Regis area and the youngest at Swanage. These Jurassic, marine deposited sediments are some of the finest in the World and yield a myriad of fossil animals such as ammonites, shells, sea-urchins, brachiopods and reptiles, which provide a clear picture of the changing conditions throughout the Jurassic Period.

The coastal exposures between Lyme Regis and Swanage are exceptional, and their study has been one of the driving forces in the development of geology as a science since the 19th century. This is recognised by the designation of the Dorset coast as a World Heritage Site for its geological interest.

Inland, the varied Jurassic strata give rise to an undulating landscape of hills, scarps and small valleys. Many of the villages have been constructed from the local rock outcrops lending the human landscape a direct connection with the rocks that have shaped its physical elements. The central part of Dorset is dominated by the rolling Dorset Downs which are formed in the Upper Cretaceous Chalk. In the east of the County, between the Dorset Downs and the narrow chalk ridge of the Purbeck Hills lie the Dorset Heaths and coniferous forests of the Wareham area. These occupy the western end of the Hampshire Basin, which is infilled with sediments of Tertiary age. These sands and clays, originally deposited on extensive river floodplains and tidal flats, now give rise to free-draining and acidic soils that support the characteristic heathland vegetation of the area.

The rocks of Dorset were folded and faulted during the earth movements that formed the Alps, some 20 million years ago. The colossal forces at work, led to the formation of geological structures such as the Weymouth and Purbeck anticlines which now form the dramatic series of ridges and valleys in south Dorset. The effects of this period of folding can be seen at locations such as Lulworth where the exposed rocks are now vertical.

The area was not directly impacted by the repeated advances and retreats of the great ice sheets of the Ice Age during the last two million years. However, the arctic climate that prevailed during glacial periods led to the development of tundra-like conditions, which would have been instrumental in shaping the Chalk Downs. Large swathes of gravel were deposited in the Poole area during this period, possibly by a large river system, the Solent River, that drained the area prior to the melting of the ice sheets at the end of the last glacial and the rise in sea-level that this caused.

Jurassic

The Jurassic (205-142 million years ago), rocks of Dorset have been known since the early days of geology as providing one of the finest marine sequences of this age anywhere in the World, with every stage of the Jurassic being represented.

Marine conditions prevailed throughout virtually all of the Jurassic, although there were marked and repeated phases of deepening and shallowing of the sea in which the rocks were deposited. The outcrop of the Jurassic occurs on either side of the chalk mass of the Dorset Downs which forms the bulk of the central part of the County. Thus, Jurassic rocks outcrop along the coast from St. Alban's Head in the east to Lyme Regis on the western border, northwards towards Crewkerne and then eastwards to Gillingham, to the north of the Downs.

The oldest Jurassic rocks belong to the Lias Group and outcrop between Lyme Regis to near Bridport and northwards to Beaminster and then westwards to the Vale of Marshwood. The lower part of the Lias is characterised by alternating clays and thin shelly limestones, between Lyme Regis and Charmouth these rocks are well know for there common ammonites and this is among the richest fossil marine reptile localities in the country. Towards the upper Lias the rocks get progressively sandier as sea level fell culminating in the Upper Lias Bridport Sand which is superbly exposed at West Bay. These sands represent the deposits of a large offshore sand bank that gradually advanced southwards during the Lower Jurassic, thus the deposits are older in the north and younger in the south.

The well known sunken lanes between Symondsbury and Chideock owe their existence to erosion by hundreds of years of cart wheels and rain of the Bridport Sands where they underlie the harder Inferior Oolite that forms the surface outcrop in the area.

Rocks of Middle Jurassic age outcrop from the coast between Burton Bradstock and Abbotsbury, behind the Fleet around Langton Herring, and then inland to Powerstock before swinging around the western extremity of the Chalk downs and north-eastwards to Sherborne. The Middle Jurassic strata give rise to a landscape of numerous hills and scarps separated by small streams, the intimate nature of the area reflecting the diverse and variable nature of the underlying geology. The earliest Middle Jurassic series is the Inferior Oolite which is a fossil-rich shallow marine limestone that varies in thickness from place to place reflecting a fluctuating sea level. The succeeding Great Oolite Group comprises the Fuller's Earth, Frome Clay and the Forest Marble.

The clays of the Fuller's Earth Rock were deposited in deeper water, while the succeeding limestones and clays of the Forest Marble contain fossils and structures that indicate that they were deposited in shallow, perhaps tidal waters. The Jurassic sea then deepened again and the rubbly, brown limestones of the Cornbrash were deposited. This fossil-rich limestone has a narrow, patchy outcrop, but occurs more extensively south of Sherborne.

A deepening of the marine basin marks the beginning of the Upper Jurassic and the deposition of the overlying marine clays of the Kellaways Beds and Oxford Clay. Both of these units are poorly exposed in Dorset, although occasional exposures occur along the shore of the Fleet behind Chesil Beach and the upper part of the Oxford Clay is exposed at Furzy Cliff, north of Weymouth. Inland, these clays, together with overlying Corallian and Kimmeridge Clay floor the lower-lying land of the Blackmoor Vale running from Yetminster to Sturminster Newton and up to Gillingham. On the coast the overlying clays, nodular limestones and sands of the Corallian Beds are exposed between Weymouth and Ringstead and the Kimmeridge Clay is superbly exposed from Kimmeridge Bay to St Albans Head. This sequence represents a progressive deepening of the Jurassic sea with shallow water bivalves and corals in the Corallian and ammonites more typical of open deeper water in the Kimmeridge Clay.

The succeeding sands and limestones of the uppermost Jurassic Portland Group outcrop on Portland Bill and at St. Albans Head where the thick-bedded limestones at the top of the sequence give rise to the dramatic cliffs at both locations. These rocks were deposited in a gradually shallowing sea and contain fossil bivalves and ammonites, including the giant ammonite Titanites. As the Upper Jurassic sea continued to shallow a series of clays, marls and limestones were deposited in a system of lagoons. These are the Purbeck Beds and they form the cliffs at Durlston Head and are also exposed at Lulworth Cove, Mupe Bay and Portesham.

Cretaceous

The beginning of the Cretaceous (142 to 65 million years ago) was marked by a marine inundation which left a deposit almost entirely composed of oysters - the Cinder bed – which is succeeded by a series of marine to freshwater limestones, which form the upper part of the Purbeck Beds. The succeeding Wealden sediments, exposed to the north of Swanage, represent the tidal delta deposits of large rivers feeding from a landmass to the west. There was then a return to marine conditions as sea level rose with the deposition of the Lower Greensand, Gault Clay and Upper Greensand. The Upper Greensand forms prominent hills, such as that on which the town of Shaftesbury stands. The Upper Greensand appears again as outliers farther west, forming the high ground above Lyme Regis, Golden Cap, and Piliesden and Lewesden Pens.

Following the Upper Greensand is the Upper Cretaceous Chalk. Initially a calcareous sand, overtime the Chalk sea deepened resulting in the deposition of the purer calcium carbonate-rich white Chalk. The Chalk forms the central and most elevated part of Dorset, giving rise to the downs that sweep in a south-westerly direction from Cranborne Chase, through Blandford, Milton Abbas and Frampton to Dorchester; westward it reaches a point just north of Beaminster. From Dorchester, the Chalk outcrop narrows and turns south-eastward through Portesham, Bincombe, to West Lulworth, and from here as a narrow ridge of almost vertically inclined beds forming the Purbeck Hills to the headland at Ballard Point and the stacks of Old Harry Rocks. Much of the Chalk was deposited as a calcareous mud and thus the fossil fauna is dominated by animals that could live on the soft seabed such as burrowing sea-urchins and flattened bivalves.

Tertiary (comprising Neogene and Palaeogene)

The south-east of Dorset forms the western part of the Hampshire Basin, which is a large structural basin filled with sediments of Tertiary age (65-2 million years ago). The outcrop extends from just east of Dorchester around Poole Harbour to Studland in the south, Bournemouth on the northern side of the Bay and up to Verwood in the north. The various clays and sands making up the Tertiary succession were laid down in shallow marine, coastal and fluvial (river) environments. The oldest clays, sands and gravels of the Reading Beds were deposited on marshy mud and sandflats crossed by river channels. The succeeding sandy clays of the London Clay were deposited under full marine conditions, indicating a rise in sea-level. Fossil plant material from the clay shows that it was deposited under sub-tropical climate similar to that of modern-day south-east Asia. The main outcrop occurs between Lytchett Matravers and Alderholt, where together with the Reading Beds it forms produces a rolling and hummocky landscape, between the Chalk downs and adjacent Dorset Heaths.

The rest of the Tertiary succession in Dorset is dominated by sediments of the Bracklesham Group. In the Wareham area these comprise the shallow water, possibly tidal, Redend Sandstone, Pipeclay Beds and Agglestone Grits of the Poole Formation and Branksome Sand. Exposed to the east, in the Bournemouth area, is the Branksome Sand, which was laid down by a large river complex flowing from the west, and which is famous for the well preserved fossil leaves and fruit of tropical plants that it yields.

These Tertiary sands give rise to the nutrient-poor, freely-draining soils which support the heathland vegetation of the area and the large coniferous forest plantations north of Wareham. South of Poole Harbour, in the Wareham area, the thick clay units within the Poole Formation give rise to the locally important 'wet' heaths.

Quaternary

Over the last two million years ago the climate of Britain has varied tremendously with periods of temperate climate interrupter by repeated advances and retreats of glaciers and ice sheets. Collectively these have been known as the Ice Age and the actions of the ice sheets have been instrumental in forming the landscape we see today. However, unlike the rest of Britain, Dorset did not experience the full effects of the repeated advances of ice sheets, as their southward movement did not extend into the County. However, during these glacial periods it is likely that much of the County resembled the tundra of today's arctic provinces.

In many areas the Tertiary succession is covered by Quaternary plateau gravels. It is possible that some of these represent marine beach deposits, laid down when sea-levels were significantly higher during warmer phases of the Quaternary. Some of the gravel deposits may, however, be related to an ancient river system, known as the Solent River that flowed across south east Dorset and southern Hampshire and into a major 'English Channel' river. The Solent River system may have developed back in the Anglian Glaciation, some 450,000 years ago, but would have been disrupted during sea-level rise in subsequent warmer interglacials when sea-level rose. Evidence from terraces of gravel around the Hampshire Basin indicate that this system was certainly in place during the late Devensian glaciation, 20,000 years ago when sea levels were as much as 120m below the present level. Rising sea levels over the last 10,000 years caused the river valley to become drowned and infilled by fluvially deposited gravels.

Unequivocal evidence for higher sea-levels during the Quaternary comes from raised beach deposits at the southern end of Portland Bill. These sandy gravels and pebbles overlie a platform cut into the Portland Limestone. Two deposits occur and represent two different phases of higher sea-level over the past 200,000 years.

Rising sea-level following the melting of the last ice-sheet also led to the development of Chesil Beach. The long bar of Chesil Beach is made up dominantly of flint with limestone pebbles an important component at the eastern end. They are graded from about pea size at West Bay to fist size at Portland. The current view of the Beach's origin is that it arose as a beach barrier bar that was pushed northwards as sea levels rose at the end of the last Ice Age.

Away from the Poole Basin, the Chalk Downs of Dorset were shaped to their present form of scarps, slopes and valleys by water erosion. Much of the erosion probably took place by increased run-off from the melting of semi-permanent ice-caps on the Chalk crest, at the end of the last glaciation.

Geological Highlights:

  • The Lower Jurassic Lias cliffs between Lyme Regis and Charmouth have yielded hundreds of extremely well-preserved specimens of marine reptiles including ichthyosaurs and plesiosaurs, and more rarely pterosaurs and dinosaurs Fossil reptiles have been collected here since the 18th century, famously by Mary Anning of Lyme Regis who is credited with discovering the first described fossil ichthyosaur in 1810 and first plesiosaur. Important finds are still being made, such as the recent discovery of an almost complete skeleton of the armoured herbivorous dinosaur, Scelidosaurus, at Charmouth. These continued finds, as cliff erosion exposes fresh rock, and the historical importance of the area, make this section of the Dorset coast the richest Lower Jurassic reptile site in Britain, and probably the world.

  • Landslides along the coastal cliffs of Dorset are a relatively frequent occurrence and are linked to the underlying geology. In West Dorset, the lower part of the cliff sections are composed of Jurassic clays and limestones, while the tops of the cliffs at Black Ven, Stonebarrow and Golden Cap are capped by Upper Greensand of Cretaceous age. Rainwater percolates through the sands of the Upper Greensand but cannot sink into the clay. Water builds up at the junction between the two and seeps out as springs. During periods of prolonged rainfall, the build up of water lubricates the clay surface and increases the mass of the cliff top sediments, causing the Greensand to slide over the clay, across the undercliff and onto the beach. One of the most spectacular landslide events of recent years took place on Stonebarrow, east of Charmouth during Christmas 2000 when hundreds of thousands of tonnes of sand and clay spilt onto the beach.

  • The Kimmeridge Clay contains oil-rich shales, which readily burn. The best example is the Blackstone, a bed less than a metre thick which has long been known as the 'Kimmeridge Coal'. It can be traced in the cliff sections near Kimmeridge and also at Ringstead Bay. At the latter locality spontaneous ignition of the shale in 1826 gave rise to a fire that lasted four years, from which the location is now known as Burning Cliff. Fires have also occurred more recently in 1973 and 2000 in the Blackstone exposed in Kimmeridge Bay.

  • Small seepages of oil have been recorded from a variety of places in south Dorset for many years, although their source remained a matter of conjecture until the late 1950s. The drilling of a borehole by British Petroleum at Kimmeridge suggested that the oil was seeping out of the Cornbrash - a layer of rock over 500m below the surface. In 1959 the Kimmeridge Oil Well of British Petroleum started production. The nodding donkey on top of the cliffs still pumps oil today at a rate of about 100 barrels a day Further exploration in the area led to the discovery that the oil originates from the Jurassic Lower Lias to the south of Dorset and migrates up through the rock sequence into the Bridport Sands which act as the major reservoir rock. The Triassic Sherwood Sandstone also acts as a reservoir rock for oil. Between them, these horizons now yield 100,000 barrels of oil per day from the Wytch Farm production plant near Wareham. The oilfield extends into Poole Bay and is the largest onshore oil field in Europe, with estimated recoverable reserves of 300 million barrels.

  • The Isle of Portland is home to the Upper Jurassic Portland Stone, one of the most widely used and famous building stones in the world. Many public buildings, including St Paul's Cathedral, are built from Portland Stone which has been used since Roman times. Today, the Isle of Portland, which is dotted with disused and active quarries, is still a major source of this important building stone.

Local sites

The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.