Natural England - Hampshire (including: Isle of Wight, Southampton, Portsmouth, Havant)

Hampshire (including: Isle of Wight, Southampton, Portsmouth, Havant)

The geology of Hampshire and the Isle of Wight although relatively simple, comprising rocks and sediments from three periods of geological time, represents one of the richest sequences of fossiliferous deposits in Europe.

Upper Cretaceous Chalk cliffs, Compton Down SSSI.

The oldest rocks in Hampshire are of Cretaceous age, and comprise the marine sands and clays of the Lower Greensand, Gault Clay and Upper Greensand that outcrop in East Hampshire and the overlying white, pure limestone of the Chalk which gives rise to the rolling downland of much of the County. The Chalk outcrop separates two areas of Tertiary sediments.

In the south are the sands and clays of the Hampshire Basin, which, where they are close to the surface, give rise to the largely free-draining, acidic soils of the New Forest. In the north of Hampshire, similar sediments outcrop and here also give rise to part of the heathland vegetation complex of the London Basin. Much of the New Forest area is capped by gravels of Quaternary age derived from the once extensive Solent River system.

The Isle of Wight is a diverse island landscape mirroring the geology of the Hampshire Basin, with Tertiary sediments dominating the north of the Island and Cretaceous (including the lower Cretaceous Wealden sediments) in the south. During the Alpine mountain-building phase, some 15-20 million years ago, the rocks of both the mainland and the Isle of Wight were folded, giving rise to the diverse structure of the area and contributing to the display of the superb range of outcrops seen on the Island.

Cretaceous

The oldest rocks present in the area are the sands and clays of the Wealden Group, which are exposed on either side of the Isle of Wight at Sandown in the east and in Brighstone Bay in the west. The various mottled, red and yellow clays and sandstones of the Wealden of the Isle of Wight were deposited across a large area of river delta-flats by extensive braided rivers. The Wealden sediments contain much driftwood, fossil plants such as ferns, pine cones and freshwater mussels.

They are perhaps best known for the dinosaur remains that they have yielded over many decades. These include large numbers of bones of the large bipedal herbivorous dinosaur, Iguanodon amongst other species of extinct reptiles. The upper part of the sequence consists of shales and thin limestones which were laid down in large freshwater lagoons. At the top of the succession evidence of marine conditions and an advancing sea is provided by brackish and marine fossil shells.

Rocks of the succeeding Lower Greensand are exposed on the south coast of the Isle of Wight and in east Hampshire, although there are distinct differences in the rocks between the two areas. On the Isle of Wight the Lower Greensand comprises a thick sequence of marine deposited clays and sands that are well exposed in the south-west of the Island around Atherfield and again on the other side of the Island to the north of Sandown. Much of the sequence is not that fossiliferous, however, there are various horizons throughout the succession that yield the fossils of marine creatures. These include horizons of nodules which contain ammonites, clusters of brachiopods and bivalves and some beds which contain well-preserved fossil crabs and lobsters.

In East Hampshire the Lower Greensand succession is also of marine origin, but comprises different rock units, mainly sandstones, which outcrop at the western end of the Weald. The Lower Greensand gives way to the Gault Clay and the Upper Greensand, which on the Isle of Wight comprise sandy clays and sandstones. The upper part of the Upper Greensand is characterised by regular courses of chert and gives rise to the conspicuous cliff that dominates the Undercliff between Bonchurch and Blackgang.

The overlying Chalk dominates much of north and central Hampshire and forms the spine of higher ground running across the Isle of Wight. The Hampshire Downs are part of the broad belt of chalk linking the Dorset Downs and Salisbury Plain in the west with the South Downs in the east, and the Berkshire and Marlborough Downs to the north. The underlying chalk gives rise to a variety of contrasting landforms including broad strongly rolling downland, both deep and shallow valleys, and distinct hilltops and ridges with minor steep scarps in places.

The very pure limestone (up to 98 per cent calcium carbonate) of the chalk was deposited in a warm shallow tropical sea around 70-100 million years ago. Inland outcrops, apart from in quarries, are not common and it is best seen along the cuttings above the M27 at Portsdown and on the Isle of Wight where it forms the dramatic cliffs at Freshwater and the Needles. Throughout its area of outcrop, wherever agricultural practice is suitable, the chalk supports calcareous grassland vegetation that is rich in many plant species including orchids and herbs.

Tertiary (comprising Neogene and Palaeogene)

Rocks of Tertiary age occur in two basins on either side of the higher ground formed by the mass of the chalk. In the north, sands and clays belonging to the Reading Formation, London Clay and Bagshot Beds outcrop in the area around Farnborough, Aldershot and Kingsclere. These sediments record a change in environments from marshy mudflats crossed by river channels (the Reading Formation) to more open, marine conditions (the London clay) and shallow water tidal sandflats (the Bagshot beds). The sands of the Bagshot beds give rise to free-draining soils which support the heathland vegetation that is typical of this part of the Thames Basin.

The same suite of Tertiary rocks occur in the southern part of the County (and the northern half of the Isle of Wight) in the Hampshire Basin, their outcrop running in a narrow band parallel to the edge of the chalk. However, here the Bagshot Beds are largely replaced by part of a much thicker sequence of marine, estuarine and freshwater sands and clays. These underlie much of the New Forest area and the coastal plain of the Solent Estuary and form the bulk of the northern half of the Isle of Wight. The clays and sands of the Bracklesham Group and overlying Barton Group were deposited under shallow marine to intertidal conditions and contain a very diverse assemblage of fossil molluscs, plant remains and reptiles (mainly turtles).

Unfortunately much of the Barton succession has now been covered by coastal defence works, but outcrops of it can still be seen on the Isle of Wight in Whitecliff Bay. The overlying marls, clays and limestones of the Solent Group indicate a shallowing of the Tertiary sea and the oncoming of more terrestrial conditions. These sediments are particularly well exposed on the Isle of Wight at either end of the Island.

The Bembridge Limestone of the Solent Group is a prominent freshwater limestone with ancient pondsnails and in the Bembridge area it forms extensive ledges. The overlying Bouldnor Formation includes a bed that yields fossil insects and a variety of mollusc shells. The variety of fossils indicates that salinity conditions varied from fresh to brackish to marine.

Towards the end of the Tertiary the formation of the Alps (about 15 million years ago) caused squeezing and folding of rocks in southern England. The consequence of this is seen on the Isle of Wight in Alum Bay and Whitecliff Bay where the rocks are vertically tilted. This is part of a large scale fold known as the Purbeck – Isle of Wight Monocline which can be traced across East Dorset and the Hampshire Basin.

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, Hampshire 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 of the New Forest is covered by Quaternary plateau gravels which are distinguished from those of the Tertiary mainly by the angularity of the flints they contain. It is possible that some of these represent marine beach deposits, laid down when sea-levels were significantly higher during some of the previous 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 interglacials when sea-level rose. Large amounts of gravel would have been carried by the river from the more upland areas following seasonal snow melt each year. 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. The river deposited vast quantities of sand and gravel throughout its flood plain.

Away from the main Basin, the Chalk Downs of Hampshire 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 plateau during seasonal snow melt and at the end of the last glaciation.

Geological highlights:

  • In 1992, in the Lower Cretaceous Wealden sediments (Wessex Formation) on the southwest coast of the Isle of Wight, the remains of a sauropod dinosaur were discovered. Though isolated suaropod bones are often found an unprecedented 40 per cent of this skeleton has been recovered making this the most complete sauropod know from rocks of this age in the UK. The sauropod was a large (approximately 15m long and 8m high), herbivorous dinosaur that is believed to have died on land and been inundated by flooding: there is evidence that the body was scavenged before it was fully buried.

  • The Tertiary clays, sands and organic deposits exposed on the Isle of Wight and the Hampshire coast are one of the most important sources of fossil vertebrates in Europe and the World. Over 45 species of fossil mammal have been described from the sediments exposed at places like Hordle Cliff and Headon Hill and include species such as tapir and other mammals indicating connections between Europe and south-east Asian faunas. These beds also yield excellent fossil reptile and bird faunas including species of turtles, crocodiles, lizards (Iguana), snakes, geese, plovers and falcons.

  • The Solent Group outcrops on the north coast of the Isle of Wight contain a thin limestone near the base of the Bembridge Marls. This is known as the 'Insect Limestone', and is the most important source of Tertiary fossil insects anywhere in Britain. Over 250 new species have been found within the limestone, including tree-ants and termites. The Insect Limestone also contains a unique flora of fossil plants, belonging to over 120 different species; 39 of these species have their type-locality here, and 20 are unique to this site in Britain. Of particular importance is the excellent preservation of the fossil plants, which allow the recognition of plant organs not usually seen in fossilised specimens.

  • The Quaternary gravels of the Hampshire Basin have yielded a large number of human-made artefacts of Palaeolithic age (500,000-10,000 years ago). These include flint hand axes, choppers and worked implements. Many artefacts have been recovered from the gravel deposits of the river valleys, or the gravel sequences exposed on the coast, where they have been transported by fast flowing glacial streams. However, a couple of sites showing occupation by man have been found. These include a recently discovered site near Ringwood, above the present-day valley of the River Avon. Here over 400 carefully made large blades and flakes, including knives, scrapers and borers have been found. A similar assemblage of tools has also been found at Hengistbury Head in Dorset, which has been dated to the end of the last glacial approximately 12,500 years ago.

  • The famous coloured sands of Alum Bay are a series of sands, clays and organic-rich layers (lignites) within the Tertiary aged Bracklesham Group. The sediments of the Bracklesham Group normally contain large numbers of marine fossils, but at Alum they are unusual for being both coloured and generally without marine fossils. The varying beds of sediment are orientated vertically, meaning that a much wider sequence of rocks is seen here than in other locations and making the variation in colours between the beds more conspicuous. The colours are largely due to their mineral content and the way in which they have weathered. Yellow and brownish sands owe their colour to limonite, which is oxidised iron. Red colouration also indicates oxidation and is associated with the weathering of iron compounds in ancient soils, although the colour could have also originated from more recent oxidation of pyrite. Green is provided by the mineral glauconite, which is common in some of the more marine units. Sands with a high quartz content can appear to be white while organic material, notably lignite can provide a black colouration.

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.

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