Natural England - Sussex - East and West (including The City of Brighton and Hove)

Sussex - East and West (including The City of Brighton and Hove)

Much of the characteristic feel and shape of the Sussex landscape can be attributed to the geological structure and makeup of the County.

Kingston

The South Downs consist of Cretaceous chalk which was folded during the Tertiary.

This is perhaps nowhere better observed than the long prominent spine of chalk which forms the South Downs and which stretches from Hampshire, eastwards across West Sussex until it is sheared off at the precipitous coastal cliffs between Seahaven and Eastbourne. The South Downs form the southern rim of the Wealden anticline, a large and eroded dome of rocks, largely of Cretaceous age (140-65 million years old) which has had a significant influence on the development of the diverse topography of the County.

With the exception of the youngest part of the Chalk, Sussex provides an almost complete sequence of Cretaceous rocks. They dip to the south with the consequence that increasingly younger rocks are exposed the further south you go. Small exposures of rocks of Jurassic age (195-140 million years old) occur on the main axis of the Weald anticline and rocks of Tertiary age (65-2 million years old) are exposed along the coast in West Sussex. During the Ice Ages no glaciers reached Sussex, but the County was under the influence of very cold tundra-like conditions at the edge of the ice sheets. Processes of erosion and deposition during this period have contributed significantly to the formation of the present landscape.

Jurassic

At the end of the Jurassic (140 million years ago) a large brackish-water embayment lay over Sussex with a connection to the sea to the south. Within this embayment were deposited the limestones and shales of the Purbeck Limestone. The Purbeck fauna is varied and includes bivalves, gastropods, reptiles, fish and insects. Rocks of this age are only exposed where millions of years of erosion has been sufficient to reach the core of the Wealden anticline along its main axis in the Brightling-Mountsfield area.

Cretaceous

Rocks of Lower Cretaceous age (140-100 million years old) form the main core of the Wealden anticline. A series of sands, soft sandstones and clays, with names such as the Ashdown Sand, Wadhurst Clay, Tunbridge Wells Sand and collectively known as the Hastings Beds, give rise to the high, broken ground of the High Weald. These sediments were all deposited in a fresh to brackish-water embayment (which covered all of Sussex) opening out into marine areas to the north (into East Anglia) and south (into France). The sands were deposited on a broad zone of braided river floodplains and channels, while the clays were deposited in quieter-water lagoons and bays, and comprise finely bedded shales and mudstones. The Wadhurst Clay contains several marsh soil beds towards its base and top indicating that the embayment at this time was very shallow.

Although not exceeding 240 m AOD, the High Weald is a hilly country of well wooded ridges and valleys. Numerous major ridges run mainly east to west, for example the Ashdown Forest Ridge and the Battle Ridge. These major ridges are deeply dissected by many tributaries of rivers which rise in the High Weald producing a network of small, steep-sided ridges and valleys, known locally as ghylls.

The outcrop of the overlying Weald Clay coincides with the low-lying broad vale known as the Low Weald. The Weald Clay was deposited within a system of shallow brackish-freshwater lagoons and tidal deltas and is mainly made up of clay with some thin sand units and freshwater limestones. The localised deposits of limestone and sandstone form gentle ridges and high points throughout the Low Weald and in many places, these are the sites of farmsteads, hamlets or larger settlements.

Bordering the outcrop of the Weald Clay, and geologically overlying it, are the rocks of the Lower Greensand. This sequence of sands and muds were deposited under marine conditions which were established as the connection of the embayment with the sea became stronger and sediments were deposited in a variety of shallow water environments. Initially, offshore muds (now shales and mudstones) of the Atherfield Clay were deposited followed by shallow marine sands of the Hythe, Sandgate and Folkestone Beds. The Hythe Beds comprise alternating layers of sandy limestone and calcareous sandstone (“rag and hassock”) in the east, becoming sandstones with chert beds in the west. The Sandgate Beds are variable and comprise sandstones and mudstones. All the sand formations show evidence for strong tidal influence in the form of large-scale structures that represent the migration of waves of sand over the sea floor.

A further rise in sea-level led to the formation of a larger and deeper sea and deposition of the Gault Clay and the Upper Greensand. These are variants of the same sequence with Gault referring to mudstones and Upper Greensand to sandstones. The Gault Clay contains phosphate-rich nodules in discrete bands and has a rich marine fauna with abundant ammonites, bivalves and gastropods. The Upper Greensand comprises a variety of sediments with fine silts at the base giving way upwards into sandstones.

The outcrop of the Lower Greensand and Upper Greensand curves around the Low Weald and gives rise to a distinctive wooded ridge which is deeply indented in some locations. Near Haslemere, the convoluted escarpment rises to a height of 280m at Black Down, the highest point in Sussex. The outcrop of the Lower Greensand gives rise to sandy, poor quality acidic soils which support mixed oak-birch-chestnut woodland and heathland on the higher ground.

A major phase of land subsidence marked the beginning of the Upper Cretaceous (100 million years ago), leading to deposition of the Chalk. The Chalk is a white limestone comprising over 95% calcium carbonate. It contains thin beds of marl and nodules of flint, either scattered or in bands. The Chalk Downs surround the Weald, with the South Downs extending from Petersfield (Hampshire) to Eastbourne. The cliffs of Sussex provide excellent sections through the Chalk, but the beds are more accessible in the many pits and quarries inland. The mass of Chalk in Sussex has been cut into separate blocks by the valleys of the principal rivers (Arun, Adur, Ouse and Cuckmere). Flat valley bottoms and a meandering river course enclosed by steep-sided slopes with minor cliffs are, in many places, typical features of these river valleys.

Tertiary (comprising Neogene and Palaeogene)

Rocks of Tertiary age (65-2 million years old) in Sussex outcrop in the far south-west of the County and form the most southerly part of the extensive Tertiary sediments that characterise the London Basin. At the end of the Cretaceous there was a major fall in sea-level and the Weald of Sussex became a low-lying land area. The reddish and mottled clays of the Reading Beds were deposited by a large river and delta system and mark the beginning of the Tertiary age in Sussex. The Reading Beds outcrop in the south-west of the County and as small areas near Newhaven and Brighton. Deposition of the overlying London Clay marks a change to marine conditions. A thin pebble bed of flints marks the base. Fossils are generally scarce, but common in certain horizons.

The sediments of the shallow marine Bracklesham Beds are the last major Tertiary exposure in Sussex and outcrop in the Selsey Bill-Chichester Harbour area. The group is divided into four units, each of which yields a characteristic fossil assemblage, including many species of fish, plants and shells.

Quaternary

Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today. The Ice Sheets did not reach as far south as Sussex, but the influence of the alternating cold and warm phases can still be seen in the landscape.

During the glacial periods, large amounts of water were locked up in the ice sheets to the north and as a consequence sea-level was up to 100 m lower than its present level. During the warmer interglacial periods, water was released from the ice and sea-level was higher than today. Coastal features such as old sea cliffs and raised beaches, produced by these sea level fluctuations, are found along the Sussex coast. A large sweep of fine silt, known as loess, occurs at the surface along the western part of the coast (e.g. the Manhood Peninsula). This material was wind transported and derived from the extensive vegetation-free plains during the colder glacials. Today, it provides a very fertile soil which supports a thriving horticultural industry.

Sea-level rise over the last 10,000 years has led to significant change along the Sussex coast. Perhaps the most obvious evidence of this is Dungeness, the largest coastal shingle structure in Europe. Here, over the past 4,000 years, shingle transported west-east along the coast has become aligned in sub-parallel ridges of differing ages with the oldest ridges furthest from the coast.

Pevensey Levels were formerly a shallow bay, which has since silted up and been artificially drained. Several small islands in the original bay at Pevensey now stand out above the levels as low hills.

Geological Highlights:

  • A number of horizons within the Weald Clay have been used for building purposes. The clay itself is used in the manufacture of bricks and tiles, while two notable layers, the Sussex Marble and Horsham Slate, are used for ornamental stone work and the production of flagstone respectively. The Sussex marble is a limestone packed with the freshwater gastropod Viviparus sussexiensis. Horsham Slate is a sandstone which splits easily to produce thick flagstone layers. Many ancient buildings in the Weald, especially around Horsham, are roofed in this flagstone. The excessive weight of these slabs often makes the roof timbers sag.

  • Ironstone in the Weald Clay has been exploited for iron smelting from about the 2nd Century BC until early in the 19th Century. During the first two centuries of the Roman occupation of Britain the Weald was the principal source of iron in the province, and more than a hundred sites of the period have been identified. In 1496 the first documented blast furnace was established in England, on Ashdown Forest in Sussex. A specialist activity of a few furnaces was gunfounding, with production being encouraged by a succession of naval conflicts and increasing British interests overseas. Dominance in this branch of the iron industry was sustained until the 1760s, but with the concentration of the iron industry on the coalfields, the last furnace in the Weald was blown out in 1813.

  • Sussex is home to the world famous Boxgrove “Man”. The remains comprise a shin bone and two teeth, found in river gravels overlying chalk in a gravel pit just outside Chichester. Boxgrove Man is thought to have been a powerfully built individual over 6 feet tall and is considered to be a representative of Homo heidelbergensis. The site has yielded a large number of flint hand axes and the bones of numerous mammals which indicate that the gravels were deposited approximately 400,000 years ago, making Boxgrove Man the oldest 'man' so far discovered in Britain. Today the site is on the flat coastal plain, several miles from the sea to the south, and a mile from the low foothills of the South Downs to the north. However, half a million years ago, the site lay at the coastline at the foot of high chalk cliffs, which have since been totally eroded. When the sea level fell, a broad grassy plain soon developed which provided a rich habitat for animals and early man.

  • Coombes are dry valleys, which have a variety of origins that dissect the escarpments and dip slopes of the South Downs. Devil’s Dyke, to the north of Brighton, is the most famous and remarkable of all the chalk dry valleys and is frequently cited as the type example. It is the largest single coombe anywhere in the chalk karst of Britain. Its exact origin is a source of debate, but it probably evolved through erosional and weathering processes during the Ice Age.

  • Sussex holds a pre-eminent position in the history of geology and in particular the study of dinosaurs. Many dinosaur remains have been excavated from the Weald Clay, including some of the earliest dinosaur finds to be named. Famously, these include the teeth and bones of Iguandon, the first dinosaur to be described. Teeth of this large plant-eating dinosaur were first observed in the gravel of a recently paved road by the wife of Dr. Gideon Mantell, a keen fossil collector, while on a visit to a patient. After a number of years searching local quarries Mantell traced the source of the rock which had held the tooth to a quarry in Whiteman’s Green, Cuckfield, where he found more teeth and other remains. He widened his search to pits in Tilgate Forest where he found further teeth and bones. Dr. William Buckland, Professor of Geology at Oxford University, thought the remains were from a large fish, while the eminent French anatomist Baron George Cuvier suggested that the remains were from a rhinoceros. On a visit to the Royal College of Surgeons in London, Mantell noticed that that the teeth he had found were almost identical to the teeth of an iguana lizard, though much larger. Mantell realised he had found the remains of an extinct giant reptile, not unlike an iguana he thought, and named his creature Iguanodon, meaning literally 'iguana tooth'. This was the first dinosaur to be described.

  • The Jurassic Purbeck Limestone that outcrops in the central part of the Wealden anticline contains commercially important beds of gypsum. This has been mined and processed at Mountfield since 1876. In the 1960s, a second mine was opened at Brightling and the Mountfield mine closed in 1990.

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.