Geography of Sussex - Geology

Geology

Sussex's geology typically runs in bands running east-west. Running across the south of the county, from Hampshire to chalk cliffs of Beachy Head lies the chalk ridge of the South Downs. 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. 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.

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. From the summit of the Downs the hilly country observed on the northern side is occupied mainly by the Hastings Beds and the Weald Clay; at the foot of the escarpment lie the Gault and Upper Greensand, while between these formations and the Wealden rocks there is an elevated ridge of ground formed by the Lower Greensand.

Between the South Downs and the English Channel is the South Coast Plain, also known as the Sussex-Hampshire Plain, part of the Hampshire-Dieppe basin, and it owes its level character to the action of marine planation. The plain is occupied partly by chalk and partly by Tertiary rocks, both much obscured by more recent deposits. The Eocene strata lying south of the Downs and west of Brighton - with the exception of some outliers of Reading Beds near Seaford - include the Woolwich and Reading Beds, London Clay (with hard "Bognor Rock"), the Bagshot and the Bracklesham Beds, which contain many fossils. Superficial deposits cover much of the coastal plain; these include glacial deposits with large boulders, raised beaches, brick earth and gravels, marine and estuarine, and the interesting Lower chalk or Coombe rock, formerly known as Elephant Beds, a coarse rubble of chalk waste formed late in the Glacial period, well exposed in the cliff at Black Rock east of Brighton, where it rests on a raised beach.

The southern side of the South Downs are deeply notched by dry valleys or coombes, which frequently end in cirques near the northern escarpment. Devil's Dyke 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.

At one time there is no doubt that the Chalk, Greensand and Gault of the Wealden anticline covered the entire area in the form of an uplifted dome, but denudation has removed the Chalk and most of the other formations as far as the North Downs, thereby exposing the underlying Wealden Beds. The oldest rocks thus brought to light along the crest of the anticline are the Purbeck Beds, small patches of shale and limestone, with some important beds of gypsum, which lie north-west of Battle. A deep boring (1905 ft.) at Netherfield, passed through Portlandian Beds and Kimmeridge Clay into Oxford Clay, but these do not appear anywhere at the surface. Above the Purbeck Beds, and covering all the north-eastern portion, of the county from the coast at Bexhill and Rye to Horsham, are sands and clays of the Lower Wealden or Hastings Beds. This includes the following local subdivisions, in ascending order; the Fairlight Clay, Ashdown Sand, Wadhurst Clay, Lower Tunbridge Wells Sand, Grinstead Clay and Upper Tunbridge Wells Sand (with Tilgate stone at the top and Cuckfield Clay at the base). The Weald Clay occupies a belt of lower ground on the south and west of the Hastings Sands, it consists of blue and mottled clays with thin sand layers and beds of hard limestone, the "Sussex marble" with the shells of Viviparus. Horsham Stone is another local hard bed. Near Tilgate the remains of Iguanodon were found by Sussex geologist Gideon Mantell in this formation.

Bordering the outcrop of the Weald Clay is the Lower Greensand Group; it appears a little north of Eastbourne and passes thence through Ringmer, Storrington, Pulborough, Petworth, Midhurst and Linchmere. It contains the following divisions in ascending order - the Atherfield Clay, Hythe Beds (sandy limestone, sandstone and chert), Sandgate Beds and Folkestone Beds. The natural gas of Heathfield comes from the Lower Wealden and Purbeck Beds. Wadhurst Clay was formerly an important source of iron ore.