The geology of West Sussex in southeast England comprises a succession of sedimentary rocks of Cretaceous age overlain in the south by sediments of Palaeogene age. The sequence of strata from both periods consists of a variety of sandstones, mudstones, siltstones and limestones. These sediments were deposited within the Hampshire and Weald basins. Erosion subsequent to large scale but gentle folding associated with the Alpine Orogeny has resulted in the present outcrop pattern across the county, dominated by the north facing chalk scarp of the South Downs. The bedrock is overlain by a suite of Quaternary deposits of varied origin. Parts of both the bedrock and these superficial deposits have been worked for a variety of minerals for use in construction, industry and agriculture.
Jurassic and older rocks
A sequence of sedimentary rocks of Jurassic age has been proved to underlie the Cretaceous succession but nowhere within the county does it appear at the surface. Beneath these again, boreholes have proven Devonian and Carboniferous rocks and possible Permo-Triassic strata.
Cretaceous
The Lower Cretaceous comprises a sequence of stratigraphic groups from the Purbeck Group through the Wealden and Lower Greensand groups to the Selborne Group. The lowermost/earliest of these are present at depth but do not outcrop at the surface. The oldest strata to outcrop at the surface within West Sussex are the sandstones and siltstones of the Ashdown Formation of the Purbeck Group which date from around 146-134 million years ago.
The younger Upper Cretaceous is represented by the rocks of the Chalk Group, overwhelmingly the characteristic fine-grained white limestone known as chalk.
Purbeck Group
This group is composed of a lower Ashdown Formation, overlain by the Wadhurst Clay Formation and an upper Tunbridge Wells Sand Formation. The last is itself subdivided thus:
'Upper Tunbridge Wells Sand' (informal term) Grinstead Clay Member (divided into an 'upper' and a 'lower' part by the Cuckfield Stone Bed) Ardingly Sandstone Member 'Lower Tunbridge Wells Sand' (informal term)
The rocks traditionally referred to as the Hastings Beds form the High Weald in the northeast of the county. These sandstones, mudstones, siltstones and clays date from the Valanginian to the Berriasian (141-133 million years ago).
Wealden Group
The Weald Clay Formation occurs throughout the Weald in the north and east of the county. It consists of interbedded sandstone, mudstone and siltstone and some ironstone units dating from Hauterivian to Barremian times (146-125mya).
Lower Greensand Group
Strata of the Lower Greensand Group occupies a tract of country parallel to and north of the South Downs though separated from that escarpment by the outcrop of Selborne Group rocks. It roughly defines the edge of the Weald. From oldest to youngest, the Lower Greensand Group consists of the Atherfield Clay, Hythe, Sandgate and Folkestone Formation which were deposited during the Aptian age between 125 and 113 million years ago. Its thickness varies from around 250m to as much as 300m. It largely consists of sands and sandstones with silts and clays in places.
The lowermost unit is the Atherfield Clay Formation consisting of silty clays and clayey silts which are overlain by the fine to coarse-grained sandstones of the Hythe Formation which is glauconitic in part and which also contains Fuller's earth and clay units. Above this is the Sandgate Formation at the base of which is the Easebourne Member. Overlying divisions of the formation include the Fittleworth, Rogate, Selham Ironshot Sands, Pulborough Sandrock and Marehill Clay members. These are generally sands, silts and clays. The uppermost unit of the group is the Folkestone Formation consisting of fine to coarse-grained sandstones with a clay unit.
Selborne Group
Stratigraphically above the Lower Greensand Group is the Selborne Group which comprises a suite of mudstones, siltstones, sandstones and limestones laid down during the Albian age between 112 and 94 million years ago. It divides into an earlier Gault Formation and a later Upper Greensand Formation. These outcrop in a thin tract of country, typically 1–2 km in width, at the foot of the north-facing South Downs scarp.
The Gault Formation comprises clays, mudstone, sandstone and limestone whilst the overlying Upper Greensand Formation comprises siltstones and fine-grained sandstones.