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.
[1] 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.
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 last is itself subdivided thus: The rocks traditionally referred to as the Hastings Beds form the High Weald in the northeast of the county.
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.
Overlying divisions of the formation include the Fittleworth, Rogate, Selham Ironshot Sands, Pulborough Sandrock and Marehill Clay members.
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 main outcrop of the Chalk forms the South Downs, the West Sussex portion of which runs broadly westwards from Brighton to the county's western border with Hampshire.
Further outcrops are concealed beneath Quaternary sediments in a discontinuous low-lying strip from Worthing, north of Chichester to the Hampshire border.
West Sussex extends across a part of the Weald-Artois Anticline, a broad east-west aligned fold associated with the Alpine Orogeny.
A similar fold structure continues east from here, but offset en echelon to the south as the Littlehampton Anticline.
[4][5] The Sayers Common and Garstons Farm faults follow a similar trend in the area west of Burgess Hill.
[8] Also common are patches of locally derived materials such as clay-with-flints and head, the latter including clays, sands and gravels which in the dry valleys of the Downs have a chalky and flinty composition.
Along the coastal zone, wind-blown deposits of brick earth, mostly silt, are widespread as are sands and gravels whose age and origin are uncertain.