The youngest solid rocks are found on the southern fringes of the National Park in the eastern extension of the Hampshire Basin and include sand, silt and clay deposited during the Palaeocene and Eocene epochs.
The oldest rocks encountered within the National Park are those of the Weald Clay Formation which stratigraphers place within the Wealden Group.
Chalk is the rock type associated most closely with the National Park and, in common with the chalk which provides other key landscape features in the southeast of England, was formed by the settling to the sea floor of myriad coccoliths (microscopic plates of calcium carbonate formed by single-celled algae known as coccolithophores) during the Late Cretaceous epoch between about 100 and 70 million years ago.
The Chalk has previously been subdivided in other ways and references to Upper, Middle and Lower abound in the literature and on geological maps.
Ranging from Cenomanian to Campanian (c.84-72 Ma) in age, these strata are (uppermost/youngest at top): The White Chalk is generally a more pure limestone and usually forms the main scarp face.
The scarp runs sub-parallel to that of the chalk as far as Combe Hill, southeast of Liss where it turns to the northeast to exit the national park near Haslemere.
The synclinal axis comes ashore at Lancing (albeit the bedrock is thickly concealed by other deposits) and can be traced WNW to the south of Arundel.
Southeast England was not glaciated during the Quaternary period i.e. the last 2.6 million years but was subject to severe climate at times that has contributed to the shape of today’s landscape.
It is theorised that many of the coombes which scallop the scarp of the South Downs were excavated by surface water flowing over perennially frozen ground thus rendering the normally permeable chalk impermeable.
[9] The British Geological Survey map numerous residual deposits across the upper surfaces of the South Downs, derived from the solution, decalcification and cryoturbation of the underlying bedrock.
The Itchen, Meon, Lavant, Arun, Adur, Ouse and Cuckmere rivers each cut through the chalk ridge of the Downs conveying water from their north to the English Channel to the south.
Rock debris at the foot of the high chalk cliffs at Beachy Head at the eastern extremity of the national park is landslide material.
Several areas of landslip occur in the vicinity of Petersfield and north towards Farnham at the junction of the Gault with the overlying Upper Greensand.