Geology of the South Downs National Park

The geology of the South Downs National Park in

• Folkestone Formation (sandstone)
• Sandgate Formation
  • Marehill Clay Member (mudstone)
  • Pulborough Sandrock Member (sandstone)
  • Selham Ironshot Sands Member (sandstone)
  • Fittleworth Member
  • Easebourne Member
• Hythe Formation (sandstone)
• Atherfield Clay Formation

Overlying the Lower Greensand are the two formations which comprise the

Chalk

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 Group which includes all of the different units which make up the succession in England, is subdivided into an earlier/lower Grey Chalk Subgroup and a later/higher White Chalk Subgroup. The Chalk has previously been subdivided in other ways and references to Upper, Middle and Lower abound in the literature and on geological maps. The Chalk is not homogeneous and within these two subgroups, numerous further units are distinguished, each with their own characteristics.

Grey Chalk Subgroup

The Grey Chalk Subgroup within the National Park is divided into two formations, an earlier West Melbury Marly Chalk Formation (within which the Glauconitic Marl Member is recognised) and a later Zig Zag Chalk Formation. All originate during the Cenomanian (100.5-94 Ma). The Grey Chalk is often described as

The White Chalk Subgroup is locally divided into six formations, the uppermost of which is further subdivided into two members. Two distinct members are also identified within the lowermost formation. Ranging from Cenomanian to Campanian (c.84-72 Ma) in age, these strata are (uppermost/youngest at top):

• Culver Chalk Formation
  • Spetisbury Chalk Member
  • Tarrant Chalk Member
• Newhaven Chalk Formation
• Seaford Chalk Formation
• Lewes Nodular Chalk Formation
• New Pit Chalk Formation
• Holywell Nodular Chalk Formation
  • Melbourn Rock Member
  • Plenus Marl Member

The White Chalk is generally a more pure limestone and usually forms the main scarp face.^[3] Amongst the exceptions are the ‘Plenus Marls’ which are a grey/green marly chalk up to 3m thick whilst the Melbourn Rock is a hard cream-coloured nodular chalk up to 5m thick.^[4] The Newhaven Chalk typically forms a secondary scarp some distance to the south of the main scarp, and much less continuous in nature.^[5] Bands of flint nodules occur throughout the chalk but are more numerous in parts of the succession than others. Flint is a variety of chert, a microcrystalline or cryptocrystalline form of quartz the mode of formation of which is still debated.^[6]

Palaeogene strata

There are no rocks recorded within the area from the last part of the Cretaceous (the Maastrichtian age) nor the earliest parts of the following Paleogene period (Danian and Selandian ages).

Lambeth Group

A succession of sands, silts and clays date back to the Thanetian (c.59-56 Ma) and Ypresian (c.56-48 Ma) ages and named as the Lambeth Group. The Woolwich and Reading formations within this Group contain some thin lignites and shell beds in places. These rocks lie unconformably on an eroded surface of the Culver Chalk Formation and in places, the uppermost Newhaven Chalk.

Thames Group

Three formations occur within the boundaries of the National Park; uppermost/youngest at top, these strata are:

• Earnley Sand Formation (sand, silt & clay)
• Wittering Formation (sand, silt & clay)
• London Clay Formation (clay, silt, sand)

Geological structures

Geological strata within the area are flat-lying to moderately dipping. The

Quaternary landscape evolution and superficial deposits

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. Frost action and solifluction will also have been contributors to their development.^[9]

Clay-with-flints and head

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. Referred to as the Clay-with-flints Formation, the deposit also contains sand and silt in places. It varies from 0 to 10m in thickness but can exceed this where it fills solution hollows.^[10] Where this and/or other material has moved down steeper slopes during the post-glacial period, it is referred to as head.^[11] Head deposits are present in the base of most of the dry valleys which penetrate the Downs.

Effects of sea level changes

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. During times of lowered sea level as during the Last Glacial Period, each river was rejuvenated and cut down into its bed. As sea levels recovered to levels similar to today, the lower parts of the valleys were inundated by the sea to become rias. River-borne sediments (sand, silt and clay) then progressively infilled the rias. In places, similar material forms not only the modern floodplain but also river terraces perched at some height above it as a remnant of an earlier floodplain when sea levels were temporarily higher than those of today.^[12]

Beach and tidal flats

Shingle, sand and gravel etc occur around the mouths of the Cuckmere and Ouse rivers. A storm beach extends from the mouth of the Ouse southeast to the end of the esplanade at Seaford. Sediments around Arundel are regarded as raised marine deposits. Each of these deposits is of Holocene age.

Artificial ground

Ground surfaces which have been raised or otherwise formed by human activity are classed as artificial. They include railway and major road embankments, together with quarry spoil and landfill for example. Small examples occur throughout the national park. Some of the more striking examples are those at Midhurst which are associated with the former Midhurst Brickworks.

Landslide deposits

Rock debris at the foot of the high chalk cliffs at Beachy Head at the eastern extremity of the national park is landslide material. Cliff falls are common on the steep to vertical chalk cliffs of this coast, though most lie just outside of the national park. 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.^[13]

Conservation and recreation

Of the many Sites of Special Scientific Interest within the national park, ten are designated for their geological interest:

• Brighton to Newhaven Cliffs
• Seaford to Beachy Head
• Beeding to Newtimber Hill
• Butser Hill
• Eartham Pit
• Horton Clay Pit
• Southerham Machine Bottom Pit
• Southerham Grey Pit
• Southerham Works Pit
• Asham Quarry

In addition to these, 50 sites have been designated as

The South Downs Way is a national trail which takes advantage of the north-facing chalk escarpment of the South Downs as it extends for the length of the national park from the margins of Eastbourne in the east to Winchester in the west.

See also

• Geology of East Sussex
• Geology of West Sussex
• Geology of Hampshire

References