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Geology of East Sussex

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Geology of south-eastern England. The Ashdown Sands and Wadhurst Clay is in lime green (9a); the Low Weald, darker green (9). Chalk Downs, pale green (6)
Geological section from north to south

The Geology of East Sussex is defined by the Weald–Artois anticline, a 60 kilometres (37 mi) wide and 100 kilometres (62 mi) long fold within which caused the arching up of the chalk into a broad dome within the middle Miocene[1], which has subsequently been eroded down to reveal a lower Cretaceous to Upper Jurassic Stratigraphy. East Sussex, geologically is best known for the identification of the first dinosaur by Gideon Mantell, near Cuckfield,[2] to the famous hoax of the Piltdown man [3] near Uckfield.

The county’s chalk has provided a world class stratigraphic marker giving a great deal of detail in Cretaceous Chalk Paleoecology and Palaeontology while in the east of the county on the Kentish border the Dungeness Foreland is important for the study of Geomorphology and Holocene sea level fluctuations.

Geological history

The geological history of East Sussex commenced during Carboniferous, with the rocks which are today basement deposited within a low swamp providing coals which were exploited to the north and east in Kent, but boreholes drilled in the 19th century failed to find this deposit in Sussex. The Carboniferous coals are overlain by Silurian and Triassic sediments.[4] The sediments where uplifted and faulted within the Variscan Orogeny, with the land now occupied by East Sussex being a low external fold belt to the main orogeny, which was located within the present day English Channel,[5] the remnants of the mountain belt can be seen today in Devon and Cornwall in what is known as the Cornubian Massif. Although unlike in Devon and Cornwall, there was little or no metamorphism.[6]

The mountain belt collapsed soon after the orogeny leading to the former northward thrusts to be reactivated as normal faults and lead to the formation of the Weald basin which developed as an extension of the considerably larger Wessex Basin [7] . The northern margin of the basin was formed by a series of normal faults, against what was then an area of land, known to geologists as the London-Brabant Massif. The Weald basin gently subsided throughout the Jurassic, Cretaceous and Early Tertiary leading to a thick succession of sedimentary rocks being deposited.

Purbeck Beds

The oldest rocks in the county are the Purbeck beds of Jurassic age which crop out in small localities north west of Battle they have a thickness of 77 to 186m in the Weald and are composed of Interbedded mudstones, limestones and evaporates (such as gypsum) of marginal freshwater, brackish and marine origin with detrital quartz.[8] These rocks were deposited in an environment of braided rivers and muddy lagoons, which periodically dried out, resulting in economic deposits of gypsum being deposited in this area,[9] The same beds crop out in Dorset. but there are difficulties in correlating the two beds which has led to increased study on these rocks. During the deposition of these rocks they were at 30°N and thus experienced a tropical climate, which has led to the Purbeck Beds host the economic supplies of 21m thick beds [10]

Ashdown Beds and Wadhurst Clay

Climber enjoying the Lower Cretaceous Ashdown Bed Sandstones of High Rocks

The upper Purbeck beds record a transition into more sand being delivered into the Weald basin, this has led to the deposition of a mixture of fine sands known as the Ashdown Beds, which along with the Wadhurst clay compose much of Ashdown Forest. The Ashdown Beds are predominantly siltstones and silty fine-grained sandstones with small amounts of finely-bedded mudstone and mudstone arranged in rhythmic units ("cyclothems") commonly divided by thin pebble beds.[11] as described by the British Geologic Survey who also describe the Wadhurst Clay as made of soft, dark grey thinly-bedded mudstones ("shales") and mudstones with subordinate beds of pale grey siltstone, fine-grained sandstone, shelly limestone, clay ironstone and rare pebble beds, which shows evidence of unconformable weathering at the top of the bed.[12] The Wadhurst clay hosted the small nodules of iron ore which was the very foundation of the Wealden Iron Industry. The ore was deposited in a tropical environment within which iron brought in from the eroding mountains in the west was altered into small nodules of ilmenite. A succession of clays and sands was deposited into the subsiding basin, with much of the source material also being delivered from the north and east as well as the west.

Greensands and Gaunt

The Greensands and Gaunt Clay best defines the Wealden Anticline, running in a broad horseshoe from Folkestone in the East, to Petersfield in Hampshire in the West and back to Eastbourne. The Greensands are divided into two units, the Lower and Upper Greensands, which sandwich the Gaunt. The three units outcrop in East Sussex along the bottom of the Downs and northward into the Vale of the Weald. Although named as such the lower Greensands are rarely sand and rarely green; the name was applied by mistake by 19th century geologists mistakenly thinking that the mineral glauconite would be found in the seams of sandstone both above and below the gaunt clay.[13]

The Gaunt clay is one of the most fossil rich horizons in the UK; yielding plentiful bivalves, cephalopod (including ammonites) and gastropods. This has allowed for a tight correlation of the age of the gaunt with other geological units in Europe, under the science of biostratigraphy. At its maximum the Gaunt clay sea grew to cover the northern landmass which had supplied the sediment for the lower sandstones; by this time Britain was at 35°N and the land and sea teemed with dinosaurs and marine reptiles, the remains of which have been found in the Gaunt.

Chalk

The Seven Sisters cliffs and the coastguard cottages, from Seaford Head across the River Cuckmere

The Chalk is the most well known rock in East Sussex, forming the Downs and where it meets the sea the spectacular Seven Sisters and geologically and biologically rich cliffs from Brighton to Newhaven. The Chalk formed in a warm, clear sea which stretched from Texas to Poland (prior to the opening of the Atlantic) and is subdivided into three units, the Lower, Middle and Upper Chalk.

Tertiary

After the end of the Cretaceous deposition continued, with the Reading and London Beds (clays and sandstones) being deposited during the Tertiary, these are no longer exposed in East Sussex, but can be seen in London and North Kent.

The structural reversal of the basin

Cross section view of Southern England featuring the Weald Basin

For much of its history the Weald had been slowly subsiding basin, but the growth of the Alpine Chain to the south during the Tertiary caused a reactivation of the Variscan basement basin-bounding faults, the rocks were arched into a broad anticline which stretched across the English Channel to Northern France, the Weald–Artois anticline. Inversion of the basin is closely correlated to compressional events within the Alps and occurred alongside deformation in Hampshire, Dorset and northern France. The basin was compressed between two 'blocks' of basement rocks, with the northward movement of the block against the London Platform;[14] the areas of land that earlier in the Weald's history supplied the sediments. The Anticline has since been eroded down to reveal the pattern of stratigraphy with the oldest rocks in the centre of the anticline forming a low ridge which runs roughly from Crowborough to Battle and onto Boulogne.

Economic Resources

The Geology of East Sussex includes a number of natural resources, at Mountfield, Robertsbridge there is a gypsum mine which produces significant quantities of gypsum extracted from the Purbeck Beds. As previously mentioned the Wadhurst Clay holds ferrous ore which was extracted up to the 17th century. Although source rocks for oil underly East Sussex and the major geologic structure in the area is an anticline, petroleum is not considered to be hosted below East Sussex. However while drilling for water at Heathfield railway station, natural gas was discovered, which was utilised to provide the first gas lighting in the UK, however it was recognised that economic oil reserves where not accessible.[15]

The large amount of clay extraction occurs within the low Weald, with brickworks extracting the Weald and Wadhurst clay. Chalk is also extracted in the south of the county, with commercial extraction ongoing near Lewes and a number of chalk extraction pits lie disused.

References

  1. ^ Blundell, Derek J. (2002). "Cenozoic inversion and uplift of southern Britain". Geological Society, London, Special Publications. 196: 85–101. Retrieved 26 September 2010.
  2. ^ Mantell, Gideon (1825). "Notice on the Iguanodon, a Newly Discovered Fossil Reptile, from the Sandstone of Tilgate Forest, in Sussex". Philosophical Transactions of the Royal Society of London. 115: 179–186. Retrieved 26 September 2010.
  3. ^ "The Unmasking of Piltdown Man". BBC News. Retrieved 26 September 2010.
  4. ^ Gibbons, Wes (1981). The Weald, Rocks and Fossils Field Guide. Unwin Paperbacks. p. 115.
  5. ^ Butler, Malcom (1990). "Tertiary structures and hydrocarbon entrapment in the Weald Basin of southern England". Geological Society, London, Special Publications. 55: 371–391. Retrieved 26 September 2010. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Butler, Malcom (1990). "Tertiary structures and hydrocarbon entrapment in the Weald Basin of southern England". Geological Society, London, Special Publications. 55: 371–391. Retrieved 26 September 2010. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Blundell, Derek J. (2002). "Cenozoic inversion and uplift of southern Britain". Geological Society, London, Special Publications. 196: 85–101. Retrieved 26 September 2010.
  8. ^ "The BGS Lexicon of Named Rock Units - Purbeck Group". The British Geological Survey. Retrieved 26 September 2010.
  9. ^ Gibbons, Wes (1981). The Weald, Rocks and Fossils Field Guide. Unwin Paperbacks. p. 115.
  10. ^ Howitt, Frank (1964). "Stratigraphy and structure of the Purbeck inliers of Sussex (England)". Quarterly Journal of the Geological Society. 120: 77–113. Retrieved 26 September 2010.
  11. ^ "The BGS Lexicon of Named Rock Units - Ashdown Formation". The British Geological Survey. Retrieved 26 September 2010.
  12. ^ "The BGS Lexicon of Named Rock Units - Wadhurst Clay Formation". The British Geological Survey. Retrieved 26 September 2010.
  13. ^ Gibbons, Wes (1981). The Weald, Rocks and Fossils Field Guide. Unwin Paperbacks. p. 115.
  14. ^ Blundell, Derek J. (2002). "Cenozoic inversion and uplift of southern Britain". Geological Society, London, Special Publications. 196: 85–101. Retrieved 26 September 2010.
  15. ^ Hewitt, J.T. (1898). "Note on Natural Gas at Heathfield Station (Sussex)". Quarterly Journal of the Geological Society. 54 (1–4): 572–574. Retrieved 26 September 2010.
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