Gallois_Geological_Map_of_Wealden_District_1965

The Lower Greensand Group is a geological unit present across large areas of

Southern England Southern England, also known as the South of England or the South, is a sub-national part of England. Officially, it is made up of the southern, south-western and part of the eastern parts of England, consisting of the statistical regions of ...

. It was deposited during the

Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...

and

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

ages of the Early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. It predominantly consists of sandstone and unconsolidated sand that were deposited in shallow marine conditions.

Lithology

The Lower Greensand typically comprises loose, unconsolidated sandstone (termed rubblestone/rubble in construction) and sands of varying grain size with subordinate amounts of siltstones, mudstones (containing

smectite A smectite (; ; ) is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secon ...

s and similar) and

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

s. The name "greensand" is derived from the presence of the green coloured mineral

glauconite Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek ...

, which forms in shallow marine conditions. In the

Weald The Weald () is an area of South East England between the parallel chalk escarpments of the North and the South Downs. It crosses the counties of Hampshire, Surrey, West Sussex, East Sussex, and Kent. It has three parts, the sandstone "High W ...

East Sussex East Sussex is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Kent to the north-east, West Sussex to the west, Surrey to the north-west, and the English Channel to the south. The largest settlement ...

the lowermost part of the group is recognised by green glauconitic clays with a basal bed of

phosphate nodule Phosphorite, phosphate rock or rock phosphate is a non- detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentox ...

s. These clays are overlain by green sandy clays and silts and finally homogeneous fine grained sands.Lake, R.D. & Shepard-Thorn, E.R. (1987) ''Geology of the country around Hastings and Dungeness: Memoir for 1:50,000 geological sheets 320 and 321''. British Geological Survey, London. The sediments are noted to become increasingly fine grained and glauconitic to the east.

Stratigraphy

The Lower Greensand Group was deposited during the latter part of the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

Period, during the

to Early

stages Stage, stages, or staging may refer to: Arts and media Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly Brit ...

.Hopson, P.M., Wilkinson, I.P. and Woods, M.A. (2010)
A stratigraphical framework for the Lower Cretaceous of England
'. Research Report RR/08/03. British Geological Survey, Keyworth. Page 15 The Group is the lowermost of two geological units that take their name from their colouration due to the presence of the mineral glauconite, the other being the

Upper Greensand Formation The Upper Greensand Formation is a Cretaceous formation of Albian to Cenomanian in age, found within the Wessex Basin and parts of the Weald Basin in southern England. It overlies the Gault Clay and underlies the Chalk Group. It varies in thic ...

. The unit was deposited in shallow marine conditions. In southern England the unit sharply but continuously overlies rocks of the

Wealden Group The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (stratigraphy), group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of wiktionary:paralic, paralic to c ...

. While elsewhere as the sea level rose it covered previously emergent highs of

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

rocks,

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

overlying them. In the

Weald Basin The Weald Basin () is a major topographic feature of the area that is now southern England and northern France from the Triassic to the Late Cretaceous. Its uplift in the Late Cretaceous marked the formation of the Wealden Anticline. The rock str ...

, the Lower Greensand can usually be subdivided to formational levels with varying properties into, in ascending order, the

Atherfield Clay Formation The Atherfield Clay Formation is a geological formation in Southern England. Part of the Lower Greensand Group it dates to the Aptian age of the Early Cretaceous. The deposit is of marine origin largely consisting of massive yellowish brown to p ...

, the Hythe Formation, the Sandgate Formation, and the

Folkestone Formation The Folkestone Formation is a geological formation in the United Kingdom. It forms part of the Lower Greensand Group, and dates to the early part of the Albian stage of the Early Cretaceous. It primarily consists of medium to coarsed grained unc ...

. North and west of

London London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...

– including

Cambridgeshire Cambridgeshire (abbreviated Cambs.) is a Ceremonial counties of England, ceremonial county in the East of England and East Anglia. It is bordered by Lincolnshire to the north, Norfolk to the north-east, Suffolk to the east, Essex and Hertfor ...

Bedfordshire Bedfordshire (; abbreviated ''Beds'') is a Ceremonial County, ceremonial county in the East of England. It is bordered by Northamptonshire to the north, Cambridgeshire to the north-east, Hertfordshire to the south and the south-east, and Buckin ...

and

Buckinghamshire Buckinghamshire (, abbreviated ''Bucks'') is a Ceremonial counties of England, ceremonial county in South East England and one of the home counties. It is bordered by Northamptonshire to the north, Bedfordshire to the north-east, Hertfordshir ...

– it is referred to as the Woburn Sands Formation. In

Oxfordshire Oxfordshire ( ; abbreviated ''Oxon'') is a ceremonial county in South East England. The county is bordered by Northamptonshire and Warwickshire to the north, Buckinghamshire to the east, Berkshire to the south, and Wiltshire and Glouceste ...

it is known as the Faringdon Sand. In North Wiltshire as the Calne Sands Formation and in parts of Wiltshire, Oxfordshire and Buckinghamshire as the Seend Ironstone Formation.

Engineering geology

The Lower Greensand is one of the most landslide-susceptible formations in the UK with at least 288 known occurrences in South-East England recorded to the year 2000.Collison , A., Wade, S., Griffiths, J. & DEHN, M. (2000) ''Modelling the impact of predicted climate change on landslide frequency and magnitude in SE England. Engineering Geology'', 55, 205-218. Of the formations within the Lower Greensand, the Atherfield Clay is the most prone to landslip.Codd, J.W. (2007) ''Analysis of the distribution and characteristics of landslips in the Weald of East Sussex''. MSc dissertation, University of Brighton. Landslides are occasional, rapid movements of a mass of earth or rock sliding along a steep slope. They tend to occur after sustained heavy rain, when the water saturates overlying rock, making it heavy and liable to slide, others occur via soil creep is a very slow movement, occurring on very gentle slopes because of the way soils repeatedly expand and contract in wet and dry periods. When the soil dries out, it contracts vertically, assisting the soil slowly down a slope.Basic Summary of Landslides

BBC The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...

Bitesize Geography A common geomorphological, chiefly dependent on the local hydrology such as

hydraulic action Hydraulic action, most generally, is the ability of moving water (flowing or waves) to dislodge and transport rock particles. This includes a number of specific erosional processes, including abrasion, at facilitated erosion, such as ''static eros ...

, at the base of the Lower Greensand is an escarpment, where the Hythe Beds overlie the Atherfield and Weald Clays, which is particularly susceptible to landslide.Gallois, R.W. & Edmunds, M.A. (1965) ''British Regional Geology: The Wealden District.'' Her Majesty’s Stationery Office, London. Most slip is attributed to massive sandstones overlying weaker shales and clays. The back part of the slip in strongly permeable locations is prone to be straight downward on a rotational slip plane. This shift leaves a steep back face, or back-scar, with a toe raised significantly less. At ‘The Roughs’ in

Kent Kent is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Essex across the Thames Estuary to the north, the Strait of Dover to the south-east, East Sussex to the south-west, Surrey to the west, and Gr ...

, where a rotational slump occurred, slips of the Atherfield Clay (and all material above) have compromised sandstone blocks of

Hythe Beds The Lower Greensand Group is a geological unit present across large areas of Southern England. It was deposited during the Aptian and Albian ages of the Early Cretaceous. It predominantly consists of sandstone and unconsolidated sand that were d ...

. Later translational slides have developed along a shear zone at the boundary between the slip material and the undisturbed underlying

Weald Clay Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock unit underlying areas of South East England, between the North and South Downs, in an area called the Weald Basin. It is the uppermost unit of the Wealden Group of ...

. This sort of rotational slip occurs regularly along the coastline between

Hythe Hythe, from Anglo-Saxon ''hȳð'', may refer to a landing-place, port or haven, either as an element in a toponym, such as Rotherhithe in London, or to: Places Australia * Hythe, Tasmania Canada *Hythe, Alberta, a hamlet in Canada England *The ...

and

Folkestone Folkestone ( ) is a coastal town on the English Channel, in Kent, south-east England. The town lies on the southern edge of the North Downs at a valley between two cliffs. It was an important harbour, shipping port, and fashionable coastal res ...

, where the

drainage basin A drainage basin is an area of land in which all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...

faces inland, exerting a steady force, where the water is subterranean, outward towards the coastal cliffs.

References

{{Reflist

External links

British Geological Survey lexicon
Geological groups of the United Kingdom Lower Cretaceous Series of Europe Albian Stage Aptian Stage Sandstone groups Siltstone groups

Lithology

Stratigraphy

Engineering geology

See also

References

External links