Lower Greensand
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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 ...
of
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 nodules. 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
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 ...
to Early
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 ...
stages.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 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 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 Hythe Formation, the Sandgate Formation, and the Folkestone Formation. 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. 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 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.


See also

* Ashdown Formation * Geology of East Sussex * Geology of Kent *
Greensand Ridge The Greensand Ridge, also known as the Wealden Greensand, is an extensive, prominent, often wooded, mixed greensand/sandstone escarpment in south-east England. Forming part of the Weald, a former dense forest in Sussex, Surrey and Kent, it ...
*
Greensand Greensand or green sand is a sand or sandstone which has a greenish color. This term is specifically applied to shallow marine sediment that contains noticeable quantities of rounded greenish grains. These grains are called ''glauconies'' and co ...
*
Hastings Beds The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentar ...
* Geology of Surrey * Wadhurst Clay Formation * Geology of West Sussex


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