Gallois_Geological_Map_of_Wealden_District_1965

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

Southern England Southern England, or the South of England, also known as the South, is an area of England consisting of its southernmost part, with cultural, economic and political differences from the Midlands and the North. Officially, the area includes G ...

. It was deposited during the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

and

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 M ...

stages of the Early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

. 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 (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

s and similar) and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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 () m ...

, 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, Sussex and Kent. It has three separate parts: the sandstone "High Weald" in the ...

East Sussex East Sussex is a ceremonial and non-metropolitan county in South East England on the English Channel coast. It is bordered by Kent to the north and east, West Sussex to the west, and Surrey to the north-west. The largest settlement in East Su ...

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 pentoxi ...

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 ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

Period, during the

to Early

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.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 thick ...

. 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 (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentary ...

. 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 Mya. The J ...

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 ...

, 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 unco ...

. North and west of

London London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary down to the North Sea, and has been a majo ...

– including

Cambridgeshire Cambridgeshire (abbreviated Cambs.) is a Counties of England, county in the East of England, bordering Lincolnshire to the north, Norfolk to the north-east, Suffolk to the east, Essex and Hertfordshire to the south, and Bedfordshire and North ...

Bedfordshire Bedfordshire (; abbreviated Beds) is a ceremonial county in the East of England. The county has been administered by three unitary authorities, Borough of Bedford, Central Bedfordshire and Borough of Luton, since Bedfordshire County Council wa ...

and

Buckinghamshire Buckinghamshire (), abbreviated Bucks, is a ceremonial county in South East England that borders Greater London to the south-east, Berkshire to the south, Oxfordshire to the west, Northamptonshire to the north, Bedfordshire to the north-ea ...

– it is referred to as the

Woburn Sands Formation The Woburn Sands Formation is a geological formation in England. Part of the Lower Greensand Group, it is the only unit of the group where it occurs, and thus is sometimes simply referred to as the 'Lower Greensand' in these areas. It was deposit ...

. In

Oxfordshire Oxfordshire is a ceremonial and non-metropolitan county in the north west of South East England. It is a mainly rural county, with its largest settlement being the city of Oxford. The county is a centre of research and development, primarily ...

it is known as the

Faringdon Sand The Faringdon Sand is a geologic formation in England. It preserves fossils dating back to the Cretaceous period. See also * List of fossiliferous stratigraphic units in England See also *Lists of fossiliferous stratigraphic units in Euro ...

. 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 which to the year 2000 had at least 288 known occurrences in South-East England.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 soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically, assisting the soil slowly down a slope.Basic Summary of Landslides

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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 county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...

, 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 ro ...

. 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 village in Canada England * T ...

and

Folkestone Folkestone ( ) is a port 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 and shipping port for most of the 19th and 20t ...

, where the drainage basin 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