Lower Greensand Group
Stratigraphic range: Aptian- Early Albian 125–110 Ma
Rag and hassock outcrop (geograph 2560799).jpg
Lower Greensand outcrop near Dryhill
TypeStratigraphic group
Sub-units
UnderliesSelborne Group
OverliesIn Southern England, Wealden Group, elsewhere Jurassic rocks
Thickness~250m in South Western Weald
Lithology
PrimarySandstone
OtherSiltstone, Clay
Location
Region England
Country UK
The Lower Greensand as a broad zone between the brick-patterned chalk (a geological term including limestone) and the interior large c-shape Weald Clay. Many towns shown are on the Lower Greensand.
The Lower Greensand as a broad zone between the brick-patterned chalk (a geological term including limestone) and the interior large c-shape Weald Clay. Many towns shown are on the Lower Greensand.

The Lower Greensand Group is a geological unit present across large areas of Southern England. It was deposited during the Aptian and Albian stages of the Early Cretaceous. 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 smectites and similar) and limestones. The name "greensand" is derived from the presence of the green coloured mineral glauconite, which forms in shallow marine conditions.

In the Weald of East Sussex 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.[1] 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 Period, during the Aptian to Early Albian stages.[2] 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. While elsewhere as the sea level rose it covered previously emergent highs of Jurassic rocks, unconformably overlying them.

In the Weald Basin, 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 – including Cambridgeshire, Bedfordshire and Buckinghamshire – it is referred to as the Woburn Sands Formation. In Oxfordshire 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 which to the year 2000 had at least 288 known occurrences in South-East England.[3] Of the formations within the Lower Greensand, the Atherfield Clay is the most prone to landslip.[4] 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.[5]

A common geomorphological, chiefly dependent on the local hydrology such as hydraulic action, 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.[6] 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.[4]

At ‘The Roughs’ in Kent, 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.[3] This sort of rotational slip occurs regularly along the coastline between Hythe and Folkestone, where the drainage basin faces inland, exerting a steady force, where the water is subterranean, outward towards the coastal cliffs.[4]

See also

References

  1. ^ 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.
  2. ^ 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
  3. ^ a b 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.
  4. ^ a b c Codd, J.W. (2007) Analysis of the distribution and characteristics of landslips in the Weald of East Sussex. MSc dissertation, University of Brighton.
  5. ^ Basic Summary of Landslides BBC Bitesize Geography
  6. ^ Gallois, R.W. & Edmunds, M.A. (1965) British Regional Geology: The Wealden District. Her Majesty’s Stationery Office, London.