|Lower Greensand Group|
Stratigraphic range: Aptian- Early Albian 125-110 Ma
Lower Greensand outcrop near Dryhill
|Overlies||In Southern England, Wealden Group, elsewhere Jurassic rocks|
|Thickness||~250m in South Western Weald|
The Lower Greensand Group is a geological unit, which forms part of the underlying geological structure of southeast England. South of London in the counties of West Sussex, East Sussex, Surrey and Kent, which together form the wider Weald, the Lower Greensand can usually be subdivided to formational levels with varying properties into the Atherfield Clay Formation, the Hythe Formation, the Sandgate Formation, and the Folkestone Formation. In areas north and west of London, including Cambridgeshire, Bedfordshire and Buckinghamshire the Lower Greensand 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.
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.
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. The sediments are noted to become increasingly fine grained and glauconitic to the east.
The Lower Greensand Group was deposited during the latter part of the Early Cretaceous Period, during the Aptian to Early Albian stages. The Group is the lowermost of two geological units that take their name from their colouration due the presence of the mineral glauconite, the other being the Upper Greensand. 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.
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. Of the formations within the Lower Greensand, the Atherfield Clay is the most prone to landslip.
A common geomorphological feature 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. Most slip progress is attributed to massive sandstones overlying weaker shales and clays. The back part of the slip in some locations moves vertically downwards on a rotational slip plane. This movement leaves a steep back face, or back-scar, with a toe raised to a significantly lesser extent.
At 'The Roughs' in Kent, where a rotational slump occurred, slips in the Atherfield Clay have undermined sandstone blocks of Hythe Beds. Subsequent translational slides have developed along a shear zone at the boundary between the slip material and the undisturbed underlying Weald Clay. This sort of rotational slip occurs regularly along the coastline between Hythe and Folkestone.