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 runs to and from the East Sussex coast, wrapping around the High Weald and Low Weald. It reaches its highest elevation, , at Leith Hill in Surrey—the second highest point in south-east England, while another hill in its range, Blackdown, is the highest point in Sussex at . The eastern end of the ridge forms the northern boundary of Romney Marsh.
About 51 per cent of the Wealden Greensand is protected as the South Downs National Park, Kent Downs Area of Outstanding Natural Beauty and Surrey Hills Area of Outstanding Natural Beauty.
Geology and soils
Geological history
The Greensand Ridge, formed of Lower Greensand, much of which is sandstone and where hardest is locally termed Bargate stone, is a remnant of the Weald dome, part of the great Weald-Artois Anticline that runs from south-east England into northern France.
The Weald dome consists of a series of geological strata laid down in the Cretaceous that have subsequently been lifted up, formed into a dome (i.e. anticline) and then deformed and faulted. The top-most and therefore youngest layer of the dome is Chalk, laid down in the Upper Cretaceous. Below it lie successively older strata of alternating clays and sandstones laid down in the Lower Cretaceous, namely Upper Greensand, Gault Clay, Lower Greensand, Weald Clay and the Hastings Beds. Differential fluvial erosion has virtually flattened the dome into a series of hills and vales. On the surface the strata of which the dome is composed crop out in a series of concentric circles, shaped like a horseshoe, with the more resistant chalk and sandstones forming hills and ridges (such as the North and South Downs, the Greensand Ridge, and the High Weald), and the weaker clays forming vales (such as the Low Weald) between them. The very resistant rocks of the Lower Greensand, in particular the Hythe Beds, have produced prominent escarpments that form an arc around the northern edge of the Low Weald, running parallel to and just south of the chalk escarpment of the North Downs. This stretch of the Greensand has become the most closely identified with the term "Greensand Ridge", and it includes the second highest point in south-east England, Leith Hill in Surrey. West of the Weald the Lower Greensand has produced a more extensive area of hills and valleys, including the highest point in Sussex, Blackdown. On the south side of the Weald the Lower Greensand also forms another arc of rather less pronounced hills parallel to and just north of the South Downs, which become less prominent the further east one goes.
Geology
The Lower Greensand is a predominantly arenaceous sandstone consisting of sediment that accumulated apparently in a shallow sea in the later part of the Lower Cretaceous. It also contains important subsidiary elements of silty and argillaceous material. Chert, ironstone and calcareous deposits occur in small amounts. When fresh the rocks have a greenish colouration owing to the presence of glauconite, but on exposure to the atmosphere this is rapidly oxidised to limonite, giving rise to a yellow or reddish brown staining.
The Lower Greensand is composed of alternating mudstones (sandy, with clay particles such as smectite) and sandstones, up to a maximum thickness of about , and is composed of a number of distinct formations, namely the Folkestone Beds, Sandgate Beds, Bargate Beds, Hythe Beds and Atherfield Clay.
Soils
The soil of the Greensand is quite varied, ranging from fertile to fairly sterile. On the fertile soils we see chestnut and stands of hazel and oak, while Scots Pine and Birch colonise the poorer soils.
Relief and drainage
Broadly speaking, the Greensand Ridge runs along the northern edge of the Weald in a west-east arc from Surrey into Kent, just south of and parallel to the chalk escarpment of the North Downs. The ridge is separated by a mixed deep and shallow, fertile depression from the North Downs referred to as the 'Vale of Holmesdale', formed on Gault Clay, and a narrow band of Upper Greensand that outcrops at the foot of the chalk scarp (ridge). In some places the clay vale is very narrow: for example at Oxted the gap between summits of the Greensand Ridge and the North Downs is less than .