Greensand Ridge

Geological map of South-eastern Great Britain

The Greensand Ridge is an extensive, prominent, often heavily wooded, sandstone escarpment and range of hills in the south-eastern counties of Great Britain. It is considered part of The Weald and runs in a horseshoe shape around the wealdlands of Surrey, Sussex and Kent. "Greensand Ridge" is not a term used locally, which knows of the Weald and the Hog's Back, but is a geological description.

The greensand reaches its highest elevation of 965 feet at the summit of Leith Hill in Surrey, which is the highest point in Surrey and in the south-eastern counties. Another hill, Blackdown, is the highest point in Sussex at 918 feet. The eastern end of the ridge forms the northern boundary of Romney Marsh.

Geology and soils

A geological cross-section, from north to south, through the Wealden dome in Kent

Geological history

The Greensand Ridge, formed of Lower Greensand sandstone, is a remnant of the Weald dome, part of the great Weald-Artois Anticline that runs from south-eastern Britain 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 (which is to say an anticline) and then deformed and faulted. The topmost 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 Surrey's county top; Leith Hill.

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 and sandstones, ranging up to a maximum thickness of about 1,300 feet, 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 Lower 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 from the North Downs by a fertile vale, the Vale of Holmesdale, formed on Gault Clay, and a narrow band of Upper Greensand that outcrops at the foot of the chalk scarp. In some places the clay vale is very narrow: for example at Oxted the gap between the Greensand Ridge and the North Downs is less than a mile and a half.

The Greensand Ridge, capped by the resistant sands and sandstones of the Hythe Beds, reinforced by bands of chert, rises steeply as a series of high, wooded escarpments between Gibbet Hill, Hindhead (892 feet), north of Haslemere, and the ridge's highest point, Leith Hill (965 feet). It then flattens for several miles, before re-emerging east of Nutfield to run eastwards as a high wooded ridge into an area between Oxted and Sevenoaks known locally as the Chartlands, where it reaches another high point at Toys Hill in Kent (771 feet). Here there are stunning views to the south of the Weald from a terrace donated in 1898 by Octavia Hill, one of the founders of the National Trust.

The ridge continues eastwards past Sevenoaks, until southwest of Maidstone it is broken by the valley of the River Medway. The ridge then continues as far as Pluckley in Kent. From there the land levels until it drops to the old sea-cliff line above Romney Marsh.

In the area around Haslemere local anticlinal features are superimposed on the main axis of the Wealden anticline, causing the outcrop of resistant Hythe Beds to widen from a mile and a half to more than 7½ miles and to produce an escarpment that is particularly marked between Haslemere and Midhurst, where Blackdown rises to 919 feet, the highest point in Sussex.

South of here the Vale of Fernhurst has been eroded down into the Low Weald by what is now a small stream following a line of a gentle west-east trending upfold. This stream, the River Lod, runs parallel to the larger River Rother which flows about seven miles further south in the lee of the chalk escarpment of the South Downs.

Valley slope processes in the Vale of Fernhurst have resulted in escarpments to the north and south that are steep enough to have collapsed by land slipping. Further east, the Lower Greensand has not produced any pronounced topographical features.

In many places along the escarpment of the Greensand Ridge erosion by wind and rain, landslips on the steep scarp face, and solifluction in glacial times have further combined to create steep-side coombes, and low hillocks below the scarp.

Economic exploitation

The Folkestone Beds consist of seams of pebbles and sand. It is from here that the stone known as chert is found, familiar in the High Chart hills around Limpsfield, Surrey. In Surrey the Sandgate Beds and Bargate Beds, which lie on top of the Hythe Beds, have yielded a distinctive yellow stone seen in many local buildings. Ironstone, from layers embedded in the Sandgate Beds, is often seen in chips (gallets) pressed into the mortar between such stones. To the east, especially around Maidstone, Kent, the Hythe Beds yield a blue-grey sandstone with a high lime content. Known as ragstone, this is seen in many local buildings.

Fuller's earth, which lies interbedded between the Bargate and Sandgate Layers, was much quarried for the cloth industry. The seam, which lies about 20 to 30 feet below the surface between Nutfield and Bletchingley, was considered the best in the country and for several centuries large quantities were excavated. Resources are now running low and little is now extracted.

Settlements

Principal settlements lying on the southern part of the Greensand in Sussex, adjacent to the South Downs, include Storrington (at the eastern end of the ridge) and Midhurst. Petersfield marks the western end, where the ridge turns north east. Settlements on the main part of the ridge, running from Surrey into Kent include Haslemere, Godalming, Reigate and Redhill, Oxted and Sevenoaks.

Recreation

Much of the ridge in Surrey and Kent is followed by a long-distance walking route, the Greensand Way. Extending for 108 miles, it starts in the west at Haslemere and ends in the east at Ham Street, Kent, on the edge of Romney Marsh. The route passes through or close to Godalming, Cranleigh, Dorking, Reigate, South Nutfield, Oxted, Westerham, Sevenoaks, Maidstone and Ashford.

Statutory designations

The south-west part of the Greensand ridge and hills is situated in the South Downs National Park. Much of the Greensand Ridge in Surrey lies within the Surrey Hills "Area of Outstanding Natural Beauty" while a section of the Greensand hills in western Kent, from Limpsfield Chart near Westerham through Sevenoaks to Plaxtol, forms part of the Kent Downs "Area of Outstanding Natural Beauty", where it is known as the Sevenoaks Greensand Ridge.

References

Books

• Brandon, Peter (2003). The Kent & Sussex Weald. Phillimore & Co Ltd. ISBN 1-86077-241-2. 
• Cowan, Bea (1997). Along and Around the Greensand Way (2nd edition). Kingston upon Thames: Surrey County Council. ISBN 1-899706-35-6. 
• Friend, Peter (2008). Southern England. Looking at the Natural Landscapes. London: Collins. ISBN 978-0-00-724743-1. 
• Gallois, R.W. (ed.) (1965). British Regional Geology: The Wealden District. London: Her Majesty's Stationery Office. ISBN 0-11-884078-9.