The Tunbridge Wells Sand Formation comprises complex cyclic sequences of siltstones with sandstones and clays, typically fining upwards, and is lithologically similar to the older
Ashdown Formation. It has a total thickness typically in the region of about 75 m. In the western parts of the High
Weald the Tunbridge Wells Sands can be divided into four separate divisions; the Lower Tunbridge Wells Sand, the Ardingly Sandstone Member, the Grinstead Clay Member, and the Upper Tunbridge Wells Sand.
Ardingly Sandstone Member A massive thick cross bedded fine to medium grained quartz sandstone separates the Tunbridge Wells Sands from the overlying Grinstead Clay. This horizon is known as the Ardingly Sandstone and occurs in thicknesses of up to 18m. It is particularly well exposed throughout the region between
East Grinstead,
West Sussex, and
Tunbridge Wells,
Kent, at localities such as; Stone Farm south of
East Grinstead; Chiddinglye Rocks near
West Hoathly; Toad Rock, Bull's Hollow and Happy Valley west of
Tunbridge Wells; and
Harrisons Rocks,
Bowles Rocks and
High Rocks near
Tunbridge Wells. At all of these places the Ardingly Sandstone forms a weathering-resistant layer, relative to the rest of the formation, which has become very popular with
rock climbers and is known locally as
Southern Sandstone. These are the closest
rock climbing crags to
London and as a result are the most heavily used in the country.
Grinstead Clay Member The Grinstead Clay comprises mudstones and silty mudstones with siltstone, ironstone and shelly limestone. This member is lithologically similar to the older
Wadhurst Clay and also has weathered red mottled clays at its top. The formation is up to 20m thick but is only present around the border of
East Sussex and
West Sussex. It can be further subdivided into the Lower Grinstead Clay and Upper Grinstead Clay. These divisions are separated by a lenticular calcareous sandstone known as the Cuckfield Stone. This is probably best known as the strata in which
Gideon Mantell discovered
Iguanodon in the early 19th century.
Upper Tunbridge Wells Sand The Upper Tunbridge Wells Sand is similar to the Lower Tunbridge Wells Sand. It comprises soft red and grey mottled silts and clays in its lower part, and alternating silts and silty clays with thin beds of sandstones. The base of the Tunbridge Wells Sand is marked by a distinct change from the predominantly argillaceous sediments of the
Wadhurst Clay to siltstones and silty sands. This boundary is often indicated on maps by spring lines and seepages, where groundwater percolating through the permeable Tunbridge Wells Sand is forced to surface at the junction with the
Wadhurst Clay. The top of the Tunbridge Wells Sand Formation is well defined in the southwest of
East Sussex but is gradational elsewhere. In the area north of
Brighton and west of
Lewes the boundary is marked by a massive sandstone, though this is not seen anywhere else. == Engineering Geology ==