Grinstead Clay

The Tunbridge Wells Sand Formation is a geological unit which forms part of the Wealden Group and the uppermost and youngest part of the unofficial Hastings Beds. These geological units make up the core of the geology of the Weald in the English counties of West Sussex, East Sussex and Kent.

The other component formations of the Hastings Beds are the underlying Wadhurst Clay Formation and the Ashdown Formation. The Hastings Beds in turn form part of the Wealden Group which underlies much of southeast England. The sediments of the Weald, including the Tunbridge Wells Sand Formation, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Tunbridge Wells Sands are of Late Valanginian age. The Formation takes its name from the spa town of Tunbridge Wells in Kent.

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. However, near Haywards Heath borehole data has proven the formation to be up to 150m thick.

In the western parts of the High Weald the Tunbridge Wells Sands can be divided into three separate members; the Lower Tunbridge Wells Sand Member, the Grinstead Clay Member, and the Upper Tunbridge Wells Sand Member.

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