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Ashdown Formation
The Ashdown Formation is a geological unit, which forms part of the Wealden Group and the lowermost and oldest part of the now unofficial Hastings Beds. These geological units make up the core of the Weald in the English counties of East Sussex and Kent.
The other component formations of the Hastings Beds are the overlying Wadhurst Clay Formation and the Tunbridge Wells Sand Formation. The Hastings Beds in turn forms part of the Wealden Supergroup which underlies much of southeast England. The sediments of the Weald of East Sussex, including the Ashdown Formation, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Ashdown Formation is of Late Berriasian to Early Valanginian to age.〔Hopson, P.M., Wilkinson, I.P. and Woods, M.A. (2010) ''A stratigraphical framework for the Lower Cretaceous of England''. Research Report RR/08/03. British Geological Survey, Keyworth.〕 The Formation takes its name from the Ashdown Forest in the High Weald of Sussex.
== Lithology ==
The Ashdown Formation typically comprises sandstones, siltstones and mudstones. In the east of the county, the formation tends to be more argillaceous in its lowermost part and fines up to a sandier division in the uppermost 30 to 50m. The clays are identified by their characteristic purple and brick-red mottled nature. In early references, these variations give rise to the division of the formation into the ‘Fairlight Clays’ and the ‘Ashdown Sands’.〔Ruffell, A., Ross, A. & Taylor (1996) Early Cretaceous Environments of the Weald. Geologists’s Association Guide No. 55, Geologists’s Association, London.〕 However, it is now considered as one due to the impersistence of the clays across the Weald. Despite this the variations of clays and sands in the formation are usually marked separately on the maps and records of the British Geological Survey. In its entirety the formation is usually found to be between 180 and 215m thick 〔Lake, R.D. & Shepard-Thorn, E.R. (1987) ''Geology of the country around Hastings and Dungeness: Memoir for 1:50,000 geological sheets 320 and 321''. British Geological Survey, London.〕
The base of the Hastings Beds and the Ashdown Formation is taken at the top of the Greys Limestones Member of the Purbeck Beds, although this boundary is not currently exposed anywhere in the Weald. The top of the Ashdown Formation is marked as the top of a massive sandstone bed known as the Top Ashdown Sandstone.〔Allen, P. (1948) Wealden Petrology: The Top Ashdown Pebble Bed and the Top Ashdown Sandstone. Quarterly Journal of the Geological Society, 104, 257-321.〕 This is overlain by an easily identified very coarse grained sandstone known as the Top Ashdown Pebble Bed, which is considered to be part of the overlying Wadhurst Clay Formation. The Top Ashdown Sandstone is the most persistent marker horizon in Early Cretaceous strata throughout the region. It comprises a fine to medium grained sandstone ranging in thickness from 1.2m to 8m.〔 Other marker beds can be found throughout the formation including the Cliff End Sandstone and the Lee Ness Sandstone.〔Codd, J.W. (2007) ''Analysis of the distribution and characteristics of landslips in the Weald of East Sussex''. MSc dissertation, University of Brighton.〕
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