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The Wadhurst Clay Formation is a geological unit which forms part of the Wealden Group and the middle part of the now unofficial Hastings Beds. These geological units make up the core of the geology of the High Weald in the English counties of West Sussex, East Sussex and Kent. The other component formations of the Hastings Beds are the underlying Ashdown Formation and the overlying Tunbridge Wells Sand 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 Wadhurst Clay Formation, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Wadhurst Clay is of Early to Late Valanginian 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 market town of Wadhurst in East Sussex. == Lithology == The Wadhurst Clay comprises predominantly medium to dark bluish grey over-consolidated clays, silts, mudstones, and shales. These lithologies often occur with subordinate amounts of pale grey silty mudstones, laminated siltstones, sandstones, conglomerate, shelly limestones and clay-ironstones.〔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.〕 When they become exposed to the elements at the surface, the mudstones often degrade over a short period of time and weather to yellowish brown and greenish grey clays.〔Codd, J.W. (2007) ''Analysis of the distribution and characteristics of landslips in the Weald of East Sussex''. MSc dissertation, University of Brighton.〕 The formation thickness ranges from 55m in the Tenterden area, to 30m near Lewes and varies in between. In Kent, the Wadhurst Clay has been proven to over 70m thick near Tunbridge Wells 〔 and in West Sussex up to 80m near Horsham.〔Reeves, G.M., Sims, I., & Cripps, J.C. (2006) ''Clay Minerals Used in Construction'', Geological Society, London, Engineering Geology Special Publication, 21, 153-175.〕 The base of the Wadhurst Clay is taken at the bottom of the Top Ashdown Pebble Bed. The base of this marker horizon marks the formational change to the Ashdown Formation. Despite its name this thin and impressistent bed comprises a coarse grained to gravelly sandstone. This horizon is best exposed at Cliff End, East Sussex, but where it is encountered elsewhere, it is usually fairly distinctive and easily identified. The Top Ashdown Pebble Bed occurs mainly in the southern half of East Sussex and is often missing elsewhere. Where this is the case, the boundary is taken at a layer of disconnected ripples.〔Lake, Shepard-Thorn, E.R., Smart, J.G.O., Bisson, G. & Edmonds, E.A. (1966) ''Geology of the country around Tenterden: Memoir for 1:50,000 geological sheet 304''. British Geological Survey, London.〕 The top of the Wadhurst Clay is marked by a distinct lithological change into the siltstones of the Tunbridge Wells Sand Formation. The uppermost clays of the Wadhurst Clay, closest to the boundary are often stained red.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wadhurst Clay Formation」の詳細全文を読む スポンサード リンク
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