The BGS Lexicon of Named Rock Units — Result Details

Ashdown Formation

Computer Code: ASD Preferred Map Code: notEntered
Status Code: Full
Age range: Berriasian Age (KR) — Valanginian Age (KV)
Lithological Description: Siltstones and silty fine-grained sandstones with subordinate amounts of finely-bedded mudstone and mudstone arranged in rhythmic units ("cyclothems") commonly divided by thin pebble beds. In southeast Sussex, around Hastings, the argillaceous parts of the "cyclothems" are well-developed and a series of clay seams, the informally named Fairlight Clays, is well exposed in the cliffs at Fairlight Cove. Here, this argillaceous part of the Ashdown Formation comprises dark grey finely-bedded mudstones and mudstones, commonly patchily red-stained, with abundant iron carbonate pellets (sideritic mudstones and sphaerosiderite nodules) at some levels. Northwards from Hastings, the Fairlight Clays become increasingly thin such that in the area of Ashdown Forest they are represented only be pebble beds overlain by thin siltstones within a generally arenaceous formation.
Definition of Lower Boundary: The lower boundary is conformable with the underlying Purbeck Group at the change from finely-bedded mudstones of the informally named Greys Limestones "member" up into siltstones and sandstones of the Ashdown Formation.
Definition of Upper Boundary: The upper boundary is conformable at the top of the informally named Top Ashdown Sandstone (see Allen, 1949) where this is identified, or at the basal bed of the Wadhurst Clay Formation, informally termed the Top Ashdown Pebble Bed. In general, the top of the formation is taken at the change from underlying sandstone into overlying dark grey silty mudstones.
Thickness: 115 to 215m in the Hastings district, but generally in the range 200 to 230m.
Geographical Limits: Occurs at outcrop in the Weald and at subcrop westward in the Wessex Basin where the succession is imperfectly known. Thins against the London Brabant Ridge to the north and the Hampshire-Dieppe High to the south.
Parent Unit: Wealden Group (W)
Previous Name(s): ASHDOWN BEDS (ASHB)
Reference Section  Cuckfield No. 1 Borehole TQ22NE19 [2961 2731] (Lake and Thurrell, 1974). 
Reference Section  High Hurstwood [TQ 499 271]. 
Type Area  Ashdown Forest area. There are no complete stratotypes but many small exposures along stream courses throughout the "High Weald". Principal partial reference sections are also listed within this definition.. 
Reference Section  Jarvis Brook Brickworks, Crowborough [TQ 531 297]. 
Reference Section  Elphicks, near Goudhurst [TQ 699 381]. 
Reference Section  Hastings Cliffs, Hastings to Pett Levels (Cliff End at the Eastern end of Fairlight Bay), [TQ 765 080 to 890 130], (Topley, 1875; Allen 1962). 
Allen, P. 1949. Wealden Petrology: The Top Ashdown Pebble Bed and the Top Ashdown Sandstone. Quarterly Journal of the Geological Society of London, Vol.104, 257-321. 
Allen, P. 1962. The Hastings Beds deltas: Recent progress and Easter field meeting report. Proceedings of the Geologists' Association, Vol.73, 219-243. 
Bristow, C, and Bazley, R A. 1972. Geology of the country around Royal Tunbridge Wells. Memoir of the Geological Survey of Great Britain, Sheet 303 (England and Wales). 
Dines, H G, Buchan, S, Holmes, S C A and Bristow, C R. 1969. Geology of the country around Sevenoaks and Tonbridge. Second edition. Memoir of the Institute of Geological Sciences, Sheet 287 (England and Wales). 
Drew, F. 1861. On the succession of the beds in the Hastings Sands of the southern portion of the Weald. Quarterly Journal of the Geological Society of London, Vol.17, 217-286. 
Gallois, R W and Worssam, B C. 1993. Geology of the country around Horsham. Memoir of the British Geological Survey, Sheet 302 (England and Wales). 
Lake, R D, Young, B, Wood, C J and Mortimore, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey, Sheet 319 (England and Wales). 
Lake, R D and Thurrell, R G. 1974. The sedimentary sequence of the Wealden Beds in boreholes near Cuckfield, Sussex. Report of the Institute of Geological Sciences, No.74/2. 
Topley, W. 1875. The geology of the Weald. Memoir of the Geological Survey of England and Wales. 
Worssam, B C. 1963. Geology of the country around Maidstone. Memoir of the Geological Survey of Great Britain, Sheet 288 (England and Wales). 
Young, B and Lake, R D. 1988. Geology of the country around Brighton and Worthing. Memoir of the British Geological Survey, Sheet 318 and 333 (England and Wales). 
Rawson, P F. 1992. The Cretaceous. 355-388 in Duff, P M D and Smith, A J (eds), Geology of England and Wales. (London: The Geological Society.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable