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 (-4440)
Ashdown Sand (-1315)
Ashdown Beds (A) [Obsolete Code: Use ASD] (AN)
Ashdown Beds (B) [Obsolete Code: Use ASD] (ASHB)
Ashdown Sand(s) [Obsolete Name And Code: Use ASD] (ASHS)
Alternative Name(s): Fairlight Clays
Part Of The Informal Hastings Sands
Stratotypes:
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). 
Reference(s):
none recorded or not applicable
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable