The BGS Lexicon of Named Rock Units — Result Details

Tunbridge Wells Sand Formation

Computer Code: TWS Preferred Map Code: TWS
Status Code: Full
Age range: Valanginian Age (KV) — Valanginian Age (KV)
Lithological Description: Predominantly fine- to medium-grained sandstone, siltstone and silty sand rhythms with finely-bedded mudstones and thin limestones. In the western High Weald (between Haywards Heath and Tunbridge Wells) the formation can be divided into three, the informally named Lower and Upper Tunbridge Wells Sand and the intervening Grinstead Clay Member. The succession commences with rhythmically bedded sandstones, siltstones and mudstones of the lower part of the Lower Tunbridge Wells Sand which pass up into the massive sandstones of the Ardingly Sandstone Member. These are overlain by the finely-bedded mudstones, mudstones and silty mudstones with subordinate clay ironstones and shelly limestones of the Grinstead Clay Member. This clay member is itself locally divided into upper and lower parts by the cross-bedded fine sandstone of the Cuckfield Stone Bed. Above the Grinstead Clay Member, the Upper Tunbridge Wells Sand comprises a generally more argillaceous rhythmic succession, including mudstones, siltstones and silty sandstones. Outside the western High Weald the Grinstead Clay Member is not recognisable and the succession is mapped as undivided Tunbridge Wells Sand Formation. There are a number of minor non-sequences or disconformities throughout the succession and a possible significant gap at the top of the formation that removes much of the Upper Tunbridge Wells Sand.
Definition of Lower Boundary: The lower boundary is conformable at the upward change from finely-bedded mudstones and siltstones of the Wadhurst Clay Formation into the thinly interbedded siltstones and silty fine-grained sandstones of the basal Tunbridge Wells Sand Formation. The boundary is commonly sharply defined because of the red staining in the upper part of the underlying Wadhurst Clay Formation.
Definition of Upper Boundary: The upper boundary is frequently cut-out by faulting but it has been observed around Crawley and Horsham where it is conventionally placed at the top of the highest prominent fine-grained sandstone beneath the mudstones of the Weald Clay Formation. In places a pebble bed is developed at this horizon. Elsewhere, in full successions the prominent sandstone is absent and the boundary is gradational.
Thickness: Generally about 75m but ranges from 46m around Hellingly to as much as 122m around East Grinstead.
Geographical Limits: Outcrops in the central Weald around Tunbridge Wells, Sevenoaks, Maidstone, Haslemere, Horsham, Tenterden, Brighton, Lewes and Hastings. Limited in the subcrop to the north by the London - Brabant Ridge and to the south by the Hampshire - Dieppe high . Its extension at subcrop to the west into the Wessex Basin is poorly known but the succession is known to thin greatly.
Parent Unit: Wealden Group (W)
Previous Name(s): Tunbridge Wells Sand Subgroup (-2508)
Tunbridge Wells Sand (-3737)
Tunbridge Wells Formation (-3736)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Cuckfield No. 1 Borehole, TQ22NE19 [2961 2731] 65.43 to 219.46m depth, where much of the Tunbridge Wells Sand Formation is described. Lake and Thurrell, 1974. 
Type Area  There is no single section that demonstrates the whole succession. There are many sections throughout the central Weald, each demonstrating part of the succession: Lower Tunbridge Wells Sand at High Brooms Brickworks [TQ 594 418], Philpots Quarry [TQ3536 3215], natural crags of Ardingly Sandstone Member between Balcombe and Tunbridge Wells, Grinstead Clay Member at Freshfield Lane Brickworks access road [TQ 3830 2642] and the Upper Tunbridge Wells Sand at Freshfield Lane Brickworks [TQ 386 264] and Hundred Acres Wood Brickworks [TQ 331 365]. 
Reference Section  natural crags of the Ardingly Sandstone Member between Balcombe and Tunbridge Wells 
Reference Section  Upper Tunbridge Wells Sand at Hundred Acres Wood Brickworks [TQ 331 365]. 
Reference Section  Lower Tunbridge Wells Sand at Philpots Quarry [TQ3536 3215]. 
Reference Section  Grinstead Clay Member at Freshfield Lane Brickworks access road [TQ 3830 2642] 
Reference Section  upper Tunbridge Wells Sand at Freshfield Lane Brickworks [TQ 386 264] 
Reference Section  lower Tunbridge Wells Sand at High Brooms Brickworks [TQ 594 418] 
Reference(s):
Allen, P. 1962. The Hastings Beds deltas: Recent progress and Easter field meeting report. Proceedings of the Geologists' Association, Vol.73, 219-243. 
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. 
Worssam, B C. 1963. Geology of the country around Maidstone. Memoir of the Geological Survey of Great Britain, Sheet 288 (England and Wales). 
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). 130pp. 
Drew, F. 1861. On the succession of the beds in the Hastings Sand in the Northern portion of The Weald. Quarterly Journal of the Geological Society, London, Vol.17, 271-286. 
Allen, P. 1976. The Wealden of the Weald. Proceedings of the Geologists' Association, Vol.86, 389-437. 
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. 
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). 
Bristow, C R 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). 
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). 
Topley, W. 1875. The geology of the Weald. Memoir of the Geological Survey of 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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E301 E304 E319 E320 E321