The BGS Lexicon of Named Rock Units — Result Details

Grinstead Clay Member

Computer Code: GRC Preferred Map Code: GrC
Status Code: Full
Age range: Valanginian Age (KV) — Valanginian Age (KV)
Lithological Description: Grey to greenish grey mudstones, commonly very silty with subordinate thin beds of siltstone, nodular clay ironstone and shelly limestone. Rootlet horizons and plant-rich beds common. Over part of its outcrop the member is divided by a bed of calcareous sandstone, the Cuckfield Stone Bed, into the informally named lower and upper Grinstead Clay. In places, the Cuckfield Stone Bed cuts out the lower Grinstead Clay entirely and rests on the Ardingly Sandstone Member.
Definition of Lower Boundary: The base of the member is sharply marked by the Top Lower Tunbridge Wells Pebble Bed or by a minor erosion surface. Where this bed is recognised there is a marked change from the massive fine-grained sandstone of the Ardingly Sandstone Member to the mudstones of the Grinstead Clay Member. Where the Cuckfield Stone Bed cuts out the lower Grinstead Clay the boundary is less clear with the finely trough cross-bedded (characteristically "festoon-bedded") fine-grained sandstone of the Cuckfield Stone Bed resting on the massive Ardingly Sandstone Member at a sharp erosive contact.
Definition of Upper Boundary: The upper boundary is placed at the sharp change from mudstones to siltstones and silty fine-grained sandstones of the informally named upper Tunbridge Wells Sand. The top of the Grinstead Clay commonly weathers to a red clay at, and emphasising, this boundary and the reddening can extend down through the whole unit to the top of the Cuckfield Stone Bed.
Thickness: Maximum of 26.9m in the Horsham area, thinner elsewhere to as little as 6.1m near Leigh to the northwest.
Geographical Limits: The Grinstead Clay Member crops out in the Central Weald within the Maidstone, Sevenoaks, Horsham, Tunbridge Wells, Brighton and Lewes geological sheet areas. The extent of the member at depth through the western Weald into the Hampshire Basin area is not well known. The subcrop of this member is limited northwards against the London-Brabant structural high.
Parent Unit: Tunbridge Wells Sand Formation (TWS)
Previous Name(s): Grinstead Clay Formation (-3441)
Grinstead Clay (-939)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  660m west of Rocks Farm, Handcross-Balcombe area. A thin succession (2.2m) of basal Cuckfield Stone and topmost Lower Grinstead Clay. Gallois and Worssam (1993, p.46). 
Reference Section  Coney Gill, Handcross-Balcombe area. An old quarry exposed a thick Cuckfield Stone Bed in an exposure of 6.0m of strata including the full lower Grinstead Clay. Gallois and Worssam (1993, p.46.) 
Reference Section  Philpotts Quarry, West Hoathly. The section shows the Cuckfield Stone Bed (1.5m) and the lower Grinstead Clay Member (7.2m) on the Top Lower Tunbridge Wells Pebble Bed. Gallois and Worssam, (1993, 47-48). 
Reference Section  Old quarry near Shovelstrode Lane, East Grinstead. Section shows "junction of Grinstead Clay and Ardingly Sandstone with Top Lower Tunbridge Wells Pebble Bed present". Bristow and Bazley (1972, p.35). 
Reference Section  Cuckfield Borehole No.1; TQ22NE/2. The full thickness of the member divided by the Cuckfield Stone Bed is encountered between 166.04m to 192.94m depth. Gallois and Worssam, 1993; Lake and Thurrell, 1974; Bristow and Bazley, 1972. 
Reference Section  Quarry 685m westnorthwest of Balcomb Place, Balcomb. The exposure shows a thick Cuckfield Stone Bed (4.5m) overlying the lower Grinstead Clay. Gallois and Worssam,(1993, p.46). 
Reference Section  Railway cutting 1.6km north of Haywards Heath Station. Complete section of the lower Grinstead Clay. Bristow and Bazley (1972, p.35) 
Reference Section  Hillhouse Farm, near Ardingley. A deep pit shows the upper contact with the upper Tunbridge Wells Sand Formation and a further 6.775m of the member including the Cuckfield Pebble Bed resting on the Cuckfield Stone Bed. Gallois and Worssam (1993, p.47). 
Reference Section  Paxhill Park Quarry. The section shows 3.15m of strata of the Cuckfield Stone and lower Grinstead Clay with a thin (0.025m) unusual limestone developed within the Cuckfield Stone. Gallois and Worssam (1993, p.48). 
Type Section  Freshfield Lane Brickworks. Section on the access road. Here an exposure with 16.2m of strata shows a "complete section of Lower Grinstead Clay, Cuckfield Stone and Upper Grinstead Clay with unusually thick development of the Cuckfield Stone. Top Lower Tunbridge Wells Pebble Bed and also the junction with Upper Tunbridge Wells Sand" (Bristow and Bazley, 1972, p.35; Gallois and Worssam, 1993, p.49). 
Reference(s):
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). 
Allen, P. 1967. Origin of the Hastings facies in north-western Europe. Proceedings of the Geologists' Association, Vol.78, 27-105. 
Gallois, R W. 1965. Horsham (302) Sheet. Summary of Progress of the Geological Survey of Great Britain for 1964. 
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). 
Worssam, B C. 1963. Geology of the country around Maidstone. Memoir of the Geological Survey of Great Britain, Sheet 288 (England and Wales). 
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. 
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. 
Dines, H G, Buchan, S and Bristow, C R. 1969. Geology of the country around Sevenoaks and Tonbridge. Memoir of the Geological Survey of Great Britain, Sheet 287 (England and Wales), 183pp. 
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. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E287 E302 E303 E318 E319 E333