The BGS Lexicon of Named Rock Units — Result Details

Clifton Down Limestone Formation

Computer Code: CDL Preferred Map Code: CDL
Status Code: Full
Age range: Arundian Substage (CJ) — Holkerian Substage (CQ)
Lithological Description: Splintery dark grey calcite and dolomite mudstones, pale grey oolitic, dark grey bioclastic and oncolitic limestones and some mudstones. Scattered cherts and silicified fossils in lower half. Sandy limestone at base in Bristol area. Deposited in a barrier/back barrier/shelf lagoon setting.
Definition of Lower Boundary: In the Bristol area taken at the sharp contact between the interbedded dolomite and calcite mudstones of the underlying Clifton Down Mudstone Formation and the basal sandy limestone of the Clifton Down Limestone Formation. The boundary needs further investigation. In Broadfield Down and the Mendip Hills it is taken at the incoming of calcite mudstones of the Clifton Down Limestone Formation above the oolitic and oolitic/bioclastic limestones of the Goblin Combe Oolite Formation and undivided Burrington Oolite Subgroup respectively.
Definition of Upper Boundary: Taken at the top of the last calcite mudstones of the formation, above which occur the dark grey crinoidal and bioclastic limestones and paler oolitic limestones of the Oxwich Head Limestone Formation.
Thickness: 266 m in the Avon Gorge at Bristol, thinning northwards to 137 m north of Bristol and thinning southwards to 150-200 m in the Mendip Hills.
Geographical Limits: Bristol [ST 60 74] area and Mendip [ST 50 53]. Passes laterally into the Hunts Bay Oolite Subgroup in the Monmouth and Chepstow area [ST 53 94].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Cheddar Limestone Formation (-758)
Upper Clifton Down Limestone Formation [Obsolete Name And Code: Use CDL] (UCDL)
Lower Clifton Down Limestone Formation [Obsolete Name And Code: Use CDL] (LCDL)
Alternative Name(s): Drybrook Limestone Formation [Obsolete: use HBO]
Partial Type Section  Great Quarry and the Portway road section, east side of the Avon Gorge, Bristol. Most of the unit (266 m) seen, apart from a gap in the lower part. Comprises mainly ooidal and skeletal limestones, and calcite and dolomite mudstones. The lower contact with the Clifton Down Mudstone Formation and upper contact with the Oxwich Head Limestone Formation are exposed (cf. Kellaway and Welch, 1993). 
Kellaway, G A and Welch, F B A. 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of the Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, p.1-21. 
Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. 
Reynolds, S H, 1921. The lithological succession of the Avonian at Clifton. Quarterly Journal of the Geological Society of London, Vol.77, 213-243. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E264 E265 E279 E280 E281 E263 E250 E251