The BGS Lexicon of Named Rock Units — Result Details

Portland Group

Computer Code: PL Preferred Map Code: Pl
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: Upper part predominantly limestone (micrites, wackestones, packstones and grainstones) including good freestones; lower part predominantly argillaceous, dolomitic sandstones/sands; some mudstones/shales, and thin beds or nodular layers of micrite.
Definition of Lower Boundary: In the type area traditionally, but not unanimously, taken at the base of the Massive Bed, which also marks the Portlandian/Kimmeridgian stage boundary. In other areas taken at upward change from silty/sandy mudstones (Kimmeridge Clay) to argillaceous sandstones/siltstones (possibly marked by a spring-line) or, where non-sequential, at the base of the Upper Lydite Bed.
Definition of Upper Boundary: The most practical definition is at the top of the highest, massive marine limestone of the Portland Stone. In the type area, the overlying beds are thin algal limestones and palaeosols (`Dirt Beds') of the Purbeck Group.
Thickness: To c.75m
Geographical Limits: Dorset, Wiltshire, Oxfordshire, Buckinghamshire.
Parent Unit: Not Applicable (-)
Previous Name(s): Portland Rocks (-4258)
Portland Strata (-4259)
Portland Formation (*931)
Portland Beds (-948)
Alternative Name(s): Portland Formation
Stratotypes:
Type Area  Coastal cliffs, and quarries on the Isle of Portland, Dorset. 
Type Area  Coastal cliffs, and quarries on the Isle of Portland, Dorset. 
Reference(s):
Wimbledon, W A. 1980. Portlandian correlation chart. 85-93 in Cope, J C W (Editor), A correlation of the Jurassic rocks of the British Isles. Part Two: Middle and Upper Jurassic. Geological Society of London Special Report, No.15. 
Arkell, W J. 1935. The Portlandian Beds of the Dorset mainland. Proceedings of the Geologists' Association, Vol.46, 301-347. 
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
Blake, J F. 1880. On the Portland Rocks of England. Quarterly Journal of the Geological Society of London, Vol.36, 189-236. 
Arkell, W J. 1947. Geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the British Geological Survey. Sheets 341, 342, 343 and small portions of sheets 327, 328 and 329 (England and Wales). 
Arkell W J, 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
Horton, A, Sumbler, M G, Cox, B M and Ambrose, K, 1995. Geology of the country around Thame. Memoir of the British Geological Survey, Sheet 237 (England and Wales). 
Fitton, W H, 1836. Observations on some of the strata between the Chalk and the Oxford Oolite, in the south-east of England. Transactions of the Geological Society of London, Vol.4, 103-378, 379*-388*. 
Damon, R, 1860. Handbook to the geology of Weymouth and the Island of Portland. [London: Edward Stanford.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E341 E342 E343 E252 E237 E282 E220 E238 E254 E219 E268