The BGS Lexicon of Named Rock Units — Result Details

Portishead Formation

Computer Code: POB Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Famennian Age (DA) — Famennian Age (DA)
Lithological Description: Red-green mudstones and marls, red siltstones and red-yellow, hard, fine-grained quartzose sandstones, with conglomerate locally.
Definition of Lower Boundary: Unconformable contact on the underlying Black Nore Sandstone (Brownstones Formation of South Wales) or on Silurian Volcanics.
Definition of Upper Boundary: Incoming of shales and thin limestones of the Lower Limestone Shale Formation which rest conformably on the Portishead Formation.
Thickness: 300 m.
Geographical Limits: Bristol - Mendip area. Splits laterally into Tintern Sandstone and Quartzite Conglomerate in Forest of Dean.
Parent Unit: Upper Old Red Sandstone Group (ORSU)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  Woodhill Bay, Portishead. Coast south of Portishead. 
Type Section  Left bank of River Avon, Abbot's Leigh, Bristol (Avon Gorge). 
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]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E279 E280 E281 E264 E250 E251