The BGS Lexicon of Named Rock Units — Result Details

Upper Old Red Sandstone Group

Computer Code: ORSU Preferred Map Code: UORS
Status Code: Full
Age range: Late Devonian Epoch (DU) — Courceyan Substage (CF)
Lithological Description: The group in south-east Wales and the Forest of Dean consists mainly of grey-green and buff, fluvial sandstones, and also contains characteristic quartz pebble conglomerate bodies. Also present locally in Powys are red to purple sandstones, some possibly of aeolian origin, and a heterolithic intertidal sequence of red sandstones and mudstones which contain a shallow marine brachiopod fauna. The Upper Old Red Sandstone Group in the Pembroke peninsula is represented by two coarsening-upwards sequences of mature, clean, fluvial sandstones and red sandstones, conglomerates rich in igneous, sandstone and phyllite clasts, and calcretes.
Definition of Lower Boundary: The lower boundary is placed at the base of the lowest bed that overlies the regional Acadian unconformity. A change of colour from red sandstones below the unconformity (although the topmost beds of the underlying formations are locally leached to pale green) to pale green or buff sandstones above is common. Where red beds also overlie the unconformity, a distinctive bed of red mudstone with quartz granules, interpreted as a mudflow, forms the basal bed. An angular discordance between the beds above and below the unconformity is visible in places on the north crop of the South Wales Coalfield. In the Forest of Dean, the lower boundary is a marked angular unconformity; the Upper Old Red Sandstone Group rests on progressively lower levels of the Lower Old Red Sandstone Group.
Definition of Upper Boundary: The upper boundary is placed at the top of the highest red bed, where there is generally a sharp junction with the overlying limestones of the Pembroke Limestone Group. In the Pembroke peminsula, there is an interdigitation of red, green and grey lithologies in a gradational passage between the Skrinkle Sandstone Formation and the Avon Group.
Thickness: Thickness ranges from 45 m on the south-east crop of the South Wales Coalfield to 350 m in the Pembroke peninsula and to 490 m east of the Severn Estuary.
Geographical Limits: Pembrokeshire [e.g. SS 08 97], Gower [e.g. SS 53 88], north crop of the South Wales Coalfield [e.g. Black Mountain [SO 74 19], east and southeast crop of the South Wales Coalfield [e.g. near Blaenavon [SO 28 08], Forest of Dean [SO 56 16], Portishead [ST 44 76], Clee Hills [SO 63 80] and in isolated outcrops flanking the Lake District, e.g. Shap Wells [NY 61 05].
Parent Unit: Old Red Sandstone Supergroup (ORS)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
none recorded or not applicable
Reference(s):
none recorded or not applicable
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232 E246 E251 E280