The BGS Lexicon of Named Rock Units — Result Details

Graig-Wen Sandstone Formation

Computer Code: GWSA Preferred Map Code: notEntered
Status Code: Full
Age range: Hirnantian Age (OH) — Hirnantian Age (OH)
Lithological Description: Massive to medium-bedded grey calcareous sandstone. Locally shelly. Thin mudstones in uppermost part. Represents transgressive shoreface sand in response to the post glacial rise in sea level at the end of the Ordovician.
Definition of Lower Boundary: Taken at the unconformable contact of the Graig-wen Sandstone with the underlying limestones and mudstones of the Dolhir Formation. The contact is locally an irregular palaeokarstic surface with the Graig-wen Sandstone penetrating through solution pits in the limestone into the underlying mudstones. It is likely the Graig-wen Sandstone Formation may be conformable with the Dolhir Formation near its feather edge but further work is needed.
Definition of Upper Boundary: Taken at the gradational (over about a metre) incoming of the mudstone dominant sequence of the Laundry Mudstone Formation above the sandstones and subordinates thin mudstones at the top of the underlying Graig-wen Sandstone.
Thickness: 0 to 12m
Geographical Limits: Meifod area (SJ 09 09), northeast Wales. Passes north and northeastwards into the lowermost part of the Laundry Mudstone Formation.
Parent Unit: Not Available (!)
Previous Name(s): Meristina Crassa Sandstone (-945)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Line of roadside quarries on the northwest side of the A495 either side of Graig-wen, 7km southwest of Meifod, Powys, northeast Wales. The section exposes the lower part of the Graig-wen Sandstone uncomformably overlying limestones and mudstones of the Dolhir Formation. Brenchley (1993). 
Reference(s):
King, W B R. 1928. The geology of the country around Meifod, Montgomeryshire. Quarterly Journal of the Geological Society of London, Vol. 84, 671 - 704. 
Brenchley, P J. 1993. The Ordovician of the south Berwyn Hills, 39-50 in Woodcock, N H and Bassett, M G (eds) Geological excursions in Powys, Central Wales. [Cardiff: University of Wales Press, National Museum of Wales.] 
King, W B R. 1923. The Upper Ordovician rocks of the southwestern Berwyn Hills. Quarterly Journal of the Geological Society of London, Vol.79, 487-507. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable