| Computer Code: |
BGL |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Llanvirn Series (OE)
— Caradoc Series (O4) |
| Lithological Description: |
Thick- to thin-bedded intermediate to mafic crystal tuffs and rare felsic tuffs, interbedded with pillowed lavas, hyaloclastites, and tuffaceous siltstones and mudstones. |
| Definition of Lower Boundary: |
Overlies mudstones and siltstones of the Ty'r Gawen Mudstone Formation, though with possible unconformity or non-sequence in places. Northwards from the Cadair Idris district it oversteps the Melau Formation to rest on the Allt Lwyd Formation in the Harlech district. |
| Definition of Upper Boundary: |
Conformably overlain by mudstones of the Ty'r Gawen Mudstone Formation in the Cadair Idris district, where it also passes laterally into the Pen y Gadair Volcanic Formation. In the northeastern part of this district it is overlain by the felsic tuffs of the Craig Cau Formation, with possible disconformity. |
| Thickness: |
To 250 m. |
| Geographical Limits: |
Not yet specified. |
| Parent Unit: |
Aran Volcanic Group (AVG)
|
| Previous Name(s): |
Basic Volcanic Group [Obsolete Name and Code: Use BGL]
(*554)
|
| Alternative Name(s): |
Pen y gadair Volcanic Formation
|
| Stratotypes: |
| Type Section |
Western slopes of Craig-y-Benglog. Allen, P M and Jackson, A A. 1985. |
| Reference(s): |
| Allen, P M, and Jackson, A A. 1985. Geology of the country around Harlech. Memoir of the British Geological Survey, Sheet 135 with part of Sheet 149 (England and Wales). (London: HMSO.) 112 pp. |
| Wells, A K. 1925. The geology of the Rhobell Fawr district (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, 463-538. |
| Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E149
E136
E135
|
