| Computer Code: |
BHVF |
Preferred Map Code: |
notEntered |
| Status Code: |
Pending Upgrade |
| Age range: |
Arenig Series (OR)
— Abereiddian Stage (O2) |
| Lithological Description: |
Massively-bedded and unbedded acid tuffs rich in embayed quartz xenocrysts. Locally, welding fabrics are preserved. Also includes agglomeratic tuffs, sandstones and mudstones. |
| Definition of Lower Boundary: |
Marked by the rapid transition from bioturbated fine-grained sandstones (Basement Beds of Jones, 1933 op.cit) to massive tuffs. |
| Definition of Upper Boundary: |
Marked by the rapid transition from massive tuffs to mudstones, conglomerates and siltstones of the Bifidus Slate Formation. |
| Thickness: |
Very variable - complicated by intrusion of dolerite sills. 40m - 420m. |
| Geographical Limits: |
Extends from coast north of Tywyn as far as Llanegryn Fault in the east. Equivalent to the Lower Acid Group (Cox, 1925. Quarterly Journal of the Geological Society of London, Vol.81) to the east of the Llanegryn Fault. |
| Parent Unit: |
Aran Volcanic Group (AVG)
|
| Previous Name(s): |
Beacon Hill Volcanic Group [Obsolete Name and Code: Use BHVF]
(-4464)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Hill section, well exposed crags. Begins in a small quarry 130m north-north-east of Cefncamberth and ends on Tal-y-Gareg Hill c.200m east of Tonfanau "Granite" Quarry. |
| Reference(s): |
| 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 |
| Pratt, W T, 1990. Geological notes and local details for parts of 1:10 000 sheets SH61SE and SH71SW, Fawnog (Cadair Idris: part of 1:50 000 sheet 149 (Cadair Idris). British Geological Survey Technical Report WA/90/39. |
| Jones, B, 1933. The geology of the Fairbourne-Llwyngwril district, Merioneth. Quarterly Journal of the Geological Society, Vol.89, 145-171. |
| Cox, A H. 1925. The geology of the Cadair Idris Range, (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, p.539-592. |
| Ridgeway, J. 1975. The stratigraphy of Ordovician volcanic rocks around the south and east of the Harlech Dome in Merionethshire, North Wales. Geological Journal, Vol.10, 87-106. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E149
|
