Cregennen Formation
| Cregennen Formation | |
|---|---|
Stratigraphic range: | |
 Acidic tuffs of the Cefn Hir Member near the summit of Pared-y-Cefn Hir, Gwynedd, Wales | |
| Type | Geological formation |
| Unit of | Aran Volcanic Group |
| Sub-units | Cefn Hir Member, Bryn Brith Member |
| Underlies | Brithion Formation, Llyn y Gafr Volcanic Formation |
| Overlies | Offrwm Volcanic Formation |
| Thickness | up to 350 metres (1,150 ft) |
| Location | |
| Country | Wales |
| Type section | |
| Named for | Llynnau Cregennen |
The Cregennen Formation is a geological formation of early Darriwilian age (late Middle Ordovician). It consists of dark grey to black mudstones containing graptolites, with interbedded volcanic units, particularly felsic and mafic tuffs.[1]
Two larger volcanic units developed are the Cefn Hir and Bryn Brith members that form the prominent ridges of Pared-y-Cefn-Hir and Bryn Brith.[2][3]
References
- ^ British Geological Survey. "Cregennen Formation". BGS Lexicon of Named Rock Units. Retrieved 22 May 2025.
- ^ Rushton, A.W.A.; Howells, M.F. (1999). "Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula" (PDF). British Geological Survey Research Report. Retrieved 21 May 2025.
- ^ Woodhall, D.G. (2000). Stephenson, D.; Bevins, R.E.; Millward, D.; Highton, A.; Parsons, I.; Stone, P.; Wadsworth, W.J. (eds.). "Chapter 6: Wales and adjacent areas Site: Pared y Cefn-Hir (GCR ID: 276)" (PDF). Volume 17: Caledonian Igneous Rocks of Great Britain. pp. 269–273. Retrieved 23 May 2025.