Cregennen Formation

Cregennen Formation
Stratigraphic range: Darriwilian PreꞒ Ꞓ O S D C P T J K Pg N
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

Prominent ridges of Pared y Cefn-Hir (to the right) and Bryn Brith (to the left) viewed from the southwest, formed of volcanic members of the Cregennen Formation

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

1. British Geological Survey. "Cregennen Formation". BGS Lexicon of Named Rock Units. Retrieved 22 May 2025.
2. 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.
3. 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.