Cregennen Formation

Cregennen Formation

Stratigraphic range: Darriwilian
Acidic tuffs of the Cefn Hir Member near the summit of Pared-y-Cefn Hir, Gwynedd, Wales
TypeGeological formation
Unit ofAran Volcanic Group
Sub-unitsCefn Hir Member, Bryn Brith Member
UnderliesBrithion Formation, Llyn y Gafr Volcanic Formation
OverliesOffrwm Volcanic Formation
Thicknessup to 350 metres (1,150 ft)
Location
CountryWales
Type section
Named forLlynnau 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.

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.

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.