Cregennen Formation
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Unit ofAran Volcanic Group
Sub-unitsCefn Hir Member, Bryn Brith Member
| 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]