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]