Ogwen Group

The Ogwen Group is an Ordovician lithostratigraphic group (a sequence of rock strata) in Gwynedd, north-west Wales. The name is derived from Ogwen Valley, a locality in Snowdonia where it outcrops.

Ogwen Group Stratigraphic range: Arenig to Caradoc Ordovician
Type	Group
Sub-units	Allt Llwyd and Wig Bach formations, Nant Ffrancon Subgroup (which includes Llanengan and St Tudwal's formations), Cwm Eigiau, Allt Fawr Rhyolitic Tuff, Dwyfach and Nod Glas formations
Overlies	Mawddach Group, Rhobell Volcanic Group
Thickness	variable, more than 2 km
Lithology
Primary	mudstones, siltstones
Other	sandstones, tuffs, oolitic ironstones
Location
Region	northwest Wales
Country	Wales
Type section
Named for	Ogwen Valley

Outcrops

The rocks occur throughout Llŷn and eastwards into Snowdonia within Gwynedd.[1]

Lithology and stratigraphy

The Group consists of about 1000 m thickness (in the Pwllheli area) of mudstones and siltstones with some basal sandstones together with tuffs and oolitic ironstones laid down during the Arenig and Caradoc epochs of the Ordovician Period.[2] The Nant Ffrancon subgroup alone exceeds 2km thickness in central Snowdonia. The subgroup as itself been classed as both a formation and a group at times.[3]

References

1. British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 118 Nefyn and 134 Pwllheli
2. "Ogwen Group". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 6 June 2020.
3. "The BGS Lexicon of Named Rock Units". Nant Ffrancon Subgroup. British Geological Survey. Retrieved 6 June 2020.