Llewelyn Volcanic Group

The Llewelyn Volcanic Group is an Ordovician lithostratigraphic group (a sequence of rock strata) in Snowdonia, north-west Wales. The name is derived from Carnedd Llewelyn, the highest peak in the Carneddau range where it outcrops.

Llewelyn Volcanic Group
Stratigraphic range: Caradoc Ordovician
Rhyolitic ash-flow tuffs of the Capel Curig Volcanic Formation exposed on the northeastern face of Pen yr Ole Wen, capped by sandstones of the overlying Cwm Eigiau Formation, which can be seen at the top of the face
TypeGroup
Sub-unitsCapel Curig Volcanic Formation, Foel Fras Volcanic Formation, Conwy Rhyolite Volcanic Formation, Foel Grach Basalt Formation, Braich Tu Du Volcanic Formation
UnderliesCwm Eigiau Formation
OverliesNant Ffrancon Formation
Thicknessapprox 1400m
Lithology
Primaryash flow tuffs
Otherrhyolites, mudstones, siltstones, sandstones, breccias etc
Location
Regionnorthwest Wales
CountryWales
Type section
Named forCarnedd Llewelyn

Outcrops

The rocks occur across the Snowdon massif and the Carneddau and within the Capel Curig Anticline.[1]

Lithology and stratigraphy

The Group comprises around 1400m thickness of ash flow tuffs, flow-banded rhyolites and breccias with a variety of volcaniclastic sediments erupted or sedimented during the Caradocian epoch of the Ordovician period. The Group comprises (in descending order, i.e. oldest last):

  • Capel Curig Volcanic Formation
  • Foel Fras Volcanic Formation
  • Conwy Rhyolite Volcanic Formation

It also includes the Foel Grach Basalt Formation and the Braich Tu Du Volcanic Formation.[2]

References
  1. British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 106 Bangor and 119 Snowdon
  2. http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=LLV (BGS on-line lexicon of rock units)


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