Fishguard Volcanic Group

Fishguard Volcanic Group
Stratigraphic range: middle Ordovician
Pillow lavas at Strumble Head, Strumble Head Volcanic Formation
Type	Group
Sub-units	Goodwick Volcanic Formation, Strumble Head Volcanic Formation, Porth Maen Melyn Volcanic Formation
Underlies	unconformity
Overlies	Aber Mawr Formation
Lithology
Primary	lavas
Other	breccias, tuffs, volcaniclastic sediments
Location
Region	west Wales
Country	Wales
Type section
Named for	Fishguard

The Fishguard Volcanic Group is an Ordovician lithostratigraphic group (a sequence of rock strata) in west Wales. The name is derived from the town of Fishguard in Pembrokeshire. This assemblage of rocks has also been referred to as the Fishguard Volcanic Series or Fishguard Volcanic Complex. These rocks are believed to be the source of the 'bluestones' which form a part of the well-known prehistoric monument of Stonehenge in southern England.

Outcrops

The rocks are intermittently exposed in a belt of country running east from Strumble Head on the Cardigan Bay coast, through Fishguard and wrapping around the northern flanks of Mynydd Preseli to the south of the village of Newport to the vicinity of Crymych.

Lithology and stratigraphy

The group comprises about 1800 m thickness of lavas, breccias tuffs and associated volcaniclastic sediments laid down in the marine Welsh Basin during the Llanvirnian stage of the Ordovician Period. The group includes (in descending order, i.e. oldest last) the Goodwick Volcanic Formation, the Strumble Head Volcanic Formation and the Porth Maen Melyn Volcanic Formation. Previous names for these divisions which may be found in the literature are 'Upper Rhyolite Division', 'Pillow-lava Division' and 'Lower Rhyolite Division. The Llanrian Volcanic Formation is recognised within the Strumble Head area. It is composed of acid tuffs.