The Fishguard Volcanic Group is an

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geology, geological science associated with the study of stratum, strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are ...

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

(a sequence of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

) in west

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

. The name is derived from the town of

Fishguard Fishguard ( cy, Abergwaun, meaning "Mouth of the River Gwaun") is a coastal town in Pembrokeshire, Wales, with a population of 3,419 in 2011; the community of Fishguard and Goodwick had a population of 5,407. Modern Fishguard consists of two pa ...

Pembrokeshire Pembrokeshire ( ; cy, Sir Benfro ) is a Local government in Wales#Principal areas, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and the rest by sea. The count ...

. 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 Bluestone is a cultural or commercial name for a number of dimension or building stone varieties, including: * basalt in Victoria, Australia, and in New Zealand * dolerites in Tasmania, Australia; and in Britain (including Stonehenge) * feld ...

' which form a part of the well-known prehistoric monument of

Stonehenge Stonehenge is a prehistoric monument on Salisbury Plain in Wiltshire, England, west of Amesbury. It consists of an outer ring of vertical sarsen standing stones, each around high, wide, and weighing around 25 tons, topped by connectin ...

in southern England.

Outcrops

The rocks are intermittently exposed in a belt of country running east from

Strumble Head Strumble Head ( cy, Pen Caer,Trwyn-câr, Pen Strwmbl) is a rocky headland in the community of Pencaer in Pembrokeshire, Wales, within the Pembrokeshire Coast National Park. It marks the southern limit of Cardigan Bay. Three islands lie off the ...

on the

Cardigan Bay Cardigan Bay ( cy, Bae Ceredigion) is a large inlet of the Irish Sea, indenting the west coast of Wales between Bardsey Island, Gwynedd in the north, and Strumble Head, Pembrokeshire at its southern end. It is the largest bay in Wales. Geograp ...

coast, through Fishguard and wrapping around the northern flanks of

Mynydd Preseli The Preseli Hills or, as they are known locally and historically, Preseli Mountains, (Welsh: ''Mynyddoedd y Preseli / Y Preselau'' , ) is a range of hills in western Wales, mostly within the Pembrokeshire Coast National Park. The range stret ...

to the south of the village of Newport to the vicinity of

Crymych Crymych () is a village of around 800 inhabitants and a community (population 1,739) in the northeast of Pembrokeshire, Wales. It is situated approximately above sea level at the eastern end of the Preseli Mountains, on the old Tenby to Cardiga ...

Lithology and stratigraphy

The group comprises about 1800 m thickness of

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

s and associated volcaniclastic sediments laid down in the marine

Welsh Basin The Welsh Basin was a northeast-southwest aligned back-arc depositional basin during the Cambrian, Ordovician and Silurian periods during which a considerable thickness of marine sediments was laid down in the area. To the southeast lay the Midlan ...

during the

Llanvirn The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

ian 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.http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=FIV (BGS on-line lexicon of rock units) 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.

References

{{Reflist Ordovician System of Europe Middle Ordovician Series Geology of Wales