The Snowdon 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, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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

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 ...

) in

Snowdonia Snowdonia, or Eryri (), is a mountainous region and National parks of the United Kingdom, national park in North Wales. It contains all 15 mountains in Wales Welsh 3000s, over 3000 feet high, including the country's highest, Snowdon (), which i ...

, north-west

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

. The name is derived from

Snowdon Snowdon (), or (), is a mountain in Snowdonia in North Wales. It has an elevation of above sea level, which makes it both the highest mountain in Wales and the highest in the British Isles south of the Scottish Highlands. Snowdon i ...

, the highest peak in Wales where it outcrops. This assemblage of rocks has also been referred to as the Snowdon Volcanic Series.

Outcrops

The rocks occur across the Snowdon massif and to its south, southeast and west, around the eastern flanks of the

Carneddau The (lit. 'the cairns'; is a Welsh language, Welsh plural form, and is sometimes anglicised to ''Carnedds'') are a group of mountains in Snowdonia, Wales. They include the largest contiguous areas of high ground (over or high) in England and W ...

and within the Idwal syncline.

Lithology and stratigraphy

The Group consists of up to 1800 m thickness of ash flow

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 co ...

s with

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

s and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

s and important

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin marg ...

s and

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

s erupted or sedimented during the

Caradoc Caradoc Vreichvras (; Modern , ) was a semi-legendary ancestor to the kings of Gwent. He may have lived during the 5th or 6th century. He is remembered in the Matter of Britain as a Knight of the Round Table, under the names King Carados and C ...

ian Epoch of the Ordovician Period. The Group includes (in descending order, i.e. oldest last): *Upper Crafnant Volcanic Formation *Middle Crafnant Volcanic Formation *Lower Crafnant Volcanic Formation The Tal y Fan Volcanic Formation occurs towards the northeast and the Bedded Pyroclastic Formation and Lower Rhyolitic Tuff Formation occur to the southwest.http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=SV (BGS on-line lexicon of rock units)

References

{{Reflist Ordovician System of Europe Upper Ordovician Series Geology of Gwynedd