The Caerfai Group is a

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

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 ide ...

(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 e ...

) 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

Caerfai Bay Caerfai Bay is a rocky cove on the north coast of St Brides Bay near St Davids in Pembrokeshire, West Wales. It is enclosed by steep varicoloured cliffs and has a sandy beach at low tide accessed by pathway and steps. The beach is situated in th ...

on the north coast of

St Brides Bay St Brides Bay ( cy, Bae Sain Ffraid) is a bay in western Pembrokeshire, West Wales. Either Skomer Island or the mainland extremity of Wooltack Point at the western end of the Marloes Peninsula marks the southern limit of the bay whilst its no ...

on the Pembrokeshire coast where the

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 e ...

are well exposed in coastal cliffs. This rock succession has previously been known variously as the Caerfai Series, Caerfai Formation and Caerfai Beds and largely ascribed to the British regional stratigraphic unit ''Comley Epoch'', though these terms are now obsolete.

Outcrops

The rock succession is exposed, along the coast south of

St Davids St Davids or St David's ( cy, Tyddewi, , "David's house”) is a city and a community (named St Davids and the Cathedral Close) with a cathedral in Pembrokeshire, Wales, lying on the River Alun. It is the resting place of Saint David, W ...

and in particular at Caerfai and St Non’s bays to the south of the city where they are seen to overlie Precambrian volcaniclastic rocks of the

Pebidian Supergroup The Pebidian Supergroup is an Ediacaran (i.e. late Precambrian) lithostratigraphic supergroup (a sequence of rock strata) in Pembrokeshire, west Wales. The term 'Pebidian' (along with Dimetian) was coined by geologist Henry Hicks in 1876 and pu ...

. They are exposed again on the west-facing coast of the St Davids peninsula facing

Ramsey Island Ramsey Island (Welsh: ''Ynys Dewi'') is an island about off St David's Head in Pembrokeshire on the northern side of St Brides Bay, in southwest Wales, in the community of St Davids and the Cathedral Close. It is in area. Ramsey means (in Ol ...

. Further outcrops though less well exposed occur south of

Croes-goch Croes-goch (also spelled Croesgoch) is a village in North Pembrokeshire, West Wales. It is situated on the A487 between Fishguard and St David's. It lies some five miles northeast of St Davids on the junction of the A487 St Davids to Fishguard ...

and north and east of

Newgale Newgale ( cy, Niwgwl) is a village with an almost stretch of beach in the parish of Roch, Pembrokeshire, West Wales. The beach is situated in the Pembrokeshire Coast National Park and on the Pembrokeshire Coast Path and has rugged coastal scener ...

and again in the Wolf's Castle area.

Lithology and stratigraphy

The Group comprises around four hundred metres thickness of green and purplish-red

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

ceous

feldspathic Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s together with mudstones and a basal

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** C ...

. The sequence of the Caerfai Group is as follows: * Porth Clais Formation * Newgale Formation * Caer Bwdy Bay Formation * Caerfai Bay Formation *

St Non’s Formation ST, St, or St. may refer to: Arts and entertainment * Stanza, in poetry * Suicidal Tendencies, an American heavy metal/hardcore punk band * Star Trek, a science-fiction media franchise * Summa Theologica, a compendium of Catholic philosophy a ...

* Ogof Golchfa Formation The basal conglomerate

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

overlies

slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic ro ...

s and

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 ...

s of the Ogofgolchfa Group of the Pedbidian. Radiometric studies have yielded ages of around 519 Ma for tuffs within the mudstone. Sedimentary structures and fossils present in the sequence suggest a shallow water depositional environment.

References

{{Reflist Cambrian System of Europe Geology of Wales Geological groups of the United Kingdom