The Cymru Terrane is one of five inferred fault bounded

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

s that make up the

basement rocks In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

of the southern United Kingdom. The other notable geological terranes are the Charnwood Terrane, Fenland Terrane,

Wrekin Terrane The Wrekin Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes in the region are the Charnwood Terrane, Fenland Terrane, Cymru Terrane and ...

and the Monian Composite Terrane.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed In this article the definition of terrane is that implying rocks associated with the composition of the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

basement. The Cymru Terrane is bounded to the northwest by the

Menai Strait Fault System Menai may refer to the following places: Australia *Menai, New South Wales Africa *Menai Bay on the island of Zanzibar, Tanzania *Menai Island, a Cosmoledo atoll islet, Seychelles Europe *Menai, an electoral ward in Bangor, Wales, UK *Menai (Caern ...

and to the southeast by the Pontesford Lineament. The geological terrane to the west is the Monian Composite Terrane and to the east is Wrekin Terrane. The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.Pharaoh, T.C., Gibbons, W., ''Precambrian Rocks in England and Wales south of the Menai Strait Fault System'', 1987 A Revised Correlation of the Precambrian Rocks in the British IslesJ. N. Carney, J. M. Horak, et al., ''Precambrian Rocks of England and Wales'', ''Joint Nature Conservation Committee.'' Geological Conservation Review Series 20 The

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

rocks of the Cymru Terrane are typified in North Wales by the Arfon Group, Sarn Complex and the Twt Hill Granite.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed The former is in the Bangor area (S & SW) with the latter two located on the Llyn Peninsula. The St Davids Granophyre and the Pebidian Supergroup are located on the coastline of St Davids Peninsula. Inferred

volcanic deposits are noted in the Bryn-Teg Borehole (

Trawsfynydd Trawsfynydd (; Welsh for "across hemountain") is a linear village in Gwynedd, Wales, near Llyn Trawsfynydd reservoir, and adjacent to the A470 north of Bronaber and Dolgellau and 10 km (6 miles) south of Blaenau Ffestiniog. The total com ...

Gwynedd Gwynedd (; ) is a county and preserved county (latter with differing boundaries; includes the Isle of Anglesey) in the north-west of Wales. It shares borders with Powys, Conwy County Borough, Denbighshire, Anglesey over the Menai Strait, and C ...

).P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdP.M. Allen, A.A. Jackson ''Bryn-Teg Borehole, North Wales.'', 1978, Bulletin of the Geological Survey of Great Britain, 61 1-52 The Padarn Tuff, which is thought to be contemporaneous with the Sarn Complex, is unconformable with the overlying Fachwen Formation and the unconformity has been constrained with dates noted of 604.7 Ma+/-1.6Ma from the tuffs and 572.5 Ma+/-1.2Ma from the Fachwen Formation.W. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323-339. Information on the

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

is somewhat sparse with no

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...

s noted to be older than 617 to 638 Ma.W. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323-339. However, there are outcrops of the Parwyd Gneiss which are retrogressed

granitoid A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

that occur with the Llyn Shear Zone.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed This is conjectured to be one of two probabilities, either as an exotic sliver, or as a metamorphic slice from the Monian Composite Terrane. The Granitoid Gneiss has Sm-Nd ages of 1350 Ma which is in the same temporal region as the Sarn Granite P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdW. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323-339.

Intrusive geology

St David's Granophyre

In the southwest of Wales is the

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 ...

Granophyre Granophyre ( ; from ''granite'' and ''porphyry'') is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image. The texture is called granophyric. The texture can be ...

the stratigraphical relationship of which was subject of heated debates towards the end of the 19th Century predominantly between

Sir Archibald Geikie Sir Archibald Geikie (28 December 183510 November 1924) was a Scottish geologist and writer. Early life Geikie was born in Edinburgh in 1835, the eldest son of Isabella Thom and her husband James Stuart Geikie, a musician and music critic. T ...

and Henry Hicks. Geikie (then director of the Geological Survey) insisted upon the Pebidian Supergroup and the high-level intrusion being placed in the Cambrian sequence with Hicks H. Hicks ''On the Precambrian (Dimentian and Pebidian) rocks of St David's'', 1877, Quarterly Journal of the Geological Society of London, 33, 199-241H. Hicks ''On the Precambrian rocks of Pembrokeshire with especial reference to the St David's district.'', 1878, Journal of the Geological Society of London. 40 507-560 taking the opposing view and suggesting that the Granophyre which cross cuts the Pebidian Supergroup should be placed in the

and by consequence placing the Pebidian Supergroup in the Precambrian. The issue was finally resolved by GreenJ. F. N. Green ''On the Geological Structure of the St David's area, Pembrokeshire.'', 1908, Journal of the Geological Society of London. 64 363-383 by revealing, in an excavation, an

unconformity 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 ...

in which an excavation of a

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...

basal conglomerate is shown to cut the granophyre.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdJ. F. N. Green ''On the Geological Structure of the St David's area, Pembrokeshire.'', 1908, Journal of the Geological Society of London. 64 363-383 The St David's Granophyre has historically been incorrectly assigned to be

alaskite Leucogranite is a light-colored, granitic, igneous rock containing almost no dark minerals. Alaskite is a synonym.trondhjemite Trondhjemite is a leucocratic (light-colored) intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites that occur in the oceanic crust or in ophiolites are usually called pla ...

due to low concentrations of

potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium: *Orthoclase (endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock *Microcline, chemically the same as orthoclas ...

but this has been shown to be an alteration product from its original calc-alkaline

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

which is noted to exhibit a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

signature.T. W. Bloxham & M. H. J. Dirk ''The petrology and geochemistry of the St David's granophyre and Cwm Bach Rhyolite, Pembrokeshire, Dyfed.'', 1988, Mineralogical Magazine, 52 563-575 Correlation of the granophyric intrusion with the Arfon Group rather than the nearby Coomb Volcanic Formation has been suggestedR. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, ''Geochemical Character of Neoproterozoic rocks in southwest Wales.'', 1995, Geological Magazine. 132 330-349 with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdP. J. Patchett & J. Jocelyn ''U-Pb zircon ages for late Precambrian igneous rocks in South Wales.'', 1979, Journal of the Geological Society of London. 136 13-19

Sarn Complex

The largest plutonic body in the terrane has limited outcrop and is sheared by the Llyn Shear Zone in the west and covered by later (

Arenig In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval. History The term was first used by Adam Sedgwick in 1847 with reference to the "Areni ...

) sediments to the east.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed Altered to

Greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

facies the

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

contains a bimodal suite of

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly

granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

.P. J. Patchett & J. Jocelyn ''U-Pb zircon ages for late Precambrian igneous rocks in South Wales.'', 1979, Journal of the Geological Society of London. 136 13-19 The Sarn Granite is

leucocratic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...

and covers an expanse of c.6 km^2 in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component has been confirmed as having a Neoproterozoic age of 614Ma+/-2Ma using U-Pb Zircon dating.P. J. Patchett & J. Jocelyn ''U-Pb zircon ages for late Precambrian igneous rocks in South Wales.'', 1979, Journal of the Geological Society of London. 136 13-19 So therefore, the shearing of the Llyn is also temporally constrained by the date.

Twt Hill Granite

This is a small fine-grained, leucocratic intrusive body that cuts the Padarn Tuff at the western end of the Bangor-Caernarfon Ridge.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

Volcano-sedimentary geology

The thick sequences of volcano-sedimentary

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

are present in North Wales (as the Arfon Group) and South Wales (as the Pebidian Supergroup) and are generally considered to be coeval but are geochemically distinct, showing acidic and basic qualities respectively.R. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, ''Geochemical Character of Neoproterozoic rocks in southwest Wales.'', 1995, Geological Magazine. 132 330-349P. J. Patchett & J. Jocelyn ''U-Pb zircon ages for late Precambrian igneous rocks in South Wales.'', 1979, Journal of the Geological Society of London. 136 13-19 Both are cut by minor granitic intrusions. The Pebidian Supergroup is interbedded basic

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 ...

s and acid

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, is cut by the St David's Granophyre.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed There is evidence presented that subaerial and subaqueous deposition has occurred in a Welsh Basin that is likely to have been undergoing varying rates of subsidence thus promoting aerial emergence.

alteration is in keeping with the

regional metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

that is characteristic of Welsh

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

rocks.R. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, ''Geochemical Character of Neoproterozoic rocks in southwest Wales.'', 1995, Geological Magazine. 132 330-349 In the Pebidian Supergroup the basal unit is largely

basaltic Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90% of a ...

and exhibits

columnar jointing Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as Joint (geology), joints, result in the formation of a regular array of polygonal Prism (geometry), prisms, or columns. Columnar joint ...

, auto

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 ...

ted lavas,

scoria Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

ceous and fine grained tuffs. The volcano-sediments have an increased acid component until the uppermost beds of the unit exhibit the basaltic influence once again. Such information is preserved in the Rhosson Group as autobrecciated lava, tuffs and scoria deposits.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

The Arfon Group

This volcano-sedimentary group incorporates over 4000m of deposits and were previously considered to be of

age. U-Pb data suggests that the whole succession is indeed Precambrian

age.A. J. Reedman, B. E. Leveridge, R. B. Evans, ''The Arfon Group (Arvonian) of North Wales.'', 1984, ''Proceedings of the Geologists Association'', 95, 313-321 Exposure of the lower unit ( Padarn Tuff) is exhibited on a ridge between Bangor and

Caernarfon Caernarfon (; ) is a royal town, community and port in Gwynedd, Wales, with a population of 9,852 (with Caeathro). It lies along the A487 road, on the eastern shore of the Menai Strait, opposite the Isle of Anglesey. The city of Bangor is ...

and also on a ridge near

Llyn Padarn Llyn Padarn is a glacially formed lake in Snowdonia, Gwynedd, north Wales, and is an example of a moraine dammed lake. The lake is approximately long (about 240 acres) and at its deepest point is deep, and is one of the largest natural lakes i ...

P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed This is noted to be a thick sequence of acid ash flow tuffs and exhibits welding and are thought to be rapid deposition of thin air-fall tuffs and

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

flows placed in a bounded

half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

(or

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

) .A. J. Reedman, B. E. Leveridge, R. B. Evans, ''The Arfon Group (Arvonian) of North Wales.'', 1984, ''Proceedings of the Geologists Association'', 95, 313-321 Isotope data shows a U-Pb zircon from the lower part of the succession as 614+/-2MaR. D. Tucker & T. C. Pharaoh, ''U-Pb Zircon ages for late-Precambrian rocks in southern Britain.'', 1991, ''Journal of the Geological Society of London'', 148, 435-433 and 604.7+/-1.6MaW. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323-339. confirms a Neoproterozoic age.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

Minfordd and Bangor Formations

These strata have an angular and discordant relationship with both formations having variable thickness controlled by bounding fault lines that are likely to have controlled the depositional regime of the rocks.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, ''The Arfon Group (Arvonian) of North Wales.'', 1984, ''Proceedings of the Geologists Association'', 95, 313-321 The formations exist to the west of the Aber Dinlle Fault. The Minfordd Formation is a sandstone dominated epiclastic and tuffaceous

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

which has a broad upward fining sequence with the presence of thick welded and non-welded ash-flow tuffs that elucidates an amount of volcanism.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, ''The Arfon Group (Arvonian) of North Wales.'', 1984, ''Proceedings of the Geologists Association'', 95, 313-321 The basal rocks of the Minfordd Formation contain clasts of

, vein

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

, quartzitic sandstone, quartzose

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

s and were thought to be

basal conglomerate. The Bangor Formation overlies the Minfordd Formation and it underlies the Llanberis Slates and is a similar lithology to the Minfordd Formation excepting the basal conglomerate.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, ''The Arfon Group (Arvonian) of North Wales.'', 1984, ''Proceedings of the Geologists Association'', 95, 313-321

Fachwen Formation

This is thought to be the lateral equivalent of the Bangor and Minfordd Formations on the eastern side of the Aber Dinelle Fault. The Fachwen and Bangor Formations are both dominated by acidic volcanic deposits but they also contain minor basic tuffs that preserve scoria textures which suggest a proximal source.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

Bwlch Gwyn Tuff

This outcrops mainly on

Anglesey Anglesey (; cy, (Ynys) Môn ) is an island off the north-west coast of Wales. It forms a principal area known as the Isle of Anglesey, that includes Holy Island across the narrow Cymyran Strait and some islets and skerries. Anglesey island ...

but a sliver of it also occurs along the Berw Fault and its properties are similar to that of the Arfon Group ash-flow tuff and as such constrain the docking of the western Monian Composite Terrane.W. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323-339.R. D. Tucker & T. C. Pharaoh, ''U-Pb Zircon ages for late-Precambrian rocks in southern Britain.'', 1991, ''Journal of the Geological Society of London'', 148, 435-433 Although evidence can be interpreted to the contrary as the Bwlch Gwyn outcrop may be faulted post-Berw and pre-Arenig.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

Boreholes

The Bryn-teg borehole is located near

in the

Harlech Dome The Harlech Dome is a geological dome in southern Snowdonia in north Wales. It extends approximately from Blaenau Ffestiniog in the north to Tywyn in the south, and includes Harlech, The Rhinogydd, Barmouth and Cadair Idris. The geological layer ...

and exhibits over 140 m of Neoproterozoic rocks. There is a sharp erosional contact with the Cambrian Dolwen Formation.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed

Palaeontology

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

has been found in the Welsh Terrane to date. The age of the Bryn-teg Volcanic Formation is constrained by Lower Cambrian

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...

in overlying beds. The ''Playsolenites cooperi'' are only otherwise known in the Placentian of SE Newfoundland.D. McIlroy, O. R. Green, M. D. Brasier, ''Palaeobiology and evolution of the earliest agglutinated Foraminifera: Platysolenites, Spirosolenites and related forms.'', 2001, ''Lethaia'', 34, 13-29

References

{{Reflist

External links

Welsh Geology
Geology of Wales Precambrian Europe Terranes Ediacaran geology Articles which contain graphical timelines