The geology of Anglesey, the largest (714 km
2) island in
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 ...
is some of the most complex in the country.
Anglesey
Anglesey ( ; ) is an island off the north-west coast of Wales. It forms the bulk of the Principal areas of Wales, county known as the Isle of Anglesey, which also includes Holy Island, Anglesey, Holy Island () and some islets and Skerry, sker ...
has relatively low relief, the 'grain' of which runs northeast–southwest, i.e. ridge and valley features extend in that direction reflecting not only the trend of the late
Precambrian
The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, 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 t ...
and
Palaeozoic
The Paleozoic ( , , ; or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of ...
age
bedrock
In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet.
Definition
Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
but also the direction in which glacial ice traversed and scoured the island during the
last ice age. It was realised in the 1980s that the island is composed of multiple
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 d ...
s, recognition of which is key to understanding its Precambrian and lower Palaeozoic evolution. The interpretation of the island's geological complexity has been debated amongst geologists for decades and recent research continues in that vein.
History of geological study
The first geological map of Anglesey was published in 1822 by
John Stevens Henslow
John Stevens Henslow (6 February 1796 – 16 May 1861) was an English Anglican priest, botanist and geologist. He is best remembered as friend and mentor to Charles Darwin.
Early life
Henslow was born at Rochester, Kent, the son of a solicit ...
. It was
Edward Greenly
Edward Greenly (3 December 1861 – 4 March 1951) was an English geologist known for his a detailed geological survey of the island of Anglesey. ''The Geology of Anglesey'' was published in two volumes in 1919 and followed by a one-inch geologic ...
who provided the first detailed map of the island in 1920, a year after the publication of a two-part geological memoir following painstaking work, during the course of which he identified and named the
mélange
In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically con ...
. Greenly's work was conducted in the era before the recognition of
plate tectonics
Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
as the broader context within which geological processes take place. It was from the 1960s that plate tectonic interpretations of Anglesey's geology were put forward by geologists such as
Dennis Wood who introduced the concept of an
olistostrome
An olistostrome is a sedimentary deposit composed of a chaotic mass of heterogeneous material, such as blocks and mud, known as olistoliths, that accumulates as a semifluid body by submarine (geology), submarine gravity sliding or Slump (geology ...
to account for the intimately mixed
lithologies
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. Lith ...
on the north coast. Through the work of Gibbons and others in the 1980s, it was realised that the island is composed of multiple
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 d ...
s, separated by faulted boundaries. Subsequently, the concept of 'underplating' at a
destructive plate margin has been used to help explain the disposition of the disparate rock sequences. More recently again, work has demonstrated structural links with the
Caledonian orogen in northeastern America, enabling Anglesey's wider tectonic context to be better understood.
Precambrian
A
Blueschist belt extends across the interior of the south - the Central Anglesey and Berw shear zones containing
Ediacaran
The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...
age rocks.
The schists of the
Gwna Group date from Ediacaran to Early Cambrian times. They extend north from
Menai Bridge
Menai Bridge (; usually referred to colloquially as ''Y Borth'') is a town and community on the Isle of Anglesey in north-west Wales. It overlooks the Menai Strait and lies by the Menai Suspension Bridge, built in 1826 by Thomas Telford, j ...
to the eastern shores of
Red Wharf Bay
Red Wharf Bay, also known as Traeth Coch ( Welsh for "red beach"), is a village and a sandy bay in Anglesey, Wales. The bay lies between the villages of Pentraeth and Benllech. It is also close to Castell Mawr Rock, thought to be the site of a ...
(or Traeth Coch) and from
Aberffraw Bay northeast to beyond
Cefni Reservoir
Llyn Cefni is a small reservoir in the centre of Anglesey, Wales which is managed by Welsh Water and Hamdden Ltd, while the fishery is managed by the Cefni Angling Association. The reservoir is located just northwest of the island's county town ...
. Smaller outcrops extend northeast from
Llanddwyn Island
is a small tidal island off the west coast of Anglesey ( Welsh: Ynys Môn), northwest Wales. The nearest settlement is the village of Newborough.
Geology and geography
The island is of geological interest with pillow lava, jasper formations a ...
and in the
Pentraeth
Pentraeth () is a village and Community (Wales), community on the island of Anglesey (''Ynys Môn''), North Wales, at . The Royal Mail postcode begins LL75. The community population taken at the United Kingdom Census 2011, 2011 census was 1,178 ...
area. Each of these areas includes igneous rocks too, including pillow lavas 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 co ...
s. The schists are also present to the north and east of
Llanfaethlu
Llanfaethlu is a village and community in the north west of Anglesey, in north-west Wales. The community population taken at the 2011 Census was 553. The village takes its name from the Church of Saint Maethlu. The community includes Llanfwro ...
, at
Mynydd Bodafon
is a small collection of peaks including which is the highest point on the island of Anglesey (although not in the county
A county () is a geographic region of a country used for administrative or other purposesL. Brookes (ed.) '' Chambers ...
and patchily along the north coast.
Though these schemes have been and continue to be subject to change, the Monian Supergroup has in recent times been considered to include the Fydlyn Felsite Formation (or Fydlyn Group), the Gwna Group, New Harbour Group (within which the Skerries Formation, Church Bay Tuffs & Skerries Grits, Lynas Formation and Bodelwyn Formation are defined), the Holy Island Group and the Rhoscolyn, Holyhead and South Stack formations. It thereby extends into the Cambrian.
The Holyhead and Rhoscolyn formations, a succession of metamorphosed quartzites and sandstones with interbedded mudstones, is between 150 and 200m thick. Its outcrop is restricted to the
Holyhead Mountain
Holyhead Mountain ( Welsh ''Mynydd Twr,'' from ''mynydd'' 'mountain, unenclosed land' and ''twr'' 'a heap, pile') is the highest point on Holy Island, Anglesey, and in the county of Anglesey, north Wales. It lies about two miles west of the town ...
and Rhoscolyn areas of Holy Island. The Rhoscolyn Formation is metamorphosed sandstones and silty mudstones with some quartzite.
Cambrian
Rocks of the South Stack and New Harbour Groups are of Cambrian age. The South Stack Formation which may be over 1 km thick in places comprises metamorphosed sandstones and silty mudstones and is considered to be of late Cambrian/early
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 ...
age. It outcrops on Holy Island between Holyhead and South Stack and at Rhoscolyn and also between Mynydd Mechell and Carreglefn inland on Anglesey itself.
Ordovician
Anglesey's Ordovician rock strata, largely interbedded mudstones and sandstones, lie unconformably on Precambrian and Cambrian basement. The main outcrop extends from Dulas Bay inland via Rhosybol and Llanbabo to the coast west of
Llanfairynghornwy with an extension southwest via Bryngwran to the coast at Rhosneigr. The basal beds are conglomeratic in nature. A second smaller outcrop extends southwest from Capel Coch (Welsh: 'red chapel', reflecting the colour of building stone), through Llangwyllog. Another stretches north from
Beaumaris
Beaumaris (; ) is a town and community (Wales), community on the Anglesey, Isle of Anglesey in Wales, of which it is the former county town. It is located at the eastern entrance to the Menai Strait, the tidal waterway separating Anglesey fro ...
and a fourth one extends southwest from Red Wharf Bay (Traeth Coch: 'red beach', reflecting the colour of the extensive sandflats at low tide).
Silurian: Mynydd Parys
It is at
Mynydd Parys
Parys Mountain () is located south of the town of Amlwch in north east Anglesey, Wales. Originally known as Trysclwyn, the high ground is reputed to have gained its current name when it was given to Robert Parys, Chamberlain of North Wales by H ...
(Parys Mountain), just to the south of
Amlwch
Amlwch () is a port town and community (Wales), community in Wales. It is situated on the north coast of the Anglesey, Isle of Anglesey, on the A5025 road, A5025 which connects it to Holyhead and to Menai Bridge. As well as Amlwch town and Am ...
, that the only outcrop of rocks of early
Silurian
The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
age can be found on Anglesey.
Old Red Sandstone
A narrow outcrop of
Old Red Sandstone
Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...
extends from
Dulas Bay
Dulas Bay ( Welsh: ''Bae Dulas'') is a small bay on the north east coast of Anglesey (''Ynys Môn''), north Wales, forming the boundary between Llaneilian and Moelfre communities. The bay is bordered by three beaches.
Overview
The north-weste ...
on the east coast, inland to the northern edge of Llangefni, the only outcrop of these strata in North Wales. A relative lack of fossils means that dating the sequence is problematic but, like the sequence in the Anglo-Welsh basin in South Wales, it is considered to straddle the Silurian/
Devonian
The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
boundary. The coastal outcrop at Dulas Bay is considered to be important both for its fine displays of cyclical fluvial sedimentation but also for being the first place where epsilon
cross-bedding
In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...
was recognised in ancient rocks. The sequence has traditionally been divided into four; a lowermost Bodafon Formation overlain successively by the Traeth Bach, Porth-y-Mor and Traeth Lligwy formations.The Bodafon Formation is
conglomeratic, the individual clasts being an assortment of Precambrian rocks of local provenance.
Calcrete
Caliche () is a soil accumulation of soluble calcium carbonate at depth, where it precipitates and binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or s ...
s (carbonate
palaeosol
In geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science.
In geology and paleon ...
s) are well developed in the middle two formations,
Carboniferous
Rocks of Carboniferous age occur in three parts of the island, the largest part of which, known as the 'Principal area', extends from the east coast between Lligwy Bay and Red Wharf Bay, narrowing southwestwards to the south of Llangefni to reach the southwest coast though is almost wholly concealed beneath younger deposits at Malltraeth Marsh (Cors Ddyga). Its southeastern boundary is largely defined by the Berw Fault. A smaller outcrop, the 'Straitside area', occupies the coastal belt southwest from Llanfairpwllgwyngyll; it too disappears under recent deposits at the southwest corner of the island. A third smaller outcrop, occupies the eastern extremity of Anglesey - the 'Penmon area' - and includes Puffin Island and is restricted to limestone only.
The basal beds of the Carboniferous succession on Anglesey are of
Holkerian to
Asbian age and are termed the Lligwy Sandstone Formation. These sandstones and conglomerates with some siltstone and mudstone record the initial influx of sediment from the
Wales-Brabant massif immediately to the south, onto an eroded surface of lower Palaeozoic and late Precambrian rocks. Their outcrop, which is up to 60m in thickness, extends from Lligwy Bay southwest to Llangefni. This succession has previously also been known as the Basement Conglomerate or Basement Beds.
Carboniferous Limestone
As elsewhere in Britain, subdivision of the
Carboniferous Limestone has undergone many changes over the years and reference is found in the literature to a confusing array of names. The first detailed descriptions of the limestone were provided by G.H.Morton between 1870 and 1901, just as the (British) Geological Survey was conducting its own survey and publication on the area. Morton's scheme distinguished the Lower Brown, Middle White and Upper Grey limestones. From the 1970s the limestone was subdivided into the various zones based on the presence of the
coral
Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
Dibunophyllum; more recently again a series of formation names has been applied within an overall
Clwyd Limestone Group
The Clwyd Limestone Group is a stratigraphic unit (a defined sequence of rock layers) of Chadian to Brigantian age (Lower Carboniferous) found in north Wales. It forms part of the Carboniferous Limestone Supergroup. Its most extensive outcrop ...
. The outcrops on Anglesey form part of a wider assemblage across North Wales which is interpreted as recording the gradual transgression of the sea southwards and the establishment of a
carbonate ramp and shelf during the early Carboniferous. In Carboniferous
palaeogeography
Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term pale ...
, this area, marginal to the Eastern Irish Sea Basin is known as the North Wales Shelf. The northernmost outcrop of the limestone forms the islet of Ynys Dulas, 1 km off the northeastern coast. Puffin Island / Ynys Seiriol provides the easternmost outcrop. In common with the rest of the Penmon area, the limestone here is divided into a Loggerheads Limestone Formation and an overlying Cefn Mawr Limestone Formation. Both formations record episodes of cyclical sedimentation reflecting contemporary changes in sea level. The latter includes some thin sandstones whilst both include palaeosol horizons and
calcrete
Caliche () is a soil accumulation of soluble calcium carbonate at depth, where it precipitates and binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or s ...
s indicating that the area lay above sea level from time to time, permitting the development of
palaeokarsts.
Upper Carboniferous strata
Mudstone, siltstone and sandstone strata within the 'Principal area', overlying the limestone, are assigned to the
Millstone Grit Group
Millstone Grit is any of a number of coarse-grained sandstones of Carboniferous age which occur in the British Isles. The name derives from its use in earlier times as a source of millstones for use principally in watermills. Geologists refer to t ...
. These are overlain by similar rocks assigned to the
Pennine Coal Measures Group
The Pennine Coal Measures Group is a lithostratigraphical term referring to the coal-bearing succession of rock strata which occur in the United Kingdom within the Westphalian Stage of the Carboniferous Period. In formal use, the term replace ...
, coal seams within which provided the basis for a former small scale mining industry. The Coal Measures and Millstone Grit strata are only found to the south of Llangefni and are largely concealed by modern tidal flat deposits which extend far inland hereabouts. Interbedded mudstones and sandstones of
Westphalian age which crop out along the western parts of the Menai Strait were traditionally known as 'Red Measures' but have more recently been designated the Plas Brereton Formation.
Geological structure
Amongst the named geological structures affecting Anglesey are the Berw, Llyn Traffwyl and Porth Nobla faults and Carmel Head and Helin Fen thrusts and the Rhoscolyn Anticline and Porth Penrhyn-mawr Syncline. Recent work (Schofield et al., 2020) recognises the Penmynydd, Aberffraw, Coedana, Porth y Felin and Amlwch terranes.
Quaternary
The most significant of the unconsolidated deposits which have been laid down in the last 2.6 million years are those associated with the last ice age and with post-glacial coastal processes.
Glacial legacy
The island was submerged during the last ice age by a southwest flowing stream of Irish Sea ice which reached a maximum thickness of 1 km. The legacy is a swarm of
drumlin
A drumlin, from the Irish word ("little ridge"), first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or groun ...
s and a thick spread of glacial
till
image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
and sporadic
glacio-fluvial deposits
Fluvioglacial landforms or glaciofluvial landforms are those that result from the associated erosion and deposition of Sediment, sediments caused by Meltwater, glacial meltwater. Glaciers contain suspended sediment loads, much of which is initiall ...
. The Irish Sea-derived till is typically red/brown coloured reflecting the Permo-Triassic sandstone and mudstone bedrock of the seafloor from which the material originated. It also contains fragments from further afield including such lithologies as
granite
Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, 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 coo ...
from
Ailsa Craig
Ailsa Craig (; ) is an island of in the outer Firth of Clyde, west of mainland Scotland, upon which microgranite has long been quarried to make curling stones. The now-uninhabited island comprises the remains of a magmatic pluton formed d ...
and
flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start ...
from
Northern Ireland
Northern Ireland ( ; ) is a Countries of the United Kingdom, part of the United Kingdom in the north-east of the island of Ireland. It has been #Descriptions, variously described as a country, province or region. Northern Ireland shares Repub ...
. In general, ice sourced from the Welsh mainland did not move travel north than the Menai Strait. In places the bedrock has been polished and striated. Global sea levels were suppressed during the ice age and it was only just over 5000 years ago that they had once again risen sufficiently to separate the island from the Welsh mainland.
Post-glacial deposits
The sands which form extensive beaches and dune systems such as that at
Newborough Warren derive from the erosion of glacial deposits and redistribution of the sediment by rising seas and by wind. Submerged forests are found in several places such as at Trearddur, Lligwy and Lleiniog. They were killed by rising sea levels around 6–8000 years ago. Alluvium (sand, silts and gravel) occupies flatter expanses of valleys and innumerable hollows around the island. There are extensive tidal flat deposits between Anglesey and Holy island, at Red Wharf Bay, Malltraeth Sands and Traeth Melynog, the last named being protected by the
sand spit
A spit (cognate with the word for a rotisserie bar) or sandspit is a deposition bar or beach landform off coasts or lake shores. It develops in places where re-entrance occurs, such as at a cove's headlands, by the process of longshore drif ...
of
Menai Point.
Economic geology
Coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal i ...
was
worked on a small scale from the fifteenth to the nineteenth century in the vicinity of Malltraeth Marsh. Igneous and metamorphic rocks continue to be worked at Hengae Quarry near Llangaffo, at Gwalchmai and at Gwyndy quarry near Llanerchymedd whilst limestone is extracted from Rhuddlan Bach and Nant Newydd quarries east of Benllech and from Aber quarry to its north. Moss peat is still exploited at Ynys Isaf near Brynteg.
Building stone
A number of now abandoned quarries have been identified as sources of building stone including those supplying Precambrian stone at Beaumaris and limestone at Penmon at the eastern tip of the island, an also at Bryn Engan, Castell Mawr, Upper Benllech and Moelfre in the vicinity of
Benllech
Benllech () is a village on the Isle of Anglesey in Wales. It is in the community of Llanfair-Mathafarn-Eithaf, which has a population of 3,382. The name of Benllech village had been removed by the time of the 2011 census with the community b ...
. The Lligwy Sandstone was worked at Creigiau to the northwest of Benllech whilst Ordovician sandstones were had at Ballog near Llaneillian. A quarry at Rhoscolyn exploited a
serpentinite
Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...
intrusion whilst further north on Holy Island, the 'Breakwater Quarries' were opened up in the 'Holyhead Quartzite'.
Geoconservation
A Geodiversity Partnership was set up in 2005 to deliver a
Local Geodiversity Action Plan and to secure the island the status of
European Geopark
The European Geoparks Network (EGN) functions as the regional organization of the Global Geoparks Network (GGN) and the UNESCO International Geosciences and Geoparks Programme (UNESCO-IGGP). Its main objective is to ensure cooperation between geop ...
, something which was achieved in 2009. Along with other such Geoparks in the UK, it became a
UNESCO Global Geopark
UNESCO Global Geoparks (UGGp) are geoparks certified by the UNESCO Global Geoparks Council as meeting all the requirements for belonging to the Global Geoparks Network (GGN). The GGN is both a network of geoparks and the agency of the United Nati ...
in 2015.
IUGS geological heritage site
In respect of the site having 'spectacular, accessible and well-preserved exposures of late Neoproterozoic-Cambrian mélange with more than 200 years of study', the
International Union of Geological Sciences
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world.
About
Fo ...
(IUGS) included the 'Ynys Llanddwyn late Neoproterozoic-Cambrian Mélange' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'
See also
*
Geology of the United Kingdom
The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geol ...
*
Geology of Wales
The geology of Wales is complex and varied; its study has been of considerable historical significance in the development of geology as a science. All geological periods from the Cryogenian (late Precambrian) to the Jurassic are represented at o ...
References
{{Geology of Wales , state=expanded
Geology of Wales