Anglesey has a great variety of rock types and ages and the complex and beautifully exposed structures have made Anglesey a classic but often controversial area of British geology. The island is used extensively for geological education and research and has recently been awarded European GeoPark status. As the topography is subdued, solid rock exposures are scarce over much of the island, with the exception of the coastline. Most of the land surface is covered with Quaternary glacial deposits but the solid exposures that occur reveal that two thirds of Anglesey is underlain by complexly folded and often metamorphically altered rocks known as the “Mona Complex”. These were described in great detail by Edward Greenly in 1919 and have, in recent years, been the focus of some academic controversy.
The Mona Complex contains rocks ranging from bedded sedimentary rock to highly metamorphosed coarse grained gneiss. Greenly distinguished the succession of sedimentary rocks, which he called the Bedded Series, from the metamorphic and igneous rocks.
The Bedded Series includes a variety of rock types; at the base, the South Stack Group includes sandstones and interbedded shales, contorted by magnificent folds and crumples, which are superbly exposed in the cliffs of Glannau Ynys Gybi SSSI at South Stack (visible from the steps down to the lighthouse). Above these strata is the thick, rather pure Holyhead Quartzite which forms the bulk of Holyhead Mountain. The New Harbour Group supersedes the quartzite and consists of an alternation of fine grits and shales with beds of jasper and spilitic lava. Recent work suggests that this group may have been emplaced in its present position by large scale earth movements and is thus not a part of the normal sedimentary succession; these ideas are still controversial, however. Vital evidence concerning these relationships can be found in the cliffs at Glannau Rhoscolyn SSSI and detailed in a booklet published by Gwynedd and Mon RIGS.
Above the New Harbour Group lie the formations of the Gwna Melange. This famous group consists of a chaotic agglomeration of blocks, strips and lenticels of all sizes of limestone, quartzite, lava, jasper, schist, etc., set in a schistose matrix of micas and chlorite. This is considered now to have formed by submarine sliding of large volumes of unconsolidated or partly consolidated sediment on an inclined sea floor, possibly triggered by earthquake shocks, and is known as an oilstostrome. Fine exposures of the Gwna Melange occur in the cliffs of the Llanbadrig - Dinas Gynfor SSSI near Cemaes. The age of these sediments has traditionally been considered to be Pre-Cambrian but recent study of microfossils suggests that they may be at least partly Cambrian.
In between the various formations of the Bedded Series occur several important units of volcanic lavas and tuffs interbedded with the sediments. Excellent exposures of basaltic pillow lava of this age occur as outcrops on the beach at Ynys Llanddwyn within the Newborough Warren - Ynys Llanddwyn SSSI.
The Mona Complex rocks become progressively altered by metamorphism in a south-easterly direction and in the southern part of the island, near the Menai Straits, occur the well-known blueschists, the largest outcrops of rock of this type in Britain. The Mona Complex gneisses are coarsely recrystallised metamorphic rocks with a complex structure; some authors consider these to be the basement to the Bedded Series but others see them as their metamorphosed equivalents. The metamorphism and deformation of the Mona Complex took place before the deposition of the Ordovician sediments, probably during Cambrian or late Pre-Cambrian times.
Lying with marked discordance on the Mona Complex are extensive sedimentary formations of Ordovician age. These form a number of isolated outliers of which the largest, centred on Llanerchymedd, covers a Y shaped area extending the full width of the island. The Ordovician sediments are principally conglomerates, breccias, sandstones and shales. These rocks were deformed and folded during the Caledonian orogeny.
Above the Ordovician rocks lie Devonian Old Red Sandstone deposits which are exposed on the north-east coast around Dulas Bay and Lligwy Bay.
A large proportion of the southern half of the island is underlain by Lower Carboniferous rocks. These are principally limestones with basal conglomerates and sandstones; a classic section is exposed in the cliffs and quarry of Trwyn Dwlban SSSI. Small amounts of Millstone Grit overlie these limestones and in places these overstep the latter and the Ordovician to lie directly on the Mona Complex.
During the Pleistocene, Anglesey was overrun by Irish Sea ice from the north-east on a number of occasions but was only marginally affected by Welsh Ice along its south-east shore. Coastal till sections are important in determining Pleistocene ice movements across the island, bearing as they do a characteristic suite of Northern erratics. Important Pleistocene sites are Henborth, Lleiniog (part of Glannau Penmon Biwmaris SSSI) and Trwyn Dwllban SSSI. An interactive map of the glacial landforms of Anglesey is available at the British Geological Survey website.
The Anglesey coastline is extremely varied as a result of the differential erosion of the many distinctive rock types which occur. The north-west and northern coast is rocky and consists of low cliffs separated by small bays whilst the north-east coastline is gentler, the bays wider and deposition predominates in sheltered areas. Along the south-west facing coast, extensive sand dune systems have built up, demonstrating both the contemporary landforms and the geosystem processes that created them. Of particular importance are Tywyn Aberffraw and the Newborough Warren - Ynys Llanddwyn SSSI, notable for their fine coastal landforms, including beaches, shingle spits, parabolic dunes and the intervening wind scoured damp slacks.
Despite the apparent contradiction of a threat to something as enduring as the rocks under our feet, important rock exposures, deposits and landforms continue to suffer unnecessary damage. Smothering of rock faces by waste material or well-intentioned "landscaping", the construction of sea defences and large-scale afforestation have all taken their toll on important earth science features on Anglesey over the years.
The conservation of Anglesey's Earth Science heritage has been addressed in part through the inclusion of nationally important sites and features, identified in the Geological Conservation Review (GCR) within the series of sites of special scientific interest (SSSI) and in a growing list of (non-statutory) Regionally Important Geological (and geomorphological) Sites (RIGS). The creation of the Anglesey Geopark designation, will help to promote the interpretation, understanding and conservation of earth science features.
Greenly, E. (1919) Geology of Anglesey - Memoir Geol. Survey. 2 Vols
Smith, B.. George, T.N. (1976) British Regional Geology: North Wales. 3rd ed. HMSO, London.