Unesco Assisted Geopark

Precambrian sites - stratigraphy

Precambrian sites - structural
Lower Palaeozoic sites
Upper Palaeozoic sites
Igneous sites
Quaternary sites
Soil science sites
Historic geology sites

Carmel Head SH 293928
This is the classic British locality for the Carmel Head Thrust plane, one of the best known structures in the Mona Complex of North Wales. Interpretation of the thrust, first described in detail by Greenly, has been the subject of important revision by Bates who has stressed the significance of the structure in controlling Ordovician sedimentation. The coastal section also covers some of the best exposures of gneisses in the Mona Complex (the Gader Inlier), the age and significance of which continue to excite interest and research. The Ordovician rocks beneath the Carmel Head Thrust are of unique interest in providing clear evidence for localised polyphase deformation within the Lower Palaeozoic succession in Anglesey. The Ordovician sediments have been mined for copper. Such a great variety of geological features along this short stretch of coastline makes the Carmel Head area one of the most significant and interesting sections in North Wales.

Carreg yr Allwyn & Henblas SH 426719
These two groups of massive quartzite/chert blocks are of regional geological importance because they represent unusual inclusions in an otherwise matrix-rich green mica-schist from the Gwna Group of Precambrian metamorphic rocks. Two, almost identical, sites are included in this RIGS as the rocks have a similar origin, and
both RIGS stand proud of the Gwna Green-schist in which they are enclosed. At Henblas, the quartzites were previously considered to be large glacial erratics or, a megalith from Neolithic times. They form significant landscape features. At Henblas, the two largest blocks are rough, angular and rise above the ground surface about 3m and 4m, respectively. They are broadly conical and lie on bases with circumferences in excess of 15m. A third block, about 4m square and 1m thick, straddles the others at a low angle. Its underside appears to be ice-smoothed and striated. Beneath these rocks, there appears to be some quartzite which is buried in the ground which could be in situ. Large isolated in situ masses of quartzite/chert occur within 0.5km of the Henblas stones near Cae Mawr Farm. These also stand proud of the surrounding Precambrian green-schists and convincingly appear to be in situ. The quartzites are thought to have been ‘rafted’ onto a mud-rich sediment, an ancient sea bed, when they were dislodged from nearby continental rocks. Later, they were dropped into a deep-sea trench, where they were metamorphosed under moderate pressure. Subsequently, the matrix and its quartzite inclusions, were metamorphosed and converted into green-schist with the harder quartz-rich rock remaining as coherent inclusions.

Castellior Farm SH 535715
This is probably one of the best examples of Precambrian-age glaucophane (crossite)schist to be found anywherein the world. The Castellior Farm site is almost certainly the only known example of the pure navy blue schist, which in places exhibits the outlines of pillows, thus demonstrating its former existence as an igneous underwater pillow lava. The mineral glaucophane is only formed under unusually highpressure/low temperature conditions. The rock has been shown to possess the geochemistry of an ocean-floor basalt and is generally considered to be its metamorphosed equivalent within the Monian Gwna Group. Recent, unpublished research explains its current location as part of an accretionary prism, that attached itself to the base of continental crust along a transcurrent fault. It would therefore be part of a tectonic rather than a stratigraphic sequence. These exposures thus provide important evidence suggesting that plate tectonic processes of similar character, have been operative throughout Phanerozoic time.

Cemaes Bay SH 373940-382950
Cemaes Bay RIGS is of national importance for its unusual sedimentary rock, the Gwna Mélange. This site was described by Greenly (1919) as the type section for this deposit. It is of great international significance as the world type-site for this deposit. It consists of a jumble of blocks, ranging in size from several tens of metres in greatest length, down to those a few centimetres across, consisting mainly of quartzite, limestone and phyllite, in a bewildering matrix of rock types one within another. It is thought to be the product of submarine slumping in a trench at a destructive plate margin. The origin of this rock has been of much debate and various scientists have regarded it as a tectonic breccia or the result of submarine sliding. Cemaes Bay is the best place in Britain to view this rock and see the evidence for the differing views on its origin. The age of the mélange and its relationship to the deformation seen in the adjacent Ordovician deposits are still uncertain. However, recent research (J. M. Horak, pers. com.) has provided dates of 650 to 700 million years for the limestone olistoliths in this deposit, putting these limestone clasts as firmly in the Precambrian and providing the oldest dates for any Precambrian rocks in Wales.

Cerrig Moelion SH 268 772
This site preserves one of the best, and certainly the most accessible, exposures of meta-gabbro and serpentinite in Wales. The interest lies in its excellent exposures ofrare Monian ultramafic rocks. These altered dunites, harzburgites and picrites are important, not only for their great rarity value but also their significance as possible remnants of ancient ocean floor caught up within the Monian subduction complex. Detailed work on the petrography and petrogenesis of these rocks has led to an important and, at present, unresolved dispute whether they do, in fact, represent true ophiolites (suites of ultrabasic rocks consisting of basalts, gabbros and peridotites associated with mud-rich sediments (pelagic) emplaced in the continent during plate collisions). This is the best site for viewing the plutonic, metamorphosed igneous rocks that lie within the New Harbour Group.

Creigiau Cliperau SH 392635
This locality contains the best exposures of pillow lavas from the New harbour Group of Precambrian rocks in Anglesey. The rocks are thought to have been erupted onto the sea bed, probably at an oceanic ridge, prior to their translocation and descent into a deep-sea trench. They are important for showing the effects of regional metamorphism on the lavas and sediments in which they are enclosed. Little of the original pillow structure remains to be seen, but traces of the pillow shape can bepicked out in the otherwise flattened and distended layers of attractive purple and green rocks. Apart from the physical alteration, the mineralogy comprises assemblages of chlorite, calcite and epidote. The rocks originated as slices of oceanfloor basalt which were scraped off a descending plate as it dropped down into an oceanic trench.

Ffynnon Badrig Grid Reference not disclosed
Ffynnon Badrig Quarry is of international and national (Great Britain) importance for its fossiliferous deposits of Precambrian age. The pseudo-oolitic limestones in Cemaes Bay are found in association with the Gwna Mélange of this area and have been found to contain singular and multiple rounded bodies identified as vesiculites, (originally cyanobacteria) and some filaments. This quarry is of historic importance as it was here that the limestones were first recognised as containing fossils. The encompassing mélange is a mud-rich sediment containing rounded boulders of varying size, which accumulated on the sea floor adjacent to a land mass. Large masses of rock, such as limestone and quartzite, became incorporated in the muddy matrix as discrete masses, some of them as large as 200m across. These large inclusions are termed olistoliths. The olistoliths and host rock eventually descended into a subduction zone (trench), where they became attached to the inner (continental) wall of a trench to become part of the Monian subduction complex. The limestone, in particular, appears to have been fairly mobile as it was often underlain by graphite schist, that acted as a lubricant, allowing the limestone to ‘float’ on the surface of the mélange. This ensured that it retained its sedimentary features and did not suffer from metamorphic change. Importantly, these rocks contain an assemblage of microfossils. Layered stromatolites, filaments and spheroidal forms have been found. Wood and Nicholls described the layers as stromatolites and the spheroidal bodies as Vesicularites (algal floating or rolling bodies) which may represent bacterial forms. They were described as Precambrian in age but later workers thought they could be Cambrian or even Ordovician fossils. However, new isotope dates have shown them to be between 650 and 700 million years old. This information has a huge bearing on the so-called sedimentary succession described by Greenly and Shackleton and recognises the Gwna succession as the oldest sedimentary group in North Wales. Ongoing research identifies these exposures to be part of a tectonic rather than a stratigraphic sequence. In addition, these rocks provide important evidence about the
nature and depth of the sea floor and the rare fossils are evidence of a time prior to the ‘explosion’ of life forms which developed during early Cambrian times.

Gadlys Quarry Grid reference not disclosed
Gadlys Quarry is of national (Great Britain) importance for its fossiliferous deposits of Precambrian age. The stromatolitic (algal) limestones in Cemaes Bay are found inassociation with the Gwna Mélange of this area. The mélange is a mud-rich sediment containing rounded boulders of varying size which accumulated on the sea floor adjacent to a land mass. Large masses of rock, such as limestone and quartzite, became incorporated in the muddy matrix as discrete masses, some of them as large as 200m across. These large inclusions are termed olistoliths. The olistoliths and host rock eventually descended into a subduction zone (trench), where they became attached to the inner (continental) wall of a trench to become part of the Monian subduction complex. The limestone, in particular, appears to have been fairly mobile as it was often underlain by graphite schist, that acted as a lubricant, allowing the limestone to ‘float’ on the surface of the mélange. This ensured that it retained its sedimentary features and did not suffer from metamorphic change. Importantly, these rocks contain an assemblage of microfossils. Layered stromatolites, filaments and spheroidal forms have been found. Wood and Nicholls described the layers as stromatolites and the spheroidal bodies as Vesicularites (algal floating or rolling bodies) which may represent bacterial forms. They were described as Precambrian in age but later workers thought they could be Cambrian or even Ordovician fossils. However, new isotope dates have shown them to be between 650 and 700 million years old. This information has a huge bearing on the so-called sedimentary succession described by Greenly and Shackleton and recognises the Gwna succession as the oldest sedimentary group in North Wales. Ongoing research identifies these exposures to be part of a tectonic rather than a stratigraphic sequence. In addition these rocks provide important evidence about the nature and depth of the sea floor. The rare fossils are evidence from a time prior to the ‘explosion’ of life forms during early Cambrian times.

Gwalchmai Quarry SH 379766-381773
Gwlachmai Quarry provides excellent exposures of the Coedana Granite and is of national importance. The Coedana Granite is one of five units constituting the Precambrian Coedana Complex and comprises the most extensive outcrop of plutonic igneous rock within the Monian Composite Terrane. This quarry contains three discrete facies, namely: 1. Porphyritic granite (large crystals within a matrix of smaller crystals) which were not exposed at the time of writing the GCR report but which are now visible at the north-eastern side of the quarry. This facies continues outside the quarried area, in the ridge to the east of Cerrig Moelion. 2. Nonporphyritic granite can be seen in various parts of the quarry but originally it formed the bulk of the original outcrop which has been largely extracted. 3. Aplite veins (fine-grained granite consisting of quartz and felspar) which were intruded into the granite mass at a late stage of granite emplacement. Original textures have been largely overprinted by a later phase of ductile (plastic) deformation that caused extensive recrystallization and the development of an impersistent rodding fabric and foliation. Geochemical data indicate that the Coedana Granite can be classified as a monzogranite (granite containing the dark mineral augite), which formed within a volcanic arc setting. A U-Pb age of 614 +/- 14Ma confirms a late Precambrian age of crystallization. The types of minerals in the Central Anglesey Shear Zone along the south-eastern margin of the granite suggest that the Coedana Complex may represent a sliver of the main Avalonian arc. This sliver was transported an unknown distance south-westwards along the Avalonian subduction complex into its present position. The granites are thought to have been derived from the melting of much older crustal rocks (1,330 - 1,443 Ma) over 30km beneath the Earth’s surface over 600 Ma ago. This site is important for magmatic studies and has great research potential.

Holyhead Breakwater Park SH 224836-SH 235837
This composite site stretches from the Holyhead Quartzite Formation which forms the Headland to the west of Breakwater Park, passing over the other rocks of the South Stack Group and finally to the mica-schists of the New Harbour Group at the Breakwater in Holyhead Harbour to the east. It includes the important worm burrows to be found in the meta-sandstones in the cliffs and hillsides in this area. The rocks have been regionally metamorphosed and faulting and thrusting is well displayed along the entire section. In addition, the chemical and physical weathering along the coast has resulted in stacks and an arch as well as many sections of wave-cut platforms, interrupted by the many faults and thrust planes to be found in the section. During the Tertiary Era, basaltic lava was injected into the rocks and a spectacular dyke can be seen in Porth Namarch which displays magnificent spheroidal weathering. The western side of this small inlet displays an excellent example of a thrust fault with mylonisation occurring at the base of the thrust.

Llanbadrig Point Grid reference not disclosed
Llanbadrig Point is of national importance for its unusual mineralogy which could be directly linked to the fossiliferous Ffynnon Badrig and Newborough Forest and Llanddwyn RIGS. The rock is a siderite (calcium- and iron-rich mineral) which occurs as an olistolith (large inclusion) in the Gwna Mélange adjacent to the fossilrich limestone of Ffynnon Badrig, some 50m away. Such iron deposits have proved to be crucial to the understanding of Precambrian rocks worldwide. They are indicative of sea-floor conditions, sea-water composition and oxygen levels in the atmosphere during Precambrian times. This particular exposure is crucial to the origin of the jaspers (iron-rich silicate minerals, also known as cherts) which have been described in the literature as containing ‘ghost’ carbonate rhombs. This shows that the siderite was originally a calcium-rich rock that has suffered total replacement of the carbonate by iron and silica. If this is the case, then this single outcrop provides the missing linkbetween the fossiliferous limestones of Cemaes and the jaspers of Llanddwyn and other parts of the Gwna where jasper occurs. It also has a bearing on the age of other Gwna deposits as they could then be linked to the known age for the stromatolitic limestones of the Cemaes Bay area. Sections of the rock do contain the traces of fossils from the parent limestone, which could explain the poor preservation of fossils identified in the jasper of Llanddwyn. This site is a crucial piece of the jigsaw in understanding the fossils and minerals in the Gwna and their relationship to each other.

Llanfair PG Bypass SH 538721-530723
This site was exposed during excavations to construct a bypass of Llanfair P.G. as part of the A55 trunk road that crosses Anglesey from Menai Bridge to Holyhead. It consists of three, 20m-long exposures of unweathered glaucophane schist. This rare rock, originally a deep-sea basalt (pillow lava), was metamorphosed at great depth below the Earth’s surface as it descended at a destructive plate margin, into a deep-sea trench and beneath the continental crust. This put the rock under the enormous pressure that converted the original mica, hornblende, olivine, augite and feldspar minerals into the rare glaucophane/crossite amphibole schists. Unusually, theseminerals indicate that the rocks did not undergo significant heating at these depths and therefore provide important information about the environment in which these rocks formed. It is thought that these are the only unweathered examples in Britain.

Llansadwrn Wall SH 558771
Important blue schist specimens can be seen as part of a wall at the crossroads of the B5109 with the Llanddona Road in Llansadwrn. The site is important because the rocks contain the largest known crystals in the blue schists of Anglesey. This rare rock, originally a deep-sea basalt (pillow lava), was metamorphosed at great depth below the Earth’s surface as it descended at a destructive plate margin, into a deep-sea trench and beneath the continental crust. This put the rock under the enormous pressure that converted the original mica, hornblende, olivine, augite and felspar minerals into the rare glaucophane/crossite amphibole schists. Unusually, these minerals indicate that the rocks did not undergo significant heating at these depths and therefore provide important information about the environment in which these rocks formed. It is thought that these are best examples of large crystals in glaucophane schist in Anglesey. The wall has been built some two miles south of the Mynydd Llwidiarth, a glaucophane schist outcrop near Pentraeth.

Maen Gwyn SH 425825
Maen Gwyn is of national geological importance because it provides two components of the Coedana Granite Complex (CGC) at their contact, namely the Coedana Granite and the host rock (hornfels). It is one of a series of RIGS in Anglesey selected to demonstrate varying pressure and temperature conditions during metamorphism of the country rocks (facies) by the Coedana Granite. The site consists of a small rocky knoll to the north-west of the farm house. There are two types of ‘baked rocks’ (hornfels) - one, a steel grey, fine-grained variety and the other with visible white mica (muscovite) flakes. The hornfels represent a quartz-felspar-rich sedimentary rock which records two pre-intrusion episodes, firstly, the original sedimentary layering and, secondly, a tectonic layering sub-parallel to the former. These country rocks also contain chlorite, muscovite and biotite minerals, indicating that they have been subjected to low temperatures (250-400) schist facies. Ages of 596-598 Ma have been given for the metamorphism of the micas, after the intrusion of the granite at around 614Ma, proving that the hornfels are Precambrian in age. The hornfels is cut by veins and larger patches of Coedana Granite which have suffered brittle deformation. The granites are thought to have been derived from much older crustal rocks (1,330 - 1,443 Ma) that melted over 30km beneath the Earth’s surface. The rock here contrasts with the high-grade metamorphic gneisses seen at Tyddyn Gyrfer RIGS. The rocks exposed at this site therefore have an important bearing on the evolution of the Coedana Complex and, in particular, on the thermal and deformation history of the country rocks into which the granite was emplaced. This site has great research potential.

Marquis of Anglesey Column SH 535715
This is the classic British locality, of worldwide international importance, for ‘glaucophane’(crossite) schists – rocks thought to have formed under unusually high pressure/low temperature conditions, deep within a Monian subduction complex. The rocks have been shown to possess the geochemistry of ocean-floor basalts and are generally considered to be the metamorphosed equivalents within the Monian Gwna Group. Any local stratigraphical significance of these exposures is, however, transcended by their position as probably the oldest-known glaucophane rocks in the world. More recent unpublished research explains their current location as part of an accretionary prism that attached itself to the base of continental crust along a transcurrent fault. It would therefore be part of a tectonic rather than a stratigraphic sequence. These exposures thus provide important evidence suggesting plate tectonic processes of similar character have been operative throughout Phanerozoic time.

Please note that the inclusion of sites in these listings in no way allows access to the sites described. The information is merely a scientific description. Many of the sites are Sites of Special Scientific Interest (SSSI), and/or on private land where permission to visit could be denied, or the site is unsuitable for safety or other reasons. Access to any site which is not in the public domain is entirely at the discretion of site owners. In no case is the collection and removal of specimens allowed from any site. Bona fide researchers require a permit from CCW to collect any specimens from GCR sites (SSSI) and these are generally allowed only under supervision. Queries regarding any of these sites should be directed to COFNOD, Parc Menai, Bangor. They will be able to answer any queries in the near future when all the relevant information is in their hands.