Anglesey--NW

See NW Wales introduction for a Time Scale and outline of the geology of NW Wales

ANGLESEY (Mon)

The main roads to Anglesey pass through the rugged alpine mountains of Snowdonia and out onto a low plain at Menai Strait. Compared to the spectacular views in the mountains, the island of Anglesey seems featureless and flat. However, this green and rolling landscape, which supports sheep and mixed farming, hides a long and complex geologic history.

Large faults divide the island into three blocks of very different Precambrian to early Cambrian rocks. The southern block contains volcanic rocks and sedimentary rocks deposited on the sea floor. The central block contains schist, gneiss, and granite that date from the continental Cadomian collision of about 600 million years ago. The northern block contains sedimentary rocks, volcanic ash and sandstone, crumpled during the Caledonian collision. Ordovician and younger rocks, including the Old Red Sandstone and Carboniferous Limestone, drape over these blocks in places.

Glaciers scoured and deepened the Menai Strait during the ice ages, but faults dictated its northeast direction. The faults crushed the rocks, making them easier to erode, and so guided the stream that first eroded a valley and the ice that later reamed it out into the strait. These crushed rocks, called mylonite, are exposed in the low cliffs along the promenade below the Anglesey Arms near the Menai Strait Road Bridge. Look for particularly fine mylonites on the island connected to Anglesey by a causeway.

Between Menai Bridge and Holyhead, the A5 follows the Roman road of Watling Street, and crosses a grassy plain about 100 metres above sea level, cut by a few small streams. The road passes many glacial features, and a few exposures of Precambrian granite.

Before crossing the island from Menai Bridge, you may want to stop at the town with the longest name in the world: Llanfairpwllgwyngyll-gogerychwyrndrobwll-llantysiliogogogoch, otherwise known as Llanfair.P.G (252 372). Immediately beyond the Menai Bridge, take the turnoff to Llanfair.P.G.. On your right is a statue of the Marquis of Anglesey (253 371) standing atop a large column (Fig. 3). He lost a leg at the battle of Waterloo.

The rocks near the base of the column are dark gray metamorphic rocks, rather nondescript, called blueschist. This may be the oldest blueschist in the world, and are Cambrian in age.

Some bands in the rock are rich in the blue mineral glaucophane, and the yellowish green mineral epidote. Glaucophane forms in the laboratory only under experimental conditions of very high pressure and rather low temperature, a rare combination of conditions in the crust. The most likely place for these conditions is thought to be in slabs of cold oceanic crust rapidly sinking through an oceanic trench into the earth's mantle. The slab sinks deep enough to raise the pressure high enough for glaucophane to crystallize before it can soak up enough heat in the surrounding crust to raise the temperature too high. Then, for the glaucophane to survive, the rocks must return to the surface before they get too hot. So making blueschist is quite a trick, which explains why the rock is so rare.

Excellent exposures of the Precambrian Coedana granite occur in the quarry and road cuttings near the A5 just west of Gwalchmai (239 376, Fig. 3). The granite intruded as molten rock, and crystallized about 610 million years ago, during the Cadomian continental collision. The road north of Gwalchmai crosses a treeless plain complete with sheep and wheat fields.

Fig. 5. Ice flowing over rock outcrops smoothes the upstream side and plucks blocks off the downstream side.

Glaciers scraped and plucked many of the bedrock exposures. Watch for outcrops that the moving ice rasped into the streamlined shapes of whales (Fig. 5). Glaciers are full of gritty sediment, so they sandpaper bedrock outcrops as they grind across them. The long dimensions of the whales trend from northeast to southwest, the direction of ice flow, and their northeast ends, which faced into the flow of the ice, slope gently. The head of the whale is at the southwest end, where the glacier plucked off blocks along joints in the rock, so the whales are swimming in the same direction the ice was moving.

The Southwest Coast Road between Menai Bridge (255 372) and Rhosneigr (232 373). 23 miles (37 km)

The A4080 between Menai Bridge and Newborough crosses flat farmland underlain by Carboniferous Limestone deposited when tropical seas flooded the land after erosion had reduced the Caledonian hills to a plain. You can see the limestone in the low cliffs along the south coast of the island where they lie on the ancient basement of Precambrian rocks.

Fig. 6. Precambrian lava flow. The irregular blobs are lava pillows that bulge out when lava erupts under water. Llanddwyn Island.

Newborough Warren has a magnificent sand dune complex in a nature reserve near the beach. Strong winds blew the sand inland rather late in Pleistocene time and dammed the small river to make the Malltraeth Marsh.

Llanddwyn Island (239 363) has the best exposures of Precambrian rocks in the southern block of Anglesey. (The section is of special scientific interest and visitors are requested not to hammer on any of the outcrops, nor collect specimens.) The exposures are a kilometre north along the shore from the car park in Newborough Warren. The rocks are basalt pillow lava of the Gwna Group (Fig. 6), which erupted beneath the late Precambrian to early Cambrian sea.

The erupting lava chilled quickly against the water to make a glassy rind. Individual blobs of lava expanded like a balloon as more lava injected, eventually breaking through it to make another balloon. The balloons are called pillows, although they are very uncomfortable to lie on. They nested like squashy balls against one another and settled into the gaps in the underlying pillows. This arrangement creates rounded tops and pointed bottoms. This distinctive shape distinguishes the top and bottom of the flow, a useful test when the layers are tilted into their present vertical position. The tilting accompanied the collision of Avalon and North America.

The causeway to the island has fine exposures of volcanic breccia, a collection of fragments that built up in front of an advancing lava front. Bacteria extracted dissolved silica from seawater to make the red chert between the volcanic blocks. Other rocks include limestone and muddy sandstones. All collected on the ocean floor.

The A4080 crosses the Malltraeth Valley between Newborough and Rhosneigr. The valley was a major meltwater channel when the great ice sheets were melting at the end of the last ice age, and presumably earlier ice ages. The wind blows sand off the river floodplain into impressive tracts of sand dunes on the valley floor.

The Berw Fault (Fig. 2) runs along the south side of the Malltraeth Valley. Movement along it during the Hercynian collision of late Carboniferous time raised the deeply buried Precambrian schist to the north and brought them into contact with the lavas of the southern block. Although Malltraeth (239 369) stands on the Precambrian schist and gneiss north of the fault, its walls and houses are built of battleship grey Carboniferous Limestone brought in from the southern block.

The rocks on the seashore at Porth Trecastell (233 371, Fig. 3) are Precambrian schist, rich in quartz, and a few bands of limestone. Some of the schist contains graphite, a slippery form of carbon that crystallizes under the same high temperatures that metamorphosed the schist. The source of carbon was probably algae growing in the sedimentary environment before the rocks were buried and heated.

The rocks here are much broken, no doubt because they are so near the major faults that separate the southern lavas from the middle block of schist and gneiss. These are tired rocks. They were buried deeply in Precambrian time, when they flowed like honey and changed from mud and sand to schist. Later, the Caledonian and Hercynian collisions raised the rocks into the upper levels of the crust. They were cool by then, and brittle. When the blocks moved, the rocks broke instead of flowing as they did while they were hot. Now they are shot through with fractures, and look as though they were squeezed through a mangle.

We do not know where these gneisses and schist came from. The central block of Anglesey is very different from the volcanic and sedimentary rocks to the north and south. Some geologists think it is a block of older continental crust carried to its present position by horizontal movements along the large faults that separate the blocks of Anglesey.

The road crosses the fault zone at Porth Nobla (233 372, Fig. 3). Here the mass of molten magma that crystallized into the Coedana granite baked and hardened the rocks. That made them more resistant to the forces that severely fractured and crushed the rock farther south.

The road between Porth Nobla and Rhosneigr crosses the boundary between the middle and northern blocks of Anglesey. Ordovician mudstone and siltstone lie on the Precambrian schists of the northern block. Although soil covers the contact between the Precambrian and Ordovician rocks, you can see Ordovician slate along the shore at Rhosneigr (233 373, Fig. 3). The closing of the Iapetus Ocean in Silurian time compressed and slightly heated the original mudstone, converting it to slate. Look for spectacular small folds in the thin sandy layers. The nearly vertical slaty cleavage bisects the opposing limbs of the folds, showing that folds and the fractures in the slate are related and both grew at right angles to the compression. The A4080 turns inland at Rhosneigr, and joins the A5 just northwest of a quarry in the Precambrian Coedana granite that dates from the 600 million-year-old Cadomian continental collision that thickened and heated the crust and melted some of it. Several road cuttings expose the granite near the quarry.

Soil and glacial till blanket the surface along the A5 northwest to Holy Island and Holyhead. The sheep outnumber the rocks.

Precambrian schist of the northern block underlies Holy Island. They were laid down as mud, sand, and volcanic ash, then transformed into schist when they were squeezed and heated during the Cadomian collision of late Precambrian time, and also by the Caledonian collision. A flaky metamorphic mineral, chlorite, provides their dull green colour. Look for these greenschists around Holyhead harbour, and on the steps that descend from the car park to the lighthouse at South Stack (220 382).

Stacks are offshore rocks or small islands left standing in the surf as the waves eroded the sea cliff inland. Spectacular folds are preserved in cross section in the rocks of South Stack. The large, tight folds resemble a giant accordion.

South Stack is also a good place to look south-east across the flat plain of Anglesey. On a clear day, you can see the Lleyn Peninsula and the mountains of Snowdonia.

Fig. 9. South Stack, Holy Island. The folds in the cliffs below the lighthouse are the younger of the two sets in the section.

Although you cannot get onto South Stack (Fig. 9), you can get a closer view of the folds by following the coast road from South Stack to Treaddur. The cliffs expose strongly folded sedimentary and volcanic rocks. For a challenging puzzle, explore the cliffs below the Cliff Hotel (225 379) north of Treaddur (Fig. 10). The rocks are volcanic ash that was heated, squeezed, and folded several times. They now resemble rolled puff pastry.

Fig. 10. Early folds in volcanic ash below the hotel at Treaddur Bay. View is 1.5 metres wide.

Most of the Precambrian rocks of Holy Island are schist recrystallized from mudstone and shale. However, on Holyhead Mountain (Fig. 7) (222 382) the original sediment was a pure quartz sand, which metamorphosed into a massive, sparkling white, quartzite.

The coast road, A5025, around the north and east side of Anglesey crosses grazing land on Precambrian schist. Many of the schists have a distinctive green tinge, which they owe mainly to the mineral chlorite. The best exposures are at Cemaes Bay and Amlwch.

Fig. 11. Geologic map of northern Anglesey. The Carmel Head fault thrusts Precambrian rocks over Ordovician rocks during the Caledonian collision that closed the Iapetus Ocean.

A side trip to Carmel Head (229 393) along back roads brings you to the only large thrust fault in the Welsh Caledonian collision fold belt. Thrust faults dip at low angles, and raise older rocks from below onto younger rocks; in this case the Carmel Head thrust places Precambrian schist on Ordovician rocks. Such thrusts are common in collisional mountain belts and are one of the ways that rocks respond to the squeezing. Thrust faults are common in Scotland, where the effects of the Caledonian collision are strongest. But Wales was outside the main collision zone, so the main effects are folding and the formation of cleavage in the mudstone.

The Carmel Head thrust is almost horizontal and is exposed along about a half kilometre of cliff. The upper parts of the cliffs expose Precambrian chlorite schist, similar to that at Holyhead; the Ordovician rocks below the thrust fault are conglomerate, sandstone and black shale. Some mixed up limestone and sandstone are probably Cambrian Gwna melange similar to that at Cemaes Bay (Fig. 11).

Amazing rocks appear on the northeast side of Cemaes Bay harbour, just below high water mark. The cliffs expose the Gwna Melange, a chaotic jumble of large blocks of limestone and sandstone as much as five metres long and which lie in a schist matrix. Indeed, a quarry just east of the village works an enormous block of limestone embedded in the schist. (The Gwna Melange is of special scientific interest and visitors are requested not to hammer on any of the outcrops, nor collect specimens.) Most geologists invoke some process of submarine landslides to create the mixture, while the rocks were still soft sediments. These slumped rocks also contain chert, blocks of lava, and some serpentine, carried in by the plate sinking beneath the continental margin.

Fig. 12. Geologic map of Cemaes Bay. The late Precambrian to Cambrian limestone slumped into a trench, leaving a wild mixture of rocks. Ordovician rocks appear in narrow faulted slivers.

The limestones are especially interesting because they can tell us something about the life of the time. Geologists have found plenty of evidence of primitive algal mounds, called stromatolites, and bacterial filaments in the limestone, but no sign of animals. The stromatolites appear to be of Late Precambrian or very Early Cambrian age.

About a kilometre east of Cemaes Bay, a road leads north to Llanbadrig Church (238 394), a fifth century building with a much newer roof. Most of the gravestones are cut from fine purple slate of Cambrian age from North Wales. Some show green spots about one to two centimetres in size, shaped like small rugby balls. They were spherical until the slate was squeezed during the Caledonian collision. The short axes of the squashed spots lies at right angles to the slaty cleavage, showing that the cleavage fractures grew at right angles to the compression.

The headland above the church shows many large limestone blocks poking up through the grassy slopes. Like the limestone blocks in Cemaes Bay, they are embedded in schist that easily weathers to a soil that supports the grass. A large block of white quartz sandstone pokes through the grass at the west end of the headland, and others appear in the cliff. From the high point on the headland, you can see the limestone quarry to the southwest. Rocks east of the headland, and beyond a small bay, are Ordovician slate and sandstone that lie on the Precambrian rocks. The Ordovician rocks are exposed along the cliff path; they were folded during the Caledonian collision.

The A5025 passes the Wylfa nuclear power station just west of Amlwch (245 393). Amlwch stands on strongly folded Ordovician slate.

The Parys Mountain copper mines (244 391) lie south of Amlwch on the B5111. The Romans worked these mines and they were opened again in 1750 and 1768 when lead sulphate (occurring as anglesite that was first named from this locality) was mined at the surface. Copper sulphide ores lay deeper and were developed into the largest copper mines in Europe in the eighteenth century, when they dominated the British and European markets. The mines employed twelve hundred people then, and the ore was smelted in Amlwch. Production declined in the latter half of the nineteenth century, as competition from European mines became more intense. The last mine closed before the First World War, but a new shaft near the road shows renewed interest in the ore bodies.

Fig. 13. Parys mine is in the core of a tight fold in Ordovician volcanic rocks.

The Parys Mountain copper deposits were probably deposited near submarine hot springs rising from volcanic rocks. Hot solutions rich in copper gushed into cold seawater. The copper precipitated around the spring. Similar springs now exist on the East Pacific rise, where lava erupts along the boundary between two separating plates.

The copper at Parys Mountain occurs in broken volcanic rocks, rhyolite, that form thick layers within the Ordovician slate. Fractures in the volcanic rocks provided avenues for the mineralizing solutions and space for the ore veins to form. The mines are worth a visit, even though you are unlikely to find any good mineral specimens. Take the path from the B5111 north of the new shaft. The view from above the main pits is like a lunar landscape.

Careful geologic work has revealed tight Caledonian collision folds in the slate and volcanic rocks in the mine. However, the rocks are so intensely altered and deeply weathered that the folds are hard to see. The acid water from the weathering of the sulphide ore minerals reacted with the rocks, converting them to red and black clays.

Southeast of Parys Mountain are the rugged hills of Mynydd Bodofon (247 385) eroded in hard quartzite of the Devonian Old Red Sandstone that rests directly on Precambrian rocks. If you want to see finer-grained sediments of the Old Red Sandstone you must go to Lligwy Bay (250 387), just north of Moelfre, where the low cliffs expose Devonian mudstone. The rocks are gently folded and show a weak cleavage, telling us that the Caledonian collision continued into early Devonian time in northern Wales. Just south of Mynydd Bodofon, the road passes onto gentler grassland developed on Carboniferous Limestone laid down on top of the Old Red Sandstone after the end of the Caledonian collision when tropical sea invaded the land.

Fig. 14. Geologic map of southeast Anglesey where Ordovician slates overlies Precambrian lavas. Carboniferous Limestone lies on the Ordovician slate.

It is worth a side trip into Moelfre (251 386, Fig. 14) to see the limestone in the cliffs around the harbour. South of Moelfre, the different rock units were so intricately faulted together during the Hercynian collision that it is difficult to sort them out.

The cliffs at Red Wharf Bay have excellent exposures of limestone laid down in the equatorial seas of Carboniferous time. The cliffs are not obvious from the village, but lie north along the beach from the hotel. This limestone is typical of the grey Carboniferous Limestone; it is well layered in parts and rich in fossils.

About one kilometre north of the hotel at Red Wharf Bay, just below a campsite, are some impressive sandstone pillars in the limestone (Figs. 15 & 16). Some are over a metre in diameter and more than two metres tall. Geologists believe the pillars fill sinkholes dissolved in the limestone when it was raised above sea level; later, sand filled the sinkholes.

The top of the limestone is a raised beach, about three metres above present sea level, cut into the limestone during an interglacial period about 120,000 years ago when sea level was higher than at present. Cold conditions returned about 100,000 years ago and frost shattering produced light-coloured debris that plasters the cliffs above the raised beach. This debris, called head, contains rock fragments shattered by frost, which then flowed over the raised beach during the spring thaws. Glaciers re-advanced from the Irish Sea and laid down a red till above the head.

Fig. 15.Left hand picture: Large sandstone pipes in Carboniferous Limestone on the wave-cut platform one kilometre north of Red Wharf Bay.

Fig. 16. Right hand picture: A section of a large pipe in the limestone. The cave, which was the lower part of the pipe, is more than two metres high and about one metre wide. The sand has recently washed out of it.

South of Red Wharf Bay, the road climbs across the Berw Fault back onto the Precambrian rocks of the southern block. These are altered volcanic rocks, buried beneath the soil. The road descends into Beaumaris after crossing another fault that brings the road back onto Ordovician slate. Slate erodes more easily than the volcanic rocks, resulting in the flat coastal plain. North of Beaumaris, the soft slate lies beneath the recent wave cut platform. Behind the platform are ancient sea cliffs cut into harder Carboniferous Limestone that lies unconformably on the folded Ordovician slate. Puffin Island to the east is a stack of that resistant limestone. On a clear day, you can see the limestone cliffs of Great Ormes Head, ten kilometres to the east.

The road from Beaumaris to Menai Bridge is narrow and mostly hugs the cliff gouged out by glacial erosion of the Menai Strait. The road cuttings are largely in Precambrian basalt that erupted on the sea floor. You can see the basalt well exposed in a good outcrop opposite a small layby (259 375) about a kilometre southwest of Beaumarais. The basalt contains good pillows, which show that the lava extruded underwater. These pillows are not as obvious as those are at Llanddwyn Island near Newborough Warren (Fig.3).

THE ROADSIDE GEOLOGY OF WALES

ANGLESEY (Mon)