Anglesey’s coastal geology – example page

Llanddwyn Island

accessed from the Forestry Comission car park at Newborough, via a walk through the sand dunes or along the sandy beach,  Llanddwyn is a magical place, best seen on a stormy day with the waves raging against wet rocks resplendent in a range of hues, though most prefer it on a hot sunny day when it feels like an island in the Aegean!  This area records an entire plate tectonic story, from the creation of the ocean floor at a mid-ocean ridge, now seen here as pillow lavas, through its journey across the ocean basin where sediments were deposited on it, to its burial and metamorphism as plates collided and the rocks sank down into a deep ocean trench.  Please note that the island is a National Nature Reserve and all hammering and collecting is prohibited (even pebbles!).

Approaching the island, you come first to a world famous exposure (well it features in many geology textbooks) of pillow lava.  This was basalt erupted on the ocean floor at a small constructive plate margin; the lava oozing out as small ‘blobs’ which cooled rapidly in the cold sea water and piled up as separate ‘pillow’ shapes, each with concave bottom as it sagged over the rounded top of an earlier one.  You will see patches of blood red jasper, greyish quartz, lime-green epidote and milky calcite – minerals precipitated from sea water – in the spaces between the oval bodies of dark green basalt. These lavas record the opening of a new section of ocean crust. The best exposures are these isolated sand-blasted rocks before you reach the island, especially the rock face with a small plaque attached, but they are also exposed all the way along the south side of the island.  We are fortunate to be able to see these rocks, they form as part of the ocean crust which is normally re-cycled back into the earth’s interior at a destructive plate margin, however these were broken off and mixed with the sedimentary rocks accumulating against the overlying continent, so we are really privileged to have this insight into deep ocean geology.  Later on, earth movements tilted all the rocks here at Llanddwyn on their side with the top facing to the west.

Cross to the steps on the far side of the causeway where the rocks are a mass of purple and green fragments – the result of explosive activity as red hot lava met the icy cold seawater where the surface ‘froze’ into a glassy skin and then shattered like shrapnel.  These fragmentary rocks have a mixture of shattered remnants and very small pillows. Following this explosive beginning, lava was then able to flow on to the seabed as pillows, rather like toothpaste being squeezed out of a tube.

Climb the steps and follow the path to the cattle grid, then take the right-hand path and go down to the beach via a small gate.  To the right of this sandy inlet are thinly bedded, fine grained, smooth sedimentary rocks (now almost vertical).  These were originally fine muds deposited on the deep ocean floor. Return to the path and cross a small summit, then walk over the grass to peer over the rocks – do not climb down, it’s too steep.  You will see alternating bands of bright green and red rocks – the green is from the later metamorphism of the muds, the red is an iron-rich silica deposit (known as red chert or jasper), typical of deep oceans far away from land.

As you pass the Cross [erected to the memory of St Dwynwen – see later], the cliffs to your right are limestone deposited as calcium carbonate amongst fragments of pillow lavas although they are better seen in the cliff below the lighthouse.  In the middle of the bay to the left of the lighthouse, an isolated mass of rock shows various types of altered pillow lavas amid red jasper and green chlorite-rich material (altered mudstone).  If the tide is low enough, pass through the narrow gap in the rocks to your left to see the jewel in the crown!  (If the tide is too high, follow the grassy path and descend further along). The gap comes out in the next small bay, Porth Twr Bach, which is a simply amazing mixture of colours and textures, christened “mélange” by Edward Greenly, the geologist who first described them.  The dark green lavas, bright rose pink quartzites, purple manganese rich shales, blue-black intrusive dolerite dykes and honey coloured limestones, best seen when wet especially with the sunlight against a dark sky – simply awe inspiring!. Deformation, chemical alteration and metamorphism show that these ocean floor rocks were dragged deep into an ocean trench (subduction zone) as this plate collided with, and dived beneath, another plate. More recent Earth movements have split the mélange and filled the cracks with lava to form dolerite dykes.

There are amazing pebbles on this beach, fragments of most of the rocks you have seen on Llanddwyn, sadly collecting is prohibited as this areas is of world importance.

Return to the main path to the Pilot’s Cottages, built to house pilots and those assisting stricken vessels, but now home to a small exhibition and the temporary summer wardens.  The first lifeboat was stationed here in 1840 but withdrawn in 1907; the current light house dates from 1846 and became automatic in 1972.

Return using the footpath which runs along the spine of the island; the ruin in the centre of the island is a sixteenth century church dedicated to Dwynwen, a 5th century Celtic princess who was unable to marry her sweetheart and chose to devote herself to God, so becoming the patron saint of Welsh lovers and gave her name to the island. This was once a thriving pilgrimage centre, two visits supposedly equalling one to Bardsey, but the reformation ended the tradition and the buildings fell into disrepair. Various small paths to the right overlook raised beaches formed as the land rose after the Ice Age, but please keep to the paths to avoid damaging the fragile sandy vegetation or disturbing ground-nesting birds in spring and early summer.  During summer months various animals might be grazing the abandoned medieval fields, sometimes small Soay or St Kilda sheep and at other times ponies.


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