Geotrail across Newborough Forest to Llanddwyn Island

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Welsh version PDF

Enjoy the fascinating geology and the magnificent views on this beautiful and spectacular walk in south-west Anglesey.

Dr John Conway (Royal Agricultural College) and Dr Margaret Wood, GeóMôn / Anglesey Geopark

Pictures: J. Conway, unless stated otherwise.

Llanddwyn Island is a magical place, best seen on a stormy day with the waves raging against wet rocks resplendent in a range of hues. On a hot sunny day, it feels like an island in the Aegean! This area records an entire plate tectonic story, from the creation of the ocean floor as a mid-ocean ridge, seen here as pillow lavas, through its journey across the ocean basin where it picked up sediments, to its burial and metamorphism as plates collided and the rocks sank down into a deep ocean trench.

Leave the Forestry car park [1][SH 40566 63439] and go on the forest track to [2][SH 39582 64225] on the map. Here you join part of the post-marked Forestry geology trail (look out for Geo posts 5 to 10) where you can see green and purple banded cherts (Geopost 5), spectacular pillows with varying stages of limestone alteration (Geoposts 6, 8, 9, 10) and one pillow (Geopost 7) looking as fresh as when it was extruded on to the sea bed nearly 600 million years ago.

Walk back to the forest road, turn right and after a short distance, cross the clearing on your left and observe the newly exposed pillows on the beach [3][SH 39344 63623]. At [4][SH 39181 63556] are the world famous lavas which erupted on to the ocean floor, piled up as separate “pillows”, with each draped over the rounded top of an earlier one. They mark the opening of an ancient ocean. Later Earth forces tilted all the pillows on their side with the uppermost (youngest) facing south-east.

Cross to the steps where the rocks are [5][SH 38996 63394] a mass of purple and green fragments — the result of an explosion as red hot lava forced its way through the Earth’s crust, met the icy cold seawater and shattered like shrapnel now surrounding small blobs of lava. Following this explosive beginning, lava was then able to flow on to the seabed as pillows, like toothpaste being squeezed out of a tube.

Climb the steps and follow the path to the cattle grid, turn right and go down to the beach via the small gate [6][SH 38754 63020]. To the right of this sandy inlet are thinly-bedded, smooth sedimentary rocks (now almost vertical), originally fine muds on the deep ocean floor.

Now proceed to the grassy area above [7][SH 38635 62851] and peer over the rocks below — do not climb down, it’s too steep. Here are alternate bands of bright green and red rocks — the green is from the later metamorphism of the muds, the red is jasper, an iron-rich silica, typical of deep oceans.

As you pass the Cross, the cliffs to your right are limestone mixed with fragments of pillow lavas (better seen in the cliff below the old lighthouse). In the middle of this bay, [8][SH 38488 62490] an isolated block of rock shows various [9][SH 38549 62394] types of altered pillow lavas amid red jasper and green chlorite-rich material (altered mudstone).

The jewel in the crown is in Porth Twr Bach [10][SH 38580 62379]. It is a simply amazing mixture of colours and textures, the “melange” of Edward Greenly, the geologist who first described them. Deformation, chemical alteration and metamorphism show that these ocean floor rocks were dragged into a deep ocean trench as one plate collided with, and dived beneath, another plate. More recent Earth movements split the mélange and filled the cracks with lava (dolerite dykes).

Return via the Pilots’ cottages and [11][SH 38655 62575] view the display, then follow the main track back past the ruins of the medieval St Dwynwen’s church and field system. Small paths to the right, overlook raised beaches [12][SH 39046 63012] that formed as the land rose after the Ice Age. Keep to the paths to avoid damaging the fragile sand-loving vegetation or disturbing ground- nesting birds in spring and early summer.

Figures

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Route map

Pillow (Geopost 7)

Pillow lavas

Detail of jasper and mudstone

View over location

Deep ocean muds

Altered Pillow lavas mixed with limestone in Porth Twr Mawr

‘Mélange’ of red chert, pink and white quartzite and (in background) green schist, intruded by later dyke

Llanddwyn lighthouse