Five favourite Anglesey rocks

Iconic pillow lavas

Best seen on the approach to Ynys Llanddwyn

It’s unusual to see these rocks on land, they most likely cover two thirds of the earth’s surface – the bottom of all the main oceans. The ones that we see here on Anglesey are indicative of a place and time far away and long ago, as far back as 5-800 million years ago and near the South Pole.

For a long time the formation of these rocks was only known theoretically until filming underwater near Hawaii accidentally caught them forming on camera. Imagine lava slowly oozing from small cracks or tubes amidst clouds of steam and cooling rapidly from red hot to dull greenish black, so fast that instead of a stream of lava oozing out of a vent, just individual blobs come out and pile up on the seafloor. The way they sag and droop over the pre-existing ones that gives the name “pillow”. In between the pillows you will find blood red jasper, white quartz and sometimes creamy coloured calcite.

Carboniferous limestone

Outcrops along the coast from Lligwy to Penmon, and inland as far as Llangefni 

A rock which is both hard wearing and easily worked, brittle but doesn’t shatter – sounds like a contradiction but it’s the rock of choice for road making for all those properties. It is relatively easy to quarry and shape, unlike granite, yet it is almost as long lasting.  

It has been quarried in many places, both along the coast and inland and has been used from the earliest times. The burial chamber at Lligwy is a block of limestone. Din Lligwy and Din Silwy are both Iron Age hill forts constructed from limestone. These are both brilliant examples of a key property of this limestone, its habit of weathering along cracks to form “limestone pavements.” These ‘pavements’ are recognised as extremely important biodiversity habitats now and are protected but before that they were the source of “waterworn rockery stone” beloved by Victorian and Edwardian landscapers. Small areas can be seen on the cliffs north of Moelfre but the two hill forts were constructed using upended blocks (‘clints’) from limestone pavements.

It forms wonderful cliff scenery from Lligwy to Penmon [and across the water, the Great Orme). This rock formed as either a chemical precipitate from warm seawater (bit like making salt) or as structures or debris from life – corals and shells. There are plenty of species of seashells to be found as fossils throughout the limestone beds, and also many species of coral – some as broken bits, others as complete colonies. Just imagine – Anglesey, a mini Great Barrier Reef some 300 million years ago.

Quartzite 

Mynydd Twr (Holyhead Mountain) and Bodafon Mountain 

Hard, white, featureless, no structures, no fossils, nothing but pure white quartz, the simplest (chemically) of all minerals yet its impact on the landscape is spectacular. Quartzite is the ultimate survivor – put a whole mix of rocks and minerals on a stormy beach and leave it for ages and only the quartz will be left. Everything else either weathers or breaks down somehow but quartz goes on forever. We have two big blocks of quartzite here on Anglesey; Mynydd Twr (Holyhead Mountain) and Bodafon Mountain. Holyhead ‘mountain’ can be seen from almost everywhere, the highest point on the island and site of a Roman lighthouse or temple depending on your views.

Bodafon ‘mountain’ is even more interesting – a small scruffy road cutting on its south side along an unimportant country road reveals a rock called “breccia” – composed of sharp, angular chunks of the quartzite but set in a reddish matrix. Bodafon ‘mountain’ was a mountain 350 million years ago, sticking up above a wide river valley in a hot desert. We are seeing a real, ancient landscape resurrected from under piles of younger rocks.

Coedana granite 

Found as pebbles on beaches along the coast from Aberffraw to Rhoscolyn

You don’t really see it in the landscape, nor much of it around anywhere other than as pebbles, it’s something of a geological puzzle. Anglesey doesn’t have a typical granite except amongst the erratics brought by glaciers, this is the nearest. Geologists have long argued over its origin – without wanting to get too technical, is it a granite or a migmatite, was it intruded or melted in-situ, how old is it, where did it come from?

Since we normally only see it as pebbles on the beach, it does have the appearance of a granite, spotty with big crystals, quartz, feldspar and other minerals.  But on a fresh surface if you break it open, it’s a dull muddy greenish-grey and looks nothing like granite. It’s far more reminiscent of the endless green schist which forms the bedrock from the Strait to Cemaes across the western half of Anglesey.

Melange 

One of the world’s top 100 rocks, found on the beach at Ynys Llanddwyn

When Edward Greenly, the last of the most famous geologists examining Anglesey’s geology, got to the very far end of Llanddwyn he was lost for words. Unable to find a proper geological name for what he saw he called it “melange” (French for ‘mixture’) and that name stuck and that became its formal name.

Walk to the very end of Ynys Llanddwyn, ignoring the fabulous pillow lavas, the wonderful deep sea red cherts, the ruined abbey, the pilots’ cottages, the two lighthouses until you reach the sea. There a riot of colour awaits you, white, cream, pink, rose, blood red, green, purple and black and a grey, spotty rock. It’s easy to see why Greenly was lost for words. Quartzite, limestone, marble, pillow lava, volcanic ash, basalt, sandstone all broken up, mashed together and then eroded by  the sea. Formed when all the various sediment, pillow lavas etc sitting on the ocean floor were scraped off as the ocean plate started to descend beneath the approaching continental plate – scraped off, broken up, jumbled up and mashed together.