Brightly coloured pebbles on the beach at Manorbier in South Pembrokeshire, Wales, with patterns of reflected light under clear sparkling water in a gently flowing stream.
These rocks are seen on the west side of The Island or St Ives Head in Cornwall, England. They form outcrops on the west side of Porthmeor Beach. I was fascinated by the sheer complexity of the colours, patterns and textures. As far as I can make out they belong to an un-named igneous intrusion (matagabbro or metamicrogabbro) composed of silica-poor magma which was later altered by low-grade metamorphism in the Devonian period between 359 and 416 million years ago.
The rock looks very different from that nearby which was featured in an earlier post. I was particularly drawn some outcrops which had an almost dark navy blue smooth surface but the majority of the rock surfaces were blue-grey and dirty yellow shades – some with smoother almost layered textures and others textured like lumpy porridge. The patterns were complex but included ancient splits and cracks that had been infilled with other materials of contrasting colours, giving paler or darker straight lines and angled cross lines – not to mention all the complicated later fracture patterns. There seemed to be large inclusions of other rock types too; this often occurs in granites. It is incredible that rocks of the same basic type can vary so much in their appearance within a few metres.
The rocks in Cornwall are certainly different from any that I’ve seen before. Generally speaking, they are mostly igneous and metamorphic in type. Often the igneous rocks themselves have been metamorphosed. The rocks shown here were seen on the beach at Porthmeor and adjacent to The Island at St Ives. They lie below a thick layer of rusty-coloured superficial deposit of glacial till. I am not absolutely certain what the bedrock is. There are a number of rock types described by the British Geological Survey in their Geology of Britain Viewer in close proximity to each other, and it is rather difficult for an amateur to decide which is which. However, this particular outcrop reminds me of basalt which I saw on the shoreline of Grand Manan in the Bay of Fundy in Canada. So, I am going to suggest that it is an un-named mafic igneous rock formed of silica-poor magma that intruded into the earth’s crust in the Devonian Period between 359 and 416 million years ago. When the magma cooled, it formed an intrusion of fine to medium crystalline rock, often as basaltic dykes and sills.
On the other hand, I could be wrong, and it might be an un-named igneous intrusion of Metagabbro and Metamicrogabbro; or even Mylor Slate Formation with Hornfelsed Slate and Hornfelsed Siltstone.
One rainy morning I walked the coastal path along the terrace at the foot of Rhossili Down in Gower. Clouds shrouded the slopes of the Down above me. Ahead of me, Burry Holms and Llangennith Burrows were part concealed by mist. Below me, the tide was going out – far out. The sand on the beach was still wet from the waves and the rain. Slowly the cloud cover thinned and allowed a filtered light to penetrate. The pale light was reflected by the shore, high-lighting the sea-sculpted rows of ridges and ripples in gentle gleaming silver. A stream cascading from the height of the Down worked its way down to the beach, where it spread out in a fan of interweaving channels that cut across the parallel ridges on its way to the water’s edge. Coloured sediments carried by the stream tinted the silvery patterns and made them seem opalescent.
Dark streaks drawn into abstract designs by the swash and back-swash of waves sometimes decorate the succession of driftlines on the sandy beach at Rhossili Bay as the sea recedes. The blue tinge of the darker areas makes me wonder whether they are composed of comminuted fragments of blue-black mussel shells.