Rocks at Porth Kidney near St. Ives

Porth Kidney Sands in Cornwall is well known for its beautiful golden beach but it also has some very interesting rocks. The natural textures, colours, and patterns of cliff strata on the west side are quite different from those previous photographed in a rock outcrop at Porthmeor near St Ives. At Porth Kidney there are two types of rock, both dating from the Devonian Period about 359 to 385 million years ago, Many parts of the bedrock show layers that have been folded in complex ways by pressures and forces in ancient times and some of these strata have been metamorphosed by heat from a major igneous lava intrusion at a later date – but still retain evidence of their sedimentary origin in the layering effects.

Slates and siltstones of the Mylor Slate Formation are found in one area and are followed further west by Hornfelsed Slate and Hornfelsed Siltone (the metamorphosed version of the rocks) nearer to Carbis Bay. There are even some spectacularly bright blue-green patches between the layers which are likely to be some derivative of copper; and an iron-bearing stream that cascades down the cliff has converted the adjacent rocks to a striking orange colour that clashes with the green algae adjacent. The upper reaches of the shore are covered with irregular beach stones and also large boulders.

Rocks at Porthmeor Beach, St Ives

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.

Rocks at Fermoyle on the Dingle Peninsula

Red Devonian sandstone rocks at the beach with fucoid seaweeds

The place where I took these photographs is marked on the map as an island but it is actually just a tiny promontory near to the village of Fermoyle, along the Dingle Way, on the north coast of the Dingle Peninsula in Ireland. I am sure that most people visit the location for its wonderful long unsullied sandy beach. However, I was drawn to this particular part, at the extreme western end of the beach, because of its fascinating geomorphology. The rocks are sandstones and conglomerates (mostly but not exclusively red) of the Glengarriff Harbour Group from the Devonian Period. The bright olive, lime, yellow and orange colours of the seaweeds, and the black, yellow and white of encrusting lichens, clash garishly with the red rocks. The rock strata are clearly defined: sometimes on-end, sometimes as flat bedding planes, and in one place a dome of strata lies cut-away and exposed. Beach stones rather than pebbles cover a portion of this area; and there are also occasional huge boulders composed of conglomerate scattered along the shore nearest the inlet from Brandon Bay.

Wet Boulders at Eype

Beach boulder in shingle at the water's edge

At Eype, blue clay cliffs slip, and subsequently unsupported rock strata above it collapse. Large boulders then roll down to the shingle shore. The variety of rock types, and sometimes the fossils within them (like belemnites), can be observed at close quarters. The newly surf-washed rocks, part-embedded in the bright orange pea-gravel and pebbles, make striking compositions with the wet surfaces revealing a greater intensity of colour, and finer detail of texture and structure.

Click here for earlier posts about EYPE.

Rock Textures at Fall Bay (1)

Limestone rock texture on the coast

The rocks at Fall Bay are arrayed like the riffled pages of a book. Layer after layer of Carboniferous Limestone is sequentially spread out across the west side of the bay. Each layer has an observably different texture; some are bioturbated with bioclasts and fossils such as fragmentary crinoids and corals. The bedding planes of some strata have deeply sculptured surfaces from weathering and bioerosion. Lichens, barnacles and limpets colonise the rocks and take advantage of the meagre shelter offered by cracks, crevices, and solution hollows.

Rock Strata from Kimmeridge Cliffs – Part 2

Detail of cliff strata from Kimmeridge Bay

The second selection of photographs showing details of the rocks in the Upper Kimmeridgian Clay cliff mudstone and shale strata with iron staining on the east side cliffs of Kimmeridge Bay in Dorset, England on the World Heritage Jurassic Coast.

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay

Detail of cliff strata from Kimmeridge Bay