Rocks at Porth Kidney near St. Ives

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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.

8 Replies to “Rocks at Porth Kidney near St. Ives”

  1. Thank you, Rh. I came across the two colourful geological features by chance but I should have expected some such as the igneous and metamorphic rocks in Cornwall are known to be rich in minerals that have frequently been a mined resource.


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