Tag Archives: siltstone

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.

The Cliffs of Moher Strata

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Rock strata at the Cliffs of Moher

The Cliffs of Moher rise to 214 metres above the sea and stretch for 8 kilometres along the west coast of Ireland in County Clare. They are spectacular. This impressive site has formed the backdrop for box-office hits like the ‘Princess Bride’ and ‘Harry Potter and the Half-Blood Prince’. The first landfall west of the cliffs is the Atlantic coast of Canada – 3000 kilometres away. You can get an idea of the vast scale of the cliffs by noting the minute figures of the people walking along the cliff-top footpath and rocks in the photographs below.

The Cliffs of Moher take their name from a ruined promontory fort known as “Mothar” that once stood at the southern Hag’s Head end of the cliffs but which was pulled down in Napoleonic times to make way for a signal tower.

The Cliffs are part of the famous Burren landscape. However, they are composed of relatively more recent Upper Carboniferous Namurian Period rocks – the Central Clare Group and Gull Island Formation, that overlie the Clare Shale Formation. These sandstones, siltstones, mudstones, and shales were all deposited on top of the more familiar Carboniferous Visean limestones of the perhaps better known eroded scenery of the Burren. The sediments of which the cliff rocks are made, were originally brought down by a large river to a huge delta system that was subsequently consolidated into rock about 300 million years ago. The Burren and the Cliffs of Moher are a recognised UNESCO Global Geopark.

Over long periods of time, some places at the base of the cliffs have been worn away by wave action to form sea caves. In fact, Ireland’s largest surfing wave (called Aileen’s) is at the base of these cliffs. Continued eating-back of the rock where these caves have been scooped out of the cliff, can lead to the formation of sea arches through which the tides can freely flow. When the arches themselves eventually collapse, isolated pillars of rock named as sea stacks are created. The final stage of erosion by the sea causes the stacks to be worn down to ever-decreasing stumps of rock. These caves, sea stacks and stumps can be seen from the cliff-top pathways.

The strata in the cliffs are almost horizontally bedded and the ledges which jut out from the cliff face, where the alternating hard and soft rock layers have been differentially weathered, form ideal nesting sites for thousands of puffins, guillemots, razorbills, fulmars, kittiwakes, peregrine falcons, and choughs. Wild flowers abound on the cliffs in the summer months. The great height of the cliffs provides an ideal viewpoint to look for dolphins and seals in the water below – and maybe even the occasional basking shark, humpback or Minke whale.

The best vantage point of all is the top of O’Brien’s Tower perched near the cliff edge. It was purpose built in the 1835 for tourists who, even then, were flocking to marvel at the cliffs. Up to a million visitors a year now come to the Cliffs of Moher and, even on a cold March day early this year when I was there, people were arriving by the coach-load to enjoy the wonder of the cliffs and to learn about their significance in the fascinating and extremely well-presented Cliffs Exhibition. Although most visitors walk no further than O’Brien’s Tower, this is enough to give them a real sense of awe at this incredible geological site.

O'Brien's Tower at the Cliffs of Moher

REFERENCES

Cliffs of Moher Visitor Experience, Co. Clare – Visitor guide leaflet.

Sleeman, A. G., Scanlon, R. P., Pracht, M. & Caloca, S. (2008) Landscape and Rocks of the Burren: A special Sheet in the Bedrock Geology 1:50,000 Map Series, published by Geological Survey Ireland, ISBN 189970257-1.

Hennessey, R., McNamara, M., and Hoctor, Z. (Compilers) (2010) Stone, Water and Ice – A geology trip through the Burren, The Burren Connect Project, ISBN 0-9567204-2-9.

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