Sunday, 21 April 2013

The Geology of Glendrian Caves

Glendrian Caves lie just under a kilometre to the east of the abandoned village of Plocaig on Ardnamurchan's spectacular north coast.  The main cave, shown, is a dark, angled cleft which enters the rock to some depth, and is only accessible as the tide retreats.

The Diary visited the caves recently with local geologist and businessman Rob Gill, who runs Geosec Slides from his house in Achnaha - and a fascinating expedition it was.

The rock of which the cliff is formed is a breccia, generated during an eruption when huge amounts of material slipped down the north-facing slope of the Ardnamurchan volcano.  So the rock is formed of pieces of earlier-cooling igneous rocks and the rocks into which the volcano originally intruded, all in a matrix of volcanic ash.

Some of the structures are difficult to interpret, like the large lump of layered rock which runs across the middle of this picture - it's about three metres across and up to a metre thick.  It looks like a finely-layered volcanic ash deposit, but it seems out-of-place in a mess of rock which tumbled down a steep slope.

Perhaps more easy to explain is the darker rock in the middle of this picture, to the right of which is a pound coin to give a scale.  This looks like a cross-section of a small hexagonal basalt column, of the sort seen today in the cliffs on Staffa.

After the volcanic breccia had cooled, it was intruded by sheets of basalt injected upwards and outwards from the volcanic centre - these are seen in this picture as dark layers up to about a metre thick.   Called cone sheets, they are commonly seen along Ardnamurchan's coastline.

Many visitors come down to the caves along this natural stairway.  It follows a dyke, a vertical sheet of molten rock which was injected into the breccia at some stage after the cone sheets.  Usually, such dykes are formed of hard rock which resist erosion, but this one has been eaten away by the sea to form this natural descent.

A map of the area is available here.  The caves are some distance from the nearest road, and are difficult to access unless you're well equipped.  The easiest approach is from Achnaha, though it can be hard going, particularly after heavy rain.

Many thanks to Rob Gill for an enjoyable and instructive experience.
Geosec Slides' website is here.

1 comment:

  1. I’ve had my eye on this for a while… it’s great to hear it’s every bit as interesting as I hoped!
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