Saturday, 13 April 2013

A Jewelled Erratic

Erratics are common on West Ardnamurchan.  As their name suggests, they are lumps of rock, sometimes weighing tonnes, which have wandered from where they belong.  The 'wandering' was caused by their being picked up and carried by the great sheet of moving ice which flowed across this landscape over 10,000 years ago.

This one lies on a hillside near Glendrian Caves on the north coast of Ardnamurchan.  It's unusual in that, unlike many of them, the source of this particular erratic is relatively easy to identify.

A close look reveals that it resembled a plum pudding, with dark 'raisins' in a lighter matrix. This rock started as a clay or mudstone which was deeply buried beneath a mountain chain and, under immense pressure and heat, altered into a schist.  The minerals of the 'pudding' are mainly micas and quartz, but the 'raisins' are a bit more exciting - they're garnets but, sadly, not big enough to be worth mining for jewellery.

A thin-section of a garnet-mica schist is seen in this video from Geosec Slides, a West Ardnamurchan business.

Garnet-mica schists are common in the Moine Schists, a formation of rock which is found to the immediate east of West Ardnamurchan, so this rock must have travelled at least ten kilometres.

Many thanks to Rob Gill for showing The Diary the erratic.  Rob's website is here.

No comments:

Post a Comment