Thursday, 9 June 2016

Geology near the Pier

There's over a hundred million years of history crammed into this small area of rock near the CalMac pier, but it's fairly typical of the many geological features which have attracted thousands of geologists to the area.

The grey rock, marked 'L', is a limestone which was laid down as a calcerous ooze on the bottom of a warm, shallow sea some 130 million years ago. It was deeply buried and solidified by the pressure of the overlying rocks.

Something like 60 million years ago, the limestone was intruded by molten rocks associated with the Ardnamurchan volcano. A sheet of this - the browner rock - pushed its way into the limestone. To some extent it followed the natural lines of weakness in the limestone, along its bedding planes, but just here it split in two, into an upper sheet marked 'S1' and a lower sheet marked 'S2' which cuts across the limestone beds before continuing to follow them. Neither of these sheets can have been particularly hot as the limestone shows no signs of baking where the igneous rock is in contact.

Since that time, the great pile of rock which buried this feature has been worn away, much of it by the ice sheets which covered the area until about 10,000 years ago. The sea has recently removed more, exposing the structure in a low cliff.

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