Saturday, 5 March 2016

The Geology of a Rocky Beach

I have never been able to understand why some beaches, as at Sanna, are almost exclusively sand, while others, such as this one to the east of Coiremhuilinn, is shingle, cobbles and rocks. Somehow, a sand beach is more appealing, but one of the pleasures of walking the rocky beaches along the south shore of Ardnamurchan is the variety of rocks which have accumulated there. Each rock tells a story, some of which are as much as a billion years old.

Many of the loose rocks have been transported in by the ice which left the area some 12,000 years ago, but most of these haven't come far. This is a metamorphic rock, a gneiss, which has probably come from the Moine Schists sequences which mainly outcrop to the east of Camas nan Geall. This one was once a sediment. The layers of rock at the bottom have been cut off by layers above, showing what is called 'cross-bedding', a characteristic of sand which has been laid down by a fast-moving current. This sediment was later deeply buried during a mountain building event over a billion years ago, and cooked, compressed and folded to form the gneiss, which was later lifted up and eroded out.

This is a relatively much younger igneous rock, something like a granite, which was intruded into the pre-existing rocks as a molten liquid and then cooled slowly underground - hence the large crystals of lighter-coloured minerals such a quartz and felspar, with some darker minerals which may be a mica such as biotite. The nearest granite to here is at Strontian.

A mere 300 million years ago, the area was covered by a warm, shallow sea - Ardnamurchan at that time was much closer to the equator - with what may have been a low-lying coastline. At times when the sea retreated, the area seems to have become very arid, perhaps almost desert. The large rock on the right looks like a breccia, a rock formed when existing rocks are suddenly eroded, as in a flash flood, and the shattered, angular pieces re-deposited quickly and, later, solidified.

This may be another desert rock, one formed in a freshwater lake which frequently dried out so the salts contained in the water were precipitated - the lighter areas - while, as other times, wind-blown dust settled in the lake - the yellower areas.

The rocks of this dry period are exposed in one place along the Coiremhuilinn beach, and very similar rocks, perhaps of the same age, are to be found along the shore below our house in Ormsaigbeg. Associated with the latter are limestones deposited at the times when the warm, shallow sea rose and drowned the desert area. These limestones contain the fossil remains of the ammonites which hunted in the warm, subtropical waters, some of which can also be found in the rocks below Mingary Castle.


  1. Thank you for that information. Here in Lewis, we have very ancient rock at the surface, visible at the Butt of Lewis. That is aged a mere 3,500 million years. Geology is fascinating.

  2. Jon, Thank you. I could read this stuff for hours. Information like this put's our short little lives and our egos into a new perspective. Ever thought of doing field-trip/study-tours/beach seminars? I would be in the queue.

  3. Agree with your comment, ADB, about geology being fascinating. It's an interest that goes everywhere with you. Envy you your Lewisian rocks.

    John - Trevor Potts already does a beach walk in the summer.