Saturday, 26 October 2013

Gneiss times

Just west of Inverpolly is a small and outwardly unremarkable bay called Lag na Saille (bay of salt - perhaps indicating that salt panning was once carried out here).  It's only when you look at the surroundings that something unusual is seen.

To the east and north the scenery is rugged and rocky, the rock type is the Archaean (Lewisian) Gneiss - pronounced "nice"- which gives Assynt much of its distinctive character.

But to the west of Lag na Saille the scenery is very different.  Low and flat, the ground is covered with grass and the rock type is Torridonian Sandstone, a sedimentary rock laid down when this part of the planet was an arid red desert. The difference can even be seen on the map, the tighter and more contorted contours of the Gneiss compares markedly with the flatter Torridonian.

In this bay, Torridonian Sanstone and Lewisian Gneiss are found in close proximity, and in geological terms Lag na Saille is a remarkable place.  It is in fact the southern limit of a contact zone between the two rock types which stretches north towards Cape Wrath.  But, it gets more interesting.....

Torridonian Sanstone is very old - it's about 800 to 1200 million years old, which makes it Pre-Cambrian rock formed when only simple life forms like Stomatolites inhabited the earth.

But Gneiss makes it seem like the new kid on the block.  If Torridonian Sandstone is very old, Gneiss is very, very old.  As in 3 billion - 3,000 million years old.

The fact that these two rock types exist in close proximity despite the massive disparity in their age is known as an "Uncomformity".  The most notable unconformity in the north west of Scotland is the Moine Thrust which has in places forced the much older Gneiss over the top of the Torridonian sediments - an idea which taxed the geologists of the 18th and 19th century from both a scientific and to an extent a theological standpoint.

Where it is exposed in outcrops, Gneiss is very distinctive.  Variable in colour and often banded, it is a Metamorphic rock which has undergone considerable change due to the effects of pressure, heat and time. It's been cooked, hammered, squashed and bent into shape and has probably also travelled a long way over vast timescales to arrive here at the western edge of Europe.

The paler bands in the rock are usually feldspars, the darker bands mica.  But Gneiss can be speckled and multicoloured or a combination of all these.  It's impermeable,  doesn't break down easily and forms a landcape of the "Cnoc and Lochan" type found in Assynt and the Hebrides.

Gneiss is, to my eye, strikingly beautiful.  But it is the sheer age of this stuff which fascinates me the most.  Three billion years is a enormous number by anyones standards, except perhaps astronomers.  But in three small ways this enormous number can be placed in a bit of context:

At 3 billion years old, Gneiss does not contain fossils because it predates life on earth.  All life.

At 3 billion years old, Gneiss is two thirds the age of planet Earth.

The "Big Bang" date postulated by scientists to mark the beginning of the Universe is only six times older than Gneiss.

To hold beautiful pebbles of Gneiss in the hand, whether banded......

....or speckled, is to hold something truly remarkable  :o)


  1. "Gneiss(ly)" written, Ian. To hold in one's hand something that pre-dates all life on the planet is extraordinarily humbling. The mind knows it can only be that way but the order of time is, of course, completely beyond human grasp. A great read! Duncan.

  2. Haha! - thanks Duncan; "Gneiss" words as ever :o)

    Kind regards