....the distant past on our doorstep
On a long car journey back from South Wales recently, I found myself listening to a CD of old Bert Bacharach songs. I hadn’t realised that he was responsible for so many fantastic hits that were written a long time ago, and will remain popular for ever – ‘Raindrops keep falling on my head’, ‘What’s new pussycat’, and ’Trains and boats and planes’. I had been on holiday to Pembrokeshire with my mother, now a good age and also still going strong. At the same time I have been reading about and visiting old rocks in the northwest Highlands; very old rocks that will still be here eons after us.
Geologically speaking, ‘very old’ means more years than we can possibly imagine. We can relate easily to our own family life span of say up to 120 years ago, and perhaps we can imagine how life has been over the last 2000 years. But it is mind boggling to try and imagine time in tens of thousands of years, or millions of years, let alone billions of years. And yet geologists tell us that planet earth was created about 4.6 billion years ago.
Shortly after the earth came into being as a hot fiery planet, rocks began to form deep in the earth’s core, and surfaced as Lewissian Gneiss (‘nice’). This type of rock is a main feature in Wester Ross, Sutherland and on the Inner and Outer Hebrides. It’s that grey knobbly rock that we see all over the place, and at 4 billion years old is the most ancient rock on the planet.
This fact alone could make our local area lay claim to geological fame, but there is so much more right on our doorstep that makes this part of the Highlands geologically unique.
Many years later, at about one and a half billion years ago, sandstone was deposited by flowing water on top of the Lewissian Gneiss. This has been named Old Torridonian sandstone. These deposits have been eroded over time but we can still see them today forming mountains such as Slioch, Lliatach and Quinag.
In contrast, much of the rest of the Highland mountains to the east of Sutherland is made up of what are called ‘Moine schists’, another dark rock formed much later, about 500 million years ago, sitting on lighter Cambrian limestones. The way these mountains were formed about 440 million years ago has caused a world-renowned effect near Loch Assynt called the Moine Thrust.
Typically, Highland mountains have been formed by two hard tectonic plates on the earth’s surface pushing together and squeezing the softer sandstone between them so that it flows up over the top of the plates.
A similar effect is seen when wet cement is squeezed up between the ends of two bricks and continues to flow over the bricks away from the two ends. The horizontal surface between the cement and the brick, in this case between the Moine schists and Cambrian limestones, is called the Moine Thrust. It is clearly visible at the lochside, and at a geological site near the main road, Knockan Crag. Apparently many student geologists visit to trace and analyse it, and generally pay homage.
And there is more to wonder at...
Near Scourie are the Scourie dykes which came into being about two and a half billion years ago. A dyke is formed when molten rock is forced up through old solid rock to form sheets of newer rock within the older. At Scourie, a dark rock called basalt was forced through Lewissian Gneiss. The basalt dykes erode faster than the surrounding Gneiss, so the dykes are visible in the form of wide trenches (up to 30 meters wide) running for a number of kilometres across the hills, a stunning and eerie effect.
Off the mainland there are remains of volcanoes which are between 50 and 60 million years old. Liquid rock, magma, is pushed up from about 35 miles deep within the earth, and collects in chambers about 3 miles below the surface of the volcano. When the volcano erupts, lava is pushed out of the top of the chamber and is replaced with more magma from below.
After these volcanoes stopped working, cooled down, and became eroded, remains of the chambers can be seen on the surface of some mountains in the form of circular arrangements of gabbro or solid magma.
The island of Rum is almost completely the remains of a volcanic chamber, and the black Cuillins on Skye are another volcano fossil.
At Ardnamurchan, it is still possible to see the entire circular shape of the volcanic chamber. Further examples are on the islands of Egg and Muck.
Back here in Achnasheen we have the prominent remains of a relatively recent glacial lake, the terraces by the lodge and the station.
Only about 10,500 years ago the whole area from Loch Maree to Loch Lomond was covered by an ice sheet which lasted for about 1000 years. Dramatically these 1000 years of ice melted in a short time span of only 50 years. Huge amounts of water were released causing substantial water flows, erosion and flooding. This is why the hills around us are deeply scarred with water-gouged ravines.
Apparently the glacier down Strath Bran presented a wall of ice at Achanalt that blocked the flow of water, causing a lake to form around Achnasheen. The water gushing in brought lots of debris that was deposited on the bottom forming what we now see as the terraces. These are made up of huge quantities of boulders, gravel and sand, and can be experienced in the ground all around us, as any person trying to dig a garden and rid it of stones knows only too well.
When the ice finally gave way, water gushed eastwards and deposited material in the Cromarty firth onto which the A9 bridge has now been built. And as the water rushed westwards, it left big rocks between here and Achnashellach, which can be seen in the form of rough ground all around Loch Sgamhain. Finer material was deposited from Achnashellach westward, making the valley all along here flat-bottomed.
I find it incredible that that we can still see today very clearly the results of the way in which the world was formed so long ago, and to acknowledge that these rocks will be here for millions, perhaps billions of years to come. As geologists say, the present is the key to the past and to the future.
And when we in Achnasheen and Garve say that something or someone is as old as the hills, we can say it with real justifiable and unique conviction.
Steve Jones
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