by John Sedgwick
Why has the landscape of the Slate Islands formed into a series of low lying rocky ridges and parallel sea lochs nearly all displaying a strong north-easterly orientated grain? Here on Seil this dominant southwest to northeast trend is seen in the orientation of Seil Sound and parallel hilly ridges that run down the length of the island of which the Ardencaple to Dunmor ridge and the Winterton to North Cuan ridge are two examples. The answer lies in the 700 to 550 million year-old bedrock which have been folded and reorientated by the “Caledonian” earth movements some 470 million years ago so that their outcrops run consistently southwest to northeast.
The rocks underlying the Slate Islands are the Easdale Slate Formation which is part of a much thicker succession of rocks known as the Dalradian named after the ancient kingdom of Dalriada. The Easdale Slates were originally deposited about 700 million years ago as a deep sea black mud around an ancient continent known as Rodinia. At this time Rodinia was situated around the South Pole! If you look closely at the Easdale Slate you will see that it is not a monotonous sequence of black slates but instead it contains layers of dark orange-brown dolomites and other sandier and siltier layers. These various layers within the slate are sometimes more resistant to weathering and thus tend to form more prominent ridges. The small ridge that runs from the Balvicar crossroads to the graveyard and on which the war memorial is situated is one of these more resistant gritty horizons.
About 600 million years ago, Rodinia was pulled apart by movements of the tectonic plates that form the outer layer of the Earth. Evidence for this continental break-up is found on Seil, where igneous rocks of the Tayvallich Volcanic Formation (dolerite and gabbro) were injected into the surrounding Dalradian sediments beneath a developing ocean called the Iapetus Ocean. These rocks have subsequently been altered to a metamorphic rock called amphibolite and these rocks are more resistant to erosion and thus tend to form prominent ridges, a good example being the ridge that runs southwest from Winterton via Kilbrandon House to Cuan. Ironically the rocks that make up the igneous intrusions and form resistant landscape features are also very easy to carveand they were the preferred material for the construction of the many Celtic crosses that are a feature of this region.
Over the next 200 million years the Iapetus Ocean widened and then contracted and the foundations of Scotland were now on the edge of this new ocean and, together with North America and Greenland, they broke away to became part of a new continent called Laurentia.
Laurentia was separated by the Iapetus Ocean from the continent of Baltica (Sweden, Finland and Russia) and from a smaller continent called Avalonia, on which the rocks of England and Wales were forming. But, some 500 million years ago, the ocean started to close and the rocks in front of the moving continents crumpled and folded, and eventually, by about 430 million years ago, the continents collided and became welded together forming a vast mountain chain. This event is called the Caledonian Orogeny. By this process the rocks were deeply buried were heated and recrystallised, growing new minerals in the process called metamorphism. Thus the rocks of the Easdale Slate were transformed from soft mudstones and shales into hard brittle slate and the sandstones of Scarba and Jura were altered to hard quartzite and the igneous dolerites and gabbros were converted to amphibolite.
The uplift of the Earth’s crust towards the end of the Caledonian Orogeny left a range of high mountains with steep unstable slopes. Seil was now just south of the equator so the climate was generally hot and dry and there was hardly any vegetation so, when it did rain there were dramatic flash floods carrying vast amounts of rock debris and the products of those floods are huge rounded boulders embedded in a hard gritty matrix to form thick deposits of the rock type known as ‘conglomerate’. These conglomerates are spectacularly displayed on the island of Kerrera and around Oban and Benderloch but on Seil there is only one small outcrop visible on the hillside above Dun Aorian, not far from the Primary School.
As the first Caledonian mountains were being uplifted, melting of rocks deep within the Earth’s crust formed magma that accumulated and rose towards the surface. In places the magma erupted as lava around 425 million years ago and these basalt and andesite lavas (the Lorn Lavas) spread out over a wide area around Oban, where in many places they can be seen resting upon conglomerates. These hard resistant lavas form the upland ridge which runs down the western part of Seil from Ardencaple to Dunmor encompassing the cliffs behind Ellenabeich and also Seil’s highest point at Meall a’ Chaise.
The geological story jumps now leaps forward to around 60 million years ago when volcanoes formed in the Inner Hebrides prior to the formation of the North Atlantic Ocean. Countless northwest to southeast trending fractures emanated from a central volcano on Mull. The fractures became filled with magma that solidified to form vertical sheets of rock up to several metres wide called dykes and these can be seen on the coast, where they tend to stand proud of softer surrounding rock.
The most recent event to have a profound effect on our landscape was the Ice Age which reached its peak about 22,000 years ago. The last glacial period halted abruptly about 11,500 years ago when our relatively warm, wet climate dominated by the Gulf Stream began. An obvious relic of the Ice Age are the raised beach platforms which have been raised up to 55 metres above present-day sea level. Their formation is due to the enormous weight of ice which caused the land to sag downwards and after the ice sheet retreated the land then rapidly rebounded upwards.
Lastly, let us not ignore Man’s impact on the landscape. The greatest effect of past industrial activity was the cutting down of forest in the 18th century to make charcoal to fuel iron furnaces. The old slate quarries on the islands of Luing and Seil have largely been assimilated back into the landscape. However, the large quarries at Easdale and Ellenabeich that have been flooded by the sea are an enduring feature and are now part of a tourist attraction, firmly rooted in the geological heritage.