Ancient folded rocks – the Scottish Highlands
North Arran and north Bute are actually part of the Scottish Highlands, made of rocks known as the Dalradian which occur in much of the region south-east of the Great Glen. The name originates from the ancient Scots kingdom of Dalriada. These rocks have characteristics which reveal a geological history quite different to that of Lowland Scotland. They are much older and, above all, they have been twisted and baked providing testimony to great upheavals in the Earth’s crust.
The Dalradian rocks started life as conglomerates, sandstones, siltstones, mudstones, limestones and even lavas. They were laid down some 600 million years ago at the edge of an ocean called lapetus, which lay in the Southern Hemisphere. To the north was a major continent known as Laurentia, forerunner to North America and Greenland as well as Scotland. Movement of the plates which form the Earth’s crust gradually widened the ocean and moved the neighbouring continent of Gondwana (parts of which are seen today in England, Europe and North Africa) further away from Laurentia.
However, by 500 million years ago these movements reversed and the ocean became narrower. Chains of volcanic islands, similar to those in the present-day Pacific, were squashed against the edge of Laurentia as the continents moved towards each other. Collision of the continents produced the great Earth movements known as the Caledonian Orogeny. Buckling and thickening of the Earth’s crust resulted in a mountain chain comparable to the present-day Alps or Himalayas. These mountains stretched for many thousands of kilometres and remnants are seen today in the Appalachians of North America, in East Greenland and in Scandinavia, as well as in the Scottish Highlands.
The rocks which were to become north Arran and Bute were buried at depths of 20 kilometres in the centre of the mountain chain. There they were heated to temperatures of up to 600°C; sandstones were altered to white quartzites, mudstones and shales to green schists and slates. The once flat-lying strata were folded, once, twice, several times over. This complex folding can be worked out on a grand, mountain-sized scale across north Arran. It can also be seen in miniature in the rocks exposed today. The pressure that produced the folding formed a new layering called cleavage; this is best seen in the way slates for roofing can be easily split along the closely spaced planes.
The mountain chain has, over many millions of years, been worn down to its roots. The hard rocks so revealed produce bare craggy uplands with thin acid soils. The hills of Bute, north of the valley of Loch Fad, show this well. These Dalradian rocks also form a fringe of lower ground with rounded hills around the granite massif in north Arran.
In Bute, the valley between Kames Bay and Ettrick Bay lies where a band of softer rocks, mainly schists and slates, has been worn down more than the harder quartzites which form the hills to north and south.