ARGYLL and the islands are richly endowed with evidence of the early human inhabitants of the area. Hunter-gatherers came to these shores some 8000 years ago.
This area was the seat of the earliest kings of Scotland, and Kilmartin Glen (between Oban and Lochgilphead) includes prehistoric archaeological sites of international importance. Some of the finest burial cairns are found here, and in 2012 an excavation site provided proof that people have occupied this land from the Bronze Age onwards. For many people today, this human history seems to represent the extreme edge of time – we can go back no further.
But study of the rocks extends that timescale by millions of years. For example, some of the earliest events in the geological history of Scotland are recorded in the rocks of Islay. The rocks of the Rhinns of Islay have been dated at around 1,800,000,000 (or 1.8 billion) years old. These are difficult numbers to make sense of, particularly as we tend to see all historic events through the prism of human timescales. But an appreciation of this extended timescale is an essential part of the study of geology.
Arthur Holmes, Regius Professor of Geology at Edinburgh University during the 1950s and 1960s, pioneered the accurate dating of rocks. His techniques delivered a degree of precision that allowed geological events to be placed in logical chronological order.
This revolutionised the study of rocks and fossils and made it much easier to correlate events in one part of the country to those in another, and indeed to those in other parts of the world. Our understanding of the sequence of events that gave rise to the geology of Argyll and the islands was much enhanced by this better understanding of time.
Jura has one of the most distinctive profiles of any Scottish island. The Paps of Jura can be seen for miles around. The form of these hills is related to the way they were moulded during the most recent Ice Age, but the material they are created from is also of interest: they are made from sandstones that were subsequently altered to quartzite. Once temperatures stabilised after a period of glaciation 635 million years ago, a thick blanket of sand was laid down in the Iapetus Ocean across a wide beach area subjected to the ebb and flow of the tides.
The thickness of the sand deposits indicates that the floor of the ocean was sinking at that time, and the shallow water depth was maintained by the rapid and copious input of sand to this wide tidal shelf area. Thick, coarser layers made up of cobbles and small boulders (conglomerate) also form part of this geological sequence, showing that this was a high-energy coastal environment. In the extreme conditions generated as continents collided and the Iapetus Ocean closed, these pure sandstones were altered to the hard-as-flint quartzites that constitute Jura’s hills today.
The Ice Age was the final natural event that shaped the landscape we see today. Over the last 2.6 million years, the climate has fluctuated between temperatures similar to those of today to prolonged cold periods where thick layers of ice and snow were a permanent feature throughout the year. During these cold periods, northern latitudes were submerged under a blanket of snow and ice. Sea levels fell worldwide as much of the water on the planet was locked in glacial ice and snow. Short spells of warmer conditions known as inter-glacials punctuated these prolonged cold periods.
For an explanation for these dramatic temperature fluctuations, we need to look to space. The Earth has an eccentric orbit around the sun, varying from a perfectly circular passage around our home star to something much more elliptical.
The period of transition from one orbit to the next takes around 100,000 years. When the planet is furthest from the sun, less sunlight falls on the Earth’s surface and this is sufficient to tip the climate into a cold phase. Variations in the Earth’s tilt and wobble as it spins on its axis also affect the amount of solar radiation that reaches ground level. During the last 2.6 million years, there have been many such transitions from warm to cold and many more are expected in the future. We currently bask in the warmth of an inter-glacial period, but at some point, perhaps as soon as in the next 50,000 years, the glaciers will return and, once again, the country will be caught in the icy thrall of the next glacial advance.
The last glaciation reached its peak around 22,000 years ago when vast sheets of ice flowed westwards across the area from the highest ground towards the sea. As the global temperatures rose slightly, the ice cover thinned and the glaciers became confined to the glens and sea lochs that had previously been carved out by the ice.
The Paps of Jura emerged fairly early from the blanket of ice, and the bedrock of quartzite was then subjected to extreme freezing conditions. These conditions had a shattering effect on the rock, and as pieces were broken off the upper slopes they tumbled down the hill to accumulate as an extensive scree slope. This jumble of rock continued to be added to in the harsh climate that persisted even after the ice had finally disappeared. The final thaw began around 11,500 years ago, allowing for the establishment of the ecosystems that we inhabit in Scotland today.
Alan McKirdy’s Landscapes in Stone series is published by Birlinn. Argyll and the Islands and Mull, Iona and Ardnamurchan are published this month, £6.99 each
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