The rocks have, over millennia, been eroded into sands which have in their turn been compressed into rocks – sandstones, called the Colonsay Group – some of which have also then undergone metamorphism, into meta-sandstones. The meta-sandstone rocks in the Rhinns are also, in the scheme of things, relatively old, about 800 million years, pre-dating the explosion of life on earth.
What’s cool in the Rhinns is that there are places where the two types of rocks, the Gneisses and the Sandstones, are thrust together cheek by jowl. And sometimes, where the junction is on the coast, the waves have cut a cleft along the fault line – like at Kilchiaran – giving us a visible ‘billion-year’ age gap.
Overview of the rest of Islay
In the north of Islay, around Bunnahabhain and Bolsa, there are 650 million- year- old limestones that contain fossilised mounds of algae that were earth’s first multi-celled lifeforms. These cyanobacteria lived and multiplied in a warm sea, the Iapetus, which existed prior to the arrangement of landmasses and oceans that we know today. The algae sometimes grew in large colonies, called stromatolites, and can be “as big as a whisky barrel” according to David. They were common in these ancient seas because,“The grazers who would normally eat algae, such as whelks and snails, hadn’t been invented yet. Islay is the best place in the British Isles to see these stromatolites,” he adds.
Thick beds of dark grey limestone, slightly older than that at the coast, occur around Ballygrant. That is where the lime for improving the fertility of Islay’s fields is quarried and ground. It is full of dead plankton, apparently. The pH of limestone is high, alkali, containing calcium carbonate. The pH of other rocks is low, acidic, as they contain more silica.
About 550 million years ago, England and Scotland were on different super-continents – separated by the Iapetus Ocean. About 470 million years ago, when there was marine life and just the beginnings of plant-life on land, the Iapetus ocean closed. This created the Caledonian mountains. You can see the drama written into the rocks in the Port Ellen area, where the rocks of different ages and origins have all been tilted and metamorphosed by the forces involved.
Then 55 million years ago, just after the extinction of the dinosaurs, the Atlantic ocean was formed along where the earth’s crust tore between North America and Eurasia. The northern shore of Islay is the place to see evidence of that event, where there are “enormous vertical sheets of frozen magma, called dykes.”