Luing and the Atlantic Islands: A Tale of Two Oceans

The dramatic story of an ocean that came and vanished, and then much later the opening of the North Atlantic Ocean, all to be read in the rocks of Luing and its surrounding islands

Folded sandstone layer amongst slate exposures on the island of Luing. Image: Alastair Fleming

The Atlantic islands of Argyll consist of a remarkable range of rocks telling the story of nearly three billion years of Earth history. Visit the geology display in the Atlantic Islands Centre in Cullipool to see how these Islands belong to different slices of the Earth’s crust.

Cullipool, Luing
From Cullipool a pattern of islands stretches out into the Atlantic – past Scarba to glimpses of Jura and Islay to the south, past the Black Isles out to the Garvellachs, with a glimpse of Colonsay beyond. In the nearer distance lie Fladda and Belnahua. In the far west is the low-lying Ross of Mull, continuing to the Carsaig cliffs and the mountains of Mull behind, and finally northwards to the other Slate Islands of Seil and Easdale.

As the ancient supercontinent of Rodinia split apart, a new ocean called the Iapetus formed around 600 million years ago, accompanied by volcanic eruptions near Tayvallich and intruded sills into the ocean-floor sediments. These hard rocks form the high ground on the east side of Luing, and consist of much-altered, grey-greenish basaltic rocks, which were used to build the Iron Age fort there. The sills are now nearly vertical due to later deformation.

The Garvellach islands
The oldest rocks in this area form Coll, Tiree, Iona, Colonsay and part of Islay, but our tale begins with the Garvellachs. The Port Askaig diamictite (a rock thought to be formed beneath ice sheets), of international geological importance, tells about the end of the period known as Snowball Earth, around 650 million years ago. You can find out more in Best Places: Islay.

Scarba and Jura
Vast quantities of sediment were eroded from the continent of Laurentia and deposited in the Iapetus Ocean; at that time, northern Scotland lay on Laurentia’s southern coast. Sand settled nearer the shore, and mud further out to sea. These sediments accumulated continuously for 330 million years building layers totalling around 26 kilometres thick. The sand formed the pale, tough quartzite rocks of Jura and Scarba. A boat trip to the famous Gulf of Corryvreckan provides excellent views of these rocks, tilted steeply to the east, which overlie the Port Askaig diamictite of the Garvellachs.

The Slate Islands of Belnahua, Easdale, Luing and Seil
As the Iapetus Ocean began to close about 500 million years ago, these sea-floor sedimentary rocks were squeezed between colliding continents. The thick layers of dark mudstone, filled with organic material and still plastic, were concertina-ed into extremely large, tight folds. As seawater was squeezed from the rock, the thin, flat clay mineral crystals became aligned at right angles to the pressure direction, producing the characteristic cleavage planes of slate. The result is the blue-grey slates of the Slate Islands; sometimes called the ‘slates that roofed the world’ because they were quarried and exported in large quantities to Ireland, the Caribbean, Canada and beyond over several centuries. The abandoned North Quarry and nearby coast at Cullipool on Luing is the best place to study these slate rocks.

The Mull Volcano
The mountains of Mull now reach their highest (about 1000 metres) at Ben More: now, imagine instead that you are looking at a mountain 5000 metres high – then it erupts! Around 60 million years ago, the Mull volcano was comparable to those of Hawaii today. What you see now are the remnants of the volcano’s roots – all that remains of a volcano active for 5 million years. This was the source of the Mull Dyke Swarm, with basalt magma filling cracks in the Earth’s crust up to more than 100 miles away; in fact, the Cleveland Dyke in North Yorkshire is one of these infilled cracks!

The North Atlantic Ocean
Like the Tayvallich volcanoes of long ago marking the opening of the Iapetus Ocean, the Mull volcano, and its siblings of Skye, Rum, Ardnamurchan and Arran, mark the opening of the North Atlantic Ocean, as the supercontinent of Pangaea broke up and the bulk of old Laurentia moved away to form North America and Greenland. But a small slice of Laurentia remained attached to Europe… we call it Scotland!

Text contributed by Alastair Fleming

Find out more
The island of Luing is just 30 minutes south of Oban by car or public bus, with a short ferry ride. The Atlantic Islands Centre is open most of the year offering food, exhibitions and events. The outlying islands are more difficult to visit, but boat trips are available from Craobh Haven, Craignish and Seil.

Geology of Seil and Easdale; John Sedgwick. 2014.  This is a private publication; copies available at the Atlantic Islands Centre in Cullipool on Luing, or from the Slate Islands Heritage Trust Museum on Seil.

Argyll & the Islands – a landscape fashioned by geology provides a great introduction to the geology of the wider area. Download from https://www.scottishgeology.com/find-out-more/publications/#argyll

Slate quarry on Luing with cross-cutting felsite dyke

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