The Isle of Arran: Scotland’s geological story all in one place

Scotland in miniature; the island where every view reflects varied geology and you can literally ‘walk through time’ across the landscapes

Fertile lowlands of Arran with the northern granite mountains in the distance. Image: Lara Reid.

The Isle of Arran, well known for its varied scenery, sits astride the Highland Boundary Fault and is therefore an excellent place to appreciate the variety of ‘Highland’ and ‘Lowland’ geology that Scotland is famous for. That’s why so many geology students go there!

There are not many places on this planet where, in a three-kilometre walk, you can see evidence of changing environments that span more than 100 million years from Devonian to Permian times. The sedimentary rocks of Arran’s east coast record Scotland’s journey across the Equator, from desert conditions, through the Equatorial swamps and back to desert again. You can literally become a time traveller, passing through the multiple environments and climates Scotland has experienced over millions of years. You can see conglomerate and sandstone deposited on land in braided stream channels during seasonal floods, followed by alternating layers of limestone, coal, sandstone and mudstone formed at the Equator, and then pass into Permian desert sandstone with dune bedding and wadi-type breccia deposits. A truly amazing assortment of geology.

In the 1780s, James Hutton visited the north of Arran and discovered the junction between these sedimentary rocks and the underlying, even older, eroded metamorphic rocks of the Highlands. This was one of his first ‘unconformities’, which he used to demonstrate the great age of the Earth and the evidence of ‘former worlds’ where different rocks had been formed by natural processes.

As if this was not enough, Arran has many other must-see sites that include a giant myriapod trail, Arthropleura, filmed by Sir David Attenborough in “Life on Earth”, one of the oldest tracks of any land creature. Early reptiles, including a crocodile-like reptile, left Chirotherium prints in Triassic sandstone layers of the south of Arran.

The interior of north Arran is dominated by a ten-kilometre-wide granite pluton formed by magma intrusion 60 million years ago, at the time the Atlantic Ocean was starting to form. This mass of tough igneous rock was in turn dissected by dykes and then eroded by glaciers, forming the majestic and very jagged skyline of today.

In the south, at around the same time as the Northern Granite was forming, there was more varied and smaller scale intrusion of magma that has resulted in a range of interesting coastal features. The Doon cliff at Drumadoon is a tremendous example of an intrusive sill that has resulted in a very imposing headland of 30-metre high columns. Close study of this headland tells the story of changing magma composition during the formation of several intrusions. Equally impressive are the sills and raised beaches of Holy Island and Pladda.
Finally, recent settlers have left their mark on Arran by taking advantage of the raw materials and structures. Sea caves on the raised beach were used as seasonal dwellings, sandstone used for standing stones and igneous pitchstone shaped into axe heads and cutting blades. More recently limestone was processed in limekilns to improve the fields, and barytes was mined from veins.
All these overlapping stories mean that Arran is a place that everyone interested in geology and landscapes must visit!

Text contributed by Stuart Blake, Lochranza Centre and Angus Miller

Find out more
Arran (Landscapes in Stone) by Alan McKirdy (ISBN: 9781780273693)
Lochranza Field Study & Activity Centre: https://www.lochranzacentre.co.uk/
Arran Geopark: http://www.arrangeopark.co.uk/

Triassic sedimentary rocks that contain reptile trackways, with the columnar igneous rocks of the Drumadoon sill in the background. Image: Angus Miller.

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