Isle of Arran, Scotland - British Geological Survey

The Isle of Arran is an island off the western coast of Scotland. The Sannox and Corrie coast lies on the north-eastern side of the island, directly east of the granite mountains that protrude to the north of the island. A ferry must be taken to the island from mainland Scotland at Ardrossan, roughly 33 miles south-west of Glasgow.

A dry stone wall in the foreground with mountains in the background — The granite mountains of northern Arran from the summit of Ard Bheinn. BGS © UKRI.

Geological background

The rocks that make up the Sannox and Corrie coast range in age from Devonian (370 million years ago) to late Carboniferous (up to 299 million years ago). What’s great about this coast is that the rocks are all exposed and unmetamorphosed, so walking along and following them tells a great story of the history of the coast and the island.

Coastal bedrock

To begin your walk at the oldest rocks, start at the north Sannox coast. The first rocks you’ll see are the Devonian interbedded sandstones. These range from sandstones with evident bedding to coarse conglomerates that were deposited during higher turbidity flows.

Beds of dark sandstone rock dipping to the right, with grass in the foreground and the sea in the background — Dipping sedimentary rock of the Sannox and Corrie coast. Although the rocks are covered in oil, thin bedding can still be seen. Mainland Scotland can be seen in the background across the Firth of Clyde. © Cameron Fletcher.

The sandstones are overlain by the Kinnesswood Formation limestones of the Carboniferous Period. They were deposited when Arran was much closer to the equator than it is today and had a tropical climate. A cycle of limestones and sandstones, which were formed thanks to fluctuating sea levels, follows the Kinnesswood Formation limestones.

If you wanted to extend your walk, the rocks just south of Corrie are red, Permian sandstones that were laid down 299–252 million years ago. Here, there is a rare fulgurite that formed when lightning struck a sand dune while the sediment was still being deposited!

A circular formation within a bed of greyish rock — Fulgurite in the Permian sandstones of Corrie. © Kotryna Savickaite.

Igneous instrusions

The coast also has multiple dykes that cross-cut the strata. Because of the dip of the bedrock, the dykes are easy to spot because they are flat and dark in colour. The dykes formed during the Tertiary Period (66–24 million years ago).

The other big geological attractions of northern Arran are the granite mountains, which formed during the same period. The mountains developed when a granite pluton was uplifted and became exposed, the highest point of which reaches 874 m today. These mountains can be accessed through a public trail just south of the Corrie Golf Club.

A mountain rises in the background above a beach covered in seaweed in the foreground — Granite mountains as seen from Sannox beach. © Cameron Fletcher.

Fauna and flora

Northern Arran is a designated National Scenic Area, to protect it from overdevelopment. It is also a Site of Special Scientific Interest, so refrain from taking any of the fossils or rocks you might find along the way (that includes the awesome fulgurite!) This area is not only conserved for its views and geology, but also for the many animals that visit or live here. For example, over 200 different species of bird have been spotted on the island, such as the golden eagle and red-throated diver. Grey seals can be seen visiting the beaches.

Boulders on a beach with seals perched on them — Seals on the coast! © Cameron Fletcher.

Because of the great change in soil and atmospheric conditions from the coast to the top of the mountains, this area also hosts a range of flora. For example, as you walk closer to the mountains and begin to see the tree line, you will come across the Arran whitebeams, which are an endemic species to the island and are endangered. But before that, as the soil becomes more acidic because of the underlying igneous rock, you will no longer see much grassland, but more alpine heath and various shrubs. These moors are home to the emperor moth and speckled wood and green hairstreak butterflies.

Further information

Take care when walking along the coast as the rocks can be slippery because of the tides or rain wetting them or moss growing on them. They are all dipping too, making them harder to cross due to their high angle, so wear sturdy boots. Also, when walking in grassland towards the mountains, keep on the tracks and out of the long grass as there are ticks.