Coastal Geology

Mighty glaciers carved out the U shaped valleys and fjords so characteristic of the West coast of Scotland

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Loch Linnhe

A Broken Coast Line

The eastern and western seaboards of the Highlands contrast starkly, with the west coast being hugely more indented than the east. This is because over the last 500,000 years, during less intense glacial episodes, precipitation was greatest over the Western Highlands and this caused more ice to build up there. Active glaciers flowed west and gouged out deep U-shaped glens which were then flooded when the climate warmed, meltwater returned to the oceans and sea-level rose again. The Lochaber coastline is so indented there is no continuous road around it. A number of large sea lochs – Loch Linnhe, Loch Leven, Loch Sunart, Loch Moidart, Loch Ailort, Loch nan Uamh, Loch nan Ceall, Loch Nevis, and Loch Hourn – bite into the land. As well as several smaller sea lochs – Loch Eil, Loch a’ Choire, Loch Aline and Loch Teachuis – there are also two major freshwater lochs – Loch Shiel and Loch Morar – which, prior to the ‘rebound’ of the land, would have been flooded by the sea.

A Wild Place

Although travel around the coast is difficult, this gives a special wild character to the area. Those prepared to walk or paddle can visit some of the most unspoilt parts of the country. The Small Isles of Canna, Rum, Eigg and Muck are also close to the mainland, and as well as having beautiful and varied coastlines themselves, they offer superb seaward vistas. The sunsets on the west coast can be special with An Sgùrr on Eigg and the hills of the Rum Cuillin offering dramatic backdrops. Though not part of Lochaber the neighbouring large islands of Skye, Coll and Mull add interest and variety to the seascapes. Smaller inshore islands – Carna, Eilean Shona and Lismore – add further character to the dramatic scenery.

Glacial Rebound

From Rum looking  to Canna

From Rum looking to Canna

So much ice built up on the land during the recent glaciations that the land itself was depressed. When the ice melted the land then started to slowly rise up again to its previous position. This process is still happening today. The land in the Arisaig area for example is some 40m higher than it once was. Loch Morar is the deepest freshwater loch in Britain although at one time it was connected to the sea near Back of Keppoch. As the land rose after the ice melted the glacial debris at Mointeach Mhòr proved to be a barrier and it was easier for the water to spill over along the very short River Morar instead. The sea at one time also extended beyond Loch Eil and linked with Loch Shiel to cut off a sizeable part of the mainland which is sometimes referred to as ‘Ardgour Island’. Even though the water in Loch Shiel is over 120m deep it is only 4m above sea-level. It is easy to imagine the sea once extending across Kentra Moss near Acharacle and linking at the head of Loch Shiel with the waters of Loch Eil. The many raised beaches around the coast show further evidence of significant land/sea-level changes.

A Landscape of Fjords

The west coast of the Scottish Highlands is often described as a fjord landscape and its character has largely been shaped by recent glacial activity. Ice-smoothed rocks can be seen in many places around the coast, most noticeably beside Loch Leven and Loch Linnhe. Rocks carried along beneath the ice created stress points in the bedrock over which the ice passed. This caused glacial striations and crescent-like ‘chatter marks’ to be formed on the bedrock.

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Recumbent fold

Deformation

Most of the rocks of Lochaber are very old and highly deformed. There are some remarkable examples of highly folded rocks in the area south of Arisaig. In several places the rocks are seen to have been pushed over into large flat-lying folds which geologists call recumbent folds. A fine example can be seen on the north side of Loch nan Uamh just where the road turns northwards and goes under the railway.

There are some small but interesting exposures of much younger Jurassic rocks where fossil hunters can pay a fruitful visit. The commonest fossil in the area is a shell called Gryphaea, more popularly known as the ‘Devil’s toenail’. It can be found on the shore of Loch Aline. Ammonites and other marine shells have also been discovered on Ardnamurchan and Eigg. The famous geologist Hugh Miller found a fossil plesiosaur, a marine reptile, on Eigg. Dinosaurs remains have been found on Skye but not in Lochaber.

 

A large Dyke

A large Dyke

Volcanic Activity

The youngest rocks seen around the coast are associated with the events just prior to the opening of the NorthAtlantic Ocean about 55 million years ago, when Europe and Greenland were still joined. Tension in the Earth’s crust caused large fissures to develop and lava erupted from these fissures and poured out onto the land. When the magma cooled and crystallised in the fissure it formed a feature called a dyke. In many cases on the west coast these dykes have proved harder than the country rock. They now stand out as small walls and narrow peninsulas running out to sea. Most of these features in Lochaber are associated with igneous activity and large volcanoes on Mull and Skye.