Isle of Eigg: Jurassic reptiles and volcanic glass

Eigg is a lovely geological gem within Lochaber Geopark, where Palaeogene basaltic lavas cap fossil-rich Jurassic sedimentary rocks, and the iconic Sgurr of Eigg pitchstone ridge dominates the skyline

The Sgurr of Eigg, at the southern end of the island, dominates the skyline above the harbour at Galmisdale. Image: Lara Reid.

Eigg is a young and dynamic island! Nearby you can find rocks that are more than 1000 million years old, but Eigg’s geology is much younger, contains evidence of one of the last-known violent volcanic eruptions in Scotland and is still being shaped by rockfalls and ongoing erosion.

The oldest rocks on Eigg are sedimentary rocks formed in the Jurassic Period. Protected from erosion by the later lava flows, they form the low ground in the northern part of Eigg and are best seen on a coastal walk. They are mainly of Middle Jurassic age, with some traces of Upper Cretaceous sandstones. The sediments were laid down in brackish, shallow water, in a coastal environment. In 1844, Hugh Miller found beds containing fossil fish debris and plesiosaur bones on the north-east coast, and evidence of Jurassic reptiles living in this area is still being found today. One spectacular feature of the Jurassic sedimentary rocks are enormous round concretions, hard bodies of cemented sandstone, found on the west coast. Nearby on the Singing Sands beach, eroded round quartz grains make a distinctive squeaking sound when scuffed underfoot.

About 60 million years ago, this area was covered by vast expanses of ‘flood-basalt’ lava flows, forming the Eigg Lava Field, which extend under the sea to the south-west of Eigg, and which also form the island of Muck. The lava flows now form steep cliffs around the northern part of the island, seen above Cleadale, and on the east coast. Within the lava flows you can find evidence of gaps in the volcanic activity and the formation of iron-rich soil horizons. These are more easily eroded than the lava flows, producing trap topography, a distinct step-like profile to many of the island’s slopes.

The Sgurr of Eigg forms a high ridge in the south of the island and makes a stunning viewpoint. It is made of pitchstone, a form of volcanic ‘glass’, with extensive dramatic, columnar jointing structures. The origin of the Sgurr ridge has been subject to speculation by famous geologists in the 18th and 19th centuries, including Sir Archibald Geikie. The most recent research, published in 2013 by David Brown and Brian Bell, suggests that the formation of the Sgurr was one of the final events of the Palaeogene volcanic episode, when pyroclastic flows from a major volcanic eruption probably originating in the Skye Central Complex covered this area in a thick sequence of ignimbrite rock. On Eigg, the ignimbrite swamped the landscape, and filled an existing valley eroded in the older lava flows. Now, it is preserved as the upstanding ridge of the Sgurr while the rest of the ignimbrite rocks have eroded away. It is also worth noting that the whole island is also shot through with Palaeogene igneous dykes from the Rum volcanic centre.

Around the north of the island there is much evidence of the recent processes of sea level change and landslips that have helped shape today’s landscape. And the process is not finished yet, for the basalt cliffs are still crumbling.

Text contributed by Jim Blair, Lochaber Geopark

Find out more
Access: The isle of Eigg is served by two ferry routes, by Cal-Mac ferry from Mallaig or by the MV ’Shearwater’ from Arisaig.
An excellent pocket field guide: “The Geology of Eigg”, by John D Hudson, Angus D Miller and Ann Allwright, published by the Edinburgh Geological Society, 2016.
A serious geological overview and a great source of references on all aspects of Eigg geology: “Geology of Rum and the Adjacent Islands”, British Geological Survey, 1997.
A very detailed recent study, with an interesting analysis of the formation of the Sgurr: “The emplacement of a large, chemically zoned, rheomorphic, lava-like, ignimbrite: the Sgurr of Eigg Pitchstone, NW Scotland”, Brown, D and Bell, B, Journal of the Geological Society of London, Vol. 170, 2013.

The Mid-Jurassic Valtos Sandstone, north end, Laig Bay. Image: Jim Blair.

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