Geology of Eigg

Eigg is a place where geology and landform are constant sources of amazement. With the oldest rock at sea level and the youngest , the Sgurr pichstone , forming the highest ground, the rocks of which the island is formed and their relationship to those of the surrounding areas tell us much about Scotland' s geological history.

Hugh Miller, the pioneering Scottish geologist, made an important contribution to that history with his discovery of plesiosaur remains on Eigg during his visits to the island in 1844 and 1846.

Even within a short time, excursions to see the geology of Eigg are extremely rewarding. The two essential walks are Laig bay, the Singing Sands and the North end or the Caves and the Sgurr

Jurassic Park at the north end
In Jurassic times, 168 millions years ago, Eigg was a shallow lagoon cut off from the open sea where ammonites thrived and plesiosaur swam around! Great influx of sand formed deltas into this lagoon. This is now the North coast between Laig and Kildonnan, famous for its Hugh Miller reptile beds and the Singing Sands beach. On that beach the pure quartz sand grains emit a shrill squeak when they are crushed underfoot in dry weather.

Hawaii in the Hebrides
61 millions years later, during the opening of the Atlantic, repeated eruption of fluid basaltic lavas (like those erupting on Hawaii today) built up layers several hundred meters thick. Accompanying stresses and stretches produced the dense swarms of dykes so particularly well displayed between Laig Bay and the Singing Sands. Weathering and erosion of the basalt layers then produced Eigg's terraced landscape.

The mighty Sgurr
58 millions years ago, a new volcano was born on neighbouring Rum. During one of its last eruptions, thick viscous lava flowed slowly along the winding bed of a river system carved in the basaltic plateau. Filling it to the brim, the pitchstone lava cooled, forming columnar joints at right angles to the cooling surface of the valley sides, creating amazing fan like patterns.

Harder than basalt, the pitchstone lava withstood erosion far better than the surrounding rock. With glaciation providing the finishing touches, the mile long pichstone ridge now stands proud as the inverse of an ancient valley, as impressive as Antrim's Giant's Causeway.

Ancient storm beaches
One million years ago, Scotland experienced a succession of ice ages. It is the waning stages of the last one , 26 thousands years ago, which has shaped the present outline of the island.

Coarse glacial debris transported by the ice created the moraine landscape of the crofting area with its flowing parallel ridges. Between these, boulder clay has provided a rich soil for tillage. Brought over by the ice, erratic pebbles and boulders from the mainland, mostly gneiss and pegmatite, are dotted around the island.

When the ice finally melted, storm-beaches formed. But as the land rebounded faster than the rate of sea level rise, they are now found at heights of 5 metres above sea-level behind Laig Bay or near Eilean Thuilm at the North end.

Today the landscape of Eigg continues to change, and the basalt cliffs at the north end are retreating. Mudslides happen regularly there, following on the spectacular landslides which formed after the last de-glaciation and are another of Eigg's striking landforms.

Further reading:
The geology of Eigg, by John Hudson, available from the Isle of Eigg Craftshop
Click here for more details

Geology of Rum and the adjacent islands, by C.H. Emeleus and others, The Stationary Office, London. 1997

For details of a summer course on the Geology of Eigg, visit the Glebe Barn website