Exploring Eigg’s geology is to take a journey through eons of time when the west coast of Scotland was situated at the edge of an ancient ocean, 1000 million years ago and was then overlaid with lava flows when volcanic complexes erupted as continental drift split Scotland from North America with the opening of the North Atlantic.
During the ‘age of the dinosaurs’, particularly the Jurassic and Cretaceous periods, Eigg was submerged beneath shallow tropical seas.
It is in the Northen part of the island, that are situated the sedimentary rocks dating from the Triassic, Jurassic and Cretaceous periods. Walking down to Laig beach, you will spy rocks formed from ancient oysters beds, dating back to a time when the whole area was shallow brackish sea, where shellfish thrived and prehistoric sea-creatures such as the plesiosaur hunted down belemnites, those ancient squids.
It was Hugh Miller, the famous 19th Centurey geologist which identified the remains of these prehistoric sea-creatures after finding fossils remains now buried under the sea.
Following the seashore to the Singing Sands, you can see the Valtos sandstone dating back to the Middle Jurassic. These fossil sand dunes are interspersed with highly fossiliferous limestones which record freshwater delta conditions alternating with periodic inundations of lagoon deltas by the sea. The Jurassic oyster Praeexogyra hebridica flourished in the brackish water conditions and shell beds in the limestones are crammed with these small shellfish ,
An impressive feature of the sandstone itself is the huge concretions, some as much as 2 metres across, which have formed by migration of calcite within the sands after burial (a process calculated on one model as taking about 5 Millions years to reach the size now seen). These protrude from the cliff face, and the rock platform below is littered with these boulders which have been eroded out of the cliff. strewn around like giant pocked marbles.
Traigh Na Bigil, the Gaelic name for the Singing Sands, means the beach of the chirping, a more accurate description of the small noise produced when scuffing the sand as you walk on it. This peculiar phenomenon of the Singing Sands is attributed to the uniformity of size of the quartz grains which form them; but they only sing when dry.
The Sgurr of Eigg, the prominent landmark that gave its name to the island, is built of a rock called pitchstone. This dramatic stump of pitchstone, sheer on three sides rises to 1,289 feet (393 metres) above sea level.
At 58 million years old it is the youngest volcanic rock preserved in Scotland. It was born in one of the last burst of volcanic activities in the west coast when an ancient valley carved out in older lavas flows, filled with viscous pitchstone erupting from a volcano on Rum. Rapid cooling of the lava – some 200 meters thick in some part, led to the remarkable columnar formation exposed by erosion of the surrounding softer rock.
The Rum volcano which is part of a long chain stretching from to Arran to Rockall, the volcanoes of Rum and Skye, represent the remains of volcanoes that formed as the crust between Europe and America split and drifted part with the formation of the Atlantic. Now deeply eroded by water and ice, the internal plumbing of these volcanoes has been laid bare, leaving only the jaggy outlines of their Cuillins.
But it is the turmoil caused by the creation of the once great volcano on Rum, which has given the shoreline between Laig and th Singing sands such an array of fantastic shapes; Where the earth crust shook and opened, lava rose from the Rum volcanic centre cutting across the Jurassic sandstones of the intertidal platform. This crisscrossing it with intrusive dikes where hot lava baked the sandstone into a rock harder to weather, fantastic shapes and archways have been created.
For the last two-and-a-half million years we have been subject to periodic warming and cooling, with the result that we have been covered by ice many times. The last of the ice only disappeared around 10,000 years ago.
It was the action of the ice which sculped the Sgurr of Eigg in its present shape, by scouring out the softer lavas surrounding it, and leaving it standing proud, x ft over sea-level. The Sgurr at one time even fed its own glacier running down towards the sea in Grulin, huge blocks of Sgurr pillars tumbling down to rest in Grulin, a name which means exactly this, the stony place!
Another glacier run from the mainland in a northwest direction, eroding a deep gash between the two mountain masses on the island and leaving behind a series of undulating hillocks separated by clay filled hollows, that distinguished the landscape of Cleadale on the north end of the island. Huge erratic blocks and boulders of granite and gneiss from the mainland have also been left here and there among such morainic deposits.
Cleadale itself – the valley of the cliffs in Old Norse – takes its name from the distinctive amphitheatre of lava cliffs eroded by the ice out of the Beinn Bhuidhe plateau, dominating today’s crofting landscape.
Some parts of Eigg are rising, and others are sliding!
Thousands of feet of ice resulted in Scotland being pressed down into the molten rock beneath its surface. When the ice melted away the land has slowly rebounded, so that beaches which were once at sea level are now tens of metres above that level.
The release of the land by the melting of the ice also caused Eigg’s raised beach at Laig, where ancient beach pebbles can be seen high up on the land, cut through by the path to Laig farm as it winds its way down towards the beach. Such ancient raised beaches can be seen as the steps on the edge of the Rum Cuillins outlines as you contemplate them from the beach.
A closer look at the slopes below Beinn Bhuidlhe, reveals how the cliffs are far from being static, with remains of old landslides in evidence by the Cleadale cliffs Weathering from the rain are still causing landslides, and the plateau is slowly collapsing on its edges, with the newest landslides occurring on the South East coast of Kildonnan.