13/06/2024
Beautiful description of one of the best geology sites in Scotland!
58 millions years ago, a major volcanic eruption occurred on neighbouring Skye. A pyroclastic flow – a hot suspension of rock particles and gases – swamped the landscape, reaching the winding bed of a river system carved in the Eigg basaltic plateau. Filling it to the brim, the suspension collapsed and cooled, becoming the pitchstone that now forms the Sgurr. Further cooling produced columnar joints at right angles to the valley sides near the pitchstone’s base and amazing fan-like patterns are seen on the summit ridge.
Volcanic rocks formed from pyroclastic flows are called ignimbrites, contrasting with lavas, such as the basalts on Eigg, that cooled from liquid magmas.
Much later the pitchstone, harder than basalt withstood erosion far better than the surrounding rock. With glaciation providing the finishing touches, the mile-long ridge of An Sgurr now stands proud as the inverse of an ancient valley, as impressive as Antrim’s Giant’s Causeway.