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The Hebridean island of Eigg is a geological jewel, with an interesting mix of sedimentary and igneous rocks and links to one of Scotland’s foremost geologists, Hugh Miller, who discovered Plesiosaur bones here in 1844.
The oldest rocks on Eigg are sedimentary rocks from the Jurassic Period. These are mainly sandstone, mudstone and limestone, and are found around the northern fringe of Eigg, close to the shore. They were deposited in shallow seas or lagoons, and contain many fossils, including molluscs, oysters and some ammonites. The most impressive fossils, Plesiosaur bones, come from a single bed of red-weathering limestone and various vertebrae and bones have been found over the years.
On top of the sedimentary rocks there is a series of basalt lava flows, which make up most of the island. These erupted from fissures about 60 million years ago, and contain in-filled gas bubbles and columnar jointing. Differential weathering of the flows has resulted in trap topography, with the more coherent central portions of lava flows forming small cliffs.
The final episode of volcanic activity, after a gap of a million years or more, was the most impressive and forms the Sgùrr of Eigg, an amazing sinuous ridge of pitchstone rock that dominates the south of the island. Underneath this final pile of volcanic rock are river deposits and pine tree fossils, showing that the last eruption filled a river valley carved out in the older lava flows. Recent analysis (Brown and Bell, 2013) suggests this is an ignimbrite, deposited in a single violent eruption of a volcano in Skye.
Brown, D.J. and Bell, B.B., 2013, The emplacement of a large, chemically zoned, rheomorphic, lava-like ignimbrite: the Sgurr of Eigg Pitchstone, NW Scotland. Journal of the Geological Society, London, Vol 170, pp 753-767.
Hudson, J and Allwright, A, 2003, The Geology of Eigg, Isle of Eigg Heritage Trust (currently out of print, but a 2nd edition is being prepared).
Isle of Eigg website – www.isleofeigg.net