Geology
The loch lies between hills just east of the Moine Thrust Belt, an unusual geological structure that runs from the Sleat peninsula in Skye on a northeast diagonal to Loch Eriboll on the north coast of Scotland. In this area, geologists found in 1907 that younger rocks from the west lay below the older rocks of the east, a discovery that helped lead to the modern theory of mountain building. The Lewisian gneisses around Loch Alsh were formed in the Precambrian period, about 2800 million years ago, while the volcanic rocks, gabbro and granite that make up most of Skye, and that in some places lie under the older gneisses, are just 55 million years old. The ancient metamorphic rocks around Loch Alsh have been heavily eroded over the years, most recently by a series of ice ages.
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