Glencoul

Whichever way you look at it, Glencoul is one of the most dramatic places in Britain. A deep fjord carved through stunning landscape, the valley gives a great section through Highland Geology. The view across Loch Glencoul to the Aird da Loch peninsula is one of the most photographed in World geology. For in that view there is the Glencoul Thrust, one of the major structures of Assynt and one of the first thrusts in the world to be recognised as such.
Even from afar it is possible to distinguish the rocks - a sandwich of Lewisian basement filled with a panel of gently dipping Cambrian strata. The upper tract of Lewisian is part of the Glencoul Thrust Sheet. The lower tract is the foreland. And by matching ancient features in the Lewisian geology (the Laxford Front) between these two tracts it is possible to estimate that the Glencoul Thrust has moved about 30 km.

Remarkably all this movement has done little to disrupt the internal order of the thrust sheet. Even the rocks beneath the thrust are not too badly effected. The Cambrian quartzites dip gently beneath. However, the softer, upper parts of the Cambrian stratigraphy have fared rather less well. The Durness limestones have been all but swept clear. The Fucoid Beds and Salterella have been stacked up on a myriad of minor thrusts (called imbricates).

Above the Glencoul Thrust Sheet lies the Moine thrust. This is exposed at a site international importance - the Stack of Glencoul. Here the thrust is very obvious, carrying Moine metasediments onto Cambrian quartzites. These quartzites are the sedimentary cover to the Lewisian basement within the Glencoul Thrust Sheet. But unlike their counterparts in the foreland, these quartzites are very strongly deformed.
The worm burrows within the Pipe Rock have been streaked out, smeared flat along the bedding plane like some kind of tectonic road kill. This shearing has imparted a new fabric in the quartzites where all the quartz grains have been flattened and streaked out.
These are mylonites. The Moine has fared no better. These rocks too are intensely streaked out. The new fabric in both the Moine and the quartzites are parallel - to one another and to the Moine Thrust itself, testimony to the high strains associated with its movement. Unfortunately there are no early features that can be used to deduce the amount of movement on the Moine Thrust - but the best guess is that the slip exceeds 100 km.

An ideal to study the geology of Glencoul is to use a boat. Regular daily cruises are operated from Kylesku by "The Statesman". Depending on the state of the sea you can ask to be put ashore on the imbricates at Eilean an Tighe [NC 266304] or on the landing causeway [NC 272300].
The tidal area between Eilean an Tighe and Eilean Ard contains a set of repeated Fucoids and Salterella and the Glencoul Thrust is nicely exposed there too. While the cruise gives excellent views of the imbricates, it's a walk up to the Stack of Glencoul. There are stunning outcrops of mylonites on the platform beneath the Stack [NC 289287]. It is possible to walk into Glencoul via its southern shore but great care should be taken passing the "bad step" at Tom na Toine [NC 260302]. There are hands-on outcrops of the Glencoul Thrust there (placing Lewisian onto Durness carbonates). Please do not take any specimens here or at the Stack - even of loose material. If there has been a lot of rainfall don't miss the chance to see Eas a'Chual Aluinn - reputedly the highest waterfall on mainland Britain.

If you'd like to arrange a cruise (apart from the regular sailings) on the Statesman, give them a call on: 01571 - 844446.

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