Clachtoll

White sand, turquoise sea and stunning views back across Assynt - what more reasons do you need to visit Clachtoll? Well, there are spectacular geological outcrops, including the oldest sediments in the British Isles that record landscape processes almost one billion years old.

Clachtoll is a hamlet about 8 km NW of Lochinver, accessed via a winding single-track road. There is an old "visitor centre" which has information on the local natural history. The geology is to be found on the low cliffs, many rising above sandy beaches when the tide is out. So it's a great area to explore. These notes only give a taste of what you can find.

The Clachtoll area contains two main units, the Lewisian gneisses and the Torridonian sediments. The sediments are part of the so-called "Stoer Group", a sequence of sandstones, mudstones and conglomerates that were deposited about one thousand million years ago. So these rocks are two hundred million years older than the main Torridonian unit (the Applecross Formation) that forms the main mountains groups of Quinag, Suilven and others.

At Clachtoll the general dip of the Stoer Group sediments is about 25 degrees to the west - so a geological succession can be built up by working from east to west. The best place to park up is at the visitor centre [GR NC 040273]. A good place to head for is the south side of the sandy cove (Bay of Clachtoll) where there are outcrops of Lewisian gneisses. This is representative of the geology inland which is dominated by Lewisian gneisses. The junction between this "basement" and the "cover" of Torridonian sediments is to be found here. This is an unconformity, representing a time gap of over 700 million years. But the most dramatic outcrops are spectacularly exposed in coves south over the headland [NC 041266]. Here the unconformity is highly irregular indicating that the land surface upon which the Torridonian was deposited was hummocky.
The Stoer Group is locally a massive, conglomerate/breccia unit, containing big cobbles of Lewisian gneiss. The largest are over half a metre long. But this unit passes quickly upwards into sandstones and mudstones. These crop out to the west on the headland and are nicely exposed on the notched rocky promontory of A'Chlach Thuill [NC037267].

Further outcrops of Stoer Group sediments are to be found on the north side of the Bay of Clachtoll towards the headland of Aird na Moine [NC 036272]. These units contain rare desiccation cracks and even thin algal limestones (the oldest evidence for simple life in Scotland). These suggest that the deposition environment included shallow lakes that dried out from time to time. The rest of the strata were deposited by streams and by flash floods. Palaeomagnetic data suggest that the Stoer Group at Clachtoll was deposited a tropical latitudes. But being so old the landscape would have been utterly devoid of multi-cellular life - no trees or other complex plants.

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