Geology of Durness

This photo shows the Geodh Smoo inlet that leads up to the entrance of Smoo Cave. This has been formed by wave erosion over thousands of years cutting inland and by gradual collapse of the Smoo Cave entrance. It is also worth noting that a large portion of the once-cave-roof lies within the eastern grassy slope of this inlet. Half way up the cliff face is the boundary between the Sailmhor Formation and the younger, overlying Sangomore Formation that is much lighter in colour and shows little evidence for burrowing. A small fault is also seen on the left hand side, displacing a thick chert horizon that runs along the western side of the inlet. The cyclic pattern of the Sailmhor Formation is also seen, seperated by white silica-rich chert horizons that preserve 'stromatolites' (laminated mounds formed by algae).

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The Sailmhor Formation belongs to the lower parts of the Durness 'Limestone' and is distinctive as it shows an unusual mottled texture (thought to be the result of burrows), also giving it the name 'Leopard Rock'. It tends to be much darker than the rest of the Durness carbonates and black coloured samples may be found. This has been the result of oil generation within these rocks. Organic-rich material trapped within these rocks has been gradually heated as the younger rocks have 'squashed' this lower formation and even more by Moine Thrust movements. This heating has formed oil which has long since escaped, but remnant staining is what is seen as the dark colourations. These rocks commonly give off a strong petroleum smell when a fresh section is broken open.

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These are medium-thickly bedded dark, bioturbated dolostones interbedded within the pale stromatolitic dolostones similar to the Eilean Dubh Formation. Most of the formation is dominated by thick shallowing cycles as seen at Geodh Smoo, thinning gradually upwards. Nodular, rounded to lens-shaped cherts up to 0.5m wide (commonly overlying thin cherty beds) are also abundant throughout the stromatolitic beds of the lower parts and preserve original lamination, hence are diagenic. These have been formed by parts of the dolostone turning into silica. At Balnakeil Bay, the nodules are milky white with black preserved laminae but within and around Smoo Cave they tend to be brown and crumbly. Both areas show an onion-like texture where nodules have been fractured, often showing orange dolostones concentrated towards their centres.