The Sailmhor Formation (Leopard Rock)

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.

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.