Geology of Durness

The Sangomore Formation belongs to the lower-middle parts of the Durness 'Limestone' and is dominated by pale grey, medium to coarse sandy grainstones, although it is common to find highly dolomitized beds with a coarse crystalline texture after dolomization. Lilac, wavy porcelain-like limestones dominate towards the top of the formation with large bed-collapse structures also seen (due to past evaporite removal). Intraclast breccias capping the limestones are also abundant here, containing dolostone and limestone clasts types.

​

Chert-rich beds are common throughout (especially at the base), as are small-scale stromatolite beds, some picked out by thin alternating dark and pale grey laminae. Small centimetre-wide nodular lenses are found, as is a distinctive 2m thick chert bed at the formation base showing deep orange staining preserving stromatolitic laminae (black coloured). Thinly bedded, beige coloured grainstones with circular pits (porous rocks) reflecting their ooidal nature are also found (as are pitted cherts) and contain some evidence of ripple lamination. These are commonly capped by thin pale grey (often red hematite stained), centimetre-thick intraclast breccias consisting of local dolostones within a coarse sandy-crystalline dolomitic matrix. The uppermost part of the formation is then dominated by interbedded dark grey to blue coloured limestones with beige sections showing evidence for large stromatolite features (at contact with overlying Balnakeil Formation below golf course at Balnakeil Bay)