Geology of Durness

The Croispol (or Croisaphuil ) Formation belongs to the middle-upper parts of the Durness 'Limestone' and consists of a thick sequence of mainly dark grey (some pale grey) bioturbated limestones (grainstones). These show little change in bed types and sedimentary structures, with the bottom of the formation being dominated by dolostones. These dolostones gradually young up into dark sandy limestones, but rare oolitic lilac limestone beds are also found, although these become lighter and more abundant higher up in the sequence. Thick beds show casts of burrows commonly up to a centimetre wide and infilled with a pale to dark, sandy dolomitic matrix. These are interbedded with normally thinner, darker beds containing very fine, vein-like burrows infilled with colourless crystalline micrite. These burrows are commonly seen as interconnecting types, including 'chrondites'. Occasional small, creamy-white nodular cherts are found throughout, some showing preserved granule beds (poorly sorted and sub-rounded). Fossils are also present throughout the formation, with sponges and ammonoid/belemnite cephalopods.