Smoo Cave is a large combined sea cave and freshwater cave in Durness in Sutherland, Highland, Scotland. The cave name is thought to originate from the Norse smjugg or smuga, meaning a hole or hiding-place.
Smoo Cave was formed within Early Ordovician dolomites of the Durness Group (also known as the Durness Limestone).
The cave has formed along the boundary between the light grey Sangomore Formation and the dark grey, mottled Sailmhor Formation (sometimes called Leopard Rock), both of which form part of the Durness Group succession.
These horizons close to the formation boundary are characterised by large and abundant chert nodules which can be found all along the inner stream chamber where they have been left behind after dissolution of the surrounding dolomite.