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 '' or '', meaning a hole or hiding-place. Geology 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. The cave was formed along two geological lines of weakness by a combination of erosion from the sea and an inland underground stream which has formed the innermost chambers. Upstream of the Allt Smoo which runs into the cave, impermeable quartzites have been faulted against the Durness Limestone, causing the stream to sink down into the carbonate rock soon after it has crossed the contact between the two different rock types. Physical description The cave is unique within the UK in that the first chamber has been formed by the action of the sea, whereas the inner chambers are freshwater passages, formed from rainwater dissolving the carbonate dolomites. Partway through the cave the waters of Allt Smoo also drop in as a high waterfall. This is mainly due to the nearby dolomite–quartzite geological boundary where the Allt Smoo stream crosses the impermeable quartzites and sinks on meeting the permeable dolomites. The cave can be thought of as two caves formed by different mechanisms that have joined together over time. The cave is composed of three main sections: a large sea cave entrance chamber, a waterfall chamber and a short freshwater passage which leads to a terminal sump chamber with some flowstone formations at the rear. The cave entrance and main chamber have been considerably enlarged by sea action to approximately wide and high, the largest sea cave entrance in Britain. The entrance is located at the end of a long tidal gorge (Geodha Smoo) which was once part of the cave, now collapsed. Several remnant pillars can be seen along the eastern side of the Geodh along with a large section of the previous roof