Smoo Cave is a large combined

sea cave A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relic ...

and freshwater cave in

Durness Durness ( gd, Diùranais) is a village and civil parish in the north-west Highlands of Scotland. It lies on the north coast of the country in the traditional county of Sutherland, around north of Inverness. The area is remote, and the parish is ...

Sutherland Sutherland ( gd, Cataibh) is a historic county, registration county and lieutenancy area in the Highlands of Scotland. Its county town is Dornoch. Sutherland borders Caithness and Moray Firth to the east, Ross-shire and Cromartyshire (later ...

Highland Highlands or uplands are areas of high elevation such as a mountainous region, elevated mountainous plateau or high hills. Generally speaking, upland (or uplands) refers to ranges of hills, typically from up to while highland (or highlands) is ...

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

. 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 The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

s 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 Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

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

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

s 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 Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...

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 Flowstones are sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave. They are typically found in "solution caves", in limestone, where they are the most common speleothe ...

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 Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

gorge A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...

(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 which has been partly buried by the grassy slope (normally covered by rocks spelling out the names of visitors to the cave). The sea rarely enters the sea cave nowadays (only during

spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...

s) as the area has undergone isostatic uplift. The present-day cave is long up to the terminal

sump A sump is a low space that collects often undesirable liquids such as water or chemicals. A sump can also be an infiltration basin used to manage surface runoff water and recharge underground aquifers. Sump can also refer to an area in a cave ...

at the rear of the third chamber/passage. The cave travels further, however, as an active stream of notable size resurges here at all times. Previous dye-testing has linked an underwater passage to an initial sink point in the Allt Smoo stream about upstream from the main waterfall, implying that the cave system is at least twice as long as once thought. Cave divers from the

Grampian Speleological Group The Grampian Speleological Group is the oldest caving club in Scotland, having been founded in 1961. It is also the largest, with members exploring caves across Britain and the world. See also *Caving in the United Kingdom References External ...

have dived this sump for a distance of about , although large volumes of silt and peat in the water have prevented further exploration. It is worth noting that the main waterfall is often dry and will only become active once this upstream sink overflows.

Archaeology

Archaeological investigations in 1995 turned up

Neolithic The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...

, Norse and

Iron Age The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age (Paleolithic, Mesolithic, Neolithic) and the Bronze Age (Chalcolithic). The concept has been mostly appl ...

artifacts but the majority of the artifacts from the cave were related to Viking/Norse or medieval activity. Fish bones, marine shells, plant remains and mammal and bird bones indicated the processing and consumption of marine and terrestrial foods. Iron slag and boat nails led the archaeologists to surmise that boats were repaired in the sheltered inlet. Four radiocarbon dates from Smoo Cave and Glassknapper's Cave provide evidence for use of these sites between the eighth and 11th centuries AD. The cave complex is protected as a

scheduled monument In the United Kingdom, a scheduled monument is a nationally important archaeological site or historic building, given protection against unauthorised change. The various pieces of legislation that legally protect heritage assets from damage and d ...

Tourism

The cave is located east of the village of Durness and is presently served by a car park, toilets, stairs and walkways. Plans are being considered for access improvements for the estimated 40,000 visitors it receives annually. Tours of the cave also operate during the summer, taking visitors to the inner chamber by boat.