Smoo Cave

Smoo is unusual in being two kinds of cave at once. The outer chamber was cut by the sea, and the inner chambers were cut by fresh water. The Allt Smoo runs off impermeable quartzite onto permeable dolomite, sinks underground at the boundary, and drops into the cave partway along as a waterfall 20 metres high. Very few caves anywhere are formed by both processes at the same site, and at Smoo the join is visible.

It sits at Durness in Sutherland, on the far north coast of the Scottish Highlands, and the outer chamber is enormous, opening straight onto a sea inlet.

The rock is Early Ordovician dolomite of the Durness Group, and the cave formed along the boundary between two formations within it: the light grey Sangomore Formation and the dark grey mottled Sailmhor Formation, which is known locally as Leopard Rock. Those beds near the boundary are full of large chert nodules, and because chert does not dissolve the way dolomite does, the nodules are left standing all along the inner stream chamber as the rock around them goes.

The name is contested and all the candidates are plausible. It may come from the Norse smjúga or smuga, a narrow cleft or hole. It may relate to the Gaelic smùid, fine spray, for the waterfall. Sir Walter Scott recorded it as Uamh as Motha, the Largest Cave. The form Uamh Smùdha was introduced for bilingual signage around 2010 and has since been picked up in Gaelic media.

The outer chamber is free and reached by a short path and steps from the car park on the A838. Getting past it to the waterfall chamber and beyond means a boat, run seasonally and weather dependent.