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Smoo Cave is a very large sea cave, but the rear part is a karst cave which formed inside limestones of the Durness Group. The Durness Group are layers of limestones and dolomites. The rocks were formed during Odovician and Lower Cambrian as shelf sediments. They are found in a narrow belt running north to south, from the area of the Smoo Cave to Ardarroch at Loch Kishorn. The karst features of Smoo cave are typical for such a small limestone area with impermeable and insoluble rocks surrounding it. Waters flowing on impermeable rock, disappear in swallow holes as soon as they reach the border to the limestone. They drain underground and reappear in karst springs and caves. Such a cave river is to be found inside Smoo cave, water from a burn which disappeared only a few meters away. The cave is formed in a band of Durness limestone which in turn is surrounded by quartzite, gneiss and grits. Originally a small swallet cave, the entrance has been much enlarged by the action of the sea. The first chamber is the large opening from the sea inlet and has been formed by the action of the sea. The second chamber has been formed by the action of fresh water. The roof holes show the difference in the forces that formed the caverns. The presence of caves in the vicinity of the Geodha Smoo, and indeed the presence of the Geodha itself, is a reflection of the character of the local geology, which is dominated by Cambrian Dumess Limestone.
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