Erosion of the soft volcanic rocks by the sea has created an extraordinary variety of caves , stacks , arches , blowholes , cliffs , voes and geos that are amongst the finest in Britain . The ' Holl o Boardie ' is a cave that passes right through the north @-@ west tip of the island . It is nearly half a mile long and wide enough to row through . Kirstan ( or Christie 's ) Hole in the south west is another spectacular cave , part of the roof of which collapsed in 1981 . Yet another is ' Francie 's Hole ' close to Hamna Voe in the west . This was the favourite of John Tudor who wrote of the island in his Victorian memoirs and described the cave as :