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 'Hol o' Bordie' is a cave that passes right through the north-west tip of the island. It is 300 metres 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: