The rocks of Fair Isle are mainly Old Red Sandstone, formed some 450 million years ago during the Devonian period...
while much of its landscape was carved by ice during the most recent glaciation, the Devensian - which ended only about 10,000 years ago.
A 'glacial erratic' - a large boulder transported from Norway to Fair Isle by the ice sheets
Around the South Lighthouse however the rocks are volcanic in origin.
Pahoehoe (p'ho-ee ho-ee), a classic sign of vulcanism, it is formed when a sheet of molten lava wrinkles as it cools on land.
This was once boiling mud - the holes are where the bubbles were