Geology
The island is composed of a variety of volcanic and sedimentary rock formations from the Devonian period. At that time the Scottish landmass formed part of the Old Red Sandstone Continent and lay some 10-25 degrees south of the equator. The accumulations of Old Red Sandstone, laid down from 408 to 370 million years ago, were created as earlier Silurian rocks, uplifted by the formation of Pangaea, eroded and then were deposited into river deltas. The freshwater Lake Orcadie existed on the edges of the eroding mountains, stretching from Shetland to the southern Moray Firth. The structure of Papa Stour is largely made up of ashes and lavas from volcanic activity associated with this period, including bands of solidified volcanic ash and lava (rhyolite), but there is also a Devonian fish bed at Lamba Banks. There are numerous large boulders deposited by Pleistocene glaciation.
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:
...in fairyland, so exquisite is the colouring of the roof and sides and so pellucid is the water... alcoves or recesses like stalls in a church.
In 1953 the spectacular headland, 'Da Horn o Papa' fell into the sea during a storm. The nearby islet of Brei Holm also has caves that can be accessed by small boats when conditions permit.
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