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 ‘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 southwest 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 being:
The nearby islet of Brei Holm also has caves that can be accessed by small boats when conditions permit.
Popular Shetland author, David Malcolm, has published the first photographic guide to the geology of the Shetland Islands. Shetland’s landscape, its hills and beaches, cliffs and stacks, are not only places of extraordinary beauty but have locked up within them secrets from an ancient past. What is particularly amazing about Shetland is that many of these treasures are so readily accessible. You can read more about Papa Stour and Shetland’s geology by supporting David through buying his book from the National History Book Service.
Age range: Eifelian Age (DI) — Eifelian Age (DI)
Lithological Description: Comprises principally rhyolite lava, rhyolitic tuff and agglomerate and basalt. In its type area on Papa Stour, the lava is largely de-vitrified. It occurs in two thick flows with intercalations of tuff, basalt and sandstone. The rhyolite is strongly banded. The basalt, which is intensely weathered locally, occurs in up to four flows. These have thick scouriaceous upper zones with chaledony-, calcite-barytes- and zeolite-filled vesicles. Similar rhyolite and pyroclastic rocks, such as the Melby Rhyolite Member, occur on the Walls Peninsula on Mainland Shetland.
Definition of Lower Boundary: In the type area, there is an unconformable contact upon sandstone and tuffaceous sandstone of the Melby Sandstone Formation. In the Melby area, it is in faulted contact along the Melby Fault against the Sandness Formation (Lower ORS) on the Walls Peninsula, or along a highly irregular contact with ‘blocks and tongues’ of rhyolite, that penetrated the underlying grey, micaceous, ripple laminated sandstone with rhyolite clasts of the Melby Sandstone Formation, to a depth of about 60 cm whilst it was an unconsolidated sediment.
Definition of Upper Boundary: In the type area, the rock at the top of the Papa Stour volcanic sequence is the youngest exposed bedrock. In the Melby area, the top of the upper unit of the Melby Rhyolite Member is a dis-conformable, irregular, brecciated contact against fine-to-medium-grained, planer bedded pink sandstone, with partings of flaggy sandstone and siltstone, and containing scattered rhyolite clasts of the Melby Sandstone Fm. The upper boundary of the lower unit is not exposed, but mapping suggests that it is dis-conformably overlain by similar sediments of the Melby Sandstone Fm to those overlying the upper unit.
Thickness: Up to at least 200 m in the type area.
The Shetland Amenity Trust also has more information on West Shetland geology here:
Papa Stour is designated a Site of Special Scientific interest (SSSI) and more information can be found here.