Fair Isle is mainly composed of 385-million-year-old sedimentary rocks and, although they are not particularly red on Fair Isle, these rocks are often known as the (Middle) Old Red Sandstone. The cliffs provide excellent, but inaccessible outcrops, but the best areas to get hands-on with the rocks are around Buness and The Haven and South Harbour/Skadan. The commonest rock type is a grey to buff, locally red, sandstone. These are locally interbedded with coarser-grained beds (pebbly grit and conglomerate) and finer-grained beds (dolomitic mudstone and siltstone). The colour is largely the result of weathering, and rock falls often result in the exposure of fresher, much redder rocks (e.g. at Lericum).
The sandstones are thought to have been laid down by an eastward-flowing, braided river system which linked alluvial fans to the west and a lake margin to the east. The finer-grained rocks, or shales, are believed to be of a lacustrine origin. These shales contain rare plant remains and a small number of fish scales and bones.
Everywhere, the beds are steeply inclined to the ESE; the dip varies from 55° to 75° . It is this angle that has led to the profusion of stacks, arches and caves.
The island is cut by a number of WNW-trending faults, most of which appear to have relatively little throw. The fault planes form belts of soft, intensely shattered rock e.g. at Finniequoy and Geo of Wirvie. The North and South Havens are thought to delineate the site of another fault, where the weakness has been exploited by the sea.
There are also some igneous intrusions (known as dykes), most of which were emplaced along these fault zones. Dykes are most common on the west and south-west coastline, and best seen from the sea. There is some mineralisation associated with the dykes, some of which contains copper ore.
Copper ore and mining
Copper is known from several inaccessible sites on the west coast. There is no evidence that copper was mined in prehistory and it is first mentioned by a visitor in 1808, several of whom later attempted to evaluate its significance.
The main site is in Copper Geo, NW of North Naaversgill, where a scapolite-calcite vein complex associated with a dyke contains patches and zones of copper and iron mineralization. Copper ores include the minerals chalcocite, chalcopyrite, covellite and malachite. Silver and gold were found in small quantities in addition to the copper.
Small amounts of copper have also been recorded at South Raeva and Ditfield.
Exploration activity peaked prior to the First World War, but press reports of "an extremely rich ore" are now known to have been an exaggeration.
It has to be said that Fair Isle is not a place where you are likely to casually find fossils. Nonetheless, important, but insignificant-looking, remains of some early plants were first found in 1969 and the island is the only site in Britain to yield a flora of this specific age. They provide an important evolutional link in a group of plants known as progymnosperms (extinct woody, spore-bearing plants) and they have been the subject of several research projects.
Early Devonian plants were pretty basic and formed no more than a low, carpet-like growth, and it wasn't until the Mid Devonian that these shrub-like forests of primitive plants developed.
Note that the entire Fair Isle outcrop is protected. As part of an SSSI, it is not possible to collect rock specimens without permission from SNH. In any event, if you find a significant fossil, they are so rare that you should consider taking it to a museum.
The primary reference to the geology of Fair Isle is contained in Bulletin of the Geological Survey of Great Gritain No. 41 published in 1972. This contains details of both the Old Red Sandstone sediments and the igneous intrusions and mineralization. This is available to view online at the BGS website.