Islands of The Clyde - Geology and Geography

Geology and Geography

The Highland Boundary Fault runs past Bute and through the northern part of Arran, so from a geological perspective some of the islands are in the Highlands and some in the Central Lowlands. As a result, Arran is sometimes referred to as "Scotland in miniature" and the island is a popular destination for geologists, who come to see intrusive igneous landforms such as sills and dykes as well as sedimentary and metasedimentary rocks ranging widely in age. Visiting in 1787, the geologist James Hutton found his first example of an unconformity there and this spot is one of the most famous places in the study of geology. A group of weakly metamorphosed rocks that form the Highland Border Complex lie discontinuously along the Highland Boundary Fault. One of the most prominent exposures is along Loch Fad on Bute. Ailsa Craig, which lies some 25 kilometres (16 mi) south of Arran, has been quarried for a rare type of micro-granite containing riebeckite known as "Ailsite" which is used to make curling stones. As of 2004, 60 to 70% of all curling stones in use were made from granite from the island.

In common with the rest of Scotland the Firth of Clyde was covered by ice sheets during the Pleistocene ice ages and the landscape is much affected by glaciation. Arran's highest peaks may have been nunataks at this time. After the last retreat of the ice sea level changes and the isostatic rise of land makes charting post glacial coastlines a complex task but the resultant clifflines behind raised beaches are a prominent feature of the entire coastline.

The soils of the islands reflect the diverse geology. Bute has the most productive land, and a pattern of deposits that is typical of the southwest of Scotland. There is a mixture of boulder clay and other glacial deposits in the eroded valleys, and raised beach and marine deposits elsewhere, especially to the south and west which result in a machair landscape in places, inland from the sandy bays, such as Stravanan.

The Firth of Clyde, in which these island lie, is north of the Irish Sea and has numerous branching inlets, some of them substantial features in their own right. These include Loch Goil, Loch Long, Gare Loch, Loch Fyne and the estuary of the River Clyde. In places the effect of glaciation on the seabed is pronounced. For example, the Firth is 320 metres (1,050 ft) deep between Arran and Bute, although they are only 8 kilometres (5.0 mi) apart. The islands are all exposed to wind and tide and various lighthouses, such as those on Ailsa Craig, Pladda and Davaar act as an aid to navigation.