A Quick Guide to the Isle of Skye, Scotland

This is an extract from the newsletter of The ‘Black Country Geological Society‘, No. 197 October 2009. This was part of the regular section entitled ‘The Dudley Bug’ written by members Alison Roberts and Chris Broughton.

A handdrawn geological map (by A. Roberts)

The Isle of Skye is the largest of the Inner Hebridean Islands with an area of 3000Km2. Portree is the largest town on the island. The northern part of the island is formed from volcanic activity which is represented by the lava flows that dominate the landscape. Basaltic columns are a popular feature here and can be seen at Kilt Rock. Macleod’s Tables display the flat hilltop topography formed by the lava flows. In some places these dip slightly, particularly at Quirang. Skye’s volcanic activity occurred during the Cenozoic Period (previously known as the Tertiary Period) and is associated with the opening of the Atlantic Ocean 60 million years ago. Central Skye is known as the ‘Skye Central Complex’ and is home to the world famous Cuillins which are formed from two very different igneous units. Britain’s youngest mountains, the ‘Black Cuillins’ are predominantly made from gabbro and peridotite whereas the ‘Red Cuillins’ consist mainly of granite.

The southern part of the island displays the sedimentary units, some dating back to the Precambrian with the youngest types being glacial deposits from the two glaciations which affected Skye between 13,000 and 26,000 years ago. The limestones of the Durness Group display some of the best limestone pavements and related features in the UK. These can be found in the Strath Suardal area, immediately south of the Red Cuillins. The majority of the dykes trend in a general NW-SE direction.

Black Cuillin, Bla Blaven (by A. Roberts)

The oldest rock found on Skye is the Lewisian Gneiss which dates back to the Precambrian when it was formed from the metamorphism of older rocks. The oldest sedimentary rock is the Torridonian Sandstone. Due to the vast amount of igneous rocks there has been a lot of metamorphism altering the sedimentary rocks. The most distinct unit to have been metamorphosed is the limestone from the Strath Suardal Formation of the Durness Group which is Ordovician in age. This has been metamorphosed not only by the volcanic activity of the Red Cuillins but also by the nearby Beinn an Dubhaich Granite. This has formed the famous Skye Marble which is currently being quarried at the Torrin Marble quarry near Broadford, the second largest town on the island. The Moine Thrust which is one of the most important faults in Northwest Scotland runs from the Shetland Islands to the Sleat Peninsula on the south tip of Skye.

Many of the geological units which crop out on Skye can be seen in the Northwest Highlands. These include the Applecross Formation and Diabaig Formation of the Torridon Group, the False-bedded Quartzite Member and the Pipe Rock Member of the Eriboll sandstone Formation and finally the Fucoid Beds Member, the Salterella Grit Member and the Ghrudaidh Formation of the An t-Sron Formation.

References
Gillen, C. 2003. Geology and Landscapes of Scotland. Terra Publishing: England. 10, 60-61, 152, 155, 194, 204 pp.
Roberts, J. L. 2004. The Highland Geology Trail. Bell and Bain Limited: Glasgow. 69-78 pp.
Stephenson, D. 1994. Skye: A Landscape Fashioned by Geology. Scottish Natural Heritage and British Geological Society: Edinburgh. 2-3 pp.

Alison Roberts
Wallheath, West Midlands

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