Druim Iosal

Also within

Ross and Cromarty County

Condition (NatureScot)

Geology & landscape

Rock Type: Gneiss

Description

DRUIM IOSAL Druim Iosal Site of Special Scientific Interest (SSSI) lies in Gleann Beag near the centre of the Glenelg peninsula. Exposures of an area of rock called the Glenelg Inlier are found here. The Glenelg Inlier is an exposed area of ancient rocks called Lewisian gneisses, surrounded by much younger metamorphic rocks belonging to the Moine Supergroup, which are the rocks that underlie much of the Northern Highlands. A major zone of shearing runs roughly north-south through the Glenelg Inlier and because the gneisses on either side exhibit different characteristics they are termed the Eastern and Western Units. The Western Unit of the Glenelg Inlier consists largely of ‘typical’ Lewisian gneiss predominantly formed from cooled molten rock. The Eastern Unit, however, contains many rock-types which are of sedimentary origin. These include excellent exposures of magnesium-rich marbles, graphite-bearing schists and a rare rock type (gneissose garnetiferous kyanite-bearing pelite) which contains the minerals garnet, kyanite and biotite. This is one of the largest and best-exposed outcrops of kyanite-bearing rock in the British Isles. The rocks of sedimentary origin are relatively magnesium-rich, suggesting they were originally magnesium-aluminium-rich mudstones and magnesium-rich limestones (dolomites) deposited in a sabkha-like environment. Data indicates that around 1080 million year ago, the rocks of the Eastern Unit were metamorphosed by burial to a depth of more than 50km during the Grenvillian Orogeny; this affected rocks in the Grenville Province of eastern Canada but few if indeed any other rocks in Scotland. The Druim Iosal SSSI is of national importance for its variety of metamorphosed sedimentary Lewisian rocks, which have unusual geochemistry, indicating an unusual origin, and also an unusual history of metamorphism.