Avernish
Also within
Condition (NatureScot)
| Unit | Feature | Condition | Area (ha) | Assessed |
|---|---|---|---|---|
| F1 | Moine | Favourable | — | 2007-02-13 |
Climate exposure
This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.
Coastal habitats are exposed to sea-level rise and storm erosion. Where hard sea defences sit behind them, there's nowhere landward to retreat to — a squeeze that keeps compressing the habitat no matter how well it's managed.
Geology & landscape
Geological Feature: Rock Outcrop · Rock Type: Conglomerate, Gneiss
Landscape processes: Coastal
Description
AVERNISH The Avernish Site of Special Scientific Interest (SSSI) lies on the north coast of Loch Alsh, below Kirkton. The site contains exposures of ancient Lewisian gneiss, which are the oldest rocks in Britain, up to 2500 million years old. They are part of the Glenelg Inlier, a large body of Lewisian gneiss lying to the east of the Moine Thrust (an area which was near the centre of the Caledonian mountains during their formation in the Caledonian Orogeny, around 430 million years ago), surrounded by much younger metamorphic rocks belonging to the Moine Supergroup. Avernish SSSI is of national importance as it demonstrates the relationships between Lewisian inliers and the surrounding Moine rocks, and enables the sequence of deformation events in rocks to the east of the Moine Thrust to be compared with that in the Moine Thrust Belt (an area which was at the very edge of the Caledonian mountains) to the west of the Moine Thrust. The Moine rock immediately adjacent to the Lewisian gneiss is a conglomerate containing Lewisian clasts. This indicates that the Lewisian-Moine contact is not a fault or other tectonic contact but is an unconformity, with Moine sediments being deposited on a landscape eroded into Lewisian gneiss. Three episodes of deformation are seen in the Moine rocks and immediately adjacent Lewisian rocks. The latest of these episodes is correlated with mylonite formation, which is the earliest phase of deformation in rocks of the Moine Thrust Belt. This indicates that Caledonian deformation in the Moine Thrust Belt, at the edge of the Caledonian mountains, occurred significantly later than deformation to the east of the Moine Thrust in the centre of the mountains. To the east of the Moine outcrop at Avernish, Lewisian rocks nearer the centre of the Glenelg Inlier are far less affected by Caledonian deformation with only localized shear zones occurring.