This is one of the largest areas of serpentine debris in Europe. The gravelly soil is made from frost-shattered fragments of serpentinite. The bedrock, originally dunite, is part of the Shetland ophiolite – a section of oceanic crust that became stranded on top of continental crust during a massive continental collision. Dunite is made of the mineral olivine, some of which was altered to serpentine during the collision to form serpentinite.
The Keen has changed little since the last glaciation ended 10,000 years ago. On the north-facing slopes, stripes of large and small stones have formed due to repeated freezing and thawing of water in the soil. This process usually occurs in high mountain areas, but here it is just 50m above sea level.
Despite Shetland’s climate, drought-like conditions are common on the Keen as water drains quickly through the shattered bedrock. Several plants have adapted to this ‘desert’ environment including Edmondston’s Chickweed - found nowhere in the world except Unst - Norwegian Sandwort and Northern Rock Cress.
In parts of the Keen there are patches of heathland rich in flowers and sedges. Beneath lies a layer of sandy, serpentine soil which was transported to the site by glacial ice
