Two contrasting landscapes sit either side of a major thrust zone at Watlee. Their character and vegetation differs due to the chemical make-up of the underlying oceanic and continental rocks.
The rocks of western Unst were once continental sediments on the fringe of an ocean called Iapetus. The rocks of eastern Unst were the crust beneath the ocean. Some 420 million years ago as the ocean closed the oceanic crust was forced up over the rocks of the continent. The contact zone is marked by a post beside the Burn of Watlee.
The hills to the west are the former sediments, metamorphosed by the heat and pressure of collision to form silica-rich schists and quartzites – part of the Dalradian Supergroup of rocks. They are acidic and weathered to produce soils with low fertility. The wet climate provides ideal conditions for sphagnum mosses to grow. The dead remains of sphagnum have built up to form a thick layer of peat. This is blanket bog – a globally rare habitat.
The exposed oceanic crust that forms the hills to the east is called an ophiolite. The harzburgite rocks of the ophiolite are increasingly crushed and deformed towards the contact. Ophiolite rocks were metamorphosed during the collision, as hot fluids altered the mineral olivine within them to serpentine. These basic serpentine rocks produce soils which support a rich heathland of flowers and sedges.
