In the quarry face at the ferry terminal, you will discover a shear zone and see rocks that were deformed during a major tectonic collision.
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 continent. This exposed section of oceanic crust is called an ophiolite.
At Belmont, you can see the highly deformed rocks at its base. This is a shear zone – a zone of intense strain. Hot fluids have circulated through the rocks altering the minerals within them. The oceanic rocks were originally harzburgite containing the mineral olivine. The olivine has been altered to serpentine to form serpentinite – a green rock with pale veins that give it the appearance of snakeskin – hence the name.
There are two intensely sheared areas in the north face of the quarry where the reaction has gone further, resulting in soft crumbly rock made of the mineral talc. Along the roadside, below these sheared rocks is dark graphitic schist formed from ocean-floor sediments which were metamorphosed by the heat and pressure of the collision. The schist gets its colour from graphite; a form of carbon derived from organic material in the sediments.
