Shaping the Landscape

The rocks that form Shetland, built up as separate blocks of the Earth’s crust over a period of almost three billion years.

These blocks, known as terranes, formed perhaps hundreds of kilometres apart and under different conditions. The terranes were brought together in their present position by vertical and lateral (sideways) movement along faults, as continents collided and mountains were built up and then pulled apart.

The Caledonian Mountain Chain, which included the Shetland ophiolite, was partly built up by compression forces acting along a series of low-angle reverse or ‘thrust’ faults that pushed rocks of one terrane up and over the rocks of another.

Once the mountain chain had been built up, the direction of the tectonic plate movements beneath changed, and the mountain chain began to be pulled apart along normal faults.

Wide valleys, known as pull-apart basins, formed between the mountains as rocks slipped vertically down past each other due to the tensional forces pulling on the crust at either side of the faults. These basins quickly filled with sediment eroded from the mountains on either side and are often known as 'Old Red Sandstone Basins'. Visit the Leaflets section to read more about Shetland's Old Red Sandstone.

These tensional forces also forced blocks of crust to slide horizontally past each other for many hundreds of kilometres along a series of strike-slip fault, which were active from about 435 to about 175 million years ago.

The major strike slip fault system that brought the terranes of Shetland together is the Great Glen/Walls Boundary Faul, which slices through Scotland and Shetland. Across Shetland there are two major off-shoots or ‘splays’ of this fault – the Melby Fault and the Nesting Fault – although the most recent movement along the Melby Fault may have been reverse.

As strain built up and then released, periodic movement along the faults triggered earthquakes and crushed or deformed and altered rocks along the fault zones.

The fault zones cut deep into the crust beneath the mountains, and as lines of weakness acted as conduits for the rising magma, this formed the intrusive rocks within the mountains or volcanoes in the sedimentary basins.

Freeze-thaw

Freeze-thaw weathering generally occurs within mountainous regions when rainwater collects in small cracks within rocks. As the temperature drops at night, the water freezes and expands, pushing the cracks a little wider. During the day the ice melts, but more water can collect within the expanded cracks. At night the water freezes again, and this cycle repeats until the rock eventually splits apart.

Freeze-thaw action also 'sorts' the stones in the soil. Water in the soil freezes and pushes the stones and smaller grains upwards. When it thaws the smaller grains drop underneath the stones, preventing them from falling back. Over time the stones are pushed to the surface and sorted into rows of different sizes.

Sea-level rise

Since the melting of the last ice cap over Shetland, approximately 10,000 years ago, water has filled the over-deepened valleys and depressions carved out by the ice to leave a flooded landscape of islands and inlets (voes).

New landforms, such as sand spits, ayres and tombolos, have been built up as the sea has reworked sediments of sand, gravel, pebbles and shingle.

Shetland is the only part of Scotland where there is no known evidence of sea levels higher than present. Throughout mainland Scotland, a process known as isostatic uplift is taking place. This means that land that was pressed down under the weight of glacial ice is rebounding upwards now that the ice has melted. The land is currently rising faster than the sea.

The weight of ice over Shetland, however, was relatively slight which means that isostatic uplift in Shetland is not keeping pace with rising sea levels.

Coastal erosion

The Shetland coastline is gradually being lost or changed as the sea continues to encroach on the land. Global sea levels are currently rising at the rate of approximately 3 mm per year and the waters continue to carve out spectacular cliff formations such as caves, stacks, arches and geos.

Ferocious storms can have particularly dramatic effects. A single storm can significantly alter a stretch of coastline, while in places, the force of the sea can tear rocks from the bedrock and hurl them many metres inland, creating dramatic storm beaches.