The geology of Shetland, stretching back some 3 billion years, is extraordinarily rich and varied.

Indeed, it is said that the geology of Shetland is more diverse than in any other county in Britain and it is certainly some of the most complex. In recognition of these riches Shetland has applied for European Geopark status and a decision is awaited soon.

A detailed geology of Shetland as a whole is beyond the scope of this summary and simplification is far from simple. Suffice it to say that most of Shetland was created during the Caledonian Mountain Building period which took place some 420-500 million years ago.

Scotland had been part of one gigantic continent that comprised most of what is now North America, while England had been part of a continent that included Scandinavia and modern Indonesia. Around 400 million years ago the Iapetus Ocean that separated the continents closed, forcing the ocean crust (ophiolite) up onto the continental crust on Unst and Fetlar, a comparative rarity.

The collision also forced the ground up into a mountain ridge - the Caledonian Mountains that run from Norway right down the Highlands of Scotland. Faulting also occurred, sometimes on a massive scale. The Walls Boundary Fault that runs up the west half of Shetland is thought to be a continuation of the Great Glen Fault in which Loch Ness sits, while Yell is separated from Mainland Shetland, and Unst from Yell, by other faults.

Once built, the Caledonian Mountains were not untouched for long, for a combination of water, frost, ice and wind has been eroding them ever since, reducing them to the mere stumps that we see today. Fault lines - weaknesses - were also exploited to produce deep inlets and sounds like Yell Sound and the Bluemull that separates Unst from Yell. Meanwhile, the sands and gravels from the eroding mountains gradually built up in layers, forming new rock like the 350-400 Old Red Sandstone found at Sumburgh Head.

However, 500 million years is comparatively young in geological terms and many of the rocks on which these Caledonian mountain stumps sit are far older - Pre-Cambrian metamorphic rock up to 3 billion years old. The intense pressure that formed and changed them created quartz and feldspar, coarse gneiss and finer schists, rocks that can be found in a number of places around Shetland.