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Geologie Shetland

14.03.2009  |  127× přečteno      vytisknout článek 

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.

Shetland's spectacular cliff sceneryA 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. Serpentine (ophiolite) Quarry, Belmont, Unst 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. 

Environmental Change Over The Last 20,000 Years

During the last Ice Age an ice-cap covered Shetland but it was much thinner than that of the Scottish mainland, giving the land a light scouring rather than the heavy grinding experienced by the former. However, evidence for the ice does exist in the form of glacial erratics (boulders carried along in the ice and deposited in “foreign” locations - one is from Norway) and moraines.

The last ice sheet left Shetland around 14,000 years ago, though a cold snap a few thousand years later brought about a return of glaciers, the last finally disappearing 10,000 years ago. 

In its wake, the ice left behind a bleak, bare landscape of rock, broken stone, gravels, sand and mud – not dissimilar to the Keen of Hamar today.

Looking towards Harolds Wick from the postglacial desert of the Keen of Hamar, Unst. Shetland.

However, this post-glacial landscape was very soon colonised by a succession of vegetation types, culminating in scrub of willow, hazel, rowan, poplar and birch which covered 50% of the island 6,000 years ago. Since then, the natural succession has been radically altered by Man and the climatic deterioration that occurred in the Bronze Age (from 2000 BC), leaving us with the landscape of today: open, peat-covered moorland and grassland with virtually no tall tree cover whatsoever.

The outline of Shetland at the end of the Ice Age was very different to today, sea levels being around 100 metres lower due to the amount of water locked up in the ice. However, since then, melting ice has caused sea levels to rise and the Shetland landmass has also been depressed as the massive weight of the ice over the continental landmass to the east was removed (as the centre bobbed up so the edges along the continental shelf sank).

Sea levels have therefore risen gradually over the past 14,000 years and by the time the first Mesolithic hunter gatherers arrived 6,000 or 7,000 years ago it was around 9 metres below current levels – peat has been found at this level at this period on the Unst coast. Rising sea levels have drowned valleys (rias) such as Bluemull Sound in the same way that they have done in Cornwall.

Massive winter storms crash onto Shetland's rocky coastlineThe sea is an enormously powerful agent of change, especially when whipped up by the North Atlantic gales that can sweep Shetland, with gusts of over 173 mph being recorded at Muckle Flugga lighthouse on 1st January 1992 (before the equipment broke). In 1967 a wind of 177 mph was recorded at the radar dome on top of Saxa Vord – again just before the equipment was blown away. This is the fastest wind speed ever recorded in Britain – though the official record is 173 mph on Cairngorm in 1986. Enormous waves crash into the shores, wearing away great chunks of rock and creating the massive cliffs, with their caves, rock arches and geos (inlets) that are so much a feature of the Shetland coast.  There are even examples of boulders weighing 5 tonnes being carried up to 50 metres inland and up cliffs 50 metres high!

Changes to the landscape since the last Ice Age are not only due to natural phenomenon, of course. Man has also played a key role and to find out more about this please see the Archaeology, History and Wildlife sections.

For a quite superb, detailed account of Shetland’s geology visit Shetland Landscapes.


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