The remote valley of Glen Roy, a few miles from Fort William in the north West of Scotland, seems at first sight no different from many similar locations – a deep trough cut into the mountains by glaciers during the last ice age. But Glen Roy, described by no less a figure as Charles Darwin (quoted in McKirdy et al) as ‘far the most remarkable area I ever examined’, is distinguished by a remarkable geological feature – the so-called Parallel Roads.
Located in the Lochaber Geopark, the Parallel Roads are an outstanding geological site and have caught the attention of visitors for hundreds of years.Clearly visible, they form parallel terraces running along both sides of the mountain at heights of roughly 260 metres, 320 metres and 350 metres. Long before the science of geology became established, people struggled to understand their origins and they are attributed in legend to the work of some mythical Gaelic giant.
The Parallel Roads of Glen Roy: Historical Theories
Gen Roy has for years been a magnet for geologists and these have included Charles Darwin. Darwin is known to posterity as a biologist but when he returned from the Beagle expedition he did so with an interest in geology. One of his earliest geological papers, published in 1839, was on the subject of Glen Roy, whose landforms he interpreted as part of tectonic theory (the physical movement of different sections of the earth).
In geology, however, Darwin’s theory didn’t stand the test of time. Earlier geologists had already postulated that the features were lake-related; and although Darwin stuck by his theory (it was a long time before he acknowledged his mistake) it wasn’t long before the Swiss geologist Louis Agassiz became involved (Rudwick 2009, see sources below).
Agassiz was in London in 1840, expounding on his then new and controversial theory that ice, in the form of glaciers, was responsible for shaping much of the present landscape. It was he who associated the lakeshore theory with the presence of ice and, provided geologists with the explanation for the Parallel Roads – that they were successive lake shores formed by fluctuating water levels of a glacially dammed lake.
The Formation of the Parallel Roads of Glen Roy
Although the details of Agassiz’s theories for Glen Roy are now considered inaccurate, with many geologists undertaking detailed study to determine the date and sequence of the Roads’ formation (Rudwick), the formation of the features is still understood to be broadly in line with his theory of a relationship between ice and water.
The lake whose fluctuating levels left behind the parallel roads was formed towards the end of the ice age during a period known as the Loch Lomond Stadial, approximately 10-11,000 years ago. Although the ice had been melting, a temporary glacial readvance blocked the entrance to Glen Spean, off which Glen Roy lies, and forming an ice dam behind which a lake was impounded (SNH; McKirdy et al 2009).
The lake extended beyond Glen Roy and was controlled by an overflow, or col, at Glen Spean, some distance to the west. The lake’s level was at an altitude of 260 m and its shoreline formed what is now visible as the lowest of the Roads. As the readvance continued, the dam grew and blocked access to Glen Spean. The lake level in Glen Roy rose and was now controlled by a different overflow, at 325m.
Repetition of this scenario with a further advance of ice meant a further rise in lake level, this time with a 350m overflow at the head of the glen – the third and highest level. As the ice eventually melted – characterised by huge periodic floods known as jokulhlaups – the pattern was repeated in reverse.
Glen Roy Today
The 260m shore is also preserved in the adjacent valleys of Glen Spean and Glen Gloy but Glen Roy is remarkable for its succession of three shores, which are best seen from a viewpoint in the minor public road which runs up the Glen. The process of deglaciation (the melting of the ice) has left the area with a range of other glacial landscapes including outwash deposits, kettle holes and morainic deposits (SNH).
Glen Roy is a protected site and is designated as a Site of Special Scientific Interest. It forms part of the Lochaber Geopark, a sustainable and scientific tourism initiative. In fact the glacial features are among the most recently formed in the ancient landscape, which also includes the volcanic relics of Ardnamurchan and Glencoe, along with evidence of both continental collision and continental rifting which occurred millions of years ago.
Sources and Further Reading
McKirdy, Alan; Gordon, John and Crofts, Roger Land of Mountain and Flood: The Geology and Landforms of Scotland, Birlinn 2009
Rudwick, Martin “The Parallel Roads of Glen Roy: In the Footsteps of Charles Darwin - A Field Guide”, Geological Society of London, History of Geology Group, 2009, darwinproject.ac.uk, accessed 7 June 2010
Scottish Natural Heritage “Glen Roy, a Landscape Fashioned by Geology”, snh.org.uk, accessed 7 June 2010
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