The Edinburgh Geologist Issue no 43

Mountain Geology

Starting at the Top – Ben Nevis by Suzanne Miller

Introduction

Ben Nevis is one of Britain's most famous natural tourist attractions, one of 284 Munros, and Britain's highest mountain, the summit standing at 1344m. It has recently been bought by The John Muir Trust and forms part of the Nevis Estate which, at 1700 ha (4158 acres), covers roughly the summit and southern slopes of Ben Nevis itself and a string of other peaks to the east with their slopes down to the Water of Nevis. The summits to the east are Carn Mor Dearg (1223 m), Aonach Beag (1234 m), and Sgurr Choinnich Beag (963 m). The area represents a huge volcanic complex, now eroded to its granite roots, that has given rise to the dramatic scenery which harbours glaciers in its north facing corries.

Ben Nevis, or 'The Ben' as it is fondly known, sits majestically at the head of Loch Linnhe, its presence dominating the landscape from all corners of Fort William and some parts of Lochaber. The dramatic effect of Ben Nevis is emphasised by the fact that it begins its rise from sea-level. From the coast, it rises like a huge whale-back shape. This ends abruptly in 2000 ft cliffs to its north-east. This two-faced form of the Ben is what makes it so popular, providing steep but straightforward walking on paths on one side and challenging climbing on the other.

In fact, Ben Nevis probably has the widest range of visitors of any mountain, from world class climbers and racers, to those who will never set foot on another hill in their lives. For most visitors it is the height (1344 m or 4406 ft) that matters. There are about 150,000 ascents every year and many industries have grown up around the fact that Ben Nevis is Britain's highest mountain.

To the south, the Water of Nevis plunges from high grass flats through the steep densely wooded Nevis gorge. This is a very different wild place with magnificently sculptured rocks and a rich native woodland containing remnants of the original Caledonian pine-forest. The gorge opens out onto the Steall flats with views across to the striking Steall Falls.

Ben Nevis, although not as high as Alpine mountains, is positioned on a more northerly latitude and the climate can be considered similar to Arctic regions. While there may be a welcoming sea breeze on the shores of Loch Linnhe, 20-30 knots of chilling wind may be evident on the summit of the Ben. Many walkers and climbers find weather conditions changing within minutes – usually for the worse – as they work their way up the mountain. Each year, Ben Nevis sees an average of 261 gales, and its summit is cloud-covered for approximately 355 days.

The first recorded ascent of Ben Nevis was made in 1771 by the noted botanist James Robertson, who was on an expedition to collect plant specimens for the College Museum of Edinburgh. He described what he saw on his ascent on 17th August:

I ascended Ben Nevis, which is reckoned the highest mountain in Britain. A third part of the hill from the summit towards the top is entirely naked, resembling a heap of stones thrown together confusedly. The summit far overtops the surrounding hills (National Library of Scotland m/s 2508, p 131-132).

A few years later, in 1774, an ascent was recorded by John Williams, who was assessing whether there were any minerals of commercial value around the summit. He discovered nothing of any great interest, but his report was the first geological account, describing the Ben rose from a base of 'elegant red granite'.

Geology

Ben Nevis lies within Dalradian schists of the Appin Group though the Ben itself, as stated by Williams, is granitic, intruded around 400 million years ago when it was part of a massive and very active volcano. The Dalradian structures and metamorphic features of the Central Highlands are complex and early workers had great difficulty in making sense of the geology of the area. However, as early as 1817, although the origin of the Ben Nevis Granite was controversial, it was recognised that granite had forced its way into the surrounding schists and was therefore the younger rock.

There was much debate as to whether the granite was of igneous or metamorphic origin. In his Ancient Volcanoes of Great Britain, Geikie (1897) did not even mention the Ben Nevis area and geological surveying only began in earnest in the late 1880s to early 1900s when James Grant Wilson of the Geological Survey was made responsible for the Loch Linnhe area. During this ground-breaking work, it was recognised that the metamorphosed sediments formed part of a spectacular recumbent fold which characterised the southern part of the Central Highlands and that these metasediments had been intruded by large volumes of magma, producing extensive metamorphic aureoles (Oldroyd & Hamilton, 2002).

There are four major intrusions - the Outer and Inner Quartz Diorites and the Outer and Inner Granites. These originated as successive and concentric ring-dykes, the product of subterranean subsidence and subsequent surface collapse. These form the Ben Nevis central ring complex (Strachan et al., 2002). It is thought that the huge granite pluton, one of the 'Newer Granites', was subsequently affected by the collapsed roof of a subterranean caldera (established during earlier subsidence), thus allowing the former cover of lavas and their underlying schists (part of a massive recumbent fold) to sink about 600m into the inner granite while it was still molten.

Development of the Ben Nevis intrusive system

Taking what is usually described as the 'tourist path' up Ben Nevis, you first climb through the grey Appin Group schists that crop out on the lower slopes of Glen Nevis. As the path begins to steepen, however, the rock appears redder and more rubbly, indicating that you have crossed the junction at the edge of the pluton into the Outer Granite. Having climbed steeply around the flank of Meall an t-Suidhe, you will reach a flat grassy area, in which lies the wild Lochan Meall an t-Suidhe. This marks the junction between the Outer and Inner Granite. The latter makes up the craggy summit of Carn Mor Dearg to the northeast together with the shoulders of Ben Nevis itself. As the tourist path zig-zags steeply, you will cross over into the subsided bedded lavas and volcanic agglomerates of Lower Old Red Sandstone Age that make up the summit dome. These volcanics also form the steep north face of the Ben so loved by climbers.

The volcanic sequence is mostly composed of rather homogeneous potassic trachytic lavas with phenocrysts of hornblende, plagioclase, magnetite, apatite and pyroxene pseudomorphs, with minor dacites (Trewin & Thirwall, 2002). A thin basal conglomerate rests on Dalradian metasediments, but this contact can only be seen in Coire Leis, accessed by heading north on the grassy plateau where Lochan Meall an t-Suidhe lies and then following Allt a' Mhuilinn up past the Scottish Mountaineering Club's Charles Inglis Clark Memorial Hut. Thin siltstones and tuffs also occur in the volcanic sequence which is metamorphosed by the Ben Nevis granitoid complex.

The monolithic scenery of this area has been carved out by the more recent action of glaciers. This ice-carved rugged landscape can be seen in the Nevis Gorge, where the Allt Coire a'Mhail cascades some 100m as a waterfall from a spectacular example of a hanging valley. When seen from the northwest, the majestic ice-quarried cliffs are revealed, their frost-riven 600m precipices unequalled in height even by those of the Cuillins. At their base extensive snow patches survive the summer thaw in the sheltered north east facing corries, demonstrating how easily glaciers could regenerate in this marginal sub-Artic environment.

Habitats and flora

Although created by dramatic geological and geomorphological processes, Ben Nevis is also of great scientific interest for its flora and fauna with several rare species and many important habitats.

There is a huge range of vegetation types within the Ben Nevis Estate. They represent full altitudinal sequence of vegetation in upland Britain, from woodland to montane mossheath and snow-bed. Carex bigelowii heath, an eastern type of vegetation which is uncommon in the western Highlands is found, and the Marsupella brevissima snow-bed vegetation on the summit of Aonach Beag is of an interesting high-montane type otherwise known only from the Cairngorms.

In Glen Nevis there are stunning natural woodlands of native pinewood, oak, birch and, closer to streams, alder and ash. Below the trees are primroses, wood anemones, wood sorrel and tiny purple violets. Small patches of mainly birch woodland also cling to steep craggy mountains, which are not accessible to sheep or deer.

On moving up the slopes, heather moorland and grasses dominate with peaty bogs on more gentle slopes. Blaeberry (bilberry), heather, mat grass, mosses, and thyme and milkwort flourish in the wetter areas. Bracken is abundant on some of the lower slopes. Higher up, a more montane habitat develops where plants such as dwarf willow, alpine saxifrage, moss campion, alpine lady-fern, alpine speedwell, mountain avens and starwort mouse-ear grow amongst the blaeberry, heather and grasses. Stiff sedge, three-leaved rush, heath rush and curved woodrush also occur.

On the rocky top of the mountain the habitat supports a wonderful array of lichens, mosses and mats of low growing alpines such as purple saxifrage, moss campion, alpine lady's mantle and the Scottish sibbaldia.

A large part of the Ben Nevis Estate is a Site of Special Scientific Interest (SSSI) because of its biological and geological importance. It is also a proposed Special Area of Conservation (SAC) due to its excellent examples of Alpine calcareous grasslands, acidic scree and plants which live in crevices on mainly acidic rocks.

The Observatory

Weather devotees will be drawn to the story of the Mountain Top Observatory, where resolute Victorian scientists collected weather data, even in heavy gales and violent winter storms. The footpath and observatory were both constructed during the summer of 1883. The contractor was James McLean of Fort William and the last rise on to the summit is named McLean's Steep in his honour. The observatory, was built to record 'the diversity of the mountain environment' e.g. temperature, wind speed, rainfall, air pressure, etc. Nominally opened on the 17th October 1883 but actually commencing operations on November 28th that year, the Ben Nevis Observatory provided invaluable meteorological data for 21 years on a continuous hourly basis. It was Britain's first high-altitude observatory and much pioneering research was conducted there during its operation. A telegraph cable was laid to connect the Nevis Observatory with the sea level observatory in Fort William. Fragments of this cable can still be seen on the mountainside today.

The Observatory was funded mostly from a list of private donations which included Queen Victoria and was managed by the Scottish Meteorological Society and the Royal Societies of Edinburgh and London. The building was manned by a superintendent and two assistants who were responsible for taking readings. During 1902 it became apparent that insufficient funds were available to continue the running of the observatory, and it was closed on October 1st 1904. Over the years that followed, the observatory fell into disrepair, this process being helped by a fire during 1932 and the actions of both weather and unthinking visitors.

During the long dark days of winter when it was operating, however, staff would brave gale force, icy winds and driving snow to carry out their labours. Inside, heat was provided by an open cooking stove in the kitchen and a closed one in the office - fuelled mostly by paraffin coke. A number of famous Scottish scientists worked at various times at the observatory, including William Speirs Bruce, who was stationed there as a meteorologist and later led many polar expeditions and Charles Thomson Rees Wilson, who invented the cloud chamber.

William Speirs Bruce (1867-1921) was a Polar explorer and oceanographer. Educated at the University of Edinburgh (where he was later the George Heriot Research Fellow 1900-01) he became the Director of the Scottish Oceanographical Laboratory. Bruce spent two winters at the observatory on Ben Nevis refining his meteorological skills before embarking on a number of polar expeditions. He was an assistant in the Challenger Expedition Commission, and a naturalist on the Scottish Antarctic Expedition 1892-93, the Jackson-Harmsworth Polar Expedition 1896 and 1897, and a number of others, and led the Scottish National Antarctic Expedition in 1902-04. His ship, The Scotia, brought back a vast collection of data, samples and specimens, corrected charts of Antarctica, and enabled the delineation of what became known as the Scotia Arc between South America, South Georgia and the South Sandwich Islands. All of this was achieved without the loss of life and vessels which dogged some of the higher-profile expeditions. He lectured on geography at Heriot-Watt College (now Heriot-Watt University) from 1899-1901 and from 1917 until his death, and at the Church of Scotland Training College, Edinburgh.

Charles Thomson Rees Wilson (1869 -1959) was a Scottish physicist and avid climber. he worked at the observatory on Ben Nevis in 1894, and his observations of optical effects caused by the interaction of sunlight and cloud led to his invention of the cloud chamber, for which he received the Nobel Prize in 1927.

When standing on the summit of Ben Nevis in the late summer of 1894, Wilson was struck by the beauty of coronas and 'glories' (coloured rings surrounding shadows cast on mist and cloud), and he decided to imitate these natural phenomena in the laboratory. His sharp observation and keen intellect, however, led him to suspect (after a few months' work at the Cavendish Laboratory) that the few drops reappearing again and again each time he expanded a volume of moist, dust-free air, might be the result of condensation on nuclei - possibly the ions causing the 'residual' conductivity of the atmosphere-produced continuously. Wilson's hypothesis was supported after exposure of his primitive cloud chamber to the newly discovered X-rays. Early in 1911 he was the first person to see and photograph the tracks of individual alpha- and beta-particles and electrons. (The latter were described by him as 'little wisps and threads of clouds'). The event aroused great interest as the paths of the alpha-particles were just as W. H. Bragg had drawn them in a publication some years earlier. It was not until 1923 that the cloud chamber was brought to perfection and led to his two, beautifully illustrated, classic papers on the tracks of electrons. Wilson's technique was promptly followed with startling success in all parts of the world and has led to fundamental advances in particle physics. Thus, Rutherford's remark that the cloud chamber was 'the most original and wonderful instrument in scientific history' has been fully justified. And all because of Ben Nevis!

References

Geikie, A., 1897. The ancient volcanoes of Great Britain, two volumes. Macmillan, London & New York.

Oldroyd, D.R. & Hamilton, B.M., 2002. Themes in the early history of Scottish geology, in: N.H. Trewin (ed.) The Geology of Scotland, The Geological Society, London. pp 27-43.

Strachan, R.A., Smith, M., Harris, A.L. & Fettes, D., 2002. The Northern Highland and Grampian terranes, in: N.H. Trewin (ed.) The Geology of Scotland, The Geological Society, London. pp 81-147.

Trewin, N.H. & Thirwall, 2002. Old Red Sandstone, in: N.H. Trewin (ed.) The Geology of Scotland, The Geological Society, London. pp 213-249.

Ben Nevis webcam: www.visit-fortwilliam.co.uk and click on: Ben Nevis Cam 


Suzanne Miller is well known to many Fellows of the Society, having lectured to the Society and led several field excursions. She works in the Department of Geology and Zoology at the National Museum of Scotland in Chambers Street and her interests in the earth sciences are wide-ranging.

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