Summer series of interest - Part 3, The creation of Ben Nevis
I was lucky enough to be part of the geological and botanical survey of Ben Nevis between 2014-2016 as a mountaineer as part of a large team structured to guide geologists and botanists around the North Face of Ben Nevis for a week at a time each year. The aim of the 3 week study was to gain data on the rare alpine plants that grow on the highest areas of the mountain to get a more accurate picture of total numbers and where they were distributed, (mostly grotty gullies, some in very grotty gullies). The other aim was to gain data on the geology of the north face to provide a more accurate data set to then conclude the original formation of Ben Nevis, this mostly involved very long abseils straight down the north face and climbing all the classic ridges, mentioned in my previous post.
This article will focus upon the formation of Ben Nevis rather than the botany or the adventures we had on the survey. If you do want to find out more there is a coffee table book produced summarising the botany, geology and mechanics of the survey, The North Face Survey, Discovering the hidden side of Ben Nevis.
The photos above show a snapshot of some of the aspects of the survey: The team, the geology app, using the app on Tower Ridge, Roddy (Midland Valley) ready for a big abseil, abseiling between the summer snow and the mountain, person for scale and taking data on Observatory Ridge.
The formation of Ben Nevis
In short Ben Nevis is the remains of a series of pyroclastic flows that originated from a volcanic vent situated about 20km to the northwest around the Glen Loy area.
Pyroclastic flows are a volcanic event where ash, gas, water and debris are ejected at large volumes and speed across the ground from a volcano, think Pompeii and Mt St Helens as recent famous flows.
More detail and background
The mountains of Scotland and the West Coast mountains that we now know of as around 1000-1300m high were once 3000-4000m high when they were first formed. We are now looking at just the base of the mountains that form the Munros. Then over the last ice age around 10,000 years ago the main mountains and Glens that we know were formed. Creating some of the famous ridges that were above the glaciers, Aonach Eagach, Glencoe, Cuillin ridge, Skye and the famous Glens and corries like Glen Nevis and Glencoe. Before this ice age the landscape would have been very different, less glens and mountains. When looking at Ben Nevis as we see it today, this scale of time and loss of height needs to be considered to understand the result of what we are looking at, the mountain as we know it now once had 3000m of mountain on top of it, built up in layers from volcanic activity.
There are 3 different types of rock formation found on the north face as we see it now. At the very base around the CIC hut level is the mudstone formation, the oldest, which consists of thin layers of mudstone and siltstone probably deposited at the bottom of a small lake with flash floods occurring, about 30 meters thick. In the middle is the Corie na Ciste member (layer) which lies on top of the mudstone which consists of fine grained ash and clay and volcanic boulders from pyroclastic flows, around 250 meters thick. Found from just above the CIC hut to around the top of Castle ridge, the small lochans and crossing Tower Ridge at halfway. Lastly is the Summit Member, about 350 meters thick starting around the level of Ledge route and the second half of Tower Ridge. This layer consists of ash flows when a lava dome in the centre of a volcano explodes scattering debris over a wide area.
All the layers are tilted from the north west to the south east at around 20 to 30 degrees but it is not known if this is the original ground angle that the flows moved across.
Some good examples of the different type of rock that is described above is via an ascent of Ledge Route. From the CIC hut there are slabs showing very obvious volcanic boulders and ash mixed together. Higher up on the approach there are boulders of mudstone formations and then once you traverse in to Ledge Route across No. 5 gully you are following the line between the Coire na Ciste member and the Summer member. Once the main ridge is gained this is the start of the Summit member which takes you to the top of Carn Dearg at around 1200m. On the plateau there are good examples of the ash blocks that were scattered from the volcano.
The first 2 photos above show the mudstone formation with the different layering. The 1st photo is particularly unique as it shows small 'waves' as a new flood happened pushing the existing mud before settling in that position from the weight of material. The 3rd photo shows the Corie na Ciste formations of the pyroclastic flows, then the next photo shows that ash and debris of the Summit member, lastly a climber on the summit formations.
If you wish to see these formations for yourself to understand the picture better, a non technical walk around the base of the north face above the CIC hut is a perfect way to see the different rock types and get a sense of the scale. Or if you like to get a rope on, making an ascent of one of the classic ridge lines like Tower Ridge or Ledge Route is a perfect way to experience the full height of the North Face whilst also getting to do an amazing classic mountaineering route.
Whist Covis-19 continues we are currently offering vouchers for activities where the date can be set in the future when things settle down, with no time scale on this. If you want more information then let us know here.