Schist, Granite and Ignimbrite Geology: the Ballachulish area, SW Scottish Highlands
In mid-June I was fortunate enough to be part of a group of 20 to be introduced to the geology of the SW Highlands in a trip lead by Professor Donny Hutton and organised by Bristol University (www.gly.bris.ac.uk/ce) which ran from June 18th through 23rd with participants who would look very much at home on a MGA event. For some it was meeting up with faces familiar from other trips, for others like me it was being launched into a wholly new group of like-minded people.
The first day was spent in glorious sunshine along the shoreline at Onich, nr Glencoe looking at metamorphosed Lower Dalradian sediments: slates, quartzite and phyllite of the Ballachulish subgroup in the Appin Group. Donny Hutton’s aim was to teach us field skills as well as see classic geology. He wanted us to tell him what we saw, he’d then help us interpret this and put it into context. From the off we were unravelling the sequence of deposition and deformation. By the end of the day we had ‘discovered’ the Appin Syncline, a major early fold in these parts, and the Fort William slide.
The second day was a little overcast and my 95% proof DET anti-mosquito spray proved
to be a particularly wise buy. We saw the contact of the Ballachulish granite, interleaved with the country rock. More interesting were a selection of mafic intrusions, local specialties, Appinites and Kentallenite (now all classed as monzonites). Even in the mist and miggies the striking coarse bronze coloured mica and rich black pyroxenes delighted us all. Two of the party who made their own thin sections became most excited. Before lunch, crossing the beach at Cuil Bay, we were told to collect as many different pebbles as we could. As a group we proceeded to discard the sedimentary and metamorphic. The remaining igneous material was then laid out according to its relative proportions of quartz, plagioclase and orthoclase feldspar, creating on the beach a very large version of the Streckheisen diagram used to classify plutonic rocks. Later on we had an introduction in identifying folded folds, upright folds and overturned folds.
Day three saw the only serious rain so we took ourselves off to the Aluminium museum in Kinlochleven. In the late 19th and early 20th Century the isolated village saw the development of a major hydroelectric scheme to smelt aluminium shipped in from Northern Ireland and latterly from further
afield. By lunchtime the day had cleared up and we mapped out the north side of Loch Leven. We identified thrusts, were shown how to use the vergence of folds and cleavage to understand the nature and sequence of deformation. With the skills picked up across these first three days we were able, with much guidance from our leader, to understand how an earlier slide and folding event was deformed by a later folding event forming the regionally significant Stob Bhan synform.
The fourth day was spent in Glencoe. This area is now considered to be of international significance geologically due to the exposure of the lower levels of the Lower Palaeozoic igneous complex. The scenery which so excited Queen Victoria and countless other tourists before and since, is a series of enormous cross sections through ignimbrites and andesites. The 2006 BGS publication by Kokelaar and Moore ‘Glencoe caldera volcano, Scotland’ is highly recommended.
On day five we took the five minute Corran Ferry ride to investigate the Strontian Granite. This Caledonian Granite has been explored inch by inch by Prof Hutton. We spent an entire day identifying the different lithologies. It is composed of tonalite and granodiorite: coming to this conclusion ourselves using what we learnt on the beach at Cuil on day two rather than just being told ‘that is a tonalite’ is strangely satisfying. The granite is foliated by the shear zone it sits in. But most striking are the abundant inclusions, called enclaves not xenoliths. These are interpreted as being blobs of mafic magma which were molten at the same time as the granite, remaining separate due to their different viscosity and density. But as with a lot to do with
granites one can sense that there is great controversy as to what they actually mean. The day concluded with a visit to the Strontian Mine from which the element Strontium gets its name. Some of you may recall that our MGA Horrocks Fund co-sponsored the production of a leaflet on these mines. The leaflet is still on sale in the visitors centre in Strontian but much of the more recent mine-workings are out of bounds – to official responsible parties in any case!
The sixth and final day took us west to view excellent exposures of the pre-Dalradian Moine succession. Most spectacular were intricately multiply folded pelites and psammites. An outcrop about the size of several tennis courts displayed a multiplicity of cross sections through these folds, showing the complex interference patterns formed. Strictly no hammers, this site is destined to become a SSSI. The final event of the trip was a written exam, taken in silence among the beautiful scenery and quite a lot of wind, to satisfy Bristol Uni that we had indeed learnt what the leader said we would.
To conclude: The geology is spectacular. We saw classic rocks which have informed geology since the science started. We also had an excellent teacher who entertained and educated us, developing in us skills we can take to other rocks in other places.
Niall Clarke
I’ve had very good reports of the Bristol University Geological trips from other sources as
well ..… worth considering for next year ? Check out their website ...Ed
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