Outer Hebrides Fieldtrip 2015

In June 2015, Lucy Campbell from the University of Leeds organized the tectonic studies group field trip to the Outer Hebrides, to study the Outer Hebrides Fault zone.

Hebrides Field Guide (right click and choose “Save As”)

Review 1: Steph Walker

The Tectonic Studies Group annual field trip this year, led by Lucy Campbell of Leeds University, was in the Outer Hebrides, and what a place for it to be! The trip consisted of a northward traverse through the length of the Outer Hebrides, focussing on the fault rocks and pseudotachylites within the Archaean Lewisian Gneisses that outcrop on the archipelago.

After a rather long and bumpy ferry from Oban to Castlebay on Barra, the fieldtrip started on a high, with some truly excellent pseudotachylite outcrops on the south coast of Barra.

Pseudotachylite TSG

Certain members of group were very excited to see some solid outcrops after the choppy crossing:

Ferry terminal outcrops

Although not as excited as Lucy when she saw this great outcrop almost entirely composed of pseudotachylite in NW Barra, not to mention the blue skies and beautiful white-sand beach!

Fault rocks

 

Of course, being a metamorphic geologist, I greatly enjoyed finding these lovely garnets with decompression-related plagioclase coronas in an example of the Scourie Dyke Swarm near Tarbert!

Garnets with Plagioclase

The field discussions as to the origins of these mid-to-lower crustal fault rocks was a fascinating insight for anyone interested in the way stresses propagate through the crust, with many an interesting point being raised from breakfast to evening beer time!

Time to leave field work and head to the pub

Despite a packed schedule, we managed to persuade Lucy to take us to the fantastic Standing Stones at Callanish – a truly amazing place to be the day before the summer solstice!

Standing stones at summer solstace

Although perhaps the BGS’s John Mendum was a little overwhelmed by the experience…

Overwhelmed

After the return ferry to beautiful Ullapool, and an eventful drive back to Oban, including an attempted rescue of a baby deer from the middle of the road near Ballachulish, some members of the party were treated to a day’s geologising on the island of Kerrera.

Despite the lack of pseudotachylites, the varied geology was more than enough to keep the remaining group interested. After the excitement of the tiny ferry from Oban, we arrived on Kerrera in style.

Kerrerra in style

Here, instead of the Archaean Lewisian gneisses of the Outer Hebrides, it was the Neoproterozoic-Cambrian Dalradian Supergroup that reigned supreme, with some fantastic folds and cryptic reverse graded beds leaving us scratching our heads for a few minutes!

Scratching heads

Also on Kerrera, is a fine example of the Dalradian-Old Red Sandstone unconformity, an important relationship helping to disentangle the end of the Caledonian Orogeny in the Silurian/Devonian

Caledonian orogeny

Other Spectacular outcrops on Kerrera included this Paleogene intrusion cross-cutting Devonian columnar-jointed basalts:

Columnar basalts

All in all, a very memorable and geologically fascinating trip to the Outer Hebrides and a short jaunt to Kerrera – two areas that I had little knowledge of, but have a renewed interest in!

Of course this fantastic trip wouldn’t have been possible without the tireless efforts of Lucy who made the trip run so smoothly; so thank you to Lucy and the Tectonics Studies Group! What a fantastic trip!

 

Review 2: Andy Emery

As someone who works in the field of pore pressure, I treated the assertion that there were rocks other than shale on this planet with some scepticism. I felt I needed the existence of these other rock types proved to me, so I seized upon the opportunity to visit the Outer Hebrides. Not wanting to confirm or conform to stereotypes often applied to geologists, but I’d been on Scottish field-trips before and four things are guaranteed: rain, sunshine, natural beauty and whisky. Plus, I’d also been on a field-trip with Lucy before, and I’d been to TSG in Edinburgh twice, both times where the whisky flowed abundantly, so I felt like I knew what was in store in this department…

After arriving in Oban a few hours ahead of schedule, I headed to the ferry terminal. Geologists are easy to pick out of a crowd, even when the majority of the crowd is similarly clad in outdoor brands. It’s something to do with their thousand-yard stares which portray their, to quote a friend, “scant regard for the petty affairs of man”. So, once the majority of the group had congregated on the upper deck of the terminal, meetings and introductions made, we boarded the ferry.

I have a love/hate relationship with ferries. I love them, but my stomach hates them. After sailing safe passage between Mull and the mainland, we broke into open waters where the Atlantic swell could buffet the Lord of the Isles around. I stood outside for four hours until fully saturated, and, despite seeing pods of dolphins, admitted defeat and headed inside to succumb to the inevitability of the lack of fresh air and constant horizon. Thankfully, the toilet facilities aboard the Lord of the Isles are ample and accommodating.

We arrived into Castlebay in a blanket of fog, trudged to our hotel, glad not to be on wobbling decks any longer. A brief evening sojourn to the Castlebay Bar provided us with our first whisky of the trip, and a locals versus tourists pool tournament which was won by the locals. I could claim we let them win in order to keep relations between the two factions amicable, however we had no choice in their victory. We were also given advice on how to behave in Stornoway on a Sunday and how best to see the Uists and Benbecula, which, according to the locals, was through the window of the minibus as you drive quickly across them. I refused to believe him…

Arrival into Castlebay in the fog.

The first field day dawned without the fog of the previous evening. Our first stop of the day was the point where we first stepped onto the island, at the ferry terminal. Is there such a thing as a Scottish island ferry terminal without some interesting geology outcropping? This was our first sight of the legendary pseudotachylyte, sitting in the footwall of the Outer Hebrides thrust, and displaying complex patterns of brecciation. Next, on to the viewpoint above Castlebay to see the relationship of the Outer Hebrides fault zone to the geomorphology of the islands. A brief lunch-stop amid the machair dunes led us to the stunning west coast of Barra, a wild, Atlantic landscape bathed in thankfully sunny spells. Rugged storm-lashed outcrops of Lewisian Gneiss lay next to gentle, inviting, gleaming white shell sands. On the beach was more pseudotachylyte (do you detect a theme emerging here?), our first Tertiary dyke, and some stunningly complex and beautiful outcrops of pseudotachylyte breccia at the beach below the golf course. After hunting for some bog-standard non-pseudo tachylyte at Earsary, where our outcrop time was extended longer than planned, resulting in some adventurous driving to make the ferry in time. Thankfully, we managed to catch the ferry to Eriskay without a hitch, and on to the second night’s destination of Daliburgh.

The main event, the pseudotachylyte.

The main event, the pseudotachylyte.

The next day was the kind of day that I probably shouldn’t admit to enjoying. After a short drive up South Uist to the Ben More parking spot, we disembarked and began walking through pleasant coastal scenery. Then it began to rain, a lot, and the wind picked up. Undeterred, we headed up the hill to two outcrops, the lower of pseudotachylyte-rich grey gneiss and the upper of Corodale gneiss. The boundary between the two, unfortunately covered by vegetation, was the basal thrust of the Outer Hebrides fault zone. Then onwards down tick-infested gullies to the coast, to see retrograde pseudotachylyte and phyllonites, although consensus amongst the group seemed to be leaning towards mylonites. After a lunch break sheltering under various alcoves, we marched the three miles back across bog, up steep gullies and more bog back to the minibus for our inevitable race against time for the ferry from Berneray to Leverburgh. South Uist, Benbecula, North Uist and Berneray passed with haste and brakes were well tested, but we made the ferry with time to spare, and enjoyed the sunshine as the ship passed its tortuous route through the reefs and skerries to Leverburgh. Arriving late into Tarbert, we grabbed some food and sampled some fine wines courtesy of John’s now-familiar open-invitation wine receptions. Topics of conversation were varied and wide-ranging, from the bunkhouse’s eclectic vinyl collection, the merits of various grapes and soils, whisky, art and politics. What an educated bunch we all were…

Typical Atlantic coast scenery.

Typical Atlantic coast scenery.

Day three saw us gathering garnets for the tame geochemist of the trip early in the morning atop the pass from Tarbert to Scalpay. Once her garnet-based rapacity was sated, we continued to Scalpay to see important timing relationships of the pseudotachylyte relative to various phases of deformation. Naturally, a group of such academic prowess is prone to intense debate (bets were made at various times throughout the trip) and time once again became a relative dimension, leading to us having a late lunch and a race against tides and weather to make it back to Tarbert in time for our booked restaurant. Our post-lunch (I hesitate to call it afternoon, more like early evening) locality comprised of more wild, rugged Atlantic coastline with gneisses, metasediments and limestones of the Langabhat belt and intrusions of the South Harris Igneous Complex. These were combined with a bit of stone-skimming across the sea and enhancing channel erosion through beach sand via foot-based bank erosion. Fine dining is a welcome respite from field sandwiches and the food served at the Pierhouse restaurant was both delicious and hearty (try the chowder there, it’s delicious). What better way to finish the evening than a trip to the Harris Hotel to try and make inroads into their collection of one hundred whiskies? I feel we had a fair crack at ticking a lot of them off between us. The opportunity to be taught about (i.e. drink a lot of) whisky was firmly grasped by our overseas visitors, with the more experienced whisky connoisseurs amongst us finding new favourites amongst the familiar classics. I forgot what they all were now, obviously.

Sunshine and fresh mountain streams. Ideal for clearing away the night before...

Sunshine and fresh mountain streams. Ideal for clearing away the night before…

Day four, grey, foggy, miserable. That was the state of both my head and the weather. Thankfully, both weather and hangover synchronised and the sun came out, clearing both away. Excellent exposures of mylonites cross-cut by pseudotachylyte were temporarily overshadowed when the eagle-eyed Lucy spotted a white-tailed eagle in the far distance. Thankfully, we were treated to a much closer flyover by the inquisitive flying barn door. Onwards towards Mangarstadh, recent road cuttings were enthusiastically scoured for interesting points, including prolonged musings on pseudotachylyte generation surfaces, then through the glacial melt-water valley and up to Mangarstadh in thick fog for some mylonites and more bets made on the timings of pseudotachylyte relative to mylonitisation. This was the point at which we were supposed to visit a recently-opened distillery, but for some reason or other, nobody seemed overly keen on the idea of the distillery tour, so we made for the beautiful headland of Aird Feinish to study the Scourie dykes and Laxfordian deformation there, and were treated to a distant view of the Flannan Isles (not St. Kilda, sorry guys!). We moved on to a tourist stop at the stones of Callanish, where it transpired that the Victorians who rebuilt the stone circle also had a keen eye for pseudotachylyte. It also transpired that John was something of a yoga master, and we were all treated to a display of his flexibility.

Mangarstadh headland from Aird Feinish.

Mangarstadh headland from Aird Feinish.

After the devastation of missing the closing time at the chipper in the buzzing metropolis of Stornoway, followed by a meal where nobody wanted bring us our drinks or take our order, and a night sleeping in a glorified cupboard, we set out for our last morning of field-work. We inspected outcrops of Lewisian which had been brittly faulted during the opening of the Minch. True to form, the brittle faults were treated with disdain by most of the group and after a passing glimpse, their attention was drawn back to the pseudotachylyte. No such luck at the next outcrop, which were Permo-Triassic conglomerates, however the oystercatcher chick drew most of the attention. Another brief stop to see more brittle faulting, then across to the Eye Peninsula for our final stop of the trip, to see the unconformity of Permo-Triassic sediments atop the Lewisian gneiss and also get some good photographs of lonely souls staring out to sea on the top of the promontory. We made our thankful presentation of a nice bottle of whisky to our trip organiser, Lucy, and after various speeches thanking both Lucy for her organisational efforts and John for his fathomless knowledge of every outcrop in Scotland, we made our way onto the Ullapool ferry. The weather was far more clement for this crossing, therefore the love/hate relationship was forgotten somewhat, as this time, both body and mind enjoyed it. A perfectly smooth, sun-drenched sailing past Shiant, Skye and the Summer Isles took us in to what must be the most picturesque ferry terminal in the British Isles, at Ullapool.

Small human on a large planet, Eye Peninsula.

Small human on a large planet, Eye Peninsula.

So that’s the end of the field-trip, right? Wrong. Who could resist a stop at Inchbae to look at the augengneisses? Not John, that’s for sure, so we all piled out of the minibus for one last outcrop, then continued on, first via a food stop in Inverness, then via a few tourist stops to look for Nessie, on to our beds for the night in Oban.

Where was this glorious sunshine we awoke to the next day when we needed it on the rest of our trip? Our group size halved as some geologists had to make their way back to civilisation, whilst others headed across on the tiny ferry to Kerrera. A splendid sunny day lay in store, with some interesting rocks, the most picture-perfect unconformity, a good deal of skimming stones in a quiet bay, and confused-looking undergraduate mapping students. I had to leave early to catch a ferry to Mull, but rumour has it that the rest of the party only caught the last ferry back to the mainland with seconds to spare and didn’t even get time to try out the tea room!

Sunshine in Kerrera.

Sunshine in Kerrera.

The Outer Hebrides did not fail to impress and the field-trip was a great success. The Atlantic threw all it could at us over the space of the week, yet we were not deterred. Don’t like the weather? Wait five minutes, as the old cliché goes. The geology was outstanding, with excellent, easily accessible outcrops of world-class pseudotachylyte. The Lewisian Gneiss, the swirling, writhing Scourie dykes, the complex interactions and billions of years of deformation have left in their wake a magnificent archipelago of bleak, beautiful hillsides, towering sea-cliffs, perfect white shelly sand beaches, homely wildflower machair and friendly towns inhabited by welcoming people. The islands have left such an impression on me that I’ve spent the last few weeks checking ferry prices and journey times in the hope of an escape back there!

I’d like to take this opportunity to speak for the whole field-work group and thank Lucy for her organisational skills and vast knowledge of rock on the islands. Without that, the trip would not have run as smoothly as it did, as, despite our best efforts to derail everything by nearly missing various ferries, things went exactly as intended and we were all very grateful of that. My thanks too to the rest of the field trip attendees; I certainly learnt a lot from the pool of knowledge I was constantly immersed in throughout the trip, as well as getting to spend time with some great people!