But that’s the foreland, you traitor!

It’s been a year since I posted! Well, that’s what happens when you’re in the lab for 10 months with nothing overly exciting happening! But I’ve been freed from the lab bench and allowed to venture into the outside world, for it’s fieldwork season!

In June, I had the great opportunity, thanks to the Tectonics Studies Group, to go to on their 2015 field workshop to the Outer Hebrides! I’d never been their before so I jumped at the chance and I’m so glad that I did. The geology was just as excellent as the company on this small workshop.

As a little bit of an outline, I work on the Caledonian rocks of Scotland. That is, the rocks that were deformed and metamorphosed during the Caledonian Orogeny that occurred as the three continents of Laurentia (Scotland, Greenland, Eastern USA/Canada), Baltica (Scandinavia), and Avalonia (England, Wales, parts of western Europe) collided between 500-400 million years ago.

However, the geology of the Outer Hebrides is in what’s called the ‘foreland’ to the Caledonian Orogeny. These are the parts of the Laurentian continent that were too far removed from the collisional centre to be affected by the orogenic cycle observed in other parts of Scotland. But because they haven’t been affected by the Caledonian orogeny, they have preserved a record of much older events in the Laurentian continent, and that’s fascinating in itself!

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 in south Barra

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

First outcrop of the week!

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!

A whole outcrop of pseudotachylite

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!

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!

Some very deep discussions by the sea!

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 Callanish

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

this is no time for yoga, John!

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

That's the ferry?!

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!

unf, check those folds!

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.

Dalradian-ORS unconformity

Yes, there’s a hammer for scale in there!

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

IMG_2483

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!

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2 Responses to But that’s the foreland, you traitor!

  1. John Dewey says:

    What a beautiful and apt epitaph to the life of John Mendum. It encapsulates his love of geology and his enthusiasm perfectly.
    John Dewey
    1st January, 2017

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