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Northwest Scotland 2019: How we learnt to map and spent a week with bog water in our walking boots

Mia: The start of the second year of our undergraduate degrees began with twelve September days in northwest Scotland. In complete honesty, it was wet, gruelling, and at the time it really didn't feel like it was ever going to end. But, eventually, it did end and we have come out the other side much better prepared for our independent mapping projects, which we'll complete next summer. Book-ended by twelve hour coach journeys, the ten days we actually spent in the field mainly concerned training in how to map and gather structural data. We thought it was important to share what we did with you because mapping is such a huge part of undergraduate geology, and we really did see some amazing things.


A geological map is a map which shows the rock beneath our feet. Even in areas mostly covered with vegetation, it is possible to work out what bedrock lies beneath. The main focus of this trip was to improve our skills in producing geological maps, both though outcrop and feature mapping. Outcrop mapping involves travelling through an area on foot in search of places were rock is exposed. This rock must be in situ, meaning it cannot be loose rubble. In these places, we can study the type of rock and its features up close, and then mark on our blank maps where it has been found and any relevant structural data we have collected there. By the time you have covered the area, you should hopefully have enough exposures marked on your map to be able to infer the geology underground.


Feature mapping involves being able to look at highs and lows in a landscape and make reasonable predictions about their geological cause. Ridges that stand out from flatter ground, for example, may indicate the presence of a harder rock type, which is more difficult to weather away than the surrounding rock. Linear ridges, therefore, often represent some kind of igneous intrusion, like a dyke. Rivers often follow faults and deep, sudden holes can be signs of limestone underfoot. This is a useful method when exposure is poor.


In this blog post, we've taken it in turns to write about different tasks, each of which covered between one and three days, and we've indicated which of us is writing in every section.


Day One: An introduction to the lithologies of the Caledonian Orogeny


Lucy: Our first day began with typical Scottish weather; overcast, freezing and drizzly.

However, even though the weather may have been on the miserable side, the Scottish landscape was stunning, which made up for being stuck in the freezing cold. We spent most of the day in the surrounding area of Loch Assynt. Here, we were introduced to the lithologies of the Caledonian Orogeny and taught how to correctly identify the different rock units using their typical characteristics. For example, a typical characteristics for the Pipe Rock Member is the presence of trace burrow fossils which resemble pipes.

We were also provided with extra information on the geological history of the area, in particular the Caledonian Orogeny. This gave us a greater understanding of the relationship between the lithologies. The Caledonian Orogeny (~390 Ma) is an event which brought together two of the largest landmasses of that time; Gondwana and Laurentia. Most of the UK once belonged to Laurentia but the north of Scotland belonged to Gondwana. This is why many geologists are drawn to Assynt; it provides some of the best evidence for the orogenic event. The area has also provided some breakthroughs in the science of geology, especially in the mechanics of thrust faulting.


Day Two: Melness: A look at the ductile deformation and metamorphism of the Moine sediments

Where is Melness? Basically go north and then keep going north.

Mia: Day two began with a three hour drive right to the very top of Scotland, where it was very, very windy; so windy it was hard to stand up in places, which lead to some not so elegant field notebook pages. The aim of the day was to look at what happened in the inner zone of the orogeny, in the place where there had been the highest pressures and temperatures. The pressures were so high in this area that the rocks behaved more like warm plastic, folding rather than breaking/faulting. Furthermore, the minerals inside the rocks aligned and changed, leaving behind altered sediments.


We started by looking at the characteristics of the rocks present in each locality, searching in particular for index minerals that would tell us how metamorphosed the sediments had been or that might be used as indicators of the composition of the original sediment; was it sand-rich or mud-rich?

We were also searching for fold structures within these metasediment layers. The last locality yielded some very good sheath folds; these are folds that, when seen in cross section, form structures that look like cat eyes. In three dimensions, they look much like the name implies; like the sheath of a sword.


We finished the day with a viewpoint stop. Honestly, when the coach stopped by the side of a road I wasn't overwhelmed with excitement and expectation about what the locality would hold, but the "multicoloured rock stop" was actually the best part of the day. We were told nothing about the rocks visible in the road cutting, but instead were left to work out what we could for ourselves. The grey base rock was the same Lewisian gneiss we had encountered on the first day, which was crosscut by picrite (very mafic) Scourie dykes and granite pegmatites. Pegmatites are very course igneous rocks that represent the final cooling stages of a melt.

The multicoloured rock stop

Day Three & Four: Our first mapping exercise in Scotland - the foreshore of Loch Assynt

Lucy: In all honesty, these two days were personally my least favourite of the whole trip. The weather was awful and I most definitely fell over more times than I care to admit.

On day three we hiked up a steep hill (my legs definitely felt the burn). We spent most of the day following a stream, inferring boundaries from exposed rock as we went along. This definitely provided insight on how to distinguish between boulders and in-situ exposure, because it genuinely is harder than it sounds. The terrain following the stream was less than pleasant; if my foot wasn’t falling between grassy mounds it was sinking into a bog. Waterproof shoes are great until your foot sinks into a bog and all that ‘lovely’ bog water overflows into your shoe and you’re stuck with wet feet for the entirety of the day. This day made me realise that Scotland is basically one massive bog, and I was not a fan.

The next day we were down at the shoreline of Loch Assynt, studying the relationship between a Scourie Dyke and the Lewisian Gneiss. The lecturers taught us how to correctly infer the thickness of units from the limited exposure in this locality. The geology here was interesting; we got to see the relationship between the dyke and gneiss which meant we were able to see some great examples of baked margins present on the gneiss, but the weather made the whole experience uncomfortable. I found it almost impossible to gain any sort of data from the rock units, even harder to write in the field notebook. After a couple of hours, once the rain, wind and temperature worsened, the day was reluctantly called off. We all raced back to the bus as quick as possible, which admittedly was not very fast as we had to traipse through boggy terrain.


When we finally reached the bus, we were all ready to get back to the hostel, but this is a university field trip, they weren’t going to let us off that easy. The lecturers decided to drive half hour to Knockan Crag to view a famous outcrop of the Moine Thrust. I think it’s safe to say we were all disappointed at the time, but looking back it was definitely worth the trip there. When we arrived at Knockan Crag, we had to hike up a hillside to reach the outcrop. When you begin the trail, you are met with a man-made rock wall illustrating the relevant units of the area. When you finally reach the outcrop, you can see the Moine Supergroup overlying the Durness Limestone. This unconformity drove geologists such as Lapworth insane; at the time, geologists could not fathom how, with a 500 million age gap, the older schist overlays the younger limestone. The Moine Thrust led to questions which in later years would help bring together the concept of plate tectonics.

Outcrop of the Moine Superthrust. A metamorphosed schist overlying the Durness Limestone.

Stratigraphy of Assynt, Scotland

Day Five: Examining the fold structures at Baghastie Bridge

Mia: After barely recovering from the rain of the previous day, and wearing still-soggy walking boots, we headed out to Baghastie Bridge to examine the fold structures within the same metasediments we'd seen on day two. In some ways, it was okay that our boots were still wet, because they spent most of the day submerged in bog water anyway.

An S fold in the metasediments of Baghastie Bridge

Baghastie Bridge, like Melness, was in the inner zone of the orogeny, meaning rocks behaved plastically and folded rather than faulted. The aim of the day was to look at small scale folding to determine the larger scale fold shape that ran across the area. Often when folding occurs on a large scale, smaller 'parasitic folding' occurs in the beds in the limbs of the fold. We can use the shape of these parasitic folds to determine the shape of the larger fold, or whereabouts we are on that larger fold. Symmetrical parasitic folds tell us that we are at the peak or trough of a fold, while S and Z folds can tell us which limb we are on.


Day Six & Seven: The Achmore Duplex is awful

Lucy:

What is a thrust fault? Thrust faults are compressional faults that dip less than 45 degrees.

What is a thrust duplex? A thrust duplex is a repeated sequence of rock (horse) that has been thrusted on top of the same sequence of rock, resulting in older rock overlying younger rock. A thrust duplex always forms with a duplex roof; they don’t reach the surface unless the overlying rock has been eroded.

Model of a thrust duplex. Image credit: The Edinburgh Geological Society

Firstly, you’ll be happy to know, I won’t be droning on about how awful the weather was. The weather was surprisingly beautiful. In all honesty, it was actually too hot as I was wearing a million layers of thermals in preparation for the horrendous weather experienced previously, but I’m not complaining. The reason the Achmore Duplex is ‘awful’ is due to the fact the roof of the duplex has been eroded away, so the sequence of lithologies is basically a big fat mess, making it extremely difficult to differentiate sequences of rock units from one another. The problem is caused by the level of erosion; it causes rock units to appear much thicker than they actually are. Two separate parts of the same rock unit but part of different horses appear next to each other, thus it is very easy to make the mistake of inferring what we see as one. That is arguably what makes this area so difficult to map, especially from a student perspective. However, it wasn’t all negative; I did personally learn how to infer boundaries using topographic features. This really is a helpful skill when the rock exposure is poor or non-existent. For instance, there were plenty of areas which had no actual rock exposure but because the area had an abundance of shake holes, it was clear that the underlying rock must be Ghrudaidh. The first day we were introduced to the duplex system as one group; we followed a stream and inferred boundaries together, before mapping the area independently the following day.


Day Eight, Nine & Ten: Independent mapping in Glencoul

Mia: Finally, these were the three days we'd been preparing for since we arrived. Three days of non-guided, completely independent mapping of a roughly 2.5km^2 area about 15 minutes coach ride down the road from the lodge. We jumped off the bus, and that was it; we were free to approach the area however we thought best. Lucy and I actually ended up working separately, so I can only give my account of how myself and my mapping partner tackled the area.

Palaeocurrent indicator: crossbedding structures within the Applecross

We started off wanting to see how quickly we could travel the whole length of the map, one side to another. Heading straight down onto the western side of the map, we were soon distracted by our first outcrop and abandoned that idea. The rock we had been distracted by was the Cambrian age Applecross Formation; an expansive layer of red sandstone that dominated much of the western half of the mapping area. The Applecross Formation represents an ancient river system, bigger than any on Earth we have today. These sediments were deposited before any plants had evolved, so the rivers had run freely across the land without being intercepted by vegetation as they are today. The rock even contained bedding features that could be measured to tell us which direction the rivers had flowed in in that spot, known as palaeocurrent indicators.

Nice views from this lunch spot

Deer tracks across our mapping area

We covered as much ground as we could on the first day, following exposures on the river that ran through our area and around the edge of the lochan (small loch). A personal highlight was my mapping partner pointing out all the dear tracks along the paths they had made, and all the fish in the river.


Over the next two days, we managed to cover the entire mapping area. We ate lunch looking at the Glencoul Thrust we'd seen earlier in the week, fell down a number of steep slopes, fought off midges and ticks, and almost lost our walking boots multiple times in the dreaded bog water. We had largely avoided the rain, thankfully, and reached the end of the trip mostly dry.


We learnt a lot in Assynt, but I'd be lying if I said I wasn't relieved to be climbing onto the coach back to Birmingham on the morning of day 12. Fieldwork can be a lot of hard work, but it is also really rewarding; I never would have been able to see the Scottish Highlands if it wasn't for my course. But I still won't miss walking through bogs everyday.

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