02 May 2022

Day on the beach measuring folds

We have this first year module that focuses on the geology of Anglesey; quite a lot of these trips I can dream. But there is one trip I have never done in its entirety. It is the trip to Lligwy Beach, where Devonian Old Red Sandstones are exposed. The previous year this trip fell on the same day as my big Welsh exam, so I had only done the first half. I'm not quite sure about earlier episodes; I suppose I must have just been otherwise engaged. And in 2020 this trip was just sacrificed to the pandemic. And there are two parts to this trip; we first look at the sedimentology (so that is where we just let Jaco loose, as he can talk about sedimentary rocks forever) and then we look at the structural aspect. As it so happens, the Old Red Sandstones have very clear bedding, and after the position, the beds have been folded by ongoing continental deformation after Avalonia collided with the continents and the other side of the Iapetus Ocean. And even though it is not a big beach, the structure is a bit too big to keep an overview just by looking at the rocks. So we would just measure the strike and dip of the bedding planes over the width of the beach and then plot the measurements on the map. That would then disclose to us what the overall structure was!

When I met Dei on the main campus he had a shock for me. After they was going to be measuring bedding planes, but he said that the University compasses had gone missing. Oh dear! What now? But he had an immediate solution; he spoke of a free app on your phone that can do the same. Amazing! I quickly downloaded it. Catastrophe averted.

What the app looks like

I hitched a ride with Dei after the students got onto the coach. It was a beautifully sunny day! And on the parking lot we met Jaco. Dei told him of the compasses, and of the solution we had found. The app doesn't only give you the measurement of strike and dip, but allows you to note the lithology, and it will plot your data on a map. Dei suggested we might be a bit quicker with this then we would be if we would be working with actual compasses! Jaco then remarked that that would give us more time to look at beautiful sedimentary rocks - that are NOT on your phone. He had a point. And then Jaco started introducing the students to the field site, and the main job of both Dei and me was done.

Jaco introduces the site

We looked at the beautiful beds and noticed the grain size differences, the dissertation cracks and the shape of the beds. We first noticed overbank deposits. A bit further north we spotted river channels. And the sun shone on us but the wind was still cold; I was wearing thin down jacket underneath my waterproof jacket, and most of the time I was wearing gloves. But it was lovely!

First stop with ORS

Beautiful stratification

Some pretty rockpool life

This was also the field site with facilities; when I needed to answer the call of nature I found a lovely accessible field. And soon after that it was time for lunch.

Field aka ladies' room

After lunch we did the structural exercise. Jaco and Dei had brought their own compasses (I hadn't; I had just counted on the University specimens) so we could compare results from analogue and digital methods. There was one student who ended up with funny measurements; he had a magnet stuck to the back of his telephone case… Yes that would pose problems. But altogether I am quite impressed with this app. And I am sure the students have no problem with getting to terms with this kind of technology. I also notice I am getting so old I am starting to need reading glasses to read off a compass rose!

Dei checks the app vs an actual compass/clino

Measurements in action

When we had measured the entire beach, Jaco plotted it live on a map he had brought. A lovely synform emerged! And then we were done. We walked back to the coaches as the drivers were afraid that by now, the parking lot would be too busy for them to be able to turn around. And then we headed back to Bangor! I think it was a success. Mainly thanks to Jaco. And now I had finally done the entire trip! It was about time…

Putting the measurements on the map

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