21 October 2021

Rhoscolyn geology

This is the fourth year we are doing our fieldwork module. And strangely enough, there was one trip out of the six we do that I never attended: to Rhoscolyn. I mainly know the area for reasons of climbing. I also hadn't been able to attend the recce that Jaco and Dei had done. I therefore wasn't very confident teaching on that particular trip. So when it came up again, and I could attend, I decided to join it without having a particular part of the teaching assigned to me. If I would just watch the others (mainly Jaco, as this particular trip is pretty much start to finish within his area of expertise) teach, I could just imbibe the knowledge, and then teach on it the next time it would come around. I did read up on the area, but otherwise I didn't have to prepare much. And Dei had offered to pick me up from Bangor.

We signed the students into the coach, and then set off together. We would first go to South Stack. That is such beautiful place! And the weather was ideal. We were lucky. And we pressed the Jaco button and enjoyed the show. He talked about the general setting, and the stratigraphy in the area. And then we went to see it for ourselves. The promontory with the lighthouse shows clearly what the area is famous for: intercalations of mudstone and sandstone, which are indicative of a deep marine setting where mud slowly accumulates, and occasionally submarine gravity flow comes and brings in coarser material. And tens to hundreds of millions of years later, all of that got deformed by tectonic processes, creating the beautifully folded strata that attract so many geologists and other interested people.

He also explained the various types of gravity flows, and how you distinguish them. And he picked one particular sand bed and showed the students how you can tell it had been a turbidity current. And he said they would look at more of those at the next location.

Jaco providing the background

The lighthouse on the folded sandstones and mudstones

Jaco pointing at turbidites. Pic by SOS

Cliffs. Pic by SOS

Walking back up. Pic by SOS

The next location would be Porth Dafarch, where we first would have lunch. There were picnic tables there and it was lovely. I personally regretted the public toilets there were closed; I had hoped they would contain drinking water, as I had had another incident of my water bag emptying itself in an unsolicited way, so I had preciously little water. That is never a situation I like to be in! I also found out that Porth Dafarch is a difficult place to find a good spot to go to the loo. But I managed.

Porth Dafarch

Deformed turbidites on the beach

Jaco holds court. Pic by SOS

After lunch we did look at the turbidites on the beach, and we did manage to get some of the students enthusiastic about being able to distinguish the various constituent parts of a turbidite in the rocks half a billion years old. We also showed them a tertiary dike cutting through all that turbidite action.

Then it was time to move onto the last location: that was Rhoscolyn proper. We walked to the lifeboat outlook post, and from there to the cliffs where you can see even more deformed turbidites. They were spectacular, and even festooned with curious seals. And we ended the day with some massive quartzites. And then it was time to go back to the cars, and then home! Dei seemed to have forgot he didn't actually want to drive back to Bangor to get me back to my car, as he lives about midway between Rhoscolyn and Bangor, but I suggested I hitch a ride back with Jaco, for whom it would be much less of a detour. I think it had been a successful day! I know that the geology we saw is not very varied, but we spent an entire day on one particular lithology and got to see it at various spatial scales. I hope the students will now appreciate deep-seated sentiments with intercalated turbidites for the rest of their lives!

Rhoscolyn proper

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