Author Topic: Nantlle Valley and the Carnedd y Filiast Grit  (Read 438 times)

Offline RobinGriffiths

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Nantlle Valley and the Carnedd y Filiast Grit
« on: May 16, 2021, 10:08:16 pm »
I had a little walk around Nantlle Lake today, hoping to 'take in' the shaft marked on the OS map at SH 5272 5279. In fact it was an adit in a very ferruginous ridge of rock. I don't doubt there actually is a shaft a bit higher up on the hillside, which I'm reserving for a wander into the valley above at some point.

But the interesting thing, when looking at the Nantlle Valley mines on a BGS map- Benallt, Simdde Dylluan, Drws y Coed (including other lesser trials from the Wilkinson Gazeteer), they all seem to be aligned with the outcrop of Carnedd y Filiast Grit. Sometimes in the underlying Marchlyn Formation, sometimes in the overlying Nant Ffrancon Subgroup, which no doubt would hit the Marchlyn Formation at depth. So my hypothesis is that in the Nantlle Valley, the Carnedd y Filiast Grit (coarse sandstone and conglomerate) acted as some sort of cap, with mineralisation forming below it in the Marchlyn Formation (mudstones and siltstones). The CYF Grit is quite a robust rock, forming escaprments in the vicinity.

Looking outside the Nantlle area there is a similar pattern in other areas where the CYF Grit outcrops. Llanberis Mine, Ceunant Mine (in the Ogwen Valley), Blaen y Pennant, Gilfach and Cwm Cipwrth in the Pennant Valley.

Ironically, I don't think my 'mine of the day' fits into this pattern. I suspect it's an analogue of the pisolitic iron mines in the Nant Francon Subgroup at Betws Garmon. (Ystrad and Silurian Mines)



Offline Cantclimbtom

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Re: Nantlle Valley and the Carnedd y Filiast Grit
« Reply #1 on: May 17, 2021, 07:39:56 am »
It's a lovely rock type. I've been sadly thinking for a while how much I miss some long mountain hill walks and scrambles and something like Atlantic Ridge (grade 2/3 scramble) is something that I've been thinking about increasingly for a while.

http://mountain-lifestyle.blogspot.com/2014/09/atlantic-slabs-carnedd-y-filiast.html?m=1
Take a look at the 4th photo in that linked article, lovely shot of fossilised plants.

I'd never thought about the iron and other minerals that's bound to be sitting under it. The quartzy bits here and there should have given me the clue, but got to admit my mind was drawn to the proximity of Dinorwig/Penrhyn. Next time I wander up there I'll keep my eyes out for the intrusive stuff

Edit: maybe it's the 5th photo and I can't count :)
« Last Edit: May 17, 2021, 07:48:44 am by Cantclimbtom »
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Offline RobinGriffiths

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Re: Nantlle Valley and the Carnedd y Filiast Grit
« Reply #2 on: May 19, 2021, 12:27:08 am »
I've been intrigued by those fossils. It's a hundred million years+ before plants arrive. They appear to be trace fossils - fossils not of a dead animal, but the imprints made in the sediment during it's life. Doing a bit of amateur geology research, they are most likely to be Rusophycus, traces made by trilobites.

Incidentally, the southern equivalent of the Marchlyn Formation - the Ffestiniog Flags appear to be the host of the Coed y Brenin porphory deposits, so maybe my hypothesis of the CYF capping the ore might have a hint of truth, even if the CYF does not play a direct role.

Offline Cantclimbtom

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Re: Nantlle Valley and the Carnedd y Filiast Grit
« Reply #3 on: May 19, 2021, 01:18:01 pm »
Wow, thanks for that reply, that gave me quite a chain of Wikipedia reading on https://en.wikipedia.org/wiki/Rusophycus and https://en.wikipedia.org/wiki/Cruziana and the like

Taken from http://mountain-lifestyle.blogspot.com/2014/09/atlantic-slabs-carnedd-y-filiast.html?m=1 their 5th picture


Even knowing that I am struggling to see it as trilobite, it is so deceptively plant like as where the "fronds" join the "stem" it is 45 degrees or something and not obviously triobity. If the Marchlyn formation is Merioneth series and late cambrian (485-500M years old) and plants arrived at that time... or some conjecture earlier (https://www.sciencemag.org/news/2018/02/land-plants-arose-earlier-thought-and-may-have-had-bigger-impact-evolution-animals#:~:text=All%20the%20analyses%20indicate%20that,multicellular%20animal%20species%20took%20off.)
And if this was laid down on top if that, perhaps they are the first plants? They look so planty.

Dunno.. I'm well out of my depth here. I'll defer to your better judgment!
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Offline RobinGriffiths

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Re: Nantlle Valley and the Carnedd y Filiast Grit
« Reply #4 on: May 19, 2021, 04:31:42 pm »
I'm no expert, but I can visualise how the raised 'leaf' 'stem' and 'ribs' correspond to the ventral line between the rows of legs, and the gaps between individual legs respectivelly when it's at rest on the seafloor. And these things came in a myriad of shapes back then.



Offline Cantclimbtom

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Re: Nantlle Valley and the Carnedd y Filiast Grit
« Reply #5 on: May 19, 2021, 05:06:03 pm »
One thing I didn't factor for and only just noticed, if they're vegetation why would all 4 in that pic be about the same length. Those leaves are all about 1 trilobite long

I think your trilobites beat my half baked leaves
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