I found this report posted today (1st Oct 2022) - new cave perhaps?

Quite an eventful journey from Capel Curig towards Nant Gwynant on the A498 today.
The road was completely blocked by a massive landslide which had recently happened as the traffic hadn’t been diverted.

Always a silver lining!

Ian wrote:
Pretty sure it's not a cave as this is in the middle of Snowdonia, where there are no caves, as there is no limestone...
I guess it could be a mine adit, but looking at the way it has formed I think it's more likely a bog burst.

Geologically the location is on a boundary of rock types, with an igneous intrusion exposed above and mudstone lower down. Both of these rocks are generally impervious to water, so a large volume of water could be held there in a peat bog, until the edge of the bog fails...

If you look at the location, there was already a small stream there and the top of the landslide coincides with a change in vegetation which could be the edge of a bog. Also, all the debris is just mud and dirt, no rocks visible really.

AditNow have no know mine sites listed any where near so I doubt its a mine, but you never know. Still it could be worth a look.

I was hoping you would chuck in a 2p ... you are the mostly likely to understand the geology

I can't see the stream you are referring to, looks more like a great gusher coming out of a hole. That's what she said anyway.

Ian wrote:
The stream is probably intermittent but is culverted under the road so will carry significant water at some times.

The geological map shows the following:

Lilac:
Cwm Eigiau Formation - Mudstone and siltstone. Sedimentary bedrock formed between 455.25 and 452.75 million years ago during the Ordovician period.
Orange:
Unnamed Igneous Intrusion, Ordovician - Rhyolite. Igneous bedrock formed between 485.4 and 443.8 million years ago during the Ordovician period.
Pink:
Unnamed Igneous Intrusion, Ordovician - Microgabbro. Igneous bedrock formed between 485.4 and 443.8 million years ago during the Ordovician period.
Yellow:
Lower Rhyolitic Tuff Formation - Tuff, felsic. Igneous bedrock formed between 458.4 and 449 million years ago during the Ordovician period.

I think the yellow (Tuff) is too far away to play any part in this event, the water is running off of the Ryolite and the microgabbro on to the mudstone and siltstone. I think the peat is on the igneous rocks as they are pretty impervious but once the hillside gets steeper the flow is on to the mudstone/siltstone and the water will be able to trickle away through the joints, so it doesn't support peat growing and so you get a more grassy soil. The actual stream channel is filled with grass and blocky mudstone/siltstone so won't be very obvious in the summer.
If the outlet from the peat bog gets blocked and the water backs up, the peat will "float" in place held there by the roots (We call them "quaking bogs" down our way, walking on them is like walking on a water bed).
I imagine with all the rain the bog was probably saturated, and then something let go and the whole bog slipped down the hill side...

A really useful resource is the BGS online geology map, zoomable and if you click it will tell you about the rock types where you clicked. For all you caver, the normal colour for limestones is blue, but not always...

Les, this is amazing.

I have read it three times and I am absolutely certain it means something.



Ian

More seriously, apart from the names of rocks from XYZ millions of years ago, I do think I have my head around it.

I knew you would understand it – you are wasted in electricity

Thank you

Ian

Ian wrote:
From a geological perspective, the mudstone/siltstone would have been deposited in a shallow shelf sea as fine mud layers, perhaps in an estuary, although they may be more open sea sediments.
The microgabbro is an intrusive rock that has formed during the closure of the Iapetus ocean. The closure of Iapetus is when Scotland (Part of Baltica then) first joined with the rest of Britain (Avalonia). As the ocean closed, subduction occurred on both sides and so caused volcanism to the south and north of the final "suture zone" which is in southern Scotland. A lot of volcanics occurred in North Wales, Lake district, and around the Midland Valley in Scotland as a result of these subduction zones and these intrusions, which were originally of basaltic composition will have assimilated some continental rock as they stope up to wards the surface and so changed their chemistry to less basaltic compositions. The microdiorite will have solidified at depth and is not much changed from basaltic, but the rhyolite is a very different igneous rock that was placed on the surface as lava...
It would have been very exciting times back then with volcanic islands and a shallow sea.
The tuff is the result of ash, from the volcanic eruptions and is another part of the story.

Once the Iapetus ocean closed the collision resulted in uplift which exposed all these rocks to erosion and subsequently the next 400 million years has allowed even the deep deposited microgabbro to be exposed on the surface.
Although I don't have the luxury of a few hand samples, I am assuming the microgabbro will be a dense crystalline rock that will not allow much groundwater to flow and the lava (rhyolite) will be similar, having solidified much quicker with very small crystal growth.
These rock types would not permit water to seep away and will hold it on the surface, within the soil.
Volcanic Tuff is generally quite porous, so ground water would be able to flow through it, but the tuff there is quite old (400mya) so may have become crystallised due to the close contact with other igneous rocks, but, as I said, not having the luxury of a hand sample...
The tuff,is further away down the valley anyway so will probably not have contributed to this event...

The BGS website is a really useful resource with all the regional memoirs, and sheet memoirs, and a comprehensive geological map, all containing masses of information, and all freely available on 'tinternet...
www.bgs.ac.uk/
I do recommend it

Ian wrote:
I also know masses about electricity having worked as a spark for most of 35 years now... so I may not be "wasted" there as it keeps me in beer...

Ian wrote:
It certainly means something...

So, what you mean is;

"It isn't a cave"

Ian wrote:
I never, ever said it isn't a cave. What I did say was I was pretty sure it wasn't a cave...
(always got to keep your options open just in case )

I was offering a summation of your teachings. I think I need to add another word ...

"It probably isn't a cave"