Was wondering if anyone knows what the lumpy yellow/tan rather nondescript looking rocks are, which are found scattered among the slate on the tips around Cwmorthin. They appear fine grained, even cement/concrete like, and consequently have probably been overlooked. but I'm sure they have an interesting geological story to tell. :smartass:

I have seen this material, or something like it underground in Cwmorthin. where it appears to infill ancient fractures or cracks, and in such cases is usually associated with quartz.

eg a band or vein of this material with quartz banding - floor D old vein, also floor 1 back vein. Photogaphs (in Cwmorthin gallery) of something very similar on floor A back vein.

A friend described the rock as "like a pumice" which I feel was a reasonable description up to a point. I think it is indeed igneous and has the light colouration which suggests a felsic material.

The association with quartz reminds me of the flinty banded "chert" up in the opencast, which suggests the yellow material may have reacted similarly to geological forces and cracked, with the cracks then getting infilled with quartz and possibly other materials.

Would be great to put a name to this stuff :)

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I guided a trip down there a few months back for members of a local geology group. The identity of that yellowish mineral was one of the questions I had. It occurs in beds like another strata of rock, where as the quartz veins are irregular as they are a later intrusion, filling the cracks in the slate created with tectonic movements.
I understand that the mineral might be quartzite, like that found on Anglesey. A metamorphic sandstone. I guess as the silty sediments destined to become the slate were deposited on the sea bed, occasional storms or sea currents brought sand too, laying as a new bed, before being engulfed in more layers of the fine mud. As the silt became mudstone, the sand became sandstone. As the metamorphic process occurred, the mudstone became slate and the sandstone became quartzite.
I'm not a geologist or expert, but that is my understanding from being with the geologists.

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many thanks for your reply PeteK. Indeed reminded me of certain sandstones/limestones. but was however still thinking along the lines of volcanos, volcanic tuff and the like.

There are also the dykes as well, running at a steeper angle than the bedding planes, where magma forcing its way though cracks became dolerite, which I think is generally a much darker coloured stone, so the mystery stone is unlikely to be this.

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We did a tour of maenofferan on the large waggons, the tour guide said that lots of fools gold is present within the slate of Wales. It contaminates the slate and has to be discarded. He told us that it was the yellow mineral which veins through the slate. Pyrite is its name if i remember right, i must admit i was nursing a sore hangover that morning!

Best fact of the day for me was them saying that every single piece of slate present on the spoil pile 'mountains' has been touched by a human hand and some point in its journey from the ground to the spoils.

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Best fact of the day for me was them saying that every single piece of slate present on the spoil pile 'mountains' has been touched by a human hand and some point in its journey from the ground to the spoils.

In general, as I understand it it's rather more than that ...

There's the rock man who won the blocks the from the face. He and his mate would split them if needed and load them onto a slab wagon. The wagon has then to be pushed or hauled to the incline.

At the mill the blocks would be split again, sawn on slate saw tables, split repeatedly to roofing thickness pieces, individually trimmed, either by hand with a knife else or by holding them up by hand to a trimming machine.

The offcuts from the saw tables and the trimming waste would be loaded, again largely by hand, into rubble wagons and taken out and tipped on the tip.

So, I guess there's a minimum of four people involved at least – anyway, that's what I tell people.

Dave

So essential each piece has been touched by at least 4 sets of human hands! Incredible when you think about the shear size of some of the welsh slate spoils, cwmorthin is an amazing site to see just from the parking spot 🙂

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back to the layers of sandy yellow coloured stone. - Was indeed trying to acertain if it ran in line with the original bedding, or at a steeper angle, but didn't have time to work this out as I would have been left behind:(

interesting point that, to my mind, the "cherty" volcanic tuffs could also be considered a sedementary rock as well, but having a volcanic origin - eg an ash fall onto the mustones, rather than an inrush or deposit of sand. And they would then be exposed to similar processes of compaction, facturing and quartz infill as the sand mentioned above. So, all in all similar processes as work, at least as I understand it.


On a related note, never really worked out what the hards above the old and back veins actually are geologically and under what geological conditions they were formed.

Also wondering what length of geological time has elapsed between the bottom of the new vein and the top of the north vein

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The rocks were supposedly sedimented in mid to late Ordovician times (Arenig to Ashgill, perhaps 465 to 445 Ma), with the material of whatever origin being sedimented in water. The cleavage would have been imposed by the orogeny at the closing of the Iapetus, cutting across the bedding. Much later, during the Tertiary rifting associated with the formation of the Atlantic, various sills and dykes would be implaced. I believe these to be the "hards" mentioned, decribed as rhyolites in some sources. At a later stage fracturing has been mineralised, here with quartz but elsewhere with metalliferous content as the ocean spread - think black smokers.
Only a speculation of course, I've slept since then...

Seen reference to the trap rock under the new vein as rhyolite, (caban magazine) but this may not necessarily mean that the hards between the old and back veins are the same material. Often wondered if they are igneous sills that have somehow forced themselves between the slate thus breaking it up into the distinct veins we know today, or if they actually represent an extrusive igneous deposit on top of the mudstones That's assuming of course they are igneous material.

There is the layer of chert above the new vein often mentioned in the quarry reports, which is compacted volcanic ash

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The clevage we see today that makes the slates is NOT bedding, so any materials along that parting are later intrusions, not sedimentary beds. These would technically be dikes as they cut the bedding, but "sill" could also be applied as they follow the partings... As they are post cleavage thay must be much younger. Concordant (same as bedding) ash layers would have settled through the Ordovician seas and lakes onto the muddy floors that later became the slate.

back to the layers of sandy yellow coloured stone. - Was indeed trying to acertain if it ran in line with the original bedding, or at a steeper angle, but didn't have time to work this out as I would have been left behind:(

interesting point that, to my mind, the "cherty" volcanic tuffs could also be considered a sedementary rock as well, but having a volcanic origin - eg an ash fall onto the mustones, rather than an inrush or deposit of sand. And they would then be exposed to similar processes of compaction, facturing and quartz infill as the sand mentioned above. So, all in all similar processes as work, at least as I understand it.


On a related note, never really worked out what the hards above the old and back veins actually are geologically and under what geological conditions they were formed.

Also wondering what length of geological time has elapsed between the bottom of the new vein and the top of the north vein

Yes these rocks are essentially tuff-turbidites - slumped in places, they are sediments made up to a greater or lesser extent of air-fall acidic volcanic ash. The stripier versions as seen in the opencut probably record spasmodic eruptive activity, between which normal sedimentation - i.e. dark grey mud and silt deposition - resumed. IIRC the volcanics are all lumped together as the Rhiw Bach Volcanic Formation. The BGS Snowdon memoir has more details.

Many thanks for all the interesting info, and will look up memoir when i have a moment. Will try and take a photo of the lumpy yellow fine grained rock, I didn't have chance to get a photo of the yellow/tan strata seen underground however

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🔗97179[linkphoto]97179[/linkphoto][/link]


I have finally got around to finding a photo of what looks like an example of another kind of strata in Cwmorthin/Oakeley. A large piece had fallen off on to the wall at base of chamber. (Oakeley floor 1) It doesn't really look like slate and appears brittle and dun/yellow/light grey? in colour but that might just be my imagination. It could well be the ceiling strata above the slate vein, but I'm unsure what that is anyway. From memory, the chamber in question is a bit odd as the retaining wall is on a corner where the level changes direction or there is a junction. I will have to look at the various plans again. The chamber is, I believe in the old vein, but there are some cross cuts in the vicinity and my understanding is that the stripy/narrow veins are quite close at that point, as is the badly disintergrated chamber 30 OV.

By the way, we visited this point again in October 2019 and pleased to say not much has changed from what you can see in the photo which as taken maybe 3 or so years ago. (that's another good reason for taking photos apart from geological information, as they can be of certain use in the monitoring of unstable areas - but that's probably a whole new topic)

Anyway, can anyone please give me some feedback on what this layer of stone might be given it's position and area in the quarry, and given what has been discussed previously. I know a photo isn't ideal, but it's probably better than nothing 

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Photo is not showing.

Did that to me.

When I signed in I could see it.

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Sorry, Photo should be viewable now!

:)


Looking up into chamber 29 floor 1 Oakeley/Cwmorthin

This chamber is one along from the heavily collapsed
chamber 30, and is also filled with fall or tip material or a mixture of both. Note short retaining
wall and pit prop. This whole area is heavily broken ground yet the levels themselves in the vicinity
of these chambers are remarkably clean.

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