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Author Rocks found on Cwmorthin tip Help with identification
John Mason

Joined: 22/09/2008

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Rocks found on Cwmorthin tip Help with identification
Posted: 23/06/2020 07:01:50
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Some of those latest illustrated samples are quite well-rounded, indicating a degree of transport - most likely in glacial drift. That means they might not be local, in a strict sense, but may be from a mile or three away.

The pinkish mineral could be feldspar. since that does occur in veins in the rocks in and west of the Moelwyn range. Some examples used to be well-exposed on the back-lane from the Oakley Arms over to Rhyd, although the heather now covers them.

The quartzy piece has obvious solution cavities formerly occupied by sulphides. There are widespread quartz-sulphide veins in the vicinity; most are minor in extent but some better-developed ones have been tried including one in that valley. Typically they consist of quartz with pyrite, sphalerite, chalcopyrite, galena and with more local arsenopyrite and pyrrhotite. Moelwyn mine and Catherine and Jane Consols are better-known mines that explored such veins (given their history, "exploited" is stretching it a bit!)...
IP: 86.139.53.229
Chrissie4224

Joined: 09/05/2020

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Rocks found on Cwmorthin tip Help with identification
Posted: 29/06/2020 10:34:53
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Hi. Had to jump in here. I live in the town and am a little knowledgable about the history of the moelwinion range of mountains. Moelwyn Mawr on the left of Cwmorthin is an extinct volcano which last erupted over a quarter of a million years ago. Moelwyn Bach to the right of Stwlan Dam is also a smaller extinct volcano but i am not sure when it last rrupted. The Cwmorthin Slate Mine is riddled with extrusive matter from mainly Moelwyn Mawr during the last eruption. Quartz Crystals of various sizes can aldo be found as well as Iron Pyrite better known as Fool's Gold. I have samples of this in my home. Hope you found my information helpfull. IP: 2.26.148.211
Minegeo

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Joined: 17/06/2008
Location: Ireland

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Rocks found on Cwmorthin tip Help with identification
Posted: 29/06/2020 13:52:37
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I think you will find your ages of the volcanics somewhat out as all of the volcanics in this area are Ordovician in age or around 450 million years old rather than your claimed "quarter of a million years". IP: 178.167.129.246
dwarrowdelf

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Joined: 09/02/2011
Location: Lost in Cwmorthin...and Oakeley too !!

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Rocks found on Cwmorthin tip Help with identification
Posted: 29/06/2020 17:40:17
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Many thanks for the interesting info everyone. Yes I was also under the impression that the geology was Ordovician, although some of the igneous intrusions in Cwmorthin/Oakeley may be later. but how much later I'm not sure, but I'm pretty certain not anything near as late as quarter of a million years.


Anyway, there are some truly massive boulders fallen from the roof in Wrysgan slate quarry, which I've clambered over, and I don't think they are slate, but volcanic in origin. I found fragments of what looked like a volcanic conglomerate near this vast fall, so maybe the boulders are composed of that. There are imposing cliffs of volcanic tuff in the vicinity of the quarry. I suspect these are all part of the Moelwyn volcanics

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'I wonder how many breakfasts, and other meals we have missed inside that nasty clockless, timeless hole?' 'The Hobbit' J R R Tolkien.
IP: 90.210.167.225 Edited: 29/06/2020 18:39:22 by dwarrowdelf
John Mason

Joined: 22/09/2008

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Rocks found on Cwmorthin tip Help with identification
Posted: 29/06/2020 18:38:32
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Rhiw-bach volcanics mostly. Moelwyn volcanics crop out a bit to the west. On the ground in front of the Stwlan Dam are some lovely lava-breccias belonging to the latter. I collected some boulders of the stuff for cutting and polishing, when we built the Wal Darwin at the Ogwen Visitor Centre in 2013, thanks to First Hydro giving me a key to the gated service-road.

Yes these igneous rocks are all Ordovician. I did find a Cenozoic dyke in Coed y Brenin a couple of years ago - the southernmost known from Wales to date - which goes to show others may be in the area too - but that's likely at least 55 million years old. The Cenozoic dyke-swarm, well-known from Anglesey and N Snowdonia, represents the final splurge of igneous activity in Wales. This past 50 million years, the place has slept peacefully.
IP: 86.137.48.34 Edited: 29/06/2020 18:42:43 by John Mason
dwarrowdelf

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Joined: 09/02/2011
Location: Lost in Cwmorthin...and Oakeley too !!

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Rocks found on Cwmorthin tip Help with identification
Posted: 29/06/2020 21:21:33
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John Mason wrote:

The banded tuffs on the tips at Cwmorthin host thin veins that occasionally have pockets of fine crystals. Quartz, albite and anatase all occur, but especially good are the rare-earth minerals, synchysite and xenotime. Specimens are very time-consuming and exhausting to collect, though! Eye-protection is essential due to flying splinters - in fact a full facial visor would not be a bad idea. Not a bad activity for a nice afternoon, when they let us out again...


full facial visor to be used down the pub after tuff smashing operation - kill two birds with one stone - stone being the appropriate word I think Laugh

To be serious - interesting research maybe for when I finally do a geology degree - The volcanics are so very interesting - and confusing - It seems to me there is so much variety even over short distances . Smile

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'I wonder how many breakfasts, and other meals we have missed inside that nasty clockless, timeless hole?' 'The Hobbit' J R R Tolkien.
IP: 90.210.167.225 Edited: 29/06/2020 21:56:05 by dwarrowdelf
Monty Stubble

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Joined: 03/04/2008
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Rocks found on Cwmorthin tip Help with identification
Posted: 01/07/2020 13:36:54
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Oh dear, my wife, who's a geologist by training ws more interested in the Welsh blanket... Blush


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The finest workers in stone are not copper or steel tools, but the gentle touches of air and water working at their leisure with a liberal allowance of time. Henry David Thoreau
IP: 31.52.196.40
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