Slate

[Iestyn Rhys Pritchard]

Can anyone explain (in simple terms) how slate is formed and how it ends up running in seams with chert? I can't seem to make any sense of what I've found on google  :blink:
Thanks!

[Danny Sutton]

i've no clue about rocks or geology...

but as i understand it..

slate is the result of the shale and mud stone been heated and compacted togeather under it's own weight.

i think the seams are formed by the direction the heat is comming from (location of the tectonic plates), the minerals in the shale and from the layers in the rock that have been caused over millions of years from ice ages and rising/dropping seal levels.

all this could be bollocks though, it's as i think it was explained to me.

[Iestyn Rhys Pritchard]

Thanks Dan!
That sounds about right to be honest.
I'm not really interested in geology, but if someone happens to ask at Llechwedd, I'm going to look like a bit of a tit if I can't answer!

[Danny Sutton]

is that your new job?

[Iestyn Rhys Pritchard]

Yep. It's a tidy job. I get to drive the train and everything. The money is just as crap as what I was getting in my last job, but I'm underground most of the day and there's a canteen, so it's heaven!

[Danny Sutton]

you'll end up like Marty though and never do any trips EVER, cos it'll be just like been in work.

[Les Williams]

Slate was once mud and silt in a deep sea. The slate is dark as it contains organic materials that haven't oxidised as they have deposited in deep anoxic (no oxygen) water.
As the depth of mud increased the sediment was compacted into mudstone, by its own weight. Over the millenia this continued with great thickness's of silt and mud deposited, along with other stuff, organics, etc.
The sea bed was eventually uplifted by tectonics and the mudstone would have remained as such except for later geologic events.

The mudstone would have been deposited in layers but these are not the layers you see in the slate today, they are something different, called foliation. At some point in the geologic past the mudstone was placed under great pressure and also some raised temperature. Slate is formed at relatively low temperatures by a process called regional metamorphism, the rock is changed by the geologic event. In this case the event was the collision between Avalonia and Baltica (England and Scotland). The collision caused some volcanism and also lots of pressure from the two colliding continents. The clay minerals in the mudstone were squashed flat and the foliation aligned with the principal stress axis (The main direction of the collision). The chert and quartz bands were formed by the mineral rich fluids within the slate being forced along the layers as the rock was de-watered by the pressure and this concentrated these minerals in bands through the rock...

If you look closely at the outcrops of Welsh slate you will see two different angles, If you look at the roofs of the chambers in Cwmorthin you will see that they are relatively smooth but inclined at an angle. The working face is at a different angle. This is because the roof represents the bedding plane of the original mudstone and the working face is the plane that the slates readily cleave along, which is the foliation...

The good slate is really only in the one bed so it is worked along the bed, this is why the workings in Cwmorthin are all inclined. In Moel Fferna the beddings are much less inclined so the workings are nearer to the level, the slates however are aligned to the principal stress axis, which bears no relationship to the original deposition layers (beddings).

Hope this is of use, If you really want to know more I will be happy to oblige over beer...  :whistle:

[Iestyn Rhys Pritchard]

Wow, thanks Les! I doubt I'll need to go into quite so much detail with people on the tours, but great thanks!

Dan, I doubt I'll not want to do any trips to be honest, cos at the moment I look at the same chambers over and over and over again....
Although they did mention having a look round unexplored parts of the mine when I took the job. so that should be fun!  :cheer:

[Les Williams]

The real problem with understanding geology is that it is full of jargon. A good geological dictionary will help decipher the almost meaningless twaddle into something more sensible and understandable.

Geology is really interesting if you can get past the jargon.

 

[Robin Jones]

Iestyn, Congrats on the new job.  Sounds like fun.  Some other questions you may want to find the answers to as someone is bound to ask them.

[ol]
  [li]How many undiscovered chambers are there?[/li]
  [li]How thick are the walls?[/li]
[/ol]

[mike leahy]

[quote="Sweetie" post=16123]Iestyn, Congrats on the new job.  Sounds like fun.  Some other questions you may want to find the answers to as someone is bound to ask them.

[ol]
  [li]How many undiscovered chambers are there?[/li]
  [li]How thick are the walls?[/li]
[/ol][/quote]
and dont get nabbed sneeking in the back door to see all the other stuffff or you 'll get the sack

[Iestyn Rhys Pritchard]

[quote="Sweetie" post=16123]Iestyn, Congrats on the new job.  Sounds like fun.  Some other questions you may want to find the answers to as someone is bound to ask them.

[ol]
  [li]How many undiscovered chambers are there?[/li]
  [li]How thick are the walls?[/li]
[/ol][/quote]

There are over 250 chambers connected by over 25 miles of tunnels, and they're on 16 floors. 7 floors are flooded. Very few of the chambers have been explored because no one seems interested in exploring them.

30ft pillars of slate are left in place to support the roof.

Mike, the temptation to start smuggling gear down there is unbelievable...

[Iestyn Rhys Pritchard]

....and the 'cathedral' chambers are somewhere between 250 and 300ft from top to bottom and they're floodlight. imagine an abseil in there  :blink: