The Friends of Cwmorthin Slate Quarry

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About Cwmorthin Slate Quarry

Cwmorthin is a substantial Victorian-era slate mine above the village of Tanygrisau, circling the slate quarrying town of Blaenau Ffestiniog. It has a long and complex history beginning in the early 1800's, with heavy underground development starting around 1860.

It was run by several different companies as a venture in its own right during the late 1800's. The earlier underground workings started at "Lake Level" by the Cwmorthin Slate Company Ltd,  ascended upwards in the mountain ultimately for 8 floors in both the Old and Back Vein. Poor working practices and reckless engineering decisions ultimately led to a substantial collapse in 1884 and the end of that company, which failed to recover, in 1888.

A new company (The New Welsh Slate Company Ltd, freshly evicted from Oakeley) took the mine on afterwards and reused Lake Level but sealed off the shattered and dangerous upper floors. Instead, they developed new workings below, going down into the mountain. Ultimately this company sunk five floors on both veins, before itself closing in 1900 due to financial difficulties, despite phenomenal extraction for just a 12 year stay and only ever using traditional hand-drilling methods. The mine came up for sale and the neighbouring Oakeley Quarry bought the rights to it, though they didn't attempt to work the lower floors initially and let it fill with water (some work was done on A and B in the Old Vein, and a new attempt was made to open up the "North Vein", previously unworked at Cwmorthin).

The lower floors flooded up to Lake Level in the Back Vein and up to C floor in the Old Vein, obviously containing an immense amount of water hundreds of feet deep, which remained until the early 1930's. Oakeley were at this time driving underneath the old Cwmorthin workings and were uneasy about having such a huge volume of water above them, so decided to drain it out. Special diamond-drilled bore holes were driven through into the deepest parts of Cwmorthin from Oakeley and the water was drained out under controlled conditions.

After the water was cleared, the mines were connected in several places and Oakeley actually re-opened some of Cwmorthin's Back Vein workings and put men to work in them on floor E. The Back Vein Incline was re-equipped and even a whole new incline was driven down another 90 vertical feet to open some more chambers on a whole new floor - Level G. The Old Vein and North Vein were abandoned.

Cwmorthin operated essentially as just another district of Oakeley right up until 1970 when Oakeley itself closed, unable to pay to keep the massive pumps running that kept the whole sprawling labyrinth dry. This marked the end of the mine's working life as a major concern, and ownership of Oakeley and Cwmorthin was once again split. Throughout the 1980's and early 1990's Cwmorthin was working on a limited scale by a small team of local men. Extraction occurred in a few chambers on Lake Level and Level 2. This endeavour too came to an end, with some unsuccessful attempts to untop the ancient Cwmorthin Slate Company workings around the year 2000 by Mcalpines PLC.

Today, almost all of the earlier upper workings in both the Back Vein and Old Vein are inaccessible and damaged. It is likely that large tracts of these workings deep in the hill remain in reasonable condition, but are cut off from our reach.

Most of the workings in the lower five floors of both the Back Vein and Old Vein can be accessed today. The Back Vein workings are in excellent shape in structurally good rock, almost all of it is still available to explore. The Old Vein workings however are in a much poorer condition due to the weaker, more fractured rock in which they were driven. Most of it can still be gotten into one way or another, but there have been many wall and roof failures and numerous chambers have collapsed entirely.

What's left to us to see today still comprises of many miles of tunnel and hundreds of enormous chambers. Within these workings can be found artefacts ranging from powder horns to timber stairways, from winches to wagons, and from cranes to bridges. Many days can be spent enjoying this exciting and awe-inspiring environment that will always remain a significant monument to the world famous Welsh Slate industry.

The "Friends of Cwmorthin" group act as custodians of the mine today, aiming to promote its preservation, documentation and exploration. Access to the mine is provided to all interested persons without cost or needless bureaucracy, by simply asking for the code to the gate. It is however not a show mine and is a very hazardous environment for the inexperienced, like most disused mines are. Please see the How to Visit page for more information about how to explore the mine.