Cwmorthin - The Lost World

An Exploration of the Cwmorthin/Oakeley boundary workings in the back vein (The "Lost World") October 2015

Lost World

In October 2015 we paid our usual visit to Cwmorthin slate quarry, this time with the objective of exploring the little known area found towards the end of lake level in the back vein which has become known in recent years as the "Lost World." Plans showing this area have proved difficult
to track down hence its name. It is in fact part of the vast workings of Oakeley slate quarry which lie to the east of Cwmorthin, and offers a rare glimpse of the remote Oakeley back vein, a mysterious tract of workings considered forever lost beyond extensive collapses. (but more of that

For a change, we ignored the the famous Cwmorthin back vein incline, to follow lake level to its furthest extremities. The route onwards however wasn't obvious as it was interrupted by an extensive fall. I speculated that we were seeing the effects of the abortive untopping scheme of the
1990's, which caused further damage to the already fragile Cwmorthin upper levels. Once over this collapse, we picked up the intact lake level passage again, by carefully popping through a hole in the rather thin chamber wall which offered a diversion to the blockage in the level. Here one can proceed along the level almost to its end which marks the boundary between the two quarries.

Along the way, we encountered retaining walls of waste slate holding back tipped material in chambers on the up dip side, designed to stop the rocky contents of these chambers from spilling onto the haulage route. The walls were reinforced with timber props, and were most likely
constructed by the old Cwmorthin Slate Company, which worked the upper levels. They were thus possibly over a hundred years old. - not something to stop and think about much, or for too long!
Anyway, for the time being at least, most had held - mute testimony to the skill of the long-ago quarry men, in an age before "health and safety" had become a household word.

Not far from the end of lake level we came across a "window" to a lower floor, which appeared at a glance to connect through to a chamber on Cwmorthin floor A below. However this window was quite deceptive: - it is all that is left of the original roofing shaft that Oakeley drove up from below to connect into Cwmorthin, and provides the only known way into the true Oakeley back vein that still remains accessible. The Oakeley back vein chambers adjoining the Cwmorthin workings are known as the Z chambers and this particular chamber has been identified as Z6. It lies in the "true" Oakeley back vein and is little visited. In contrast, the Z chambers on floor G, which Oakeley worked deeper under Cwmorthin, are well known.

On why Oakeley decided to break through into the Cwmorthin workings at this point, we can only speculate: - for a supposedly significant event it seems to be strangely undocumented. The plans available to me seem to suggest it took place towards the end of the 19th century where an actual roofing shaft is indicated in the correct position on a plan of Cwmorthin dated 1899, but nothing is shown on a plan dated 1884. (although it may simply have been omitted: - the quarry plans are notorious for being confusing - but that's another story!)

Some have suggested it provided an escape route between the two quarries, (there is a substantial original ladder positioned against the working face) or a means of improving airflow in the remote western reaches of Oakeley, (the draught certainly draws through this connection now) or a
preparation for Oakeley's eventual takeover of Cwmorthin. (Oakeley was to work parts of Cwmorthin's lower back vein much further west during the 1930's although there is no evidence this route was used then)

A short abseil from this "window" dropped us down the working face into a medium sized chamber coming up from Oakeley level 3. Part way down, the sturdy ladder (mentioned previously), came into view. We found it to be in reasonably good condition, although understandably rather fragile after its long and lonely existence under the mountain (touching it is not recommended).

Lost World original ladder

From this chamber we proceeded into a level through the eastern wall to access another chamber of moderate size (Z5) which from a quick glance, didn't look as if it roofed through to anywhere above. Artefacts such as a powder horn, a kettle and a couple of shovels had been placed to one side, probably by previous explorers to prevent them from being trodden underfoot. They made for an interesting photograph.

Lost World artifacts

Such artefacts as those shown below are scattered through this remote and little touched area.

From Z5 the level continued on east, to meet a junction with a roughly northward heading level, which might cut across the drop fault which is known to intersect these workings. We knew this fault to be a significant feature further west in the Cwmorthin back vein, where the chambering is
consequently divided into two sections north and south of the fault. (It also cuts through the old vein workings below)

Leaving this inviting tunnel until later, we continued to proceed east to meet yet another chamber somewhat higher in elevation than those previously encountered. (Z4) This contained a selection of large slate blocks which appeared to have been cut ready for haulage. One was attractively split, another tipped at an angle. I know of nowhere else in Cwmorthin or Oakeley, where one can find blocks like these. Maybe something to do with the age of these workings and thus the methods used to work the rock?

If only these fine slate blocks could tell their story!

Lost World slate blocks

The chamber floor was covered with the usual array of jumbled, irregular fallen slabs one normally encounters. We proceeded over these towards the steep working face to look up into the cathedral like elevation of Z4 which like the others appeared closed at the top. A rope hung down
in the far corner and it occurred to me that this was the point where previous explorers had originally scaled the chamber with ladders to investigate if progress could be made on the floor above which was thought accessible from this point.

Level 3 leads yet onwards from this chamber deeper and deeper into Oakeley, and with much enthusiasm we proceeded in this direction - but only for a few feet! I knew what to expect however from previous descriptions: - the level was almost blocked by an assortment of slate slabs, probably
indicating the presence of chamber Z3 which was either filled with tipped material or had collapsed. Interestingly it looked possible to carefully wriggle through a gap between the slabs and the roof of the level but there was no knowing how stable or otherwise this fall or backfill actually
was, so we left it well alone. I had previously read that actually not that much can be gained by doing this as another blockage is soon met with (probably marking the position of chamber Z2) which has so far been successful at halting any further exploration.

Retracing our steps we turned our attention to the cross cut tunnel. If this lead across the fault, as seemed possible, we would expect to find substantial back vein chambering on the other side (north of fault) The chambers we had so far encountered might therefore actually be south of fault back vein chambers. (see rough sketch) This level appeared stable, but there appeared to a slight change in the rock: - if one looked carefully at the walls, discolourations appeared, seemingly forming a band. It can just be made out in the photograph below. Whether this suggested the presence of the fault, I'm not sure, but the rock does indeed seem to change at this point. Maybe this discoloured, course, rather crystalline banding has been caused by grinding action at the fault boundary which may have been progressing on an infinitesimal scale since the mountain itself was formed.

Lost World

On the far side of the cross-cut we met a junction. I don't remember much of the eastward heading level, I think it was blocked by a impenetrable collapse. However, in the other direction, the level was open and led roughly north west to a smallish chamber where one met with a "Moria-like"
parting of the ways: - a number of tunnels branched off in different directions. One level soon went blind, east terminated in another fall, but west led into an isolated area of small partially worked chambers. None of these were what I expected for this supposed north of fault area, and
one gained the impression that the quarry men weren't particularly pleased with the rock they encountered here. Hob nail boot prints were much in evidence. And what tale did they tell? Did they belong to the rock men who worked these chambers, the miners who drove the levels, or quite
possibly the surveyors who left their distinctive circular survey marks painted on the walls? (these marks, one of which is found above the "window" into the "Lost World" made me believe that the area is indeed on a survey somewhere).

Lost World bootprints

This was a lovely hidden place, so far off the beaten track, with delicate frost like white crystals furring the rock. In addition iron formations like curtains oozed from the hanging wall and ochre puddles adorned the floor. At the side of a passage we found a well preserved lantern designed for
holding a candle. It nestled in a bed of crumbly crystalline rock. I am still undecided as to exactly what vein we were in here. Part of the north of fault back vein? another vein lying beyond it? ( The North vein situated north of the back vein was also worked in places by Oakeley) Or somewhere
which was not even in a true slate vein?

Lost World passage

A long level headed deeper into this area, but frustratingly it was leading away from Oakeley. After some seemingly aimless meandering and some half hearted excavations it came to a blind end. The miners had given up at this point, and the rock men had abandoned their small and unproductive chambers. This corner of the quarry evidently did not yield good quality slate, unlike the main back, old and new vein workings to the east.

Returning thus, the way we had come, we too departed the "Lost World" to leave its remote byways to brood in the quiet and empty dark.

Below is a rough plan of the "lost World" we put together, showing the area of the Oakeley back vein that still remains accessible. (Note: Plan drawn from memory, some of the annotations were added later in light of the 1904-10 back vein survey).

Lost World map

Lost World lantern

Lost World

Shortly after our lost world trip, a plan of the western end of the Oakeley back vein adjacent to Cwmorthin, was made available: -

Putting the "Lost World" into context: - the 1904-10 Oakeley back vein plan

The lost world in the context of the first quarter (approximately) of a plan of the Oakeley Quarries Back Vein workings based on what was the last survey dated 1904-10 produced by Thomas Jones & Son:-

The "Lost World" section, (a rather small part of this plan, I may add) more or less matches the rough plan we drew up on our visit, and also provides many fascinating insights to what may now be lost forever. The plan seems to suggest that this area of workings actually seems to split into three sections, rather than the two areas divided by the cross-cut I originally identified and which are easily explorable. Firstly there is the section where one can proceed through three chambers (Z6, Z5 and Z4) in an eastward direction, and view a selection of artefacts. Notably chamber Z4 has a collection of big regular slate blocks scattered on the floor. The first collapse encountered in this direction, appears to be in chamber Z3 which is a shame,as the 1904/10 plan shows this chamber to be the first one to be worked up to the floors above. There is a report by explorers of a a run in level at the top of ( I believe) chamber Z4, which appears to be the beginning of a cross cut through to the old vein, but this doesn't appear to be indicated. It looks now as if these series of chambers may be south of the drop fault. The roughly northward heading cross cut is clearly marked, and the rather isolated section of smaller workings, which we explored are clearly shown, although I still can't work out if these are actually part of the back vein or not. There is collapse eastwards here too, but the plan indicates that there is not much beyond. Of particular interest is the extensive area of chambering in the "central section" also accessed at one time by the cross cut. Maybe these are
the main back vein workings north of the fault. If so, they were sadly inaccessible. I think I remember catching a glimpse of a fall in the east heading level immediately beyond the cross cut, possibly indicating the position of chamber Z4 in this north of fault section. Therefore a significant
area of chambering may lie hidden behind this collapse, something I was unaware of at the time.  But the annotations "tops down, signs of pressure" isn't an encouraging sign, if that was the state of these workings even when the plan was drawn up.

Intriguingly the 1904/10 plan seems to suggest that at one time one could have followed floor 3 Oakeley in a round trip through these hidden back vein workings, cross cutting across the fault from one section to another and back again, although it is highly unlikely this would be possible now even if a way is eventually found through the collapses we encountered.

However part of me still wonders if beyond these falls there are small sections, or even whole tracts which have managed to stay relatively intact. I was surprised that the "Lost World" itself is in such reasonably good condition, considering that it is indicated as lying roughly below the dried up reservoir, (Llyn Bach) with its deep cracks, as well as being split by the fault. So who knows?

Lost World slate

Author: Siriol Richards, with Peter Hamilton, Annette Price, Neil Montgomery, David Roberts (October 2015)
Photographs by Peter Hamilton
(The 1904-10 Oakeley back vein plan was made available by Graham Isherwood and can be viewed on


Siriol Richards