An introduction to the Geology of OFD

Author: Helen Nightingale

Course Date - 28th October 2023

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Andy Freem, accompanied by Prof Peter Kokelaar, ran a 1 day course introducing us to the geology of OFD. It was a truly fascinating day exploring Top Entrance, teaching us about the local geology. Any factual errors in this blog are entirely mine.

Not quite geology but a fascinating example of fungus growing from a dead Herald moth (Scoliopteryx libatrix) near the entrance.

OFD is mainly contained within a bed of Dowlais limestone, which is about 100m deep, and is laid down on a tilt, hence the cave has significantly greater depth from top to bottom than 100m. Above the Dowlais limestone is a thin bed of honeycombed sandstone, 1-2m thick, then above that is Penderyn Oolitic limestone.

Andy explains the significance of Dowlais limestone on the surface above top entrance of OFD.

Before going underground, we talked about numerous landscape features important to the formation of the cave. You will have to imagine for now, that there is no Tawe river, no river valley, and there are mountains another couple of Kilometres higher surrounding us. Imagining what is no longer there is quite a skill.

Attendees of the geology course, standing where the entrance passage used to be.

As soon as we entered the cave, we saw interesting things. Pause a moment at the gravel pile just inside the entrance. As you let your eyes adjust to the darkness, consider the fact that all that gravel was carried into the cave by glacial flow via the chimney above. When the cave was active, if you could stand at the gravel pile, you would not be near the entrance, but mid-passage.

The gravel pile at the entrance we all walk past without a second thought!

The passage would have extended out into the now-invisible mountain range. Not much further in, we can see dissolution tubes high on the walls of the cave.

Dissolution tubes high on the walls.

The sandstone and oolitic limestone are not conducive to cave formation, and the Penderyn bed on the whole makes a strong ceiling for the cave and is readily visible as flat exposures in the ceilings. The strong rock which makes the distinctive flat ceiling is described as "competent", good at not collapsing. Very different to a few mines I've been in, with highly incompetent ceilings.

Gnome passage demonstrating the strong ceiling, where the Dowlais meets the Penderyn bed.

Andy discussing geology. Here, very unusually, the cave has formed upwards into the Penderyn. There is very visible evidence of the highly bituminous limestone here.

The large entrance passage would have formed, then filled with relatively static post-production flood waters. The water at the surface was more acidic, so more corrosive, at the surface of these lakes. Flutings can easily be seen on the dissolution grooves, showing vertical rippling at the lake surface.

Evidence of fluting.

Dissolution characteristics are also readily found in the cave ceilings. We saw the main roof tube running above us through our route through the cave, created from phreatic pipes followed by the erosion of the limestone beneath. Also, this unusual collection of anastomosing dissolution tubes. If you haven’t seen this feature before, go have a look for it.

Roof tubes, the anastomosing dissolution tubes forming the cross like pattern in the foreground of the ceiling.

People often talk about the power of water in the formations of caves, but we don’t talk enough about the power of ice. We must consider that blue ice played a huge part in creating OFD as we now know it. Many of the blocks we are very familiar with were formed by ice effectively chiselling blocks away from the walls, moving a great mass of limestone against gravity.

Discussing the origin of blocks of stone.

An interesting feature to look for if you are loitering near any avens, is broken off stall. Domes of ice encased them, breaking off anything hanging in the way, then transporting it elsewhere.

Photo up Aven, where there used to be more stals.

Mud is also a very important feature in cave development that we rarely value enough. Whole passages would have been filled with mud or silt filled water, for thousands upon thousands of years. We saw mud banks made of numerous striations, showing each occasion where the silt or clay fell out of suspension and settled onto the floor.

Andy explaining the origins of this mud deposit.

In a location near Arete, there is a wonderful cut-away into the side of a mud bank, very clearly showing the many different layers, and spanning half a million years.

We had a look at some of the fossil beds, and talked through death beds and living beds - death beds being less dramatic than the terminology suggests, just fossils that were already dead when covered in sediment, rather than those in living beds - these had a rather more rapid ending. I do need more practice at telling the two apart. It's a bit like trying to tell if a Norwegian Blue parrot has croaked or not, if you don't know what you are talking about. I believe we saw mainly bivalves in the area we looked at with Andy, but coralline fossils can also be found in other areas of the cave

Schrödinger's coral, dead or alive?… definitely old!

This describes, very briefly, just a fraction of the interesting things we saw, and is greatly over simplified. I think all of us came away with a greater appreciation of what we can see if we explore the cave at a slow pace, and a greater understanding of the cave. Everybody went away enthused and wanting to know more. As we walked down the hill towards Penwyllt, clouds rolled in, covering the no-longer existing mountains of a past era. The geological complexity of Cribarth remained bathed in beautiful sunlight.

The walk back to the club, with renewed appreciation of the landscape.

It had become easier to imagine flood pulses ripping through the now remnant passages of Top, and spilling out from a long-gone entrance, sending torrents down the path we were walking. For me, it remains harder to imagine the desert plains of Penwyllt, complete with Pterodactyls in flight instead of crows, or indeed the more Alpine glacial scenery of Penwyllt – past. Perhaps don’t imagine a Dan-yr-Ogof show cave complex in an icy winter though!

Caving in Ireland, County Clare

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In July 2023, Claire and I spent a week in the Burren area of County Clare, Ireland and squeezed in a few caving trips…

Through the power of Facebook we had reached out to local caver Pat Cronin to help us locate and possibly guide a few trips. Pat provided a wealth of advice but due to existing commitments was unable to actually lead any trips. This task fell upon his friend and other local caver Paul McGrath.

The Burren at the coast, limestone paving for as the eye could see.

The SWCC library has several books for the region which can be lent out to members (or you can just go buy them), we used the modern Caves of Mid-West Ireland. Although an excellent book it was no replacement to having Pat and Paul dispense local knowledge!

We flew direct to Shannon airport from Heathrow and had hired a car to get around. On Pat’s advice we stayed at the Rainbow Hostel in Doolin which is about 1 hours drive from Shannon airport. The Hostel was an excellent base for exploring the area and they were no strangers to cavers hanging muddy kit out on the washing line in their back garden. A pub called McDermott’s was 30 seconds walk away. Good food, plenty of Guiness and a live band ensured it was packed out every night we went there!

As we flew to Ireland we had minimal caving kit but this did not stop us from visiting several caves. We had arrived in Ireland after a storm which meant the two big systems in the region were essentially off-limits and we were advised to avoid those as they were most likely flooded out. Wasn’t really a big issue as Paul was able to take us to 3 other cave systems.

Cullaun 2

Our first cave was Cullaun 2, an active stream cave located a few miles North East of Lisdoonvarna, this map locates the entrance.

Easy going, no equipment required made for a great first cave in Ireland.

Claire in the streamway of cave Cullaun 2 after recent heavy rain.

Paul our Irish host, Cullaun 2.

Claire admires some of the roof formations, Cullaun 2.

Faunarooska

Our second cave was Faunarooska. This was a bit more tricky to find as Paul’s memory of the location of the entrance was hazy. We actually started a rather cool trip into Hawthorne cave that came to an abrupt end so we exited and then went in search of Faunarooska. Thankfully it was not too far, this map locates it. Again a system that required no extra kit.

Duncan looking out of Hawthorne Cave, the first one we found.

Following the streamway in Faunarooska.

Claire under a curtain in the Faunarooska streamway.

Duncan admiring formations in Faunarooska.

Claire admiring formations in Faunarooska.

No trip to the Burren should be without a visit to one of the two show caves in the region. We went to the Doolin show cave with the ginormous stalactite. The tour guide was suitably humorous and it made for an entertaining trip.

The mighty stalactite, Doolin show cave.

On one of the few days the weather promised to behave we visited the Burren National Park and did the blue Mullaghmore loop. It’s a fairly easy walk with spectacular scenery, definitely worth the effort!

The spectacular Burren National Park.

Poll na gree

Alternative spelling is Poll na grai, was to be our final cave. Similar to the other two caves, linear in nature with an active stream it has been extended. Part of the route is a hands and knees high level traversing. Not as scary as the midnight traverses in OFD but sufficiently worrying that you would not want slip and get wedged in. The end of the first section ends with a ridiculously tight, up-hill, slippy crawl with no hand holds. Needless to say I failed to get past this choke point whilst Claire breezed it. Credit to Paul, he managed to get through and it looked like it took Herculean strength to negotiate the flat-out crawl! (It was the tight U-bend further on that proved arduous).

A muddy Claire having returned from the by-pass to sump 2

An equally muddy Paul. The crawl for sump 2 is very arduous.

Sign identifying cave entrance.

That was it. Our remain day was rained out with us escaping to Galway for some city culture. We barely scratched the surface of the caving scene in Ireland and with stunning scenery and great people, Ireland is a fab place and I can’t wait to go back and may be tick off the large systems. We are indebted to Pat and Paul and hopefully we can return the favour by leading them on some choice trips here in the UK!

The trouble with Tribbles\Cobbles

About a month ago, I and three others did a through trip in OFD, in which we came out of the Cwm Dwr entrance. I was at the rear of the team as we crawled our way through the challenging restricted section known as Dim Dwr. The three in front had passed through pushing cobbles towards its exit and when it came for me to pass through I was faced with a wall of cobbles. Unable to crawl over, or push them ahead I resorted to screaming for help until one of the team finally came back and dug out enough at their end so I could push the remaining cobbles out of the way to exit. Would be fair to say it was an extremely unpleasant experience getting stuck there!

The bags storing the cobbles from a previous clearance had failed and were simply spilling their contents back into the crawl. I’m speculating this was accelerated by the big rescue in November 2021 which would have seen hundreds of people passing through that section of the cave system in a very short period of time.

Fast forward a month and I had decided to do something about it. Originally I had planned to do nothing more than bag as much as I could in situ. Thankfully the Freem’s offered to help and turned my amateurish thoughts into a far more productive setup. On Tuesday 19th April in just under three hours we absolutely smashed it! Bagging, dragging and stashing the cobbles away from the crawl.

Before we started the work we shoved Andy into the crawl to put things into context, this was about as far as he could reverse into it.

Antonia filming Andy. Note all the cobbles.

Andy for scale, demonstrating just how filled the crawl was.

Andy looking relieved to be out of the crawl!

Looking into the crawl, Andy’s foot for scale, so pretty snug…

We took it in turns to dig and bag up the cobbles. Then we turned into a 3-man haulage team dragging the bags using ropes to a point just beyond the stal boss one has to crawl pass. Once the cobbles were stashed we returned to the crawl and repeated this process 4 more times. 24 bags in total.

Bagging the cobbles.

Antonia, the secret weapon! Small enough to get into the crawl and kick out the big stuff at the tightest point.

More bagging.

Hauling the bags of cobble back towards the entrance. We stashed the pebbles in a small hole on the left some 30m back from the crawl.

Andy passes the stal boss, carrying the scrapper used to push the cobbles into their final resting place.

The end result, the crawl cleared of cobbles with Andy being able to reverse fully into the crawl.

The crawl is now open to a wider range of caver sizes and could potentially offer an escape route for a lesser injured person? Anyone reading this who has never been through cwm dwr should be aware that this is a restricted space with a tight tube beyond this crawl, if you can get through this you should be able to do the rest.

So no more troubles with tribbles…

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Llethryd Swallet Reopened!

Digging team: Luke Ashton, Andy Freem, Antonia Freem, Duncan Hornby, Peter Kokelaar, Gareth Smith & Claire Vivian

Other contributors: Val Bednar, Alun Freem, Piers Hallihan, Paul Tarrent & Phill Thomas

Photos by: Andy Freem, Antonia Freem, Duncan Hornby & Claire Vivian

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Llethryd Swallet is a cave found on the Gower peninsula that takes an active stream and became blocked in 2003. A team has been working on gaining access into the system for the last 3 years. Initial attempts focussed on an alternate way in, which became known as Barns Cave and is documented in an earlier blog. Although Barns Cave offered a promising alternative route into the main system a line survey revealed that it was trending in the wrong direction.

Attention was then focussed on the original entrance. A concerted effort was made to join the main streamway by finding an alternative higher route that avoided the blocked section.

Pushing alternate routes took the team into some rather tight spaces!

After several digging sessions led by the local cavers a breakthrough was made in August 2021, regaining access to the main stream and the way on to the impressive main chamber.

None of this could have been achieved without the permission of the landowners as there is no right of way to the cave entrances. Access arrangements have been set up for permitted caving groups, more details on access can be found at the end of this article.

Recent flood events indicate many passages flood to the roof as indicated by leaf litter, so this is a cave that is not to be entered under wet conditions especially if weather reports forecast heavy rain. The swallet entrance can back up several metres to the height of the cavers door entrance.

The swallet in low flow conditions. The cavers door entrance is just off to the right of this picture.

The cavers door entrance. Under high flow conditions this entrance gets submerged!
Note all the debris.

After any severe flooding the caver's door often needs to be dug out.

There are two technical sections through the entrance crawl that require great care, this is not a trip for novice cavers. The first challenge is under a scaffolded multi-tonne boulder named the S-bend. Small people can get into the void and get round the bend head first, the rest of us have to reverse around the corner legs first up a slope. So a nightmare going in but much easier going out! Large people will likely find this obstacle very difficult if not impossible to pass.

Andy entering the S-bend. The scaffolding is securing a large boulder above.

The second challenge is the Grim Reaper, a large boulder seemingly held up by nothing and the current way on is directly below it dropping onto a chock stone that appears to be holding everything together. The digging team is planning to shaw this up with scaffolding because my pants can’t take anymore of this!

After carefully navigating the somewhat concerning Grim Reaper everything gets considerably easier having regained the original main stream. Along here are some crawls and walking which eventually leads into the climb up into the main chamber. The ground becomes covered in a fine silt making it very slippy to walk on.

The cave takes on a dramatic change in size, initially passing a formation called the mushroom.

The atomic mushroom!

You enter the lower section of the main chamber and heading off to the left you pass many silt covered stals before entering the awesome main chamber.

Just before entering the main chamber the roof dips and you pass under various straws and helictites. One spiral shaped helictite makes a “zzzzzit” sound. On the first time I passed this formation I could not hear it but Claire swore blind it was making the sound. On a second trip the sound was louder and I could hear it but Antonia could not! Seems like the older you get the harder it is to hear.

The helictite in the roof that makes the zzzzit sound.

We managed to record the sound which you can hear by clicking on the link below. You’ll hear a tapping sound in the background, that was water collecting in a container. But in the foreground you will hear a very faint “zzzzit” sound at about 1 second into this 3 second recording. You might need to listen to it several times before you hear it and bump up the volume on your computer. But it is there I promise you that.

The sound the helictite was making. Note there is no associated video with this sound recording.

The main chamber is a large sloping boulder filled void, with an impressively large curtain dominating the roof. The formations have a grandeur comparable to those often found in other European countries.

Claire in the main chamber.

Luke at the stunning curtain in the main chamber.

At the top of the chamber is the formation known as the church and steeple which has been abused by past cavers climbing on it, no doubt for that glory photo shot.

This has become a centre of attention for restoration. I have to admit I was somewhat sceptical thinking the damage was done and whatever we could do was too little too late.

I could not have been more wrong.

Originally Andy had brought in a hand pump sprayer and I was amazed at the transformative power of this washing the muddy hand prints off the stalactite. What was dull and muddy suddenly burst into colour showing its translucent edges.

Duncan uses the hand pump sprayer to do the initial cleaning of the stalactite, note the muddy edges on the background stalactite, that's all gone now.

All I could think of was, I have a battery powered jet washer I use at home for cleaning my bike/car, that would blast off the mud!

We have had 2 subsequent trips where I have brought my jet washer into the cave and using water sourced from the stream we have begun transforming the damaged formation into something that frankly looks awesome!

Duncan using his jet washer, note for comparison the mud covered front of the base of the formation, much of that has started to be removed.

This formation has suffered significant abuse in the past and it will take several further sessions to restore it to its former glory for future cavers to enjoy.

The jet washer used was a battery powered worx hydroshot.

If you know the route, the main chamber can be reached in about 30 minutes from the entrance. No ropes\ladders are required to get to the main chamber.

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Access restrictions:

Since the cave blockage in 2003, land ownerships have changed and all previous access arrangements (e.g., in Tim Stratford’s book) are now obsolete.

• Llethryd Swallet can suffer total flooding, sumping the entrance series for extended periods. Passage instability exacerbated by regular violent flooding can add to the excitement. Cavers entering the land and caves do so entirely at their own risk and risk assessment. The caves must not be entered immediately after a period of heavy rain, or when significant rain is forecast. In winter, water backs up from ‘terminal’ sumps and submerges all access passages for long periods.

• The issuing of a permit is not a guarantee that access will be physically possible.

• Due to the seriousness of the cave no novice cavers are permitted. Several tight/sinuous sections in the new entrance sections have a ‘size limit’. Rescue from flood entrapment or injury would be problematic.

• All cavers are required by the landowners to hold BCA cavers’ insurance.

• The caves (within an SSSI) are gated and locked ( NRW and Landowner stipulations). A key will be made available on-site on the booked days only for pre-booked, permit holding parties (1 per day). Groups are asked to be considerate to others in the caving community by not booking multiple days at this stage in the re-opening process. Evening trips will be possible.

• Keys must be returned to the on-site key safe immediately after each trip.

• Access is free. We hope that groups will support and comply with the arrangements making this possible.

• To avoid conflict with other landowners, access must be via the car park at the south end of Green Cwm (Parkmill), walking up the valley and crossing into the Llethryd Barns land at the south end. Detailed instructions will be given.

• There is no right of way to the cave entrances and the agreed paths are only for use by permitted caving groups.

• Full updated instructions and a (conditional) permit for Llethryd and Barns caves can be obtained by email. An illustrated instruction sheet showing surface access, routes inside the caves, and current issues will be sent with each permit.

• Please use this address for enquiries: llethrydswallet@llethrydbarns.co.uk