Course Date - 28th October 2023
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.
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| 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.
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| 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.
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| 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.
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| 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.
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| 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.
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| Gnome passage demonstrating the strong ceiling, where the Dowlais meets the Penderyn bed. |
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| 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. |
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| 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.
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| 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.
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| 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.
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| 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.
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| 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
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| 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.
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| 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!
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