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Pant Mawr Pothole

by Graham Wilton-Jones

We arrived at Penwyllt prepared for a photographic trip into O.F.D.  However, the day was warm and the sky cloudless.  These conditions prevailed for weeks, and the moor top peat was dusty dry.  We decided to take the opportunity for visit Pant Mawr Poy, which is about three miles from Penwyllt and tow miles from Cwm Pwll-y-Rhydd.  Since the latter route starts with a steep climb from the Nedd Fechan, we took the easier, though longer route.

It is easy for follow the old quarry railway track up a gentle slope past O.F.D. Column Hall area, past several limestone quarries and up the Byfre valley to the sandstone quarries around Pwll Byrfre.  The sink here is not spectacular, the water from this stream sinking at several points in mud and boulders.  Undoubtedly this section of O.F.D. is solidly blocked with washed out moraine.

By following the east-west wall above Pwll Byfre we eventually arrived, hot and thirsty, at a particular high point on Pant Pawr from which the view was magnificent.  The whole of the lower, northern side of the moor is covered in holes, many of them quite large, and some in very obvious rows.  Clearly, there is much to be found here for someone who doesn’t mind plenty of walking and, perhaps, plenty of digging.  None of the hollows are obviously stream sinks.

To the north of the wall, there is one large and obvious sink.  Pant Mawr Pothole is less than a hundred yards south of this, but was easy to find in the clear weather as the fence posts around the pot appear white against the heather.

We left the path, and headed straight for the pothole through the thick heather, which was buzzing with bees and small insects and alive with little spiders.  Ravens were roving the skies above while grouse lurked around the damp peaty hollows.  However, the sound of cool fresh water at the bottom of the pothole lured us below.

The top half of the pot is roughly conical with a very steep southern side and a slightly less steep northern side, dropping down thirty feet or forty feet to a ledge.  From this ledge, there is a sheer drop of fifty five feet.

Using handline and lifeline, we dropped down to the ledge and inserted two rawlbolts.  On unrolling the ladder, we realised that we had only forty five feet.  However, we set up tackle and I descended first, being very dehydrated by now.  The ladder was ten feet short – I wondered what cavers do when they use the recommended fifty foot ladder? Fortunately, at thirty five feet there is a wide ledge.  By swinging on the ladder I got a foot on the ledge and a hand into a vertical crack. Once on the ledge, I found an easy climb down to the floor of the pot.  A thirty fife foot ladder would be sufficient, with a ten foot length of cord at the bottom to tie the ladder to the ledge.  Ever tried jumping onto a ladder which is hanging five feet away in space?

We took a look at the upstream series first.  The water comes through a narrow rift which has some superb shelving – unusual in Wales.  Beyond this is a waterfall in a side aven but the passage goes on a little way as a rift. This can be climbed to a bedding plane. In turn, this leads to the top of the waterfall in one direction and away below the moor, close to the surface in the other.

Downstream, the passage is large and is possible to walk for most of the way.  There are roof falls and consequent boulder chokes in a few places. One fall is a direct result of limestone breaking off the overlying shale bed.  Here, the roof is visibly bowed towards the centre and further falls are imminent!  Another fall has been caused by phreatic tubes spreading into a bedding plane development a feet or so above the existing roof.  Again, there is a danger of further collapse, although this is not so acute here.

Climbing over one of these boulder chokes, we passed through a long but low chamber, well decorated with straws and small stalactites.  From this point onwards, the stream passage, which forms most of the cave, is well decorated with stal, though much of this is rather muddy.  This is not the fault of cavers but is due to frequent and extensive sumping.

From the boulder choke downwards, the cave is liable to severe flooding and though it appeared to be safe in certain places, it would not be wise to rely on these.  A soft, black, organic mud clings to the roof and walls in many places as a warning.

There are some large chambers off to the west of the main stream passage, but we did not visit these. Although they constitute an important section of the cave geomorphologically – considering the past connection with O.F.D., they are not a large percentage of the total passage length. Instead, we continued down to the fire hydrant where a torrent of water, as much as the main stream, issues from an impenetrable fissure in the low roof at the west side of the main passage. From about thirty feet back up the passage, from a hole up in the west wall, a stream could be heard.  We persuaded Bert to enter the hole and have a look, and forced him literally upwards and inwards.  After a quarter of an hour, there was still no sign of him, so Buckett climbed up and disappeared after him.  For three quarters of an hour I pottered about while Bert and Bucket followed a narrow stream passage trending northwards.  They did not reach the end, as time, energy and enthusiasm wore out.  There were a number of cross rifts, some un-entered.  Surprisingly, there is no surface stream of catchment area that could give rise to such a large stream, so its source remains a mystery.  The extraction of the pair of explorers back into the main passage was most amusing as they both returned head first.  It should be mentioned that the hole through which they had to emerge was squeeze size and almost eight feet pup a sheer wall. The rest can be imagined!

From here, we continued downstream.  The passage rapidly became narrower and lower with a gravel floor.  Eventually it degenerates into a crawl and, with the sump not far off, we decided to return.  The journey back to the surface took less that half an hour.  The total time underground was three and a half hours. If anyone wishes to visit Pant Mawr Pot, you need a letter of permission to walk over the moor, so it is wise to write in advance.