Ogof Tarddiad Rhymni (Rhymney) description

Description credits

Mark "Tarquin" Wilton-Jones, Brynmawr Caving Club.


Last updated: 15-Nov-2016


Trefil, South Wales, U.K.

1'608 metres
Entrance grid reference
SO 0850:1410
Entrance altitude
260 metres

The cave is situated in the Cwar yr Ystrad quarry. From the village of Trefil proceed north for about 5 km from the Tafarn Ty Uchaf, along an increasingly poor condition road, which slowly swings west. After passing some quarries, the road - now little more than a dirt track - splits. Take the right fork, towards a larger quarry. This area suffers badly from car crime, so either leave someone in your car, or park somewhere out of sight. The quarry has two main entrances, both blocked to traffic.

The cave entrance is located at the bottom of the far wall, on the left side of the quarry, and can be a little difficult to find. There is a large depression in the quarry floor immediately in front of it, behind a tall rock pile, which contains some intermittent pools. The entrance itself is about 2 to 3 metres off the surrounding quarry floor, and is usually concealed with a dry stone wall. Please replace this wall after your visit.

A short cave, Ogof Gwybedyn, is located a little further to the right, and is a slightly more obvious hole, dropping into a very tight passage.

No control is placed over access.


The cave is taped to protect its formations and mud deposits, and the club that found the cave request strict adherence to the conservation policy. Please respect the tapes. In the interest of conservation, please do not dump carbide - spent or unspent, leave cigarette ash or other litter within the cave.


A survey has been conducted but is not known to be publically available.


The entrance opens almost immediately onto a 1.5 metre freeclimb down into the main passage. The passage is fairly typical of South Wales caves, being a few metres wide and high, with a normally rocky floor, and occasional small formations. It continues this way for 200 metres, with the taped route leading through an undercut oxbow at one point, to a larger section which swings to the right, and reaches a junction.

In the left wall of the larger section is a short, decorated side passage, that is fairly tight and awkward. At the main junction, the passage continuing up a slope ahead is the way to Nant Criban, while the main passage continues down the left. This descends a rocky slope with an inlet on the right. The inlet becomes too tight after 20 metres, but the main passage continues for another 300 metres, with some sandy sections, to reach a choke.

The route through the choke is fairly obvious, and reaches a climb up after about 10 metres, back into the main passage again. This enlarges quickly to its largest size, before funneling down suddenly into a narrow rift, Nant Rhymni. This soon lowers to become a flat out crawl in the stream, and lasts for about 250 metres, before becoming too low to follow. The stream has been traced to the Rhymni Risings. About 50 metres before its end, a hole up enters a series of rift passages, Ffordd Swnd, which can be followed for about 50 metres.

The way into Nant Criban begins comparatively small, requiring crawling over sediment. This passes some reasonable formations and enlarges as it enters a chamber. Part way along this section, one of the holes in the floor can be descended with care to enter the larger passage of Nant Lwyd. In one direction, the stream flows out and becomes too tight with a visible connection to the inlet in the main passage. Nant Lwyd can also be followed upstream for about 50 metres.

The chamber has a dig to the left, and a small passage to the right, which is the way on. This is a fairly tight 80 metre crawl, which turns left, then slowly loops right, passing over some narrow rifts, before suddenly entering the side of a much larger stream passage; Nant Criban. This can be followed to the left for about 50 metres to reach a choked section, where the way on reaches a junction. To the right this regains the passage, which finally becomes impassable after 40 metres. To the left is a much tighter, more awkward passage. This enlarges after 50 metres when it enters an aven. All possible ways on close down after a maximum of 40 metres.


Whilst every care has been taken in the compilation of this cave description, neither the author nor his fellow cavers can accept responsibility for loss or injury arising from any errors, omissions or inaccuracies in this cave description.

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