Ogof Ffynnon

NGR SN 9341 1292

Description with History

The entrance to the cavelies on the east side of the Mellte Valley, about 7km to the west, and a little south of, Porth yr Ogof. A line of deep shakeholes indicates the presence of a major, fault, which crosses the.moorland on a NNW/SSE bearing (3300-1500); the cave entrance is concealed in the base of one of these shakeholes.

The 'walk-in' size opening leads directly to a collapse chamber formed under the grit> The floor is a rubble of limestone and grit boulders and the limestone and shale walls are deeply fluted by dripping percolation water — in short, all the features which typify an unconformity cavern. At the north end of the chamber a route down through boulders leads, via a flat-out crawl to another, long? chamber. This has one solid. 'wall along its major axis; the floor, roof and other walls are comprised of massive jammed boulders.

The cave so far described has certainly been known sinoe 1974, when a U.B.S.S. survey was published. The pioneering of the next section, down to a sump, appears to have been achieved around 1979r although more, than one group claims that this breakthrough was theirs. It is probable that later explorers re-excavate the route down following collapse of The boulder ruckle; the clean washed nature of the passage at the bottom means that little impression is made by visiting cavers.
The exit from the long chamber is a vertical squeeze at the far end. Below this, a route can be divined down amongst further boulders, until an exit is made from the base of the choke into a superb, solid passage in white limestone. All ways off this were choked, except one which is sumped.

Over the winter of 1985 a party of S.W.C.C members visited this passage and found the 'sump' dry; a steady draught blowing into the recently vacated passage, was followed for an estimated 500m. A successsion of muddy crawls, even muddier chambers, and a duck was revealed before the party turned round. The passage continued beyond.

Another S.W.C.C. visit on 23rd February 1986 found the sump drained once more. This time an estimated further 150m was added, before a 20ft pitch was reached, at which point they turned back.

On 1st March I joined Sam Moore and Jerry Peat (S.W.C.C.) on the follow-up trip where, armed with a survey kit, lump hammer7 belay stake and ladder we rigged and descends the 20ft pitch. The cave, beyond sported a selection of muddy squeezes and chambers for all of 30m, at which point a boulder ruckle with parallel squeeze forced an about turn. Both of these obstacles would probably yield to a little determined pushing but the most promising is the bouldery option - the draught was blowing strongly into this.


The entrance lies in the unicomformity between the basal grit and underlying Upper Limestones (Llandyfan Limestone Beds) and owes its existance to massive collapse along a fault line. Descending through the, approximately, 25m vertical range of the entrance chokes, a series of clearly defined shale horizons is evident.  The main passage below the chokes is probably developed in a light colitic limestone, but i did not stop to check this.  The half kilometer or so extension passage follows the line of the fault zone; evidence of this, in the form of bedding displacement, calcite (barytes?) infilling and lengths of unusually straight passage, is observed  as the passage corsses adn re-crosses the fault zone.  Much of the passage and, in particular, the fault-guided chambers, are aligned along the 330 to 150 degree axis and, therefore, are parallel with the fault on which the entrance lies.  A major shale band appears to be the primary stratigraphic control; the phreatic 'swtchback' profile of the passage follows the band either immediately above or below it.


Water sinking at the entrance disappears into the underlying boulders and reappears 100m lower down in Porth Inlet in Porth yr Ogof. The sump, which usually bars access to the extension, reportedly resurges water in wet weather to join the water sinking from the entrance.  It is clear that the whole length of the extension sumps under such conditions, although the source of this water and its point of ingress into Ogof Ffynnon is a complete mystery.  The sump is only known to drain after prolonged (at least three weeks hard frost; access is denied, therefore, for all but a couple of weeks per year.


The route through the entrance chokes is extremely dangerous. The boulders here have recently been suffering regular major collapse - it takes more than caution to descend in safety The extension passages are best described as "Daren Cilau entrance series" plastered with "Otter Hole mud", and the combination proves very tiring. The possibility of flooding is another factor to consider.


Lloyd, 0.C.  "The Hepste River Caves and a Study of the Hepste-Mellte Area" Proceedings University of Bristol Spelaeologi-cal, Society 15(2) pp.107-127 1979.

Jon Young