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Beneath Llangattwg

by Graham Wilton-Jones

The 1976 extension to Ogof Craig y Ffynnon (Rock and Fountain Cave) raises some interesting questions concerning past and present drainage under Mynydd Llangattwg.  A recent visit to Ogof Craig y Ffynnon prompted me to have another look at Ogof y Darren Cilau, which lies further along the north-eastern outcrop of limestone towards Agen Allwedd.  Perhaps some notes on Ogof Craig y Ffynnon and Ogof y Darren Cilau would be useful.

To begin with I shall refer to Wig's article in B.B. No. 356, December 1977, to comment on Ogof Craig y Ffynnon.   The small rising (IP 2) is not the main rising for the cave, which is actually Ogof Capel (see also IP 8).  This is situated at the bottom of the Clydach Gorge, 500 yards west-south-west of Ogof Craig y Ffynnon entrance.  On my visit to Ogof Craig y Ffynnon the cave was wet.  Between the boulder chokes were deep pools concealing flooded lower passages which can be entered in summer.  These carry Ogof Capel water.  The entrance to Ogof Craig Ffynnon is a rubble rift, the sides of which show superb scalloping, and must once have been part of an impressive streamway approaching a resurgence in the Clydaoh area, or does this section of cave pre-date the valley (see below - Clydach rejuvenation)?  The limestone continues below the coalfield south of the Clydach and I believe that some caves there actually head under the coal.  Ignoring the scarp outcrop to the east, the next place the limestone is seen is in the coast districts, close to sea level.

The lower stream series (IP 4) is not that difficult, and is reminiscent of the more complex parts of O.F.D. One of the streams we pushed (at least, J.D., the wellie-booted worm did) to a choke.  This was under a dripping aven in the other passages of this series.  The sources of the streams down here have not otherwise been traced, but I would venture to suggest that the Ogof y Darren Cilau stream deserves further attention in this respect.  Dye tests have been made, but it should be borne in mind that negative results are not indicative of no connection hydrologically.

The end of Ogof Craig y Ffynnon (IP7) lies after two miles of fairly straight passage (with obstructions) equidistant from Agen Allwedd terminal sump (I or IV, I don't know) Eglwys Faen and the end of Ogof y Darren Cilau.  It is in the same beds as Agen Allwedd, i.e. the Oolitic, having risen up through the Dolomitic (IP 7 and 8) and is similar in character to Agen Allwedd, especially Main Passage, St. Paul's, etc.

I will return to the subject of Ogof Craig y Ffynnon.later.  What of Ogof y Darren Cilau?  For those who do not know this cave, and no doubt, you are many (sensible people) a brief description may be useful (then you won't have to go yourselves).

At the base of the cliff, behind the old limekilns above Whitewalls, is a low, wet entrance, one of eight at the base of the outcrop between here and the valley be the old sheep dip.  It is a taste of things to come.  The lowness and wetness, and narrowness continue for a thousand feet.  One thousand feet of very technical, grovelling, with only a few short stretches of walking.  Finally this small streamway breaks into larger passage, and the stream disappears under the edge of this.  The larger passage leads to a fault guided rift with stal, some old and massive, at this end, and a grey, shaly conglomerate breakdown at the other end, several hundred feet away.  This breakdown is also the end of a huge phreatic passage: remarkably similar to Agen Allwedd Main Passage, but almost immediately filled to the roof with mud. However, a further passage leads from here, zig-zag rift which goes to the final chamber.  This chamber is several hundred feet long and tens of feet wide, formed entirely of collapse (into what would be interesting to know) and floored with boulders and glutinous mud.  There is one similar, but smaller chamber off to one side.

Several interesting thoughts come to mind:

What are the relative altitudes of the caves mentioned?  Unfortunately ‘Caves of Wales and the Marches’ does not give these.  However, following the Tram Road on the 2½ map is helpful.  Near Brynmawr it is at 1175' OD.  At Eglwys Faen it has dropped, and varies between 1100' and 1125' OD. At Agen Allwedd the track is lost, but Aggie entrance seems to be 1275' OD.  (Is there really a 150' climb from Eglwys Faen to reach it?)  Perhaps this is more accurately indicated on the new survey. Eglwys Faen must be 1125' - 1150' OD. Ogof y Darren Cilau, way above the Tram Road, must lie at 1300' OD or more.  Ogof Craig y Ffynnon, 200' below the Tram Road, must be at about 975' OD, while Ogof Capel and Elm Cave, the Agen Allwedd resurgence, must be round about 750' OD.  Someone must be able to find more accurate figures for these.

Several passages run in from the escarpment - Ogof Pen Eryr, Ogof y Darren Cilau stream, Aggie entrance passage - on the strike apparently, or is the dip at the edge of the hill west instead of south?

What is the relationship between the large passages so far known? Are they parts of the same cave, did they undergo similar conditions of formation, or is it simply that they are formed in the same rock?  Ogof y Darren Cilau seems to be too high up in the beds to have any relationship with the other caves, but is it?  The entrance altitude suggests it is.  So does its breakthrough into shale.  However, in the entrance series of Ogof y Darren Cilau and in the long (crawl in Ogof Craig y Ffynnon there is a band of green limestone.  Are these the same beds?

The final chamber in Ogof y Darren Cilau is totally dissimilar to the big chambers in the other caves, though perhaps it is a large phreatic passage.  Why does it have such wet mud in it?  Is it because there is no draught to dry it out - we noticed no draught here or because it has an occasional humid draught?

Why should Ogof Craig y Ffynnon be thought not to be a fossil part of Aggie? (IF 8).  I would have thought that that is just what it is. It rises eventually from its entrance (250' above the Clydach) into the Aggie beds.  Why should it not, perhaps, be a continuation of Aggie Main Passage? It has not yet reached near there according to the surveys available.  Pete Bull has done a great deal of sedimentological work in Agen Alwedd and this, more than anything else, seems to be helping to date the passages of Aggie, and to demonstrate the relationship between them.  Similar work in Ogof Craig y Ffynnon to show a comparison would be invaluable here.

Was the Clydach Gorge a product of rejuvenation following glacial deepening of the Usk valley? Cave streams seem now to be the major erosional influence in the gorge.  Could a fossil extension of the present Aggie streamway exist somewhere above the base of the Clydach, possibly on a level similar to that of Ogof Craig y Ffynnon, but maybe to the west of this?

What happens beyond Aggie terminal sump IV?  It would seem that there must be pitches, or at least ways down like those in Ogof Craig y Ffynnon which take Ogof Capel water, except that these for Agen Allwedd would still be taking all the water.  Only a small stream actually goes down those in Ogof Craig y Ffynnon.

How much do the Llangattwg system pre-date the present topography?  Projecting the northern end of Summertime takes it straight out of the hill.

The questions and postulations are endless.  Several of the former can be answered easily by a geologist or someone with more access to relevant information that I have.  The reward for the studious could another Key to Llangattwg - or have written (as we say in my Norfolk homeland) a lood o’ ol’ squit!

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All members are reminded that it their responsibility to ensure that the Belfry is always kept locked - remember there have robberies in the past.  Will all members make a special point to ensure that the Library is kept locked at all times, certain items in the collection are quire rare and extremely useful for reference purposes.  Library keys are obtainable from any Committee Member.

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Time is going on again – The AGM 1977 AGM Minutes will be included with the B.B.