From Its entrance at NGR.SO164126, the tunnel extends at a bearing of 332 ° Grid for 2026 metres, rising over this distance 7.78 metres (the entranceas a point of light Is visible from 2026m inside testifying to the expert workmanship in digging the tunnel so straight).
In so doing it passes through boulder clay for the first 125m, then Upper shales of coal and ironstone for 70m. Millstone grit for 1050mand the Dowlais limestone for 820m. It is brick lined for the first 1300m apart from small sections here and there and carries water from the surface for the first 900m. The tunnels only use in modern times is as a flood bypass for the lower Carno reservoir, water being taken from the surface stream linking the two reservoirs and passed through the tunnel to exit below the lower reservoir and become the headwaters of the river Ebbw Fawr. When water is flowing in the tunnel access is denied as the depth and flow of water make progress up the tunnel impossible.
Cave features become evident in the tunnel almost immediately it enters the Dowlais limestones at 1200m distant. At 1290m a side passage on the right is met which can be entered through a hole in the brick lining; it is 10m long and phreatic in character with shelving and a cap mud floor. It heads at 90° to a mud choke and is very reminiscent of parts of Agen Allwedd in its overall shape. A few metres further a cave passage in the roof of the tunnel is met, 23m long and heading roughly parallel with the tunnel. This passage again has phreatic shelving and a cap mud floor and looks as if it is part of the same passage as the previous remnant. The roof passage also ends in a mudchoke after a 20° turn to the left.
During a visit, members of Chelsea SS. excavated an entrance to a crawl on the right of the tunnel just beyond the roof passage. This crawl headed for 23m back towards the other side passage ending at a mud choke in a small chamber. Phreatic shelving is also evident in this crawl and the Warren in Eglwys Faen springs to mind when you enter the passage. Several other small entrances to passages are present in this area of the tunnel but all have been blocked with rough cast concrete and have yet to be examined by myself or other cavers, however we have them in mind for the future.
200m further on in the tunnel, in a small brick-lined section (most of the tunnel from entering the Dowlais limestone being unlined) lay the first really interesting feature, a strongly draughting hole in the wall. A study of the draught, its dependence on outside conditions and its lack of moisture or surface smells has prompted us to widen the hole and dig in the chamber beyond, especially as the Geological Report by Lapworth in 1912 mentioned the presence of a draughting fissure in this area of the tunnel.
95m beyond the draughting hole is the first floor feature of the tunnel, a small area of collapse which takes water in wet weather. The next 120m has intermittently pools of water on the rough boulder strewn floor and an active sink at its northern end, water being taken away from the tunnel in a small diameter pipe cemented into the floor, it was at this sink that the dyed Lycopodium was introduced to give the positive trace to Ffynnon Gisfaen.
Just beyond the sink access can be gained through a hole in the brick lining to a dripping aven some 6m high and 2m in cross section at its lower end. The aven is closed. at the top with water issuing from a small crack, cascading down the aven and flowing along the tunnel to the sink. The survey of this part of the tunnel indicates that the aven is only a few metres away from a large doline on the surface near to and east of the forestry plantation which covers the land between the two reservoirs. We estimate the vertical difference in height tunnel to surface could be 65m in this area.
The dip of the limestone is very evident in this area of the dripping aven and seems exaggerated at 11° to the south. There are also thin layers of fine grey shale between the beds with some needle crystals embedded in them; they may be of silica by appearance but no chemical tests have been done to confirm this.
At 2026m the tunnel bends for the first time to a bearing of 315° and remnants of a rift cave in the roof of the tunnel are seen for some 30m. It seems that the original diggers used the rift and followed it for some way until it terminated, where upon they proceeded at the new bearing towards the Claisfer springs. A further 505m into the tunnel the first large shale band, is encountered, a grey shale which marks the uppermost level in the Llanelly shale beds found throughout the local limestone outcrop. Immediately after entering the shale the tunnel becomes once again brick lined and continues so for the next 195m. Taking information from the Lapworth Report it seems it passes through 50m of grey shale and 90m of a rubble band followed by 45m of grey green marl behind the brick lining.
100m beyond the start of the brick lined section a side passage on the left leads for 80m at a bearing of 235° to a blocked end. The geological report indicates that this side passage was dug to try and intercept water flow that was thought to be present in the shale bands, the bands being impervious and there being small caves carrying water at this level elsewhere in the locality; no water was encountered. Another smaller side passage is met 10m further on, again in the left hand wall, this continues for 15m at 240° to a blocked end, again no water was met.
50m. beyond the second side passage a large hole in the floor of the tunnel is met. It is 3m. long and the whole width of the tunnel, able to be traversed using narrow shelves on either side. The hole was dug to s depth of 12m. to inspect the quality of the limestone below the shale bands. The Lapworth report recommended the hole be dug to 100ft. to the very bottom of the limestone but it was abandoned at 40ft. since when it has filled with percolation water from the shale bands. In dry weather the water level is some 3m. from the tunnel floor, in wet weather the water flows down the tunnel to the sink previously mentioned nearer the entrance . However the flow is never very great and the 1912 report states that a maximum of 60,000 gallons a day was extracted from the hole in the early days - not enough to be of any real use to Ebbw Vale.
Beyond the hole the tunnel continues brick lined for a further 30m. It is noticeable that extreme pressure and strain has been put on the brick in this area, presumably by the surrounding shale beds because many lateral cracks are present in the bricks and the floor has sunk several centimeters in places.
Immediately the brick lining finishes the tunnel enters a collapse area where the shale walls have rune in and successive collapse of the thin beds in the roof have made tunneling difficult. Wooden props are in place here and there, some with substantial chambers above them where the roof has broken away Progress through this area, which goes in for 95m. is dangerous and should be undertaken with great care. After 95m. the tunnel, continuing at the same bearing, enters the Gilwern Oolite beds directly below the Llanelly beds. There is evidence of standing water here as the floor, which is rough stone chippings has a coating of mud on it and a mud stain climbs about 0.5m. up the walls. The tunnel ends after a further 110m. at a solid stone wall containing a shot hole 3m. deep in its centre. This shot hole had a 3m. long piece of wood in it when we first arrived at the scene, it has since broken into two pieces. In very wet weather a trickle of water enters through a small solution tube in the right hand wall and sinks in a shallow stony depression in the floor near to the tunnel end.
There is no draught however in the area and prolonged visits result in stale air being produced.
The location of the tunnel end according to the survey is at GR.146153 and a depth below the surface of 120m. Had, they progressed a further 1400m. they would have arrived at Ffynnon Cae Rhos in the Claisfer valley - their original objective.
To complete the account of the tunnel and its surroundings I will give some figures from the Geological Report by H. Lapworth (1912).
He estimated the overall dip of the beds at 1 in 17 to the South
East.
As a result of the Lapworth report the other springs quoted
are now used as water supply and a pipe carrying water from the
Claisfer now traverses the surface of Mynydd Llangynidr to feed
the reservoirs in Cwm Carno; it follows approximately the line
of the incompleted tunnel beneath the moor and was laid in 1942
. It can be seen plainly in places on the moor and is a good landmark
for cavers trying to locate Ogof Cynnes when walking from Blaen
Onneu,
Access to the tunnel is controlled by the Water Authority and
is denied to all but the individuals registered as digging in
the cave passage inside. If the cave digs result in substantial
amounts of passage no doubt revised negotiations with the Authority
will ease access for cavers. But for the time being any pirate
trips could close the tunnel to cavers and deny us the possibility
of cracking open Ogof Mynydd. Llangynidr.
There is a centreline survey of the Carno Adit that accompanies this report. Unfortunately, it doesn't come out too well on-line. It can be downloaded for viewing from here though in .zip format, by selecting the following link.
aditsurvey.zip (23kb)
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