Friday, June 25, 2010

Geological features of the Melte and Nedd Fechan Valleys

This walk in the southern part of the Brecon Beacons National Park, South Wales has a great deal to offer those interested in geology and its effects on landscape and the natural world.

Two major rivers, the Afon Mellte and the Afon Nedd Fechan, rise on the Old Red Sandstone slopes of Fforest Fawr and flow south across the limestone outcrop, here two to three kilometres wide, before uniting at Pontneddfechan. Both rivers show clear evidence of relatively recent rejuvenation, with steep-sided gorges incised into the floors of more gently graded V-shaped valleys. This rejuvenation may be due to isostatic uplift following the removal of the great weight of ice which once covered this area. Under low flow conditions water for both rivers sinks close to the junction of the limestone with the Old Red Sandstone, ascending stratigraphically through the limestone succession to resurge where the limestone passes beneath the overlying Millstone Grit further to the south. In flood, surface rivers continue across almost the entire limestone outcrop although both rivers have short sections over which surface flow has been abandoned permanently.

1. Church Sink (SN 932133)
In dry weather this is the sink for all of the water from the upper reaches of the Afon Mellte, although it is often choked with drifted vegetation and silt and might easily be overlooked. During prolonged periods of dry weather the river bed carries no surface flow for the next kilometre as far as the entrance of Porth yr Ogof.

2. Porth yr Ogof (SN 928124)
Porth yr Ogof is a classic example of a river cave and has the largest and most spectacular entrance of any cave in Wales, almost 20 metres wide and three metres high. In dry weather the water from Church Sink enters the main cave via a much smaller passage some distance inside the entrance, but in flood the river flows straight into the Main Entrance, sometimes almost completely filling the cave. The scale of such floods is obvious from the large tree trunks littered through the system and also from the small dissolutional scallops on the roof; these demonstrate flood velocities of more than one metre per second which, when combined with the cross-sectional area of the cave entrance, indicate a discharge of more than 50 cubic metres per second in extreme floods! The main river passage meanders gently southwards for nearly 300 metres but the route straight ahead from the entrance is soon blocked by a pool. In dry conditions the non-caver can cross the river to a large wide opening on the right hand wall a short distance in from the entrance. With the aid of a torch this can be followed, ignoring smaller side passages, to where light pours down a natural shaft from the surface. On the far side of the skylight chamber a lower passage can be followed towards the sound of running water. A few paces of 'gorilla walk' lead to where the roof rises into the splendour of the Great Bedding Cave, more than 30 metres wide. The wide, roofed shape here and at the Main Entrance are due to development on a series of closely-spaced bedding planes.
Upstream of the Main Entrance a short gorge, littered with large limestone blocks, has formed by collapse of the former upstream extension of the cave. Several small entrances on the east bank upstream of the Main Entrance give access to the underground flow from Church Sink and provide a very watery connection, suitable only for properly equipped cavers, with the main cave.

3. Dry valley and Porth yr Ogof resurgence (SN 927122)
Overlying the cave to the south of the Main Entrance lies a small rocky gorge. This is the former surface course of the Afon Mellte, prior to its capture underground, and can be followed for several hundred metres to the resurgence exit of Porth yr Ogof (see front cover). Several major collapse dolines along the route connect with the cave beneath and hint at its ultimate fate. The resurgence itself lies some 700 metres upstream of the faulted margin of the limestone and is free-draining, a simple river outfall.
N.B. Only experienced cavers should attempt to enter or exit the cave via the resurgence. This site has claimed more lives than any other cave in Britain.

4. Glan Mellte (SN 926116)
The Afon Mellte cuts through a small rise in the limestone floor of the valley to form a small gorge some ten metres wide and up to six metres deep. The walls are vertical and the river bed is littered with large blocks. There is at least one small cave in the side of this gorge and it is possible that the gorge itself is, in part, a collapsed cave analogous to Porth yr Ogof. The Carboniferous Limestone dips beneath the Millstone Grit a short distance downstream of the gorge.

5. Sgwd Clun-gwyn waterfall (SN 925110)
Although developed on the Millstone Grit rather than limestone, this waterfall illustrates the effects of geological controls on drainage. The waterfall has developed on a fault which downthrows the harder sandstone bands some 15 metres to the south, on its downstream side. The easterly dip of the sandstone beds is causing the river channel to shift eastwards, undercutting the east bank and leading to abandonment of the west side of the channel except in flood.

6. The Millstone Grit ridge
The watershed or interfluve between the Afon Mellte and the Afon Nedd Fechan to the west of Sgwd Clun-gwyn lacks any obvious karstic features since it is developed on Millstone Grit with the limestone more than 50 metres below the surface.

7. Pwll Du (SN 912121)
This pool within the rock amphitheatre is the flood resurgence for all of the caves further upstream in the Afon Nedd Fechan. In dry weather the pool is static and all of the underground flow issues from gravel on the opposite bank. To reach these two risings the water from the Nedd Fechan caves must pass to a depth of at least 30 metres beneath a downfaulted block of Millstone Grit. A short distance up the path to the north of Pwll Du an elliptical, joint-guided phreatic tube, Ogof Cagoule, descends a few metres into the crag. It may have acted as a resurgence during an earlier period of the caves history before the river had incised to its present level.

8. Pant Mawr Rising (SN 912123)
To reach this point may require some scrambling down a steep bank which is rather treacherous
in wet weather. In dry weather water can be seen to resurge from bedding planes and gravel banks
in the river bed at the junction of the limestone with the overlying Millstone Grit. Water resurging here is derived from Pant Mawr Pot, four kilometres to the north-north-west, rather than from the Nedd Fechan underground drainage to the north-east.

9. Nedd Fechan Gorge (SN 911135)
The Nedd Fechan has incised a narrow and remarkably straight gorge more than a kilometre long in the floor of a much broader V-shaped valley. This gorge is developed, at least in part, along a fault. Towards its northern end the gorge is entirely in limestone. Small patches of rounded limestone pavement, formed below a former soil cover, lie adjacent to the footpath above this end of the gorge. At its southern end the gorge has cut through the Millstone Grit into the underlying limestone. Several collapse dolines are developed in this grit cover to the west of the gorge (SN 899132) and have been caused by dissolution of the limestone beneath the grit cap.

10. Pwll y Rhyd and White Lady Cave (SN 911137)
Pwll y Rhyd is a large gaping rift in the bed of the Nedd Fechan, formed by collapse into a phreatic rift. In dry weather Pwll y Rhyd and the riverbed for more than 300 metres upstream is dry, but in flood the river plunges into the rift and passes underground to emerge at White Lady Cave. A small, narrow gorge carved through the limestone continues on the surface downstream of Pwll y Rhyd. This is the former surface course of the river prior to its capture by the cave passages beneath, but now it is completely abandoned by the surface river; it carries only a trickle of water fed by small percolation inlets. At several points bedding planes in the walls of the gorge form undercuts, widened by dissolution when this was the main flow route. At the lower end of the abandoned gorge a two metre descent, climbable only in dry conditions, gives access to the entrance of White Lady Cave, a large phreatic tunnel which has been breached by downcutting of the gorge. Deep pools just inside the entrance prevent access to non-cavers. A short distance downstream on the west bank the narrow rift passage of Town Drain is another inlet to the main cave system under the eastern bank. Under normal flow conditions much of the surface river sinks at a bend in the river just upstream of the bridge to enter the Little Neath River Cave system, containing almost nine kilometres of explored passages accessible to experienced cavers. Other inlets to the system lie at several points upstream and downstream of the main sink. The cavers' normal entrance lies in the riverbank upstream of the bridge and is a small, twisting passage accessible only in low flow conditions and clearly formed by relatively recent capture of the surface flow. A second entrance, located in a large depression just downstream of the bridge, leads into the much larger passage of Bridge Cave. This connects with Little Neath River Cave via an 18 metre flooded section. The difference in size between the two entrance passages illustrates the progressive upstream capture of the surface river by the underground drainage. A short distance further downstream, at the foot of a small crag on the west bank, lies another major sink, though often choked with flood debris.

12. The Carboniferous Limestone ridge
Returning eastwards across the Carboniferous Limestone outcrop to Ystradfellte the topography is very different from that seen when the Millstone Grit ridge further south was crossed. The surface is pock-marked with subsidence dolines where the limestone is blanketed by only a thin glacial till cover, allowing drainage into and dissolution of the limestone beneath. On higher parts of the ridge, bare limestone is exposed at the surface to form small crags and patches of rounded limestone pavement.

Christopher Spry
Wimbledon, London
21:01, 25 June 2010
Updated 18/01/2021

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