The Mellte Valley Part 1 - Physical Geography


The Afon Mellte is the largest and most mature of three tributury streams which flow from the southern slopes of the Brecon Beacons in South Wales to form the River Neath. In common with the other two streams, the Afon Nedd Fechan (or Little Neath River) and the Afon Hepste, it rises on the Old Red Sandstone ridge between the valleys of the Afon Taff Fawr and the River Tawe. From here it flows southwards across the northern outcrop of the South Wales Carboniferous Limestone, before continuing onto the Millstone Grit and Coal Measures of the South Wales Coalfield. The three streams coalesce in the vicinity of Pont Nedd Fechan (or Pont Neath Vaughan) [fig. 1], whence the unified River Neath veers south-westwards to discharge into the Bristol Channel at Neath.

The disappearance underground of all these tributaries for part of their course across the Limestone outcrop has attracted travellers etc. since early times and, more recently, spelaeologists. Of the three streams, none disappears in a more spectacular fashion than the Afon Mellte, whose valley is abruptly interrupted by a high cliff, at the base of which the entire river is engulfed by the cavernous entrance to Port-yr-Ogof, 17.5 metres wide and 5 metres high.

The valleys of all three headwaters of the River Neath are encompassed by the scattered Parish of Ystradfellte. The centre of the village is situated in the valley of the Mellte, not far from Porth-yr-Ogof, and it is here that, for many years, Croydon Caving Club has had its caving base. It is this affinity of the Club with the Mellte valley which has prompted this six part study, which attempts to describe the valley in its geological context and to identify and describe the sites of spelaeological interest contained within the study area. It does not purport to be either exhaustive or scholarly, but aims to draw together existing information and provide an accurate basis upon which others, it is hoped, may build in the future.

The Study Area

This study relates to the area of land lying between the National Grid lines 215000 and 211500 to the north and south, and the lines of the surface watersheds between the Mellte and the Hepste and Nedd valleys to the east and west respectively. This area encloses most of the Carboniferous Limestone outcrop in the Mellte valley, and has an area of approximately 9.2 square kilometres.

In the north-east part of the study area the valley is flanked by the steep scarp of Gwaen Cefn-y-Gareg, Its gritstone summit cliffs creating an impressive backdrop to tile village of Ystradfellte. Below this, the side of the valley levels to a narrow limestone pavement, before again dropping steeply to the river. To the south-east is a lower gritstone moor (Gwaen Hepste) , which is now under a Forestry Commission plantation To the west of the valley is a high limestone plateau, known as the Shar Wlad (Common Land), which rises gradually from the river to the crags of Carnau Gwynion. This is a curious, undulating area of dry valleys and dolines and is of great Spelaeological. interest. In places it is capped by thin outliers of gritstone, which produce distinctive plateau hill tops, such as Plas-y-darren.

The Mellte Valley

The valley of the Mellte has great charm situated, as it is, in tranquil isolation amidst the bleak moorland which surrounds it. The well drained limestone soils are quite fertile, in contrast with the poor quality acid soils of the Surrounding grits and sandstones, and a chequered landscape of small fields and farmsteads has developed in the valley floor. At its centre stands the ancient church of St. Mary, surrounded by enormous yew trees and attendent buildings which include a pub, post office and a handful of cottages including Godre Pentre, the Croydon Caving Club base. To the north is the manor house, Mellte Castle and the remains of the ancient Castell Coch which indicate that the valley was of importance in medieval times as a royal hunting estate. In more recent times however, the valley became a quiet backwater, all but forgotten except by the occasional discerning traveller or cave hunter. Only very recently has it been rediscovered by tourists and school journey parties, who now flock into the valley during the summer months to visit its waterfalls and caves.

The Afon Mellte is, by virtue of its size and tile maturity of its profile, the major tributary of the River Neath. It rises on the peat-covered moorland of the Fforest Fawr as two branches: the Afon Llia and its partner the Afon Dringarth. The latter rises between the summits of Fan Fawr and Fan Llia to flow, via the Ystradfellte Reservoir, into a narrow valley scattered with isolated farmsteads. The Afon Llia rises to the west of Fan Llia, beneath the slopes of Fan Nedd, and descends steeply to its confluence with the Afon Dringarth, just below the twin road bridges at Mellte Castle. From here the united flow runs as the Afon Mellte, crossing onto the limestone outcrop a short distance south of the confluence. Downstream the river is bordered by alternating flat alluvial meadows on the one side and by a steep river cliff on the other, as it winds its way towards Porth-yr-Ogof.

In conditions of low flow the river sinks underground into impenetrable fissures on the east bank near the church, just upstream of the old ford at Pont Rhyd-rhiwllan (now bridged) from which the village derives its name (Ystradfellte = way across the Mellte). The remaining 1 km or so of streambed as far as Porth-yr-Ogof is usually dry. In winter, or during summer storms, however, the Church Sink backs up and this dry river bed can become a deep flowing torrent. The word Mellte means "lightning", and any witness to the speed with which this transformation can happen will appreciate that the name is well deserved.

The water which goes underground at Church Sink is next seen emerging from a sump in Port-yr-Ogof, and it is here that it merges with the water from the surface streambed. After only 250m underground the water re-emerges from a deep pool into a permanent surface course. Above the cave the old, deserted river bed can be seen as a shallow overgrown gorge which today only takes water in very exceptional circumstances.

Downstream, after a brief spell of tranquillity, where again it is bounded by alluvial meadows and steep banks, the river runs into a small limestone gorge before crossing onto the Millstone Grit and plunging over Scwd Clun-gwyn, the first of a series of spectacular waterfalls. By way of these falls the river rims through a wooded valley to the River Neath confluence.

Solid Geology

General Geology

The rocks which outcrop in the study area are sediments of Upper Palaeozoic age which have become exposed along the northern flank of the South Wales Coalfield [fig 4]. This complex synclinal structure underlies the whole of the South Wales industrial area and has its main axis trending east-west. It developed as a result of the Hercynian earth movements of the late Carboniferous period, which marked the end of the Palaeozoic era. The younger rocks of the Carboniferous, such as the coal measures, are preserved at its centre but, along the northern and southern flanks of the structure, these younger rocks have been eroded so that progressively older rocks are exposed.

On the northern rim of the coalfield the predominant dip of the strata is about 10 degrees to the south, and the succession of Upper Palaeozoic rocks outcrops here in chronological order from north to south, in narrow bands trending east-west along the strike. The comparative resistance to erosion of these older bands has resulted in the formation of the highland ridge of the Brecon Deacons, whose precipitous northern scarp marks the northern edge of the Coalfield Basin.


The oldest rocks to be seen in the Mellte Valley are the Upper Old Red Sandstones. These outcrop briefly at the northern edge of the study area and consist of mar15 and conglomerates of Devonian age, having a distinctive red colouration which gives them their name. The Devonian rocks are generally overlain conformably by rocks of the Carboniferous Series, here represented by Limestones of the Dinantian Series and Millstone Grit of the Namurian Series.

The limestones of the northern outcrop of the South Wales Coalfield form a complex series. The varying sea levels and climatic conditions in which they were deposited, were caused by earth movements during the Dinantian Period, which resulted in the differing composition of the rocks across the outcrops. Phases of erosion of the landsurfaces between the depositional stages have also resulted in unconformable contacts between some of the beds.

The Dinantian limestones occur in a series of recognisable beds identified by their dominant fossil types. The Lower Series consists of the Lower Limestone Shales (Cleistopora:K Zone) , the Lower Dolomites (Lower Caninia, Zaphrentis:C,Z Zone) and the Lower Limestones (Upper Caninia, Lower Seminula:c,s1 Zone). The Upper Series contains the Upper Main Limestone (Upper Seminula:52 Zone) and the Upper Limestone Shales (Dibunophylluxn:D, ,fl2,D3 Zones). The Upper and Lower Series are separated by a well marked unconformity which persists through most of the length of the outcrop.

Earth movements during the Carboniferous Period seem to have resulted in a general subsidence of the south-eastern edge of the main land mass that is thought to have existed in those times (St. George's Land). The effect of this is that towards the east the whole of the Lower series is well represented, but the Upper Series is completely overstepped by the Millstone Grit. Towards the west the Lower Series is progressively overstepped by the Mid-Avonian Unconformity, but the majority of the Upper Series sequence is present below the Millstone Grit.

Within the study area the C2S, zone is almost completely absent, as are the D3 rocks. The K zone shales outcrop in a narrow band just below the Old Red Sandstone near Mellte Castle and, above this, the Z and C, zones are briefly represented by rocks consisting mainly of dolomite and platey limestones.

The major limestone outcrop of tile valley is the ~2 Main Limestone. These rocks are composed, predominantly, of dense, grey, standard limestones, which are mainly oolitic in character but also contain a few intermittent bands of mudstones and sandstones. Above the S2 zone limestones of the D1 and D2 zones can be found; these consist mainly of coarse oolites and calcareous sandstones.

The contact between the limestones and the overlying Millstone Grit is clearly marked by the Post-Dinantian Unconformity, which will be shown later to be a significant factor in the spelaeogenesis of the area.

The Millstone Grits themselves consist of coarse brown quartz conglomerates with a siliceous matrix, interspersed with beds of grey shales. The basal grit is particularly resistant to erosion and is prominent in the scarps which crown the hilltops of the area.

The Hercynian earth movements which were responsible for the downwarping of the South Wales Coalfield also resulted in extensive faulting in a NW/SE direction across the strike. These faults show substantial lateral movement, which has considerably complicated the simple, east-west strip pattern of the limestone outcrop in this area. The vertical displacements are much less marked. In tile Mellte Valley four roughly parallel faults can be traced, mainly to the west of the river.

To the south of the study area, and running normal to this faulting, is the Neath Disturbance - a much more extensive zone of faulting and contorted strata, which is thought to have developed gradually throughout the Carboniferous Period. The rocks on the line of this disturbance have been very much more susceptible to erosion, and the River Neath has cut down its present course along this line. Although this disturbance does not appear in the Mellte Valley it has played an important part in the development of the drainage pattern in the region.

Development Of The Mellte Valley

It is generally thought that the predecessors of the present streams at the head of the River Neath found their origin on an ancient Mesozoic land surface which was probably formed on Cretaceous chalk. Progressive uplift of the area during the Tertiary era caused the more or less complete stripping of these rocks, exposing the older carboniferous rocks below. The effect of this was that the ancestral streams, which flowed generally to the south-east on this land surface, were superimposed onto to the newly exposed outcrops. Valleys developed, cutting straight across the Carboniferous succession with little regard for its structure. The detailed mechanism of the formation of the present day stream pattern has been described by North in his classic work, The River Scenery at the Head of the Vale of Neath, a brief r5suiné of which is given below

After the re-emergence of the Carboniferous rocks, new streams began to form along the strike of the newly exposed strata to complement the superimposed drainage routes. As time went on these strike streams became dominant and their tributaries captured the old superimposed drainage by diverting it to flow along the dip into the strike stream valleys [fig. 2]. The remnants of these superimposed drainage routes are represented by the present day Llia and the upper reaches of the Little Neath. Before capture, the Llia flowed on to the south-east, its route being marked by the shallow valley between Gwaen Cefn-y-Gareg and Gwaen Hepste and the dry notch in the hills between Foel Penderyn and Twyn-y-Glog. The Little Neath probably also continued in a south-easterly direction, crossing the present Neath/Mellte watershed in the vicinity of Heol Fawr, and continuing to join the Lia as a head-water of the ancestral Cynon. The major strike stream was developed along a line which corresponds to the present day Pyrddin and Sychryd valleys. One of the capture tributaries that developed on the dip slope was the ancestral Mellte.

The Alpine earth movements of the Miocene manifested themselves in the South Wales area as general uplift of the whole land mass. This resulted in a large scale rejuvenation of the streams and, in particular, in the rapid development of a new stream along the line of the Neath Disturbance. In this region of comparatively weak rocks, it cut down its bed very rapidly and, in the process, captured all tile headwaters of the Cynon, diverting them into the River Neath.

Below Pont Neath Vaughan the Neath has developed into a broad, mature valley but its headwaters have been left perched at high level, behind a barrier of harder, less broken, strata over which they cascade in a series of waterfalls and gorges.

Over time, the Mellte has developed as the major drainage route in the area. The other ancient superimposed stream, the Little Neath; a new dip stream, the Hepste; and streams formed on the line of the ancient strike valley, the Pyrddin and the Sychryd, have all become hanging valleys above tile mature valley of the Mellte. Even the Llia is a hanging valley above the Mellte which has developed a new primary tributary on the slope of the strata, the Afon Dringarth.

Drift Deposits

The solid geology of the study area is largely obscured by Quarternary drift materials. The most recent of these is alluvium, which underlies the meadows at the inside of the meanders along the river. On the west of the valley, notably to the north of the village centre, is a terrace of older river gravels which appears to complement the limestone river cliff that has formed on the east of the river. The most dominant drift material, however, is Boulder Clay, which is derived from the Pleistocene ice ages that ended about 10,000 years ago. It consists mainly, of cobbles of Old Red Sandstone in a brown clay matrix.

The glaciation in this part of Wales was not severe but the general flow of the ice appears to have been across the line of the valleys. The result is that these valleys, which were already well established at the onset of the glaciation, quickly became completely choked with ice and debris and their profiles were not significantly modified by the ice above. The intermediate ridges between the valleys, however, must have been subjected to scouring by the ice, and the prominent gritstone scarps and flat hilltops are probably, at least in part, the result of this action. In spite of subsequent removal of the greater part of the boulder clay (which once filled the valley) remnants still flank the sides of tile valley in thick banks: for example the shallow trough between Gwaen Cefn-y-Gareg and Gwaen Hepste is still choked with drift deposits, as are many of the depressions on the western moor land.

The remaining five parts of this study will be published in successive issues of Pelobates

Allan Ockenden