The evolution of the Neath-tawe Drainage system, South Wales

530

THE EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM, SOUTH WALES. By R. O. JONES, Ph.D., M.Sc., F .G.S. [Received 30th June, 1939.] [Read 7th July, 1939.]

I. INTRODUCTION. has long been recognised that the courses of the rivers in the eastern part of South Wales bear little relation to the geological structure: flowing in a general parallel direction south-south-east, they cut across the grain of the Armorican syncline and have all the characters of being superimposed. Strahan [20 and 21 *J suggested that this river system was initiated on 3 surface of Upper Cretaceous rocks, by which all features in the Palaeozoic rocks were blanketted, and that these Cretaceous strata were at some stage generally tilted from north-north-west to south-south-east. He pointed out that the River Neath (Nedd), the River Tawe, part of the River Loughor (Llwchwr) and the River Towy runs almost at right angles to the other South Wales rivers. For considerable parts of their courses these rivers follow belts of powerful disturbance and he suggested that movement occurred along these lines in Miocene times, giving rise to corresponding ridges in the Mesozoic cover which caused the deflection of the drainage to the WSW. Trueman [22 and 23J has pointed out that while the Neath and the Tawe flow approximately from north-east to south-west, most of their tributaries have a direction which is more nearly north-to-south. He suggested that if the rivers of the eas t of South Wales were developed on a tilted cover of Mesozoic rocks, the tilt may have changed slightly in direction westwards from the Neath valley. He further suggested that it is possible that the Neath and Tawe have worked back along the belts of shattered rock and captured streams which had continued to flow north-to-south after the Tertiary faulting. This theory differs from that postulated by Strahan, who stated that the disturbances which had an uplift to the south caused the north-to-south rivers to be deflected into a south-westerly direction when the drainage system developed and while there was still a Cretaceous covering. A point of great importance in the consideration of the two theories is that neither the Vale of Neath nor the Tawe (Cribarth) disturbances continue to the sea, and so it is not clear why either of these rivers should have cut through the southern Pennant escarpment.

IT

*

For list of References see p. 564.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

531

a. T. Jones [11J has considered that the development of the drainage in South Wales may have been due to the effects of two distinct movements, one, the earlier, which gave a prevailing south-easterly trend to the streams, and the other, a later one, to which was due the southerly or south-westerly tilt. The present writer [12J has shown that the history of development of the Tawe drainage basin is more complicated than can be readily explained on the simple invocation of regional warping or of superimposition from a Mesozoic cover. North [18J ends his description of the river scenery of the Vale of Neath by suggesting that the River Neath (Nedd) has captured the headwaters of the Taff, but states that this is the subject for another account. Dr. North did not follow this point further, and in correspondence in 1933 he expressed his wish that the writer should take up this matter. In a recent edition of the work he has, however, dealt in a general way with the evidence that the upper Neath tributaries once drained into the Taff. II.

DETAILED DESCRIPTION OF RIVERS.

(I) River Neath ' (or Nedd). The Neath Valley, like the Tawe Valley, is a markedly flatbottomed glaciated trench, its upper tributaries all being valleys hanging at a height of about 700 feet above a.D. As far as can be determined the river follows the Vale of Neath disturbance for a great part of its course; but the solid valley floor is driftcovered, in parts at least to a great depth, and details of structure are unknown. The river follows the disturbance as far as Resolven [15b, p. 209]. As it maintains a rectilinear course to Aberdulais there seems every probability that the disturbance continues, but possibly with less severity, as far at least as that village. From the town of Neath to the sea the river follows a north-south general direction, that is, a parallel direction to the lower part of the Tawe. The officers of H.M. Geological Survey [15d, p. 146J suggest that the Neath river had its outlet to the sea to the west of its present mouth, that is, west of the Pennant Sandstone hill of Coed-yr- Iarl!. According to Codrington [3J a number of borings were put down near the mouth of the river varying from 36 feet to 95 feet below a.D. in depth and only one reached solid. This one was at a point near the mouth of the river below Briton Ferry, 350 yards west of the present low-water channel, and here the solid I The River Neath (Nedd) is here considered to occupy the Caledonoid fault-determined valley course from a point about 300 yards above Pont-Neath-Vaughan (Pont Nedd-Fechan) to the sea.

53 2

R. O. JONES,

was reached at 53 feet below G.D. It is possible that the Neath and Tawe [7a] valleys are rock basins buried by drift. There is evidence of a great deal of glaciation in the Keath valley, and at one period at least the ice reached about 1,000 feet above G.D. in the lower part of the valley [15a], boulder clay of the main glaciation being found at this level on the west side of Foel Fynyddau. A very marked dam belonging to a halt in the recession of the Newer Drift is to be seen at Aberdulais, This is similar to the ones at Glais in the Tawe valley, and at Llantrisant in the Ely valley and, like the latter, it seems at one time to have dammed the river to form a lake. Upstream of this morainic dam, the torrential streams flowing into the Neath have in many cases deposited deltaic fans of coarse gravel upon the alluvial flat of the Neath. The deltas of two left-hand tributaries, one near Clyne (Resolven) Tinplate Works, and the other near Ynysdyfnant Farm, have been truncated by the river: their flat top suggests that they mark the margins of a former lake. A terrace-like feature in the drift, similar to that described by the writer [12 and 13J as occurring in the Tawe valley, is very noticeable in this valley. A terrace in solid rock occurs between Pont-Neath-Vaughan and Pontwalby on the left side of the river, at a height of 300 feet above G.D. and about 50 feet above the river. This has been mentioned by North [17J as evidence of uplift. In the neighbourhood of Pont-NeathVaughan there is a great deal of river gravel, particularly on the right-hand side of the valley. A short distance above this village, on the right-hand side of the Mellte, gravels are seen resting on solid rock 10 feet above the river which is here flowing in a gorge. There is also a great deal of terrace gravel near the town of Neath itself at about 50 feet above G.D. Similar post-glacial gravels are found in other valleys in South Wales at the same altitude.

(2) River eynon and some Upper Tributaries of the River Neath (Nedd). (a) R i v e r C y n o n .' This right-hand tributary of the Taft rises on the south of the faulted Quartz Conglomerate (Millstone Grit) ridge of Cefn Cadlan. It flows mainly in a south-westerly direction across Millstone Grit and Carboniferous Limestone to Penderyn. It crosses the Vale of Neath disturbance and, although for a distance below Wern-las its direction is parallel to the general line of disturbance, the river itself crosses and re-crosses the actual fault-line. From Penderyn to Hirwaun it flows south-south-east across Millstone Grit country. At Hirwaun the river turns east-south-east and later follows a 1

This is also spelt as Cynnon.

PROC. GEOL. ASSOC., VOL. L. (1939).

MAP SHOWINO PAST AND PRESENT DRAINAGES IN A PORTION OF 'OUTH WALES.

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[To fau p. 532.

EVOLUTION OF THE NEATH-TAWE DRAIXAGE SYSTEM.

533

general south-east direction to join the Taft at Abercynon, flowing across Lower Coal Series to Penrhiwceiber and thence across Pennant Sandstone. The valley is filled with a great deal of boulder clay from near its source to Aberamman. The amount of glacial material in the neighbourhood of Hirwaun is very extensive and often mound-like in character (and now covered with peat bogs) as would naturally be expected considering that here the Brecknock ice was diverted by the Pennant escarpment of Craigy-Llyn, part going along the Neath and part into the Taft [5 and 15b]. Another patch of boulder clay occurs near Mountain Ash, but downstream the glacial drift becomes more gravelly. Gravels of post-glacial age forming a terrace at about 15 feet above the river and 400 feet above a.D. are seen on the right bank of the river at Penrhiwceiber. On either side of the valley at this point there is a terrace-like feature in the drift at about 30 feet above the river, similar to that at Mountain Ash. At Abercynon, patches of gravel forming a terrace are to be seen on either side of the Taft and the Cynon at about 65 feet above the river, or 320 feet above O.D. The Taft cuts through these gravels in a gorge. From its source to near Penderyn, the gradient of the Cynon is fairly steep, but downstream the gradient is small and the river is today generally flowing on its own alluvium. In its course from Penderyn to Aberdare in particular, the river is a very obvious mis-fit. To the north of Penderyn there is a gap between Foel Penderyn (1,218 feet) and Cader Fawr (1,592 feet) at about 890 feet above O.D. The gap has all the features of a wind gap, and for it there are no apparent geological reasons. To the west-northwest of Hirwaun there is another very marked gap between the Pennant Sandstone ridge of Craig-y-Llyn (1,969 feet) and Foel Penderyn (1,218 feet), its centre being about 700 feet at a point one mile north-east of Rhigos, where it attains the highest altitude (see Fig. 48). It occurs mainly in Lower Coal Series. These gaps will be referred to respectively as the Penderyn gap and the Rhigos gap. The existence of the twa gaps and the marked mis-fit of the River Cynon indicate a former more extensive river system draining into the Cynon from the north and west. There is evidence that the original course of the River Hepste, Afon Llia and Afon Dringarth was through the Penderyn gap and that a system of rivers from regions now drained by the Pyrddyn' and the Nedd Feehan (Upper Neath or Little Neath) and other I

This is also spelt as Purddyn, I'erddvn, or Pyrddin,

R. O. JONES,

534

rivers flowed through the Rhigos gap. Each of these is, therefore, considered here. (b) Afon Llia and Afon Dringarth. Afon Llia rises at Bryn Melyn (1,550 feet above O.D.) and flows across Old Red Sandstone, in a general south-south-east direction, to join the River Mellte at a point about one mile north of Ystradfellte. To the north-north-west of the head of the valley is the very deep upper part of the Senni valley which seems to have captured the headwaters of the Llia (as the Tarell appears to have done with the headwaters of the Taff Fawr) , leaving a gap between Fan Nedd and Bryn Melyn at about lA70 feet above O.D. Boulder clay is present in the Llia valley near its source in the Foel Penc:ieT"l(ft

Rhigos Ga.?

2000'

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FIG. 48.-SECTlON ACROSS RHIGOS GAP

(from N.N.E. to

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neighbourhood of Maen Llia, while further downstream there is extensive glacial material, particularly on the west side of the river, where at times it forms a typical lateral moraine. At a point about 1,300 feet above O.D., upstream of more boulder clay, there is present in the river bed an apparent knick-point, while at about 1,175 feet the Llia becomes rejuvenated and fast-flowing to join the Mellte. The Dringarth stream rises on the south side of Craig Ceriggleisiad and flows more or less due south to join, near Mellte Castle, the Llia which occupies a well-marked valley in the Old Red Sandstone between Fan Llia and Fan Fawr. Glacial drift is present in parts of the valley, but on the whole is not as extensive as in the Llia. It forms a terrace-like feature near

535

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

Blaen Dringarth on either side of the river, similar to those found in the Neath and Tawe valleys. A very marked gravel terrace of re-distributed Old Red Sandstone occurs in the Dringarth and Mellte valleys in the neighbourhood of Ystradfellte. This terrace is clearly seen on the right-hand side of the valley and is about 4 feet above the alluvial flat of the river and about 860 feet above a.D. As in the other valleys the gravel was formed by the great deal of material brought down by the Dringarth and Llia into the Mellte valley in early post-Glacial times when the Beacons (Fans) were not yet covered with vegetation. From Ty-canol downstream the Dringarth and, later, the Mellte river (combined. Llia and Dringarth) flow south-southwest, and although at first the latter valley has the general wide character of those of the Dringarth and Llia, it soon becomes narrow and gorge-like, and the river is rejuvenated when it crosses the Carboniferous Limestone and Millstone Grit to join the main Vale of Neath river near Glyn Neath Powder Mill. The dry valley and underground drainage near Ystradfellte, and the waterfalls developed in the Millstone Grit area have been so well described by North [18J that it would serve no useful purpose here to give details of the beautiful scenery. If the general direction of the Llia is followed across the Mellte valley, Carn-yr-Arian is reached. This is almost a horizontal tract of Quartz Conglomerate (Millstone Grit) covered with drift and forming lower country than the ground to the north at Gwaen Cefn-y-Gareg, which rises to about 1ASo feet above a.D., the highest point in the Carn-yr-Arian neighbourhood being 1,133 feet (see Fig. 49). The author believes this spur,

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Assoc.,

VOL.

L.,

PART

4,

1939.

35

R. O. JONES,

making a platform-like extension from north to south and being largely a drift-covered hollow, is part of an old gap, the north side of which is preserved, but the south side has been denuded by the rejuvenated rivers of the Mellte and lower Hepste, the two having their confluence just south of Cilhepste-coed. The centre of the original gap was probably south of Carn-yr-Arian and was, therefore, lower than its present remnant. It is seen from Fig. 50, however, that even when allowance is made for the lower altitude of the gap, the stream that drained through it was not graded to the present Penderyn gap. (c) River Hepste. This river rises in two tributaries, the Afon-y-Waen and Nant Cwrier, to the south of Fan Fawr and drains southwards. On reaching the Carboniferous Limestone, the combined streams take a more south-westerly direction. About half-a-mile north of Caer Howel the river turns sharply to the south-east and near the latter swings round to a due west direction and plunges down by the Scwd yr Eira (or Erw) Waterfalls and a series of rapids through a deep gorge to join the Mellte. Above this well-marked bend, the river flows in a fairly gently graded valley on boulder clay and occasionally on its own alluvium, and is seen to be graded to the Penderyn gap. The drift in the area between the Hepste and Dringarth contains Old Red Sandstone boulders. The reach (a mile or so in length), where the river flows underground in the Carboniferous Limestone, and the waterfalls and rapids which characterise rejuvenation, have been described by North [18]. The Penderyn gap to the east of the Hepste appears to have been the original course of a combined Llia-Dringarth river, which flowed across Carn-yr-Arian and, joined by the Hepste, continued in a south-south-easterly direction to Hirwaun. The south-eastern part of the marked bend in the Hepste, near Caer Hywel, was probably connected with the direction of the original valley, although the rejuvenated river now does not necessarily flow in the centre of that old valley. To the east of the present rejuvenated valley there is an extensive patch of drift filling the old valley. It would seem, therefore, that the source of the Cynon originally was in the Fans in the form of two main tributaries, the Dringarth and the Llia, flowing in a general south-south-easterly direction and joined by a lefthand tributary flowing in a south-westerly direction, which developed as a subsequent stream (the River Hepste). A comparison of the heights of the Carn-yr-Arian and Penderyn gaps shows clearly that the Llia and Dringarth were diverted in a direction other than through the Penderyn gap long before the latter was forsaken by the River Hepste. The three rivers, the Dringarth, Llia and Hepste, were later captured by the

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R. O. JONES,

River Neath developing north-eastwards along a line of disturbance. The two stages in the diversion of the Llia and Dringarth are shown by a study of their thalwegs (see PI. 36). It is seen that the first rejuvenation is to an S50-foot level at Ystradfellte and the second rejuvenation to the present River Neath. The significance of these rejuvenations will be dealt with later (p. 541). (d) The Ned d Fee han R i v e r. This rises in Fan Fraeth and flows in a general southerly direction across Old Red Sandstone, Carboniferous Limestone and Millstone Grit to near Pont-rhyd-y-cnau. From the ford downstream, the river has a slight westerly trend and soon becomes markedly rejuvenated, passing over the Ddwli and other waterfalls, and joining the Pyrddyn between Ton-y-Gilfach and Glan-yr-Afon. Afon Senni rises at practically the same point as one of the Nedd Feehan tributaries and flows along a very deep narrow valley, Cwm-Blaen Senni. The Senni is here capturing some of the headwaters of the Nedd Feehan, which it will continue to do until at least the rejuvenated part of the latter cuts back far enough upstream. During most of its course, the Nedd Feehan flows on boulder clay; this deposit covers an extensive part of the Carboniferous Limestone country, and in the Old Red Sandstone country it is present at heights of over 2,000 feet above O.D. between Fan Fraith and Fan Nedd, suggesting that ice passed through this col. Drift covers the lower land near Plas-y-Daren, where there is a suggestion of a gap between this valley and that of the Mellte. At a point about 1,300 feet above O.D. there is a definite change in the river's gradient and a suggestion of a knick-point in the river bed, but as this is masked by boulder clay it cannot be regarded as a definite feature. On the whole, until the beginning of its rejuvenation, the Nedd Feehan is in a fairly gently-graded wide valley like those of the Dringarth and Llia. (e) The River Pyrddyn. The Pyrddynrisesnear Coelbren and has a number of left-hand tributaries having a general south-west direction and rising in Millstone Grit country. Its source is only 1,15° feet above O.D., which is very much lower than that of the other Neath tributaries. The Pyrddyn flows in an east-south-east direction as an obvious strike stream until it joins the Nedd Feehan when its course becomes more southeast. The river has a steep gradient on the whole, but is evenly graded to about 750 feet above O.D.; at 650 feet it becomes markedly rejuvenated and plunges over a number of waterfalls, such as Scwd Einion Gam and Scwd Gladys in the Millstone Grit country.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

539

A well-marked gap floored by drift is clearly seen at 900 feet above a.D. between Bryn-y-Bugeiliaid (1,254 feet) and Hirfynydd (1,579 feet). Beyond this gap is the deep Tawe valley which has a hanging right-hand tributary-Nant Haffes. This tributary shows a most marked knick-point at 1,500 feet (see PI. 36, B). (f) The River Sychryd. The Sychryd rises at the foot of Craig-y-Llyn and flows in a northerly direction as a small stream (Nant Gwrangon) without a well-defined valley across Lower Coal Series. Then the valley becomes a ' cwm' (Cwm Wyrfa) and the river continues along a steeply graded gorge in a west-north-west direction, ultimately swinging round to a south-west direction, after passing over some waterfalls on Millstone Grit, to flow slowly and join the Mellte near Glyn Neath Mill. (g) General Discussion of the history of the Rhigos Gap. As already suggested it is clear that the Pyrddyn, Nedd Feehan and the area now drained by the Sychryd at one time formed a river system that flowed into the Cynon. They did so in an east-south-east direction through Rhigos gap. The Nedd Feehan flowed across the area (now drift-covered) in the neighbourhood of Glan-yr-Afon and Hendre-ffydd towards Rhigos, probably joining the Pyrddyn stream here.' This drainage through the Rhigos gap developed as a tributary to the Llia-Penderyn stream near Hirwaun and, flowing along the outcrop of the Pennant Sandstone, drained the regions of the Pyrddyn and Haffes, and the Nedd Feehan. The present Pyrddyn down to 850 feet above a.D. is graded to this gap, but downstream of that point the rejuvenation due to the Neath capture is seen. The original Pyrddyn that drained through the Rhigos gap had its source in the Upper Haffes towards Fan Sir Gaer. This original river flowed across the present Tawe-Neath watershed between Mynydd-y-Drum and Bryn Bugeiliaid to the north of the present lower course of Nant Llech (a left-hand tributary of the Tawe). The rejuvenated part of Nant Llech has an obsequent direction, but originally its upper course joined this river draining towards Rhigos. This Haffes-Pyrddyn-Rhigos stream in the neighbourhood of the present Pyrddyn valley developed along the outcrop of the Pennant Sandstone, which, of course, today does not appear until well to the south, owing to the formation of a wide valley by the denudation of the earlier stream. The stream flowing through the Rhigos gap not only drained the Nedd Feehan region, but also caused the deflection cf the Dringarth and Llia along the Mellte. The rejuvenation of the Mellte tributaries to the Ystradfellte level of today (about 850 feet above a.D.) is due to this early diversion into the Rhigos

540

R. O. JONES,

gap. They thus abandoned the Carn-yr-Arian-Penderyn course while the Hepste was still draining through that gap . The thalweg of the Nedd Feehan river suggests that possibly its course upstream of 1 ,300 feet abo ve a .D. is grad ed to a gap which may be present (as suggeste d by North [18J) in the driftfilled area to the sout h of Pl as-y-Daren. In that case its upper part flowed through the Carn-yr-Arain and Penderyn gap s t o the Cyn on at a very early period in the drain age history. There is not enou gh evidence, however , to pro ve this. The story of the river development in the upp er part of the Vale of Neath area is th at of the more rapid growth of the strike stream through the Rhigos gap than the superimposed consequent drainage through the Penderyn gap and the capture by the strike stream of the western tributaries of the latter drainage. The River Neath working back along a line of disturbance captured the head -waters of the Rhigos gap river which drained into the Cynon and Taft. The Tawe also working headwards captured the upp er headwaters of this drainage at about the same time. As a result of the Neath capture, the P yrddyn became rejuvenated and developed, in its lower cour se, possibly a little south of the origina l valley. The P yrddyn being now a short beheaded r iver , the reju venation has worked back along mu ch of its course, as is shown by the pr esent vall ey being a narrow one within a wider valley in this part . Similar reju venati on is seen in the ot her tributaries. The capt ure of the Hepst e by the Mellte ma y ha ve resulted from the reju venation of the latter upon its capture by the River Neath . As the Neat h cut back and eroded its valley , an obsequent stream (River Sychryd) developed, draining down the old valley side on the south of t he R higos gap and then plunging t o the Neath vall ey. The actual valley of the Rhigos gap has, naturall y, been greatly mod ified by the intense glaciation, as shown by the deflection of the Brecknock Ice and the corrie lak es (Llyn Fach and Llyn Fawr) at t he foot of Craig-y-Llyn. (h) F eat u r es in th e t h a I w e g o f th e a ri g ina I Cynon and the datin g of the ca p t u re by th e N e a t h R i v e r. an reference t o PI. 36 it is seen t hat a number of the present tributaries of the Neath which are believed to have originally drained into the Cynon valley, bear indications of at least one knick-point [24J at about 1,200 t o 1 ,3°0 feet above a.D. This is seen in the Llia, Dringarth and Nedd Feehan riv ers. There is also clearly present a most marked point of increasing gradient at about 750 feet a.D. (at 725 feet in the Mellt e and Hepste, at 750 feet and 650 feet respectively in the Nedd Feehan and at 750 feet in the Pyrddyn). These streams, therefore, display evid ence of two episodes of rejuvena-

EVOLUTIOX OF THE NEATH-TAWE DHAINAGE SYSTEM.

541

tion, one at about 1,ZOO feet and another at about 750 feet. The first is connected with the initial capture of many. of the streams draining through the Penderyn gap by the strike stream flowing through the Rhigos gap. The second is shown by the height that the valleys are hanging above the River Neath (proper) today; for the latter river, following a line of disturbance, has become graded to base-level much more rapidly than these captured upper tributaries; moreover, it naturally contains far more water than the individual tributaries to effect erosion. A study of the Cynon shows that it rises at a much lower level than the other upper tributaries of the Taff, and that it has a more mature valley than the others. It shows a knickpoint at 850 feet above a.D., where the river enters the southern continuation of the Penderyn gap-that is, where, slightly hanging, it once joined the major stream. Apart from a very minor break in the general gradient at 450 feet above a.D., the river is perfectly graded to its junction with the Taff. The Taff, below a knick-point at 350 feet, is perfectly graded with the Cynon. This clearly indicates that the Cynon was not originally the minor stream that it is today, otherwise the Cynon would be in the hanging valley endeavouring to grade to the Taff, whereas it is seen that the thalweg of the Cynon is the more mature. The Cynon appears to be graded to the zoo-foot platform for the major part of its lower course, while the Taff is not graded to this base-level to the same extent. This means that the Cynon must have still had a great deal of the waters of its original upper tributaries draining into it when the zoo-foot platform was the beach-level, otherwise it could not have graded its course faster than the Taff itself above the confluence. This, therefore, dates the major part of the actual capture of the Cynon headwaters by the Neath as having occurred after the uplift of the 400-foot platform, but, possibly, before the zoo-foot platform was uplifted from beach-level. The fact that the upper tributaries of the River Neath are hanging at about 400 feet above that river lends strong support to that view, while the remarkable coincidence in level of the wide, but now dry, Rhigos gap, indicates almost with certainty that the tributaries were still flowing into the Cynon when they and the latter river were graded to a base-level about 400 feet above that of the present day.

(3) Lower Tributaries of the River Neath and neighbouring rivers. (a) River Clydach (Neath) and its captured headwater s. The River Clydach is a right-hand tributary which

542

R . O . JON E S ,

joms the River Neath about a mile south-west of Neath. It rises on the north-west side of March Hywel and flows southwestwards and then southwards along a minor valley within a gap, but on the east side of it during part of its course. Beyond Rhos it flows along the centre part of the gap and continues on the Upper Coal Series between the Duffryn and Rhyddings Faults. The river is slow-flowing at first, but becomes rejuvenated near its confluence with the Neath valley. The rejuvenation can be traced from a height of 175 feet above O.D . During most of its length to a point a mile below Bryncoch the river is flowing on boulder clay, but in the lower part of its rejuvenated course, it is on solid rock. On either side of the river in the boulder cla y, the t errace-like feature seen in other streams is again clearly present. Th e gap at Rh os was discu ssed by the author in an earlier paper [12J. It wa s then shown that the gap is really the confluence of two gaps, namely the Rhos (Alltwen) gap and Rhos (Gellinudd) gap, the former of which was once occupied by the Upper Clydach, the latter by an extension of the Upper TwrchCwmdu streams. A point of intense rejuvenation being found in the Upper Clydach valley at 2 6 2 feet above O.D., it was suggest ed by the author that the Upper Clydach ri ver , prior to it s capture by the Tawe, could not have flowed through the Rhos ga p unless the relative lev el of the latter was lower then than now; in other words, that there has been an uplift on the east side of the Tawe [12, 14]. After fur ther examinat ion of the thalweg (see PI. 36, B) and other features of the valley, he is now of the opinion that the rejuvenation near Gelligron H ouse is not the point of commencement of rejuvenation of the Upper Clydach (or Gorse) consequent upon its capture by the Tawe. This point is really the rejuvenation head resulting from the more intense glacia tion suffered by the Tawe valley than by the Upper Clydach valley , the Tawe valley now being a rock basin partly filled with drift at this point [7a, 12]. The point of actual rejuvenation of the Upper Clydach, resulting from it s capture by the Tawe, can be seen in Pl. 35, A at a height of 310 feet (in the solid rock). Owing to the presence of glacial material in the valley , the rejuvenation or kni ck-point cannot be so easily seen. The earliest traceable signs of rejuvenation in the Gorse are at a height of 480 feet oppo site Garth Farm, and in the Egel at 550 feet. Continuing the mature gradient of the Gorse, it will be seen that an earl ier stream could have flowed through the Rhos gap at its present altit ude without any difficulty. No very marked water-parting occurs betw een this valley and the streams flowing north into the Amman valley, there actually being a wide gap at about 5 20 feet abo ve O.D. between

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EVOL UTION OF THE NEATH-TAWE DRAI NA GE SYS TEM.

543

Bane Cwm Helen and Mynydd Ucha f, opening out to the almost flat common of Gwaun Cae Gurwen. On the slopes of Bane Cwm Helen, there rises a little stream, the Garnant, which plunges into this gap in an easterly dire ction. After flowing across the gap for about 2 0 0 yards, it turns northwards and flows down from it in a deep narrow vall ey (crossed by two viaducts ) to Garnant, where it joins the Amman. At th e confluence with the Amm an , a delta has been deposited . The Amman has two right-hand tributaries joining it in this neighbourhood; namely . the Pedol and Garw st reams. The Pedol rises in the Millst one Grit hill s to the east of Tair Cam and flows at first south-south-westwards, then pract ically north-south and finall y again south-south-westwards to join the Amman at Garnant. where it has formed a well-m arked delta of very coarse gravels. From 10450 feet to 1,150 feet above O.D. the Pedol valley is fairly wide and the gradient not as steep as further downstream. At 1,150 feet a knickpoint occurs in the thalweg of the river (see PI. 35, A). Near Bryn Pedal, at 750 feet on the left side of the valley, a terrace covered with boulder clay is to be seen in the Lower Coal Series. Along the dir ect ion of the middle course of the Pedol, the Gwaun Cae Gurwen gap can be clearly seen directly in lin e. A min or kni ck-point occurs in the river's thalweg at about 650 feet above O.D . The P edol occupies a drift-covered valley in th e middle part of it s course. The Nant Garw rises in the Bla ck Mountains in Millstone Grit country to the south-cast of the Carboniferou s Limestone hill of Foel Fawr. It flows in a general southerly dir ection pa rallel to , but about half a mile west of, the Cwmlly nfell Fault. Th e valley is st eeply gra ded like that of the Pedol, but is more ext ensively covered with bould er clay of local origin . Kni ckpoints occur at compara ble alt it udes t o the Pedal , namely , at 1 ,2 00 feet and 750 feet respecti vely , above a.D. Opp osit e the point where this strea m join s the Amman is the east ern part of the wide Gwaun Cae Gurwen gap . A stream having its Source in the upper part of the pr esent Pedol valley flowed in a general south dire ction; it had an eastern upper tributar y, possibly flowing west of south. It flowed slightly west of Bryn Pedol {where a terrace is now seen ) and through the Gwaun Cae Gur wen gap. Near the latter place it was joined by another tributary coming from, the dir ection of the Nant Garw valley. There is no evidence th at thi s latter stream eroded a valley as big a s that of the original Pedol valley. In both valleys there a re knick-points at ab out 7 0 0 feet above a.D. which mark the height by which they are hanging above the Amman a t the present time.

544

R. O. JONES,

Both these streams were captured by the back-cutting of the Amman along the soft Lower Coal Series at the base of the Pennant Sandstone. Since their capture the Garnant has developed as an obsequent stream. As with other initial streams in the present Neath, Tawe and Loughor basins, the original drainage through the Rhos gap of the rivers Twrch-Cwmdu and Pedol-Gorse was mainly northsouth, suggesting the same general direction of the slope on which this early drainage originated. On this analogy the drainage about Gwaun Cae Gurwen and Cwmllynfell evolved on an emerging land surface; the present Upper Clydach, fed by streams (the Garw and Pedol) now diverted into the Amman-Loughor system, thus originally continued south-eastwards via the Rhos (Alltwen) gap to form the upper part of the present Clydach (Neath) river. Again, the Upper Twrch, with the Gwys and the upper Llynfell, before diversion to Gurnos and the River Tawe, flowed along the Cwm Du valley and across the Rhos (Gellinudd) gap to form the headwaters, confluent with the one-time Upper Clydach, of the Clydach (Neath) river (see PI. 33). There is some evidence that there was a stand-still in the emergence of the land, upon which this system developed, at about 550 feet to 600 feet below present sea-level. This is suggested by terraces on the south side of March Hywel above Rhyddings at 625 to 650 feet above O.D. ; on the north side of Mynydd y Drumau, and at Craig Llangiwg and Pen-y-lan Farm above Pontardawe, at 650 to 700 feet in each case. Terraces probably related to the period of planation of the 400-foot platform are seen at such points as Glynmeirch Farm near Pontardawe, and south of Cilybebyll to the west of Plas Cilybebyll, at 450-500 feet above O.D. (Part of the latter terrace is driftcovered, the present-day surface being over 500 feet). Since the base of the Rhos gap (in solid) is 253 feet above O.D., it is clear that drainage across it continued after the period of planation of 400-foot platform had been followed by uplift. (b) The G i e d d - D u I a isS y s t em. The Giedd has its source at practically the summit of the dip-slope of the Old Red Sandstone escarpment within a short distance of the head-waters of the Twrch. It flows south-south-west for the upper portion of its course, becoming more north-south in direction farther downstream. Where it crosses the outcrop of the Carboniferous Limestone, the drainage becomes underground, but after the water reaches the surface again (through a Millstone Grit covering), the river flows in a fairly wide mature valley. At about 700 feet above O.D. it becomes rejuvenated (see PI. 36, B) and the valley sides become steep in their lower part, forming a deep narrow valley within a wider high-level one.

EV OLU TI OX OF T H E NEATH- TAWE DRAI NAGE SYSTE M.

545

Th e st ream is frequ ently flowing on drift until the point of confluence with Nant Llyn. From here to its junct ion wit h the Tawe the rejuvenat ed river is flowing on solid, although drift is occasiona lly pr esent on either side of the valley. Where the river ente rs the Tawe va lley, there are traces of a delt a as in the case of other rivers such as th e Twr ch, Cwmdu and Upp er Clydach. The Rhos Common gap, on the opposite side of the Tawe valley, is clearly in line with the main part of the Giedd valley. Th at the Giedd once flowed through th is gap has already been suggested by the writer [12J. On the south side of the gap flows a very sma ll strea m in a wide ma ture valley-a clear case of a misfit. At abo ut a mil e above Crynant, this st ream joins the Dul ais river, which is a tributary of the Neat h. The Dulais has it s source in the drift- and peat-covered flats on the Lower Coal Series near Onllwyn, and flows betw een the hills of Lower Coal Series of Mynydd y Drum and the Pennant heights of Hirfynydd in a south-south-west erly direction, lat er becoming almost du e sout h between the latter mount ain and Myn yd d March Hywel. It flows in a well-developed valley, the solid generally covered with drift below Seven Sisters; frequ ently the stream is flowing on its own allu vium . In the drift, at times, well-developed terrace-like features are seen. Above Seven Sisters th e va lley itself is free of drift, although there are widespr ead pat ches of it on th e flat areas around. Bould er clay on Rh os Common shows that ice passed through the gap. At a point downs tre am of Cefn-coed Colliery, there is a most marked hillock of drift. In the neighbourhood of Cilfrew, the river shows signs of rejuvenation, its valley being drift-free for a short distance. It join s the Neath at Aberdulais. On either side of the valley, between March Hywel and Hirfynydd, there is a te rrace at a height of 650 t o 700 feet ab ove O.D . In lin e with the lower pa rt of the Dulais, on the other side of t he Neath valley, is a very marked gap (the Bw1ch) between Mynydd y Gaer (1,028 feet ) and Foel Fynydd au (1,218 feet ) at a height of about 750 feet a bove O.D. To the south is the Avan valley. It is clear that the origin of this gap must be ascribed to a time when the land was at a much lower level , when it was occupied by a stream running north to south and having its source probably in the dir ection of the Dulais. . Thi s stream would ha ve graded it s valley in the region of Bwlch, more or less to the sea-level of that time and , therefore, the land her e mu st ha ve then been relati vely 700 feet or so lower.

R . O . JONES,

A stream flowing south through the gap apparently formed part of the early Avan dr ain age. It was also part of the original valley along the Dulais-Giedd course, an early high-level stream along that direction having drained through the gap before the River Neath came into being. Since the Crythan (a left-han d tributary o f the Riv er Neath) and the Pelenau (a righ t-hand tributar y of the Avan) are connected with the drainage hist ory of this area , they are discussed here. The Pelenau rises on the west side of the Pennant region of Cefn Mawr and flows, first in a sout h-west dire ction and then in a west-south-west direction, across Pennant Sandstone as far as Gefail (or Efail) Fach, where the river turns sharply south-south-east to join the Avan at Pont-rhyd-y-fen . At first the Pelenau flows in a fairly wide valley, but soon , through Cwm Blaen Pelenau, the valley becomes deeper and in its south-south-east course, the river flows at the bottom of a ver y deep narrow trench-like gorge, the valley sides rising from about 250 feet to goo feet on the north, and to over 1,0 0 0 feet precipitously on the south . At 780 feet there is a knick-point in the river' s thalweg, whil e re juven ati on is seen to set in at a bout 350 feet (see PI. 35, B). In the major part of it s westsouth-west course, the rive r flows on drift. Thi s is mainl y gravelly and of local origin . To the sout h of the river , there is a great deal of bould er-clay , rising to heights of goo feet above O.D . at Myn ydd P en-Rhys. In the upper part of it s east south-eas t course , the vall ey ha s some glacial gravels, but farther downst ream it is free of glacial drift , where, owing to its small gradient, it flows on its own alluvium. A right-hand tributar y, the Gwenffrwd , flows southwards across Pennant Sandsto ne , frequently on dr ift ; it also shows rejuvenati on in it s lower cour se, a kni ck-point being presen t at 650 feet. Near Twyn- y-Pandy , a small right-hand t ributar y of the Pelenau plunges down in the vall ey side from Foel Fynyddau and has cut a ravine in the mountain side . Near it s entry into the Pelenau, a mineral-railway bridges the stream and between this bridge and the main river there is an extensive a lluvial fan produced by the small stream in flood periods within recent times. A similar ravine and fan occur with a little t ributary of the Crythan which flows down the west side of Fo el Fynyddau just to the north of the Bwlch gap. South-westwards along the general line of Cwm Blaen Pel enau , a small gap occur s at Gyfylchi betw een Mynydd Pen-Rhys and Pen Moelgrochl efr at a height of about 750 feet . A mu ch more marked gap (th e Efail Fach) at a height of 500 feet above O.D., occur s between the Pennant San dst one

EVOL UTION OF THE N E AT H - TAW E DRAINAGE SYSTEM.

547

hills of Cefn Morfydd (1,000 feet) and Foel Fynyddau . The lower part of this gap , and extensive areas to th e northwest, are covered with boulder clay, and higher ar eas to the south, with glacial gravels. The solid floor of the gap is, therefore, well under 500 feet, being probably about 420 feet. A small stream flows eastwards from the direction of the gap and plunges down to join the Pelenau at Efail Fach. In th e neighbourhood of Crynallt F arm, the Pennant Sandstone sta nds up above the level of the boulder clay , forming a hill of over 500 feet. The Crythan Brook rises to the north of the Efail Fach gap and flows at first northwards in a small valle y. Before reaching Crythan Farm, it turns west-s outh-west and flows on glacial drift in a wide valley for a time, and then on solid rock in a rejuvenated valley to join the Neath river near Felin Crythan. (c) His tor y 0 f the G i e d d - D u I a i s - Pel e n a u Dr a ina g e . It appears that the drainage in this area developed on an emerging tilted plateau surface, th e predominating direction of tilt being north to south as in areas farther to the west. On this surface the main drainage established itself through the Bwlch gap very early in the hist ory of the uplift. Before the 600-foot platform st adium the Bwlch gap was abandoned by the drainage of the present cycle , this being deduced from the fact that the present gap level in solid is about 720 feet above O.D . As tributary to this earl y dr ainage and before the abandonment of the gap, there developed a subsequent stream from the dire ction of Pontrhyd yfen . By the time of planation of the 600-foot platform, a subsequent strea m of the Twrch-Upper Clyd ach-Clydach (Neath) river had work ed it s way back towards Aberdulais. This tributary-an early part of the River Neath-s-cap tured the Giedd-Dulais st ream that had been draining through the Bwlch gap, and consequently left the gap dry. Possibly the change of slope in the Dulais valle y side, on Mynydd March Hywel, at 1,000 feet above O.D., represents a terrace that resulted from the rejuvenation of the riv er upon this capture. I The Giedd-Dulais stream at this period was draining through the Rhos Common gap from the Old Red Sandstone and Carboniferous Limestone country of the eastern part of the Black Mountain, but it is unlikely that the drainage has not developed further headwards since this early part of its history. The upper part of the Dulais evolved as a tributary of this stream. There is no definite evidence that the Dulais ever reached back as far as Nant Llech Pellaf and that the latter was captured I It is, of course, not sugg ested th at th e bed of th e river that flowed th rou gh th e Bwlch ga p was a t th e sa me heigh t a s th e terra ce referred to at tbi s poi nt.

R.

0. JOXES,

by the Tawe-Pyrddin-Cynon stream to be later captured by the Tawe itself. This, however, is a distinct possibility. With the development of part of the Tawe along a line of disturbance, a tributary of the Giedd-Dulais river was captured to become the Lower Twrch, and the Giedd itself became a Tawe tributary, leaving the Rhos Common gap. The rejuvenation in the Giedd, consequent upon its capture by the Tawe, is seen in the river's gradient at 750 feet (see Pi. 36, B). A higher knick-point at 1,425 feet is the result of a change of base-level before the Tawe capture which is possibly connected with the rejuvenation of the system upon capture of its original lower part by the Neath river, or with rejuvenation following uplift of the (present) 600-foot platform. The rejuvenation in the Dulais (see Pi. 36, A) is due to the more rapid erosion of the River Neath through its greater volume of water and the line of disturbance, but it may be partly due to the scooping out of the latter valley by ice and its overdeepening in glacial times. In the neighbourhood of the present town of Neath, and after the capture of the Dulais-Bwlch gap stream by the lower Neath, there apparently developed another tributary of this drainage on the eastern side of the new valley. This tributary worked its way back in the direction of the Crythan-Pelenau area to drain the Cwm Gwenffrwd region through the Efail Fach gap. The drainage in the Avan area followed the south-southwest direction of what appears to have been a tributary which developed to the original north-south stream (i.e., the river from the Giedd-Dulais region through the Bwlch gap). The tilting of the original plateau surface, on which the drainage developed, apparently resulted in that direction showing itself in the more north-south trend of the river as it cut back. The river was situated east of the present valley at Pontrhydyfen, as can be seen from the terrace at 600 feet above O.D. on the north side of Moel-y-Fen, At Craig Gyfylchi a left-hand tributary to this river developed along what may be another line of stucture similar to the Tawe and Vale of Neath lines to capture the headwaters of the Llynfi (the Corrwg and that part of the Avan north of Taren Rhiw Llech) that previously flowed through the Blaencaerau gap. The lower Avan (that is, the Avan below Craig Gyfylchi) to which the upper part of the Avan apparently developed as a tributary, developed another stream which seems to have cut its way back between Foel Fynyddau and Mynydd Pen Rhys, forming at first a col in the Pelenau-Crythan valley side. That this tributary existed fairly early in the history of the drainage is indicated by the fact that the meander caused by it in the Avan's course developed when the river was 600 feet

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

549

above present O.D. as is shown by the shoulder on the hillside of Moel-y-Fen, There is some doubt whether the headwaters of the then Pelenau-Crythan stream were captured by this Avan (Lower Pelenau) drainage before the Western Drift Glaciation. The writer is of the opinion that they were. His reasons for believing this include the fact that the rejuvenation of the Crythan with the rapid down-cutting of the Neath has resulted in but a very small valley, whereas if the major part of the Pelenau waters were entering the Crythan until Glacial times, its rejuvenation would undoubtedly have formed a bigger or deeper valley. In any case there is evidence of ice having over-ridden from the Neath drainage into the Avan along the lower part of the Pelenau-between Cefn Morfydd and the Crynallt outcrop of Pennant Sandstone. This over-riding occurred during the Western Drift Glaciation' and with the recession of that ice sheet, an overflow channel developed here due to the damming of the Neath valley by the Western Drift. This overflow could have cut down the col (if one was still in existence) enough for the Upper Pelenau to have flowed along it into the Avan, assuming the diversion had not already taken place; and during the long inter-glacial period, there would have been enough time for the gorge between Efail Fach and Pont-rhydyfen to have been cut to practically its present level. There is evidence that the depth of the gorge in Newer Drift times was more or less as it is today, the gravels of that ice being present near the river bottom at Twyn-y-Pandy, three-quarters of a mile downstream of Efail Fach. These gravels contain very few Old Red Sandstone and Carboniferous Limestone pebbles and are mainly of local origin. The absence of a drift of more foreign origin in the deeper part of the valley does not prove that the valley did not exist in Western Drift times, as local ice would in any case be filling its lower portion during this glaciation. It is clear, however, that the Lower Pelenau could not have attained its present dimensions by the overflow during the retreat of the Irish Sea Ice as the overflow channel (between the Neath valley and the Pelenau-s-the Efail Fach gap) is not graded to the same base-level as the lower Pelenau. The dimensions of the Gyfylchi gap shows that no great amount of water passed through it before the headwaters were captured by the Pelenau to become the Cwm Blaen Pelenau part of this stream. The knick-point in the Pelenau at 780 feet above O.D. possibly marks the upper limit of the grading of the I There is coarse glacial material on either side of Mynydd Hawdref and south of it, linking the Neath and Pelenau valleys. The drift contains a great deal of Old Red Sandstone pebbles and reaches a height of 1,000 feet above O.D. (see p. 532), The presence of so much Old Red Sandstone pebbles at this great altitude proves that it cannot belong to the Newer Drift, the southern limit of which was not far away.

550

R. O. JONES,

drainage through the Efail Fach gap into the Crythan and Neath valleys. There is evidence that ice over-rode the GyfyJchi gap at one period. (d) The Lower C I Y d a c h - L I a n - Lower Taw e S y s tern. In an earlier publication [12J it was mentioned that the Lower Clydach upstream of Craig Cefn Pare once flowed through a gap at Rhuddwen (Rhydd-Waun)" between Mynydd Gelliwastad and Mynydd y Gwair. A left-hand tributary of the Lower Clydach, Nant y Capel, which rises on the south-west side of Mynydd Gellionen and flows southwestwards in a rather wide drift-covered valley, also flowed through the Rhuddwen gap. Forming a minor valley within the south-western part of this gap is a very small stream which flows south-westwards to join the River Llan at Rhydy-Pandy. The River Llan itself rises on the west side of the summit point of Mynydd y Gwair and flows south-south-westwards in a small valley through a wide expanse of drift-covered country, the drift being clayey in the upper part, but becoming more gravelly downstream. It is mainly of local origin, but Old Red Sandstone pebbles are occasionally found, particularly in the higher part, suggesting over-riding of ice in that area at heights of goo feet above OiD. [15c]. About a mile south of Cynghordy the river swings round to an easterly direction until it joins the tributary at Rhyd-yPandy. The stream in its easterly course flows in a minor valley on drift, while there is a major drift-covered valley-like depression continuing in a general southerly direction. At Rhyd-y-Pandy the Llan takes on the direction of the little tributary from the Rhuddwen gap and so flows south-southwestwards and then south-westwards towards Bryntawod, about half-a-mile north of Llangyfelach, leaving a gap on the left-hand side of the valley almost in line with its upper (more or less southerly) course. The gap runs from Pontlasau through the south-west part of Cwm-Rhyd-y-Ceirw to Morriston, and will be referred to as the Morriston gap. This gap has a maximum height of about 340 feet above a.D., but is extensively covered with drift and so the level of the solid rock is probably under 300 feet. The land to the north-west of the gap rises to the Pennant Sandstone mountain of Gelliwastad (700 feet) and to the south to 500 feet near Clase House (on drift) and at Llewelyn Park, Morriston. The Llan to the north of Llangyfelach still flows in a broad valley which is more like a wide basin than the valley of a small I

This was incorrectly called Rhydygwin in the

1931

paper.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

55r

stream. The river itself meanders across this tract, flowing at times on its own alluvium. The whole area is still extensively covered with drift, which is frequently gravelly in character, particularly west of Llangyfelach. It forms a morainic ridge from near Tirdonkin (Tirdwyncyn) to Tredegar Farm, through which the river has cut a gorge. In looking along the river in the wide hollow-like valley north-west of this gorge, it is frequently difficult to determine the point where the stream escapes and, therefore, why the hollow is not a lake. On examining the drift in the neighbourhood of Bryntawod, it is found to be a fine sand that covers an extensive area. This sand is so widespread that the farmers always refer to the 'sandy nature' of their soil, while the name of the hamlet (Bryntawod or Bryn-tywod) means 'the hill above the sand.' The writer sampled two horizons near the railway station, the higher being separated from the lower by a thin layer of clay at this point. These samples were submitted to Dr. ]. Cedric Griffiths, who kindly analysed them and found that they contained only minerals of Welsh origin, and that on mineralogical evidence both belong to the Newer Drift. His conclusions were either (r) that the lower sand is derived more directly from a mixture of Coal Measure Sandstone, Millstone Grit and Old Red Sandstone, while the upper is restricted to Pennant, or (2) that the upper is a very much re-washed example of the lower material in which mineral sorting has been more complete. This sandy flat is about 200 feet above O.D. and around it alluvial fans are traceable at a height of nearly 300 feet near Dorglwyd Farm, near Felin Wen (the Mill) and a few hundred yards south of Aber-gelli. These alluvial fans and the sand (frequently covered by the river's present alluvium) in the central part of the hollow, prove that this area was once a lake in late glacial or early post-glacial times, the lake overflowing across the morainic dam in which it has now cut a gorge at Penlle'r-gaer. It is interesting to note that if the lake level had been 40 feet or so higher it would have overflowed along the Morriston gap into the Tawe valley. Beyond this gorge the Llan flows, mainly on drift-covered Lower Coal Series, to join the River Lliw. In line with the north-south course of the upper part of the Lliw there is an extensive drift-covered flat tract of land southwest of Rhos Fawr Farm suggestive of a pre-glacial gap in the solid rock. The height of this feature in the boulder clay is 460 feet above O.D., so that the height of any gap in the solid cannot be over 430 feet. At 480 feet there is a knick-point in the thalweg of the Lliw. To the north of the Town Hill Pennant Sandstone ridge at Swansea there is a very well-marked dry valley running west to PROC. GEOL. Assoc., VOL. L., PART 4, 1939·

36

552

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east through Cockett Station-Cwmdu and Br ynhyfr yd. Thi s is a gap (the Brynhyfr yd gap) between the Loughor and Tawe drainage. It is now largely filled with drift , but even including this deposit its highe st level is only 340 feet above O.D . The thickness of the drift at Cwmbwrla is mentioned by the Officers of H.M. Geological Survey as being 36 feet [15d]. The gap is a water-cut cha nnel, but its form has been modified by ice passing through it since its original formation . There is a minor overflow cha nnel across the Townhill ridge, at about 400 feet above O.D . at Cockett , as mentioned by the Geological Survey [15d]. With the recession (and possibly advancement) of the Irish Sea Ice, there was undoubtedly an impoundiug of the waters in the Llan-Loughor area, but there are no definite traces (in the form of strand lines, etc .) of the existence of the high-l evel glacier lake that overflowed along the Brynhyfryd gap. As the ar ea was extensively covered with ice in Newer Drift times it is not surprising that such features as strand lines are abs ent. With the further recession of the Western Drift a lower level glaci er-lake overflowed through the gap at Dunvant , which shows clear evidence that it is water-worn. The base of the Dunvant gap is about I70 feet above O.D. To the south of the Dunvant through-channel is the Clyne valley, the source of that river being well to the west of the gap on its south side. The fact that the Brynhyfryd gap has such a rounded shape and is drift-covered indi cates that it developed to its present form through the over-riding of later ice (Newer Drift ?) along it . This chann el is west -east similar to the Efail Fach gap , part of the Ely vall ey, and the Nantgarw gap (between the Taff valle y at Nantgarw and the Rhymney valley at Caerphilly). It seems that all these gaps ma y have been formed or modified with the recession of the Older Drift and that during stages in that recession there was a drainage of a series of glacier-lakes eastwards, but these lakes were not , of cour se, all in existence simultaneously. The earliest element of the drainage in the lower Tawe area appears again to have been that of a stream dev eloping in a north to south direction on an emerging plateau tilted southwards. It had its source in the upper part of the Lower Clydach t errain and flowed southwards through the Rhuddwen gap along part of the Llan valley through the Morriston gap and southwards through the south limb of the Pennant Sandstone between Town Hill and Kilvey Hill (see PI. 33). The main stream had tributari es, including a right-hand one near Pontlasau which had upper streams draining from the dire ction of Mynydd y Gwair (now followed by part of the

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

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Upper Llan) and from the Lliw region (via gap near Rhos Fawr Farm), and a left-hand tributary near Rhuddwen from the direction of Nant y Capel. A subsequent stream developed near Morriston and worked back along the line of the Cribarth Disturbance (the southern limit of which has been traced to Morriston, but not farther south [20J). This subsequent stream, the Tawe, as it cut backwards, may have captured the headwaters of a high-level stream which combined with tributaries from Mynydd y Drumau (Nant Bran and the Craig-y-Perchyll stream) probably flowed in the direction of the present Crymlin Bog. A tributary of the Tawe developed along the upper part of this captured stream and with the rejuvenation consequent upon the Tawe capture it worked headwards ultimately to deflect the Nant-y-Capel and the Lower Clydach into its course. The Rhuddwen gap was thus abandoned by the two last-mentioned rivers, while the capturing of the earlier drainage and the swift down-cutting of the Tawe since have resulted in a marked rejuvenation in the Lower Clydach in the neighbourhood of Craig-cefn-Parc. The formation of a deeper valley inside the older wider valley has left a clearly-defined terrace at 500 feet above O.D. between the river and Mynydd Gellionen at the foot of Bane yr AlIt. This terrace is covered with boulder clay. After the capture of the upper waters of the initial river by the lower part of the Lower Clydach, the Llan still flowed into the lower Tawe through the Morriston gap. Having lost most of its headwaters, it was now a mis-fit unable to denude its valley rapidly. Possibly the Llan also lost some of its drainage to the Lliw and the Loughor in the neighbourhood of Rhos Fawr Farm. A tributary of the Loughor cut backwards along the strike of the Upper Coal Series and across the Pennant Sandstone inlier along a fault near Penlle'r-gaer to capture the Llan, which thus abandoned the Morriston gap soon after its headwaters had been diverted from the Rhuddwen gap. Hence the difference in heights between the Rhuddwen and Morriston gaps is very small. Having once been a region where three streams joined (the upper part of the Lower Clydach, Upper Llan and part of the Upper Lliw), and being on Upper Coal Series, the valley near Llangyfelach became wide and open, while downstream where an inlier of Pennant Sandstone occurs the river could only cut a narrow valley. The Lower Clydach entering on the western side of the Glais dam probably kept that from being a complete barrier and from forming a lake in the Tawe valley [15d], although the wide nature of the latter valley north of the Mond Nickel Works

554

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has the appearan ce of a silt ed-up lake and wha t are apparent ly glacial grav els form te rraces at Ynys ymond and n ear Gwra chy-llwynau F arm. There is no indication that t here was a pre-glacial valley where the Brynhyfryd cha nnel now exist s. Becau se t his form ed t he dip slope of the sout hern Pennant Sandstone outcrop, however, it is natural t ha t t he land would ha ve been more eroded (especially if covered with Upper Coal Series) t han t he outcrop of the harder rock barrier to the south. Th eref ore, a n eas t to west overflow from a glacier lak e would be expected to follow this line. The development of the T awe, therefore, is that of a north to south consequent stream forming a valley across the southern out crop of the Pennant Sandstone and developing a subsequent tributary along a disturbed belt. The lower part of the Tawe (north to south in direct ion an d not along a line of disturban ce) is a consequent stream whil e the re st of the valley, the N.N.E.S.S.\ V. course, is a subsequent stream . There is an indication of a zoo-foot terrace on the west side of the valley in the gorge which the river ha s cut through the Pennant escarpment an d at pl aces on the south side of Town Hill.

III. GENERAL DISCUSSION AND SUMMARY. There are t hree maj or prevailing dir ecti ons followed by the rivers in that area of the South Wales Coalfield east of the Loughor va lley, nam ely , N.\\' . t o S.E .. N . t o S. and N.N.E. to S.s.W. Th ese directi ons are not strictly adh ered t o by every stream classed as belonging to one or other of the grou ps; for exa mple, a stream which is referr ed t o as a north-south typ e may have a dir ection approxi ma t ing N .N.W.-S.S.E . While it is not maintain ed , therefore, that one or other of these three direct ions is followed by all rivers in th e region, it is maint ain ed that all the st rea ms can be definitely linked up wit h a system having one or ot her of the three direct ions. The north-south streams form a maj or group in the weste rn par t of the area while the n orth-west t o south-east streams form a similar group in the easte rn part of the region. These gro ups belong to drainage sys tems which developed mor e or less concur rently. As they show no relation to the geological struct ure along part at least of their cours es, the presence of tw o such drainage directions in tw o different regional ar eas definit ely points to an essential initi al difference in these region s, but which was n ot connect ed with t he present geological st ructure . This is thought to be a difference in the tilt of a plateau up on which the strea ms originally developed , namely , a north-south warping in t he west ern par t and a N.W .-S.E . warping in the eas te rn

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

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part, these general slopes merging one into the other in the neighbourhood of a line from Maesteg to Brecon. On this warped plateau as it emerged from the sea-floor there developed part of the present drainage system. The primary consequent streams occasionally took advantage of lines of structural weakness when such lines occurred parallel with the direction of the tilt of the plateau, but lines of weakness cutting across the general slope of the planed surface did not influence the initial drainage to any great extent, though they were often followed by tributaries that developed early in the drainage history. As these points are well brought out by a study of the Twrch-Upper Clydach-Clydach (Neath) system and of the upper Taff and its original tributaries, it is proposed to discuss only these two groups.


The North-South and North-West-South-East Streams. (a) The T w r c h - U P per C 1 Y d a c h - C 1 Y d a c h ( N eat h) d r a ina g e. This system belongs to the northsouth group. Part of the stream took advantage of the north-south Duffryn fault and to some extent of the Rhyddings Fault. On the other hand the Upper Clydach developed between two faults, having no structural assistance in the major part of its course. Later, however, it developed a tributary along a line of disturbance at Pontardawe to capture the TwrchCwm-du stream. Still later, the whole system was beheaded by subsequent rivers developing along definite structural lines; the Tawe (along a line of fracture), the Amman and the Lower Twrch (originally flowing along the junction of the Pennant Sandstone and the Lower Coal Series). It therefore appears that in the case of the north-south drainage, lines of structural weakness were not utilised by the rivers except at a very limited number of points. (b) The Upper Taft and its original streams. The Vale of Neath disturbance did not affect the consequent drainage direction of the Taff as it (the disturbance) was at right angles to the general tilt of the surface and, the surface having been planed to a platform, there were no fault scarps. Subsequent tributaries took advantage of such a line of disturbance as, for example, Nant Callan and Nant Car Fawr. With dissection of the plateau as it emerged and the development of tributaries, the structural features certainly aided in determining the direction of some of the drainage of the area, for example, the middle Pyrddyn- Rhigos-Cynon stream developed along the junction of the Pennant Sandstone and the Lower Coal Series. The main stream here, however, had developed along the major tilt of the plateau surface which, in this ground, had

R. O. JONES,

a more north-south direction (being west of the Maesteg-Brecon line) ; examples are provided by Nant Llia aud DringarthCarn-yr-Arian-Penderyn stream, the Upper Neath (Nedd Fechan) and the upper tributaries of the original Pyrddyn-Rhigos stream in the Glyntawe-Pen-y-cae area. (c) G e n era I Con cl u s ion. From a study of these drainage systems and others it follows that whereas the streams frequently show no apparent relation to the structure (that is, the rivers are superimposed) there has been occasionally an adaptation to structures in the consequent drainage. The fact that the rivers at times show a definite relation to geological features appears to the writer to suggest that there was little or no Cretaceous rock covering the region when a great deal of the present cycle of drainage developed. This conclusion is not in agreement with earlier writers such as Strahan. An emerging marine platform warped during the earlier uplifts would have sufficed for the development of most of the consequent systems mentioned and would explain how the structural features occasionally aided the rivers' development or caused a marked change in their general direction (as in the Pyrddyn-Cynon region).

(2) The Development of the North-North-East-SouthSouth-West Rivers. These streams belong to a later period of the drainage history than that of (or at least than the major part of) the north-south, and north-west-south-east consequent streams. They developed along lines of disturbed strata as subsequent tributaries to consequent streams. This is well shown in a study of the rivers Tawe and Neath and also seems to be shown in part of the Loughor drainage and the central part of the Avan. The lowest part of the Tawe is the remains of an earlier consequent north-south stream to which the Tawe developed upstream as a subsequent tributary from the point where the north-north-east-south-south-west line of disturbance came into contact with the consequent valley. The Tawe developing along this line of fracture (the southern extension of the Cribarth Disturbance) apparently captured first the headwaters of a number of small high-level tributaries of a little north-south consequent stream that had developed to the east. One of these, after its capture, appears to have caused the .growth of the lower part of the Lower Clydach as a right-hand tributary at Clydach-on-Tawe and the ultimate deflection of the upper streams of the earlier consequent drainage (Nant y Capel and the upper Lower Clydach) into it here. The little tributaries of the initial north-south stream to the east also probably con-

EVOLuTION OF THE NEATH-TAWE DRAINAGE SYSTE:\1,

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tributed to the production of the wide Tawe valley above Clydach-on-Tawe. Before the Tawe cut back as far as Pontardawe, there is evidence [12J that this line of fracturing had already caused a deflection of the Twrch-Cwmdu-Rhos (Gellinudd) stream along it into the Upper Clydach-Rhos (AIltwen) stream, probably by the development of a subsequent stream of the latter along this disturbance. The back-cutting of the Tawe caused the Upper Clydach to become its tributary and to abandon the Rhos (Alltwen) gap, from which a small obsequent stream (Nant Llechau) now drains into the Tawe. With further cutting upstream the Tawe captured a tributary of the Giedd-Dulais (Neath) system which had already developed as a strike stream near the Pennant Sandstone-Lower Coal Series junction well back in the direction of the Upper Twrch-Cwmdu drainage. I Upon its deflection into the Tawe this stream was rejuvenated and, as the Lower Twrch, captured the Upper Twrch, Gwys, and Llynfell streams which were the headwaters of the TwrchCwmdu system. Further upstream again the Tawe captured the Giedd, the main headwaters of the Giedd-Dulais (Neath) north-south consequent stream. The continued headward cutting of the Tawe caused the capture of the Haffes and the earlier higher level Tawe north of Pen-y-cae (north of the main disturbance), both of which formed consequent tributaries of the Pyrddyn-Cynon system, here having a more north-south than a north-west-south-east direction. The knick-point in the Haffes thalweg at 1,500 feet above O.D resulting from this capture is clearly seen in PI. 36, B, and terraces showing earlier higher levels of the Tawe valley are seen in the Craig-y-Nos neighbourhood. The River Neath also developed as the tributary of a north-south stream, namely, the Twrch-Upper Clydach-Clydach (Neath) river. The latter consequent stream, of which the lowest part of the Neath river itself was part, apparently developed a tributary early in its history, while some of the north-south drainage was still in its infancy. This tributary worked its way headwards fast enough to cause the capture of a very early stream that seems to have flowed from the Dulais direction through the Bwlch gap. A subsequent stream to this system developed along a line of disturbance until it reached the upper tributaries of the Taff drainage basin and captured the earlier streams that flowed through the Rhigos gap, with the formation of an obsequent tributary, the Sychryd. I The fact that the Rhiwfawr (Twrch-Cwmdu) gap is only 50 feet higher than the Rhos Common gap clearly denotes that they were abandoned very nearly at the same time. Therefore the Giedd-Dulais tributary in the Lower Twrch neighbourhood must have already been welldeveloped before its capture by the Tawe for it to have beheaded the Twrch-Cwmdu system so Soon after its deflection into the north-north-east-south-south-west drainage.

558

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A study of the Tawe and Neath rivers, therefore, clearly indicates that the two north-north-east-south-south-west streams were later developments than the other two groups of streams in this area. Their tributaries and their own lower and upper courses belong to one of these other earlier drainage groups, while their north-north-east-south-south-west course may be the linking up of two or more subsequent streams that developed independently at first along these lines of disturbances; for example, the original Tawe between Ynysmeudw and Pontardawe with that farther downstream.

(3) The Towy and Usk Tributaries Associated with the Rivers being considered. The area under discussion in this account is bounded on the north side by the Towy and the Usk drainages and there is evidence that these have captured part of the headwaters of the rivers in the region, and certain tributaries of the former rivers are still actively beheading some of this drainage at the present time. Tributaries of the River Sawdde, a left-hand tributary of the Towy, are cutting headwards into the Loughor and Tawe drainage; for example, Afon Clydach (Sawdde) is working back in the direction of the River Garw, a tributary of the AmmanLoughor river, and Afon Mihertach is capturing the headwaters of the Twrch Feehan stream which is a tributary of the Twrch-Tawe drainage. In both examples the streams are cutting into the Old Red Sandstone scarp. Without necessarily putting forward any suggestions regarding the development of the Usk drainage itself, it is clearly seen that this river and its tributaries are beheading part of the drainage to the south [1]. The Afon Senni has a very deep trench-like valley to the north of the Afon Llia (River Neath tributary) and has possibly captured the headwaters of the latter; in any case, it is the major valley and any further river capture in this area will certainly mean the yielding of parts of the Llia to the Senni drainage. Another Usk tributary, the Crai (Cray), has captured some of the headwaters of the Tywyni, leaving a high-level gap at Bwlch Pen Rhudd between Cefn CuI and Fan Gihirych, It is interesting to note that the formation of the Cray Waterworks by the Swansea Corporation has resulted in the conveying of some of the Crai's headwater by pipes through a tunnel back in its original direction! Afon Hydfer is working headwards towards Llyn-y-Fan and the source of the Tawe.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

559

The south to north direction of a number of the Usk tributaries such as the Upper Usk, Hydfer, Crai, Senni and Upper Tarell that are clearly capturing, and have already captured, part of the drainage to the south, suggests strongly that there was an early north to south drainage in this area and that these streams are obsequent to the earlier drainage. The drainage region described in this paper is, therefore, seen to be gradually decreasing owing to the activity of the Towy and, particularly, the Usk. The streams of the area have generally a shorter course to the sea than the Usk or the Towy from the points where the capture is taking place. Yet the NeathTawe system being superimposed for a great part of its course meets with more resistance to swift denudation than do the waters of the Usk or Towy which flow for the most part along the strike. As the tributaries of these two streams are northward flowing, they were helped in their later back-cutting by the development of corries during the Ice Age on the scarp slope of the Old Red Sandstone.

(4) Platforms. Besides the evidence of an earlier warped platform upon which the present streams developed, there is some proof of high level horizontal platforms in this area [7b, 8, 9 and 10J, but it seems to the writer that those over 600 feet above O.D., whether they were marine or subaerially denuded, are merely notching the general land surface and their formation has not had much influence on the drainage. When, however, we get to the 600-foot and lower platforms, far more definite evidence of their existence is found and there is clear indication of adjustment of the drainage to these older shore-lines. The higher level of the Bwlch gap, through which, apparently, drained the original north-south Giedd-Dulais (Neath) stream, at a point so near to the present sea-shore, suggests that the stream existed when the shore-line was over 600 feet higher than it is at present. The fact that the present head tributaries of the Neath hang above the major valley by approximately 400 feet, suggests that the former had adjusted themselves in this particular part of their course to a coast-line at 400 to 600 feet higher than the present coast-line before they were captured by the River Neath. In the eastern valleys of South Wales, which are far less complicated than the ones discussed in this work, the writer has found clear indication that the initial drainage adjusted itself to a coast-line which remained stationary at 600 feet higher than the present. There is, however, some evidence of this in the complex valleys under discussion. The knick-points in the

560

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valleys at high levels show the downstream break in the rivers, grading to this standstill in the uplift, namely, in the Nedd Feehan (Upper Neath) at 1,350 feet above a.D., in Giedd at about lAOO feet and in the Upper Tawe (in existence earlier than the N.N.E.-S.S.W. capture) at 1,300 feet.' The evidence of the knick-points is supported by the existence of high-level terraces which are again better seen in the eastern valleys, but traces of them are found in a number of places such as (a) the Dulais (Neath) valley between Hirfynydd and March Hywel (at about 700 feet above a.D.); (b) in the Clydach (Neath) valley on the south of March Hywel near Rhyddings (at 6z5-65o feet) and near the Rhos gap (see Plate 34) on the side of Mynydd Drumau (at 650-700 feet); (c) in the Upper Clydach valley at Craig Llangiwg and Pen-y-lan (at 650700 feet). The grading of the rivers above the knick-points mentioned and the continuation of such grading in the remnants of terraces referred to, plus a great deal of evidence in other rivers in South Wales between the Towy and the Usk recently studied, clearly indicates to the writer that there was a standstill in the uplift of South Wales at a 600-foot coast-line. Miller [16), however, has advanced somewhat different conclusions concerning the country farther west in Carmarthenshire and Pembrokeshire. There is testimony of a ' 400-foot platform' standstill of the coast-line in the drainage development. The knick-points mentioned as breaking the earlier grading of the drainage to the , 600-foot' coast-line, of course, mark the head point of the grading of the rivers to this later sea-level. The terrace at Glynmeirch in the original Upper Clydach-Rhos (Alltwen) valley at 450 feet above a.D. belongs apparently to the remains of the valley graded to this coast-line. The grouping of some of the lower Tawe gaps at about 300350 feet above a.D. suggests strongly that the development of the Neath and Tawe rivers along their lines of disturbance was subsequent to (or possibly during the later part of) the 400 ft. standstill. During the time the earlier river systems were adjusting themselves to the uplifts ending with the zoo ft. standstill, they were captured by the N.N.E.-S.S.W. streams (Neath and Tawe). awing to the more rapid development of the N.N.E.-S.S.W. streams they captured the upper streams of the earlier drainage before the adjustment of the thalwegs of the latter to the , zoo-foot coast-line,' and in some cases to the 400-foot platform. In other words, when the consequent streams were captured by the upper parts of the subsequent streams, their thalwegs I This point may, however, mark the rejuvenation head resulting from the deflection along its present course from its Pyrddyn-Rhigos course.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

56r

in these parts were still those being graded to the 4oo-foot base-level or graded to the 600-foot platform. Farther downstream there were possibly knick-points resulting from the zoo-foot platform base-level. This would result in these upper tributaries hanging at from 400 to 600 feet, and in some cases even higher, above the present level of the north-north-eastsouth-south-west rivers. There is considerable evidence that the rivers in South Wales were graded to the zoo-foot platform and that the last uplift (to present level) has resulted in the rivers finding their way across this platform in a meandering fashion, their actual direction being decided by minor features (e.g., in the Vale of Glamorgan). In the thalwegs of the mature rivers there is generally no clear trace of knick-points associated with the break in the curve of the river profiles at the head of the grading to the zoo-foot platform base-level or by the head of the grading to the present sea-level. The reason for this is the intense glaciation which these valleys have undergone, several now being rock basins buried in drift and completely obscuring any evidence which may exist. In the Taft (adjusting itself to the eynon) there is a knick-point at 350 feet above G.D. which may be the rejuvenation head connected with the zoo-foot base-level. There is clearer evidence in the Avan where there is a knick-point at 400 feet above G.D., which is the head of the adjustment to the zoo-foot platform, while the terrace at the mouth of this valley is a remnant of the zoo-foot platform, as is the terrace at the eastern end of Town Hill in the Tawe valley. It seems, therefore, that the earlier rivers (the north-westsouth-east or north-south) developed on an emerging warped platform. The emergent tilted platform had a standstill period, when the coast-line was relatively 600 feet above present day G.D., long enough for a great deal of the drainage to adjust itself to that coast-line. It appears possible that the platforms at higher levels (i.e., those above the 600-foot platform) merely denote very short standstill intervals in the uplift of the warped major platform. In that case these stable periods were so short that no traces of the halts are preserved in the drainage which did not have time to get adjusted to the brief standstills, although the streams were probably in existence. If the warping was part of the uplift up to the t ooo-foct standstill' it is admittedly difficult to see how these higher-level platforms are such perfect horizontal planes as claimed by other writers [8, 9, 10J. The 4oo-foot standstill was long enough to cause terracing and the grading of part of the rivers' longitudinal profile to that coast-line. It also appears that with further uplift the northnorth-east-south-south-west drainage was initiated along

56 2

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lines of disturbances. The back-cutting along these lines was apparently delayed until the tilted surface of the original platform had been severely enough dissected to allow the development of subsequent streams at angles to the original warp. Before the planation of the 200-foot platform, but later than the initiation of the 400-foot platform (possibly during a ' resting' point in the uplift at 300 feet above present O.D.) the northnorth-east-south-south-west streams asserted themselves sufficiently to cause the capture of many north-south streams and possibly the headwaters of some north-west-south-east streams. With the further uplift to more or less present day altitude, no extensive areas of plain was exposed in the region now being considered and, therefore, great differences in the lower courses of the rivers such as are seen in rivers farther to the east are seldom present. On the other hand there is evidence of the additional rejuvenation due to this last uplift occasionally to be seen in the western rivers; for example, the Avan has cut a gorge through the zoo-foot 'bar' near its mouth and there is a knick-point in the Avan thalweg at 225 feet above O.D., while there are a number of zoo-foot hillocks (with the river ' gorging' between two of them) near the mouth of the Neath. As with the' zoo-foot standstill,' glaciation has destroyed most of the knick-point evidence of the grading to this stationary period. Cox and Heard mention [4J that the Vale of Neath was initiated by the uplift which terminated the zoo-foot platform shore-line. The writer, however, is not in agreement with this, but believes that although the development of this valley above Aberdulais is later than the 400-foot platform coast-line, it had certainly asserted itself prior to the zoo-foot platform being uplifted above beach level.

(5) Gravels. Delta gravels are very often present in major valleys at points where hanging valleys join them as (a) in the Tawe at Pontardawe, Ynysmeudw (near Bryn Works), Gurnos and at Ystradgynlais; (b) in the Neath valley at Resolven, at the mouth of Rheola Brook, at Blaengwrach and at Aberpergwm; (c) in the Amman valley where the Pedol and Garnant streams flow into it. There are also river gravels in the Vale of Neath near Pont-Neath-Vaughan. These gravels are all largely due to the greater amount of water and consequently of flooding in early post-glacial times, before the growth of extensive vegetation. This resulted naturally in a very marked increase of denudation, particularly by the rejuvenated streams.

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At about 50 feet above a.D. there are gravels seen in the Tawe near Ynystawe and in the Neath valley where the town of Neath is built. No marine shells have been found in these gravels, but it seems from the character of their flat surfaces that they represent a deposit laid down along the shore of a stretch of water (presumably the sea) at 50 feet above present a.D. These may possibly be related to a raised beach at this altitude in Gower [6a and 6b]. Near Port Talbot and at Swansea there seems to occur a gravel terrace at z5 feet.

IV. CONCLUSIONS. (1) The Neath-Tawe drainage system seems to have been initiated on a warped platform having a southerly tilt. The original surface being a plateau, possibly covered by later rocks in parts, the river development was not directly and immediately influenced by the present geological structure, and so the earliest elements were superimposed in great parts of their courses. There is, however, no evidence of a very extensive Mesozoic covering and the location of early tributaries of the initial drainage is seen to have been controlled by geological structure. (z) The north-north-east-south-south-west streams developed later as subsequent tributaries to older rivers along lines of disturbed strata. Their development was accompanied by extensive river capture. (3) There is some evidence of earlier base-levels to the rivers at 600 feet, 400 feet and zoo feet respectively above present a.D. The north-north-east-south-south-west parts of the Neath and Tawe streams developed later than the planation of the 400-foot platform, but before or at the latest during the early period of planation of the zoo-foot platform and were in an advanced stage long before the uplift of the zoo-foot platform above sea-level. (4) There is evidence that although glaciation caused such minor modifications and deflections of the drainage as overflow channels, moraine-dammed lakes and overdeepening of valleys, the ice did not affect fundamentally the major features of the drainage pattern. The ice frequently spilled over old wind-gaps connected with earlier drainage stages. (5) River gravels form deltas at junction of hanging tributaries with major streams and form terraces in wider parts of the valleys; these features bear evidence of greater denudation in the swifter-flowing parts of the rivers under more or less flood conditions. Most of this gravel is of immediate post-glacial age while, of course, some alluvial fans are still growing. There are flat-topped gravels at 50 feet above a.D., suggestive of a very late glacial or early post-glacial base-level at this altitude.

R. O. JUKES,

The writer is indebted to Dr. S. W. Wooldridge for his guidance while this work was pursued; to Professor Trueman, who encouraged him to commence a study of the rivers systems in South Wales; and to Professor T. Neville George for loan of instruments, company in the field, frequent helpful suggestions during discussions and for critically reading this paper in manuscript. Other people to whom the writer is indebted are Dr. ]. C. Griffiths for analysing certain glacial sands and clays, and to Messrs. ]. Lee and H. P. Gerrard for making inked copies of many diagrams. The author also wishes to express his thanks to the Royal Society for a grant to cover a portion of the expenses connected with the examination of the river systems in the larger region between the Usk and the Towy valleys, which includes the area dealt with in this paper.

1. 2. 3. 4. 5. 6a. 6b. 7a. 7b. 8. 9. 10. II.

12. I3. 14.

REFERENCES. Notes on the Glaciation of the Usk and Wye. Rep. Brit. Assoc. for 1906 (1907), pp. 579-80. CHARLESWORTH, J. K. 1929. The South Wales End Moraine. Quart. Journ. Geol, Soc., vol. lxxxv., p. 335. CODRINGTON, T. 1898. On Some Submerged Rock Valleys in South Wales, Devon and Cornwall. Quart. Journ. Geol, Soc., vol. liv., p. 251. Cox, A. H. and A. HEARD. 1937. Report of Week-end Field Meeting in the Cardiff District. Proc. Ceol. Assoc., vol. xlviii., PP·5 2 - 60 . DAVID, EDGEWORTH. 1883. On the Evidence of Glacial Action in South Brecknockshire and East Glamorganshire. Quart. Journ. Ceol. Soc., vol. xxxix., p. 39. GEORGE, T. N. 1932. The Quaternary Beaches of Gower. Proc. Geol. Assoc., vol. xliii., pp. 291-324. - - - - - 1933. The Glacial Deposits of Gower. Geol, Mag., vol. lxx., p. 208. - - - - 1936. Geology of the Swansea Main Drainage Excavations. Proc. Swansea Sci. and Field Nat. Soc., vol. i., pt. !O, pp. 33 1-359. 1938. Shoreline Evolution in the Swansea District. Proc. Swansea Sci. and Field Nat. Soc., vol. ii., pp. 23-48. GOSKAR, KATHLEEN, L. and A. E. TRUEMAN. 1934. The Coastal Plateaux of South Wales. Geol, Mag., vol. lxxi., pp. 468-477. - - - - 1936. The Form of the High Plateau in South Wales. Proc. Swansea Sci. and Field Nat. Soc., vol. i., pt. 9, pp. 305-312. HOLLINGWORTH, S. W. 1938. The Recognition and Correlation of High-Level Erosion Surfaces in Britam : A Statistical Study. Quart. Journ. Geol, Soc., vol. xciv., p. 55. JONES, O. T. 1930. Some Episodes in the Geological History of the Bristol Channel Region. Brit. Assoc. for the Adv. of Science for 1930 (1931), PP' 57-82. JONES, R. O. 1931. The Development of the Tawe Drainage. Proc. Ceol. Assoc., vol. xlii., pp. 305-321. 1931. Note on a Supposed Terrace in the Tawe Valley. Proc. Swansea Sci. and Field Nat. Soc., vol. i., pt. 5, p. 145. LAKE, PHILIP. 1934. The Rivers of Wales and their Connection with the Thames. Science Progress, No. II3, 1934, pp. 25-39.

CARTER, W. L.

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

565

15.

Memoirs of the Geological Survey: The Geology of the South Wales Coalfield. Pontypridd and Maesteg (Sec. Ed. 1917). (a) Part IV. Merthyr Tydfil (Sec. Ed. 1933). (b) Part V. (c) Part VII. Ammanford (1907). (d) Part VIII. Swansea (1907). 16. MILLER, A. AUSTEN. 1937. The 600-foot Plateau in Pembrokeshire and Carmarthenshire. Geog. Journal, vol. xc., No.2, pp. 126-147. 17. NORTH, F. J. 1929. The Evolution of the Bristol Channel. Nat. Museum of ~Vales. 18. NORTH, F. J. 1930 (New Ed. 1938). Studies in the Origin of the Scenery of Wales: 1. The Scenery at the Head of the Vale of Neath. Nat. Museum of Wales. 19. PRINGLE, J., and T. N. GEORGE. 1937. British Regional Geology: South Wales. (H.M. Stationery Office). 20. STRAHAN, A. 1902. Origin of the River-System of South Wales and its connection with that of the Severn and the Thames. Quart. [ourn, Geol. Soc., vol. Iviii., pp. 207-225. 21. 1910. South Wales. Geology in the Field, Part IV. Chap., xxxiii., Geol. Assoc. Jubilee vol., p. 826. 22. TRUEMAN, A. E. 1924. The Geology of the Swansea District. Proc. Geol. Assoc. (Excursion Handbook), vol. xxxv., pp. 2833 08. 23. TRUEMAN, A. E. and S. W. RIDER. 1929. South Wales. A Physical and Economic Geography. (Methuen & Co. Ltd.) 24. WOOLDRIDGE, S. W., and J. F. KIRKALDY. 1936. River Profiles and Denudation: Chronology in Southern England. Geol. Mag., vol. lxxiii., pp. 1-16.

DISCUSSION. Professor T. NEVILLE GEORGE (contributed). I regret my inability to be present at the reading of Dr. Jones' paper and to offer him in person my congratulations on the accomplishment of a notable piece of research work: for his analysis of the development of the local drainage systems is a convincing and essentially adequate solution of some of the problems that have confronted geomorphologists since Ramsay first attempted to explain the origin of the land forms of South Wales. In particular Dr. Jones has demonstrated that the initial drainage pattern bear all the features of superimposition, and that the major streams at the present day-the Tawe and the Neath-are later developments adjusted to the geological structure which have captured the headwaters of the original consequents. In terms of the chronology of coastal elevation (presumably of Pliocene age) the extreme modernity of these two rivers is shown by the fact that the Neath was at best but a few miles long when the' 4oo-foot ' platform was being carved, while the Tawe was virtually non-existent at even so late a date as the time of emergence of the' zoo-foot ' platform. It is not without interest to note that work I have recently carried out on the Cothi-Towy-Loughor drainage system has shown the lower reaches of the Towy to be equally youthful, the Cothi having been diverted from its southward course into the Loughor, probably with emergence of the' zoo-foot ' platform. The rapidity of differential erosion along these three structural lines (faults in the cases of the Tawe and the Neath, a faulted anticline in the case of the Towy) is clearly seen to be due to rejuvenation, and it is improbable that there was renewed concentric tectonic movement along them as envisaged by Lake. An important result of Dr. Jones's work is the recognition of knickpoints in the long profiles of many of the rivers indicating grading to

566

EVOLUTION OF THE NEATH-TAWE DRAINAGE SYSTEM.

base-levels higher than the present. Of particular interest is his conclusion that the upper reaches of some of the streams are graded to a base-level about 600 feet above present sea-level, for this confirms the findings of O. T. Jones in the Upper Towy, and provides further evidence of the existence of a ' 6oo-foot ' platform distinct from, and older than, a " 40o-foot ' platform: especially is this the case where a lower rejuvenation head indicates grading to the' 400-foot • platform. Mr. AUSTEN MILLER congratulated the Author on the careful research he is carrying out on the rivers of South Wales. He looked forward to examining at close range the evidence, supplied by the river profiles, for earlier and higher sea-levels, especially the one at 600 ft. for which evidence is scanty further west. He enquired whether the Author had considered the effect of such elevated sea-levels on the position of the mouths of the Neath and Tawe, and enquired whether they might have been sufficiently far" inland" from the present coast to have assisted the encroachment of the subsequent streams along the Vale of Neath and Cribarth shatter lines, which do not extend as far as the present coast. THE AUTHOR, in reply, said he was interested in the results of Professor George's work on the Towy-Cathi-Loughor drainage. The author had himself been examining recently the relation of the Towy and Cennen to the Loughor in the neighbourhood of the LJandebie gap. In reply to Mr. AUSTIN MILLER, stated that the 4oo-foot and 200foot coast line could not have been very far inland in the Swansea and Neath area. The later uplifts certainly caused rejuvenation in the lower parts of the earlier consequent streams and this, undoubtedly, contributed to the greater activity of the subsequent streams and the rapid growth of those that were developing along lines of disturbances.

Note with reference to scale of Plates 35 and 36 and Figs. 48 and 50. Horizontal Scale, I inch = I mile; Vertical Scale, I inch = 1,000 feet.

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