CYNGOR CEFN GWLAD CYMRU
COUNTRYSIDE COUNCIL FOR WALES
SITE OF SPECIAL SCIENTIFIC INTEREST CITATION
POWYS
DYFFRYNNOEDD NEDD A MELLTE,
NEATH PORT TALBOT
A MOEL PENDERYN
RHONDDA CYNON TAFF
Date of Notification:
1954, 1965, 1978, 1979, 1999
National Grid Reference:
SN 907100, SN 921090, SN 937088
O.S. Maps:
1:50,000 Sheet number: 160
1:10,000 Sheet number: SN 80 NE, SN 90 NW, SN 91 SW
Site Area:
421.1 ha
Description:
Dyffrynnoedd Nedd a Mellte, a Moel Penderyn is of special interest for its extensive and diverse
semi-natural woodland, important populations of several flowering plants and supporting
outstanding assemblages of mosses, liverworts and lichens. The site includes a range of
geological features, well-exposed in the cliffs and rocky river beds. These include exposures at
Moel Penderyn, Craig y Ddinas and Bwa Maen and geomorphological features within parts of the
valleys of the Hepste and Mellte are also of special interest.
This site includes the wooded valleys of the rivers Nedd and Mellte, and their tributaries above
Pontneddfechan, as they pass through a Millstone Grit and limestone plateau, and Moel
Penderyn, which lies to the east. The plateau lies at about 300 m, the rivers having eroded deep,
narrow valleys with gorges, river cliffs, block scree and waterfalls.
GEOLOGY
The old quarry faces and rock outcrops at Craig y Ddinas and Moel Penderyn provide important
exposures of geological structures formed during the Variscan Orogeny, late in the Carboniferous
and early in the Permian periods of geological history. The Carboniferous Limestone layers are
steeply tilted on Craig y Ddinas, as a result of folding, and small tight folds can be seen in the
banks of the River Sychryd. The exposed Dinas fault, a major dislocation of the crust, is marked
by a belt of shattered limestone associated with several minor fractures. Numerous other small
structures occur, including faults, fractures, veins and folds. This complex assemblage of
structures makes up a narrow belt called the Neath Disturbance, which extends from the Welsh
Borders to the Gower Peninsula. The Disturbance probably lies above an ancient line of
weakness in the deep crust reactivated during the Variscan Orogeny. These localities provide the
best exposures of this important feature of the geological structure of South Wales.
The best exposure in South Wales of the rock strata of the
Gastrioceras subcrenatum Marine
Band (below the Farewell Rock) in its full development occurs in the valley of the Nedd Fechan.
The full range of environment types is represented in the marine band section, including brackish
water with a restricted fauna and off-shore marine environments with a goniatite-rich fauna. The
site is important in understanding how this brief marine incursion occurred, which, in turn, is
important because the marine band, the expression of this event in the rocks, is taken to indicate
the start of the Westphalian Epoch of Carboniferous time.
The Nedd, Mellte and Sychryd, forming the headwaters of the Afon Nedd, provide key exposures
of Carboniferous rocks. These include extensive and spectacular exposures of the whole
Namurian of the North Crop. In particular, the Basal Grits are well exposed and present the best
available section in the Main South Wales Basin. The predominantly shaley sequences of the
higher Namurian are also well exposed here and their study has been of great importance in the
elucidation of the stratigraphy of the Namurian. Additional importance attaches to the presence
of a varied and abundant Basal Grit flora. These are outstanding, nationally important sections of
the Carboniferous.
The river system is exceptional in showing the combined effects of geological controls and
changing sea levels in river landform development. Faulting has juxtaposed rocks of varying
resistance and influenced the form and evolution of waterfalls. Thus the landforms of this area
reflect a variety of interacting controls, and both the total assemblage as well as the individual
features are important.
BIOLOGY
This site supports one of the most extensive and diverse areas of semi-natural woodland in
Wales. The predominant woodland vegetation communities are characteristic of the uplands of
north and western Britain, with extensive areas of oak
Quercus spp. woodland, and smaller areas
of ash
Fraxinus excelsior woodland. The distribution of the particular communities reflects the
distribution of soils of low, moderate and high base status, respectively. More locally, however,
the aspects and microclimate have allowed the development of the southern and eastern lowland
counterparts of these free-draining oak, birch
Betula spp. and ash-dominated woodlands.
Large areas of the site have a woodland canopy dominated by oak intermediate in character
between the sessile oak
Quercus petraea and the pedunculate oak
Q. robur, with a little downy
birch
Betula pubescens and small-leaved lime
Tilia cordata in places. Here, the shrub layer is
absent or sparse with a little hazel
Corylus avellana or rowan
Sorbus aucuparia. The field layer
is typically dominated by wavy hair-grass
Deschampsia flexuosa, with bilberry
Vaccinium
myrtillus present and, where grazing is relatively light, quite abundant. The ground layer has a
very high cover of mosses, including
Rhytidiadelphus loreus,
Polytrichum formosum,
Dicranum
majus,
Plagiothecium undulatum,
Hypnum cupressiforme,
Mnium hornum and
Sphagnum
quinquefarium. Elsewhere, large stands of similar canopy and shrub layer composition have a
ground layer typically dominated by wavy hair-grass with common bent
Agrostis capillaris and
some sweet vernal-grass
Anthoxanthum odoratum. Purple moor-grass
Molinia caerulea is
present occasionally and is sometimes abundant, while bilberry is absent or very rare except on
inaccessible ledges and cliffs. The ground layer often has carpets of the bryophytes mentioned
above.
Another woodland community that occurs extensively in some parts of the site is dominated by
sessile oak and forms intermediate with pedunculate oak, or downy birch, or a mixture. Other
canopy trees are rare and the shrub layer is typically sparse, but in some areas dense hazel and
scattered hawthorn
Crataegus monogyna are present. The field layer is grassy, with common
bent, sweet vernal-grass and creeping soft-grass
Holcus mollis often abundant with patchy wavy
hair-grass. Other species that occur here are wood-sorrel
Oxalis acetosella, bluebell
Hyacinthoides non-scripta, bracken
Pteridium aquilinum and heath bedstraw
Galium saxatile and the mosses
Rhytidiadelphus squarrosus and
Polytrichum formosum.
Small stands of woodland, particularly along steep tributary stream valleys are dominated by ash
with a dense shrub layer of hazel, a little hawthorn and locally frequent rowan. No single species
is dominant in the field layer, but wood-sorrel, false brome
Brachypodium sylvaticum, common
bent, lady-fern
Athyrium filix-femina and rough meadow-grass
Poa trivialis are all locally
frequent. In the ground layer, mosses and liverworts are prominent, with
Plagiomnium
undulatum,
Mnium hornum,
Atrichum undulatum,
Pellia sp., and
Ctenidium molluscum all
frequent and often abundant. The ground flora in these areas can be extremely rich and includes
species that are scarce in mid and south Wales, such as wood fescue
Festuca altissima.
The canopy in one area of the site is composed mainly of intermediate oak, with a little downy
birch in places. The shrub layer is sparse, with only scattered hazel and holly
Ilex aquifolium.
The field layer is a carpet of wavy hair-grass with few other species present, although bracken
occurs in places. Mosses are sometimes quite prominent in scattered tufts, especially
Polytrichum formosum and
Mnium hornum.
In another area, the canopy mostly comprises intermediate oak and occasional ash with a
relatively dense shrub layer, primarily of hazel, but with scattered holly and rowan. The field
layer is dominated by three species: bramble
Rubus fruticosus, bracken and creeping soft-grass
with frequent wavy hair-grass.
Small patches of woodland on open, steep cliffs and screes, particularly of limestone, are
dominated by ash and are relatively species-rich, with many calcicoles. On shadier steep cliffs,
similarly ash-dominated patches are relatively species-poor, with abundant ivy
Hedera helix and
some calcicoles. In small areas of woodland at the base of shadier steep cliffs, scattered field
maple
Acer campestre occurs in the shrub layer, beneath a canopy of ash. The field layer
includes species such as dog's mercury
Mercurialis perennis, herb-Robert
Geranium
robertianum, common nettle
Urtica dioica, cleavers
Galium aparine and, where bouldery, hart’s-
tongue
Phyllitis scolopendrium.
Elsewhere, soils with impeded drainage or flushing support an extensive area of alder
Alnus
glutinosa-dominated woodland, in a range of topographical situations. Wet flushes within this
type of woodland provide the most southerly known locality in Britain for marsh hawk’s-beard
Crepis paludosa. There are small patches of woodland where alder is dominant, with a little ash
and downy birch over a shrub layer of hazel. Here, the field layer is generally dominated by
remote sedge
Carex remota with many other species present in small amounts. Elsewhere,
extensive stands of alder-dominated woodland have a relatively dense shrub layer, primarily
composed of hazel but often with ash saplings, hawthorn and downy birch saplings. The ground
layer is dominated by tufted hair-grass
Deschampsia cespitosa, with wood-sorrel and lady-fern.
Further areas have a canopy mostly of alder with occasional downy birch and little of a shrub
layer except scattered ash saplings. The field layer is a mixture of meadowsweet
Filipendula
vulgaris, creeping buttercup
Ranunculus repens, false brome, soft-rush, great horsetail
Equisetum
telmateia and bramble. In the ground layer, the mosses
Eurhynchium praelongum and
Thuidium
tamariscinum are frequent.
Locally, where woodland extends onto the drift-covered plateau, stands of a community
dominated by alder with occasional downy birch, rowan and ash are present. There is no shrub
layer and the field layer is dominated by purple moor-grass, with frequent bramble, soft-rush
Juncus effusus and broad buckler-fern
Dryopteris dilatata. Scattered, relatively small patches of
a similar type of woodland have the same canopy trees and a typically sparse or absent shrub
layer. The field layer here is always dominated by purple moor-grass, with soft-rush, tormentil
Potentilla erecta, creeping soft-grass, tufted hair-grass and common bent typically present. In the
ground layer, bog moss
Sphagnum recurvum and hair moss
Polytrichum commune are usually
present.
The overall botanical diversity is outstanding, with more than 600 species of plant having been
recorded from the site. This includes a very large proportion of the bryophyte flora of mid and
south Wales.
The high humidity of much of the woodland has a strong influence on its botanical diversity.
Trees and rocks support a diverse assemblage of plant species largely confined in their
distribution to the Atlantic Seaboard of Europe. These Atlantic species include green spleenwort
Asplenium viride, Tunbridge filmy-fern
H. tunbridgense, hay-scented buckler-fern
Dryopteris
aemula, all of which are rare in south Wales, liverworts, such as
Bazzania trilobata,
Jubula
hutchinsiae and the scarce
Anastrophyllum hellerianum,
Colura calyptrifolia,
Jamesoniella
autumnalis and
Sphenolobopsis pearsonii, mosses, such as
Isothecium holtii,
Dichodontium
denudatum and the scarce
Bartramia hallerana and
Seligera acutifolius, lichens, such as
Enterographa hutchinsii,
Micarea alabastrites, the rare
Micarea hedlundii, and the scarce
Micarea stipitata and
Phyllopsora roseii. These contrast with the local abundance of more
continental species such as, on logs, the scarce moss
Dicranum flagellare, on gritstone cliffs, the
scarce lichen
Chrysothrix chlorina and, on bark, the scarce lichen
Micarea pycnidiophora.
Boulder screes, cliff faces, springs, seepages, decaying logs and mature and overmature trees
provide some of the specific habitats required by several scarce lower plants demanding
conditions that are subject to little change with time, including the lichens
Leptogium plicatile and
L. subtile, and the liverwort
Cephalozia catenulata. Despite recent high levels of
atmospheric pollution, the epiphytic
Lobarion lichen community persists in these sheltered
valleys and includes species such as
Lecanactis premnea,
Biatora sphaeroides,
Parmeliella
triptophylla and
Dimerella lutea.
The rivers, with their fine waterfalls, are an integral part of the site with the splash zones and
spray providing wetness and humidity necessary for many species. River courses support a
notable riparian flora dominated by liverworts, and mosses, including the nationally scarce
Fissidens rufulus and
F. rivularis, and
Tetrodontium brownianum, which is threatened in Europe.
The woodland in the valleys is interspersed with several more open habitats that provide
additional interest. As well as the cliffs, screes and rivers there are extensive areas of grassland
dominated by common bent and sheep’s fescue
Festuca ovina.
There are also patches of less
acidic grassland in the valley bottoms and,
at
Craig y Ddinas and Moel Penderyn, the limestone
supports calcareous grassland. Wet grassland dominated by purple moor-grass or rushes (
Juncus spp.) occurs in a series of flushes along the valley sides and bracken-dominated glades are
widespread. Patches of dry heath and wet heath characterised by deergrass
Scirpus cespitosus and cross-leaved heath
Erica tetralix can be found in Hepste valley.
Other
habitats of interest include areas of beech
Fagus sylvatica plantation, grey willow
Salix
cinerea and
bramble
scrub
and
nutrient-rich ponds.
The fauna of these valleys is well developed, and includes birds such as breeding dipper, grey
wagtail, goosander, pied flycatcher, redstart, wood warbler, woodcock, buzzard and
sparrowhawk.
Remarks: Part of this site lies within the Brecon Beacons National Park.
The Forestry Commission and the Brecon Beacons National Park Authority own parts of this site.
