Historic Landscape Characterisation - Dolgellau Sites of Special Scientific Interest

Physical setting and environment

This area is rather specialised topographically, because its main feature is its dissected nature at an internode of valleys and routes with relatively little land available for agriculture or settlement. Lower Palaeozoic sedimentary and igneous rocks of Ordovician age dominate the bedrock geology of the Cadair Idris massif. The igneous assemblage comprises extrusive basic and acid volcanic rocks, including pillow lavas and ash-flow deposits, together with a variety of intrusive rocks such as dolerites and microgranites. Interbedded with the igneous rocks are mudstones, fine sandstones and, on the north-facing scarp of Cadair Idris, an oolitic ironstone which was worked locally as a source of low-grade iron ore. Locally superimposed upon the bedrock geology are hummocky glacial deposits (moraines) which were deposited during the last of several major phases of ice-sheet glaciation which culminated approximately 18,000 years ago. This glaciation caused deep dissection of the landscape, creating an impressive assemblage of landforms including cirques and U-shaped valleys. Post-glacial times witnessed the development of alluvial fans and screes, some of which are stratified, whilst glacial over-steepening of the southern flank of the Tal-y-Llyn Valley resulted in a major landslip that dams the valley at the south-western end of the lake.

The Cadair Idris massif provides excellent exposures through a thick succession of bimodal acid-basic volcanic rocks of Llanvirn-Caradoc (Ordovician) age. Such bimodal volcanic successions are characteristic of the marginal basin environment which existed in parts of Wales during Lower Palaeozoic times. The site incorporates a well-exposed south- to south-easterly-dipping succession of volcanic and associated sedimentary rocks, many component units of which show considerable lateral variation. The varied volcanic products consist predominantly of basic lavas and related pyroclastic rocks in the lower part of the sequence. Higher in the sequence acid lavas and tuffs predominate. Of particular importance are exposures of a thick granophyre sheet which evidently broke surface to form an effusive flow. This large site is a classic locality for the study of the palaeo-volcanic rocks of Wales.

The area of Cadair Idris and Tal-y-Llyn is outstandingly important for glacial and periglacial landforms. It contains a number of glacial and nivation cirques, including Cwm Cau which has been described as the finest cirque in Britain. This cirque shows a very clear relationship to geological structure and opens out onto the Tal-y-Llyn valley, a classic U-shaped valley developed along the structural weakness of the Bala Lineament. In addition to large-scale features of glacial erosion, the area is also renowned for a range of depositional landforms asociated with mass movement and periglacial processes. Most spectacular of these is the bar impounding Tal-y-Llyn, formed by a huge landslide from Graig Goch. The Tal-y-Llyn Valley also contains very fine examples of stratified screes, well exposed near Maes-y-pandy. Other periglacial interests include protalus ramparts, notably at Graig-y-llyn, and a large debris fan or blockstream near Bwlch Llyn Bach.

Furthermore no fewer than five still extant woods are listed by Edward Lhuyd's Parochial Queries (1696) as being in Llanelltyd Parish, including Koed y Ganlhwyd, Koed y Berthlwyd, Koed Dol y Melynllyn, Koed y Hengwrt and Koed y Vanner (Linnard, 2000). Some of these are in the project area.

Sites of Special Scientific Interest

The area is particularly important in natural environmental terms, which is reflected in the fact that there are nine SSSIs, varying in size, either partially or fully within the study area. These are: Aber Mawddach (CCW, 31WVS), which covers a central part of area 06; Bryn y Gwin Isaf (CCW, 31WVX), which lies within area 04; Cadair Idris (CCW, 31WMT), a huge area which covers the whole of area 17 and also parts of area 16; Coedydd Dyffryn Wnion (CCW, 31WNY), which lies in the lower parts of areas 15 and 16; Glasdir Copper Mine (CCW, 31WHD), which touches on the western edge of area 11; Llwyn-iarth (CCW 31WGD), which occupies a substantial part of area 04; Penmaenuchaf Hall (CCW, 31WVP) in area 04; Llyn Gwernan (CCW, 31WHG) in the western part of area 04; and Mwyngloddiau Wnion ac Eglwys Sant Mark (CCW, 31WLZ), which occupies two small sites within area 15.

Brief descriptions of these areas are given, as this natural environmental interest will influence any future management of the character areas.

Aber Mawddach (Mawddach estuary) covers 1340.6 ha, which includes the whole of character area 06. It was designated for its biological features, including the estuary itself, a large shallow estuary which drains south-west into Cardigan Bay, and a series of adjacent habitats. The overall management objective for the SSSI is to retain the habitat- and species-related interest of the area, which includes several BAP species.

Cadair Idris, a large upland site extending to some 5500 ha (making it one of the largest SSSIs in Wales), is of special interest for biological, geological and geomorphological features: it includes the whole of character area 17. The summit of Cadair Idris rises to 893m, and there are cirques, summit ridges, steep scree slopes and cliffs found on the massif itself. On the eastern side there is the large U-shaped valley which contains Tal-y-Llyn. On the western side of the site there is a long, steep-sided ridge running from Craig Las to Mynydd Pennant, and a large basin lies to the south of the summit. The special biological features of the site include blanket bog, wet and dry heaths, lichen-bryophyte heath, tall herb and fern ledges, vegetated natural rock exposures, standing water, broad-leaved woodland, calcareous, acid and marshy grassland, flushes and springs. The site is also of special interest for its assemblage of higher plants, lichens, bryophytes and montane invertebrates. The area of Cadair Idris and Tal-y-Llyn is outstandingly important for glacial and periglacial landforms. It contains a number of glacial and nivation cirques, including Cwm Cau, a particularly fine example.

Coedydd Dyffryn Wnion SSSI is situated on the south side of the Afon Wnion, extending from the outskirts of Dolgellau for 6km towards Rhyd-y-main and Y Bala. Two main areas of woodland are included. The first of these is centred around Coed Maes yr Helmau (also known as Torrent Walk) and the second is an area of woodland between Brithdir and Bryn Coed Ifor. Both the woodland areas are north-east-facing, with deeply incised and humid gorges. The SSSI includes a 2.7km section of the Afon Clywedog and 0.7km section of Nant Helygog. The woodlands include land shown as woodland on the Ancient Woodland Inventory.

The bedrock geology at this site is dominated by a variety of lithologies, which are late Cambrian (Merioneth Series) to early Ordovician (Tremadoc-Llanvirn Series) in age. Rock types occurring both in the subsurface and at outcrop include mudstones, siltstones and sandstones of the late Cambrian Mawddach Group, and basic and acid volcanic rocks of the early Ordovician Rhobell Volcanic and Aran Volcanic groups. The latter groups are also associated with numerous basic dolerite intrusions.

The soils are predominantly brown podzols (strongly leached) on the steeper slopes, and stagnogley/stagnohumic gley (slow draining, base accumulating) soils on the gentler slopes.

Bryn y Gwin Isaf (SH717178) is a grade II-listed, early 19th-century 'Plas', situated 1.5km west of Dolgellau, overlooking the Afon Mawddach. The SSSI designation extends to 12.3ha in extent. It is of special interest as an important breeding roost of the lesser horseshoe bat, a European stronghold for this species. The site comprises the main house, which supports the nursery roost, and roof spaces of associated dwellings which are used as satellite and night roosts. The grounds and associated woodland, which are used as feeding habitat by lesser horseshoe bats, are also within the site, and the roof space of the main house is also used as a breeding roost by a colony of brown long-eared bats.

Glasdir Copper Mine (SH 740223) extends to 3ha and was selected as an SSSI for its geological interest. The site is important for the good exposures of a mineralised breccia pipe, the details of the genesis of which remain a subject of continuing research and debate.

Llwyn-Iarth (SH 709173 - entirely within area 04), extending to some 56.8ha, consists of a number of enclosures and unfenced land. It is of special interest for its species-rich neutral grassland and for its large population of wood bitter-vetch. Also of special interest are the extensive areas of flush and fen, and the mixture of habitats, which includes wet heath, marshy and acid grassland. The acid flushes are dominated by soft rush and/or by bog mosses with a patchy cover of purple moor-grass, bog asphodel, common sedge, carnation sedge and star sedge. There are small areas of neutral flushes on the western side of the site, where acid-loving bog mosses are found mixed among more base-demanding species of bog moss. Marsh St. John’s-wort and bog pondweed are locally abundant along flush lines or soakaways. The wet heath is characterised by frequent cross-leaved heath and heather with associates such as tormentil, heath milkwort, and lousewort.

There is an extensive area of fen vegetation close to the stream near the southern boundary, where tall tussocks of purple moor-grass rise out of a carpet of more base-demanding mosses. Bog myrtle forms a patchy canopy over the tussocks. Less prominent but still frequent are various sedges and other species, including round-leaved sundew, cross-leaved heath, common cotton grass, dioecious sedge, bottle sedge and bogbean. The presence of the nationally scarce bog moss Sphagnum platyphyllum, as well as lesser bladderwort, oblong-leaved sundew, many-stalked spike-rush, and white beak-sedge, is notable. On the southern and western edges of the site lie two other areas of marshy grassland dominated by purple moor-grass with large patches of bog myrtle. A narrow band of purple moor-grass along the edge of one of the northern fields includes marsh hawk’s beard. Other vegetation represented at Llwyn-iarth includes rush pasture with frequent sharp-flowered rush and common marsh bedstraw, small stands of meadowsweet and wild angelica, and western gorse heathland.

Penmaenuchaf Hall (SH699184) is a 19th-century country house situated 3km west of Dolgellau, at an altitude of 40m, overlooking the Afon Mawddach. The SSSI is 0.1ha in extent, of special interest as a breeding roost of the lesser horseshoe bat. Penmaenuchaf Hall is used during the summer months by the bats in order to raise their young. Lesser horseshoe bats return annually to the same site and Penmaenuchaf Hall has been used by the species for many years. The building also supports a breeding roost of brown long-eared bats. Natterer’s bats, pipistrelle bats and whiskered/Brant’s bats have also been recorded at the site.

Llyn Gwernan (SH703159), 3ha in extent, is an important geological site, notable for an unusual thickness of Devensian Late-glacial organic deposits.

Mwyngloddiau Wnion a Eglwys Sant Marc was selected for its biological interest and is located in the Afon Wnion valley near Brithdir, Dolgellau. The sites consists of two adits and a church and is of special interest for hibernating and breeding bats, particularly lesser horseshoe bats Rhinolophus hipposideros. The complex of mines around Brithdir was worked during the 19th century for various metals including gold, lead, silver, copper and zinc. St. Mark’s church, Brithdir, was built in the 19th century by owners of the Caerynwch estate, and was fashioned on the lines of a northern Italian country church.


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