Glannau Tonfanau i Friog

Also within

Gwynedd County Eryri / Snowdonia National Park

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Geology & landscape

Geological Period: Cambrian, Ordovician, Silurian

Landscape processes: Coastal, Glacial, Volcanic / Igneous

Birds & fish recorded

Birds

Description

Description This site is of special interest for both its geological and marine biological features. Glannau Tonfanau i Friog is a stretch of coastline, of approximately 11 km, situated in the northern part of Cardigan Bay, Gwynedd. Fairbourne lies just beyond the northern boundary and Broadwater SSSI forms the southern limit of the site. The northern shore is backed by high rocky cliffs, while the majority of the coastline is made up of eroding soft sedimentary cliffs, which descend almost to sea level at the southern end. Glannau Tonfanau i Friog is of special interest for its structural geological and mineralogical features which provide important evidence regarding the age of the Dolgellau Gold-Belt. This coast is also of special interest for its biogenic reefs, specialised rockpools and its extensive, natural, mixed substrate shore. The cliffs and intertidal zone between Tonfanau and Friog are composed of folded and faulted, south-easterly-dipping sedimentary rocks which range in age from the Middle Cambrian (St David’s Series) to Lower Ordovician (Tremadoc Series). The shore is strewn with boulders derived from both the cliffs and re-worked glacial material. Extending off-shore from the southern end of the site is Sarn y Bwch, one of three Sarnau which are unique reef features within Cardigan Bay. They were formed by the action of glacial deposition from the mountains of Snowdonia and are composed of boulders, cobbles and pebbles which provide an unusual habitat for marine life. GEOLOGY The geological interest is located south-west of Friog in a 1.4 km section of coastal cliffs together with the wave-cut platform beneath. The section is underlain by a south-easterly dipping succession of sedimentary rocks which consist of interbedded mudstones, siltstones and sandstones belonging to the Clogau and Maentwrog formations. These rocks are of Middle to Upper Cambrian age and were deposited in a marine sedimentary basin, the Welsh Basin, approximately 520 million years ago. The section is located at the south-western end of the Dolgellau Gold-Belt and displays Cambrian rocks intruded by minor sills and cut by numerous quartz sulphide veins. The latter are strongly enriched in arsenic and bismuth, a geochemical signature which is characteristic of veins within the Gold-Belt, and clearly exhibit a variety of deformational textures including folding, boudinage and fracture cleavage. These important observations provide critical structural evidence which confirms that emplacement of the Gold-Belt veins predated the end- Silurian phase of Caledonian folding and cleavage formation. This conclusion is contrary to the long-held and well-established belief that the veins were emplaced after the Caledonian Orogeny and represents an important discovery with regard to the relative timing of Gold-Belt mineralization and deformation. BIOLOGY The coast here is of special interest for its extensive natural mixed substrata shore, its nationally important honeycomb worm Sabellaria alveolata biogenic reefs, and its associated highly diverse coralline rockpools. The Tonfanau to Friog coast is north-west facing and moderately exposed to wave action. The mixed sediment shore (boulder, cobble, gravel and sand), derived from glacial debris, is typical of this part of Cardigan Bay, and is the second most extensive. Many of the marine intertidal fauna and flora found here are characteristic of these substrata. The barnacle Verruca stroemia is found on the undersides of cobbles and boulders, as are the sea slugs Onchidoris bilamellata and Doris pseudoargus. The peacock worm Sabella pavonina is found living in the sand between the cobbles which protect it from wave action, and the sand tolerant red alga Gracilaria longissima is common. A community of dense, turf-forming red seaweeds including Furcellaria lumbricalis, Gracilaria species, Rhodomela confervoides and Polysiphonia elongata, occur towards the south of the site where the lower shore is influenced by sand scour Close to Sarn y Bwch, brown seaweeds such as bladder wrack Fucus vesiculosus dominate the boulder shore and in places the seaweeds are replaced by a dense covering of edible mussel Mytilus edulis. Sarn y Bwch itself has a sparse covering of barnacles on its cobble substrate and on the south side of the Sarn a small mussel bed is present. The honeycomb worm occurs on the mid and lower shore in extensive reefs. These reefs are made up of tubes of coarse sand grains cemented together, forming rounded golden-brown hummocks, sheets and mounds. The reefs have an important stabilising influence on the shore by consolidating the cobbles and boulders and provide an important habitat for diverse communities of algae and animals. They are the most extensive honeycomb worm reefs in Cardigan Bay. Species associated with the reefs include the sponge Hymeniacidon perleve, the breadcrumb sponge Halichondria panicea, snakelocks anemone Anemonia viridis, dahlia anemone Urticina felina, the barnacle Balanus crenatus, and the bryozoan Flustrellida hispida. The brown algae Cladostephus spongiosus and toothed wrack Fucus serratus and the red algae Gelidium pusillum and Gymnogongrus crenulatus are frequent. The biogenic reef areas are occasionally interspersed with expanses of mobile sand which support communities of burrowing polychaete worms, such as the lug-worm Arenicola marina, the tube-worm Scolelepis squamata, Spirorbis tridentatus and the sand mason worm Lanice conchilega. There are also amphipods including Haustorius arenarius. Where the reefs are interspersed with boulders, these are dominated by bladder wrack and lava bread Porphyra linearis, or mixtures of barnacles such as Semibalanus balanoides and Chthamalus montagui. Species rich shallow rockpools, formed within the biogenic reefs, are the most extensive within Cardigan Bay. These rockpools support a specialised community of coralline algae including species such as Corallina officinalis and Lithophyllum incrustans. Other species include butterfish Pholis gunnellus, shanny Lipophrys pholis, green leaf worm Eulalia viridis and the starfish Asterias rubens. The pools support a variety of red algae including Ahnfeltia plicata, purple lava Porphyra purpurea, carragheen Mastocarpus stellatus, pepper dulse Osmundea pinnatifida and Dumontia contorta. Bladder wrack and C. spongiosus are the dominant brown seaweeds. The pools also provide shelter for numerous invertebrates. These include the barnacles S. balanoides and C. montagui, the shrimp Crangon crangon, the hermit crab Pagurus bernhardus and beadlet anemone Actinia equina. The shellfish include a diversity of species such as the dog whelk Nucella lapillus, the littorinids Littorina littorea and Littorina obtusata, the flat top shell Steromphala umbilicalis and chitons including Lepidochitona cinerea. Upper shore rockpools of varying salinity support communities of ephemeral green algae, along with periwinkles and common limpet Patella vulgata. Hard cliffs in the north of the site are characterised by the orange lichen Caloplaca marina, black-tar lichen Verrucaria maura and small periwinkle Melarhaphe neritoides. Areas of the upper shore influenced by freshwater runoff are dominated by ephemeral green seaweeds such as Enteromorpha species and the scarce Prasiola stipitata. Shell fish such as the grey top shell Steromphala cineraria and rough periwinkle Littorina saxatilis are also present. Of more local interest is the maritime grassland at Tonfanau which includes the following frequent species, common bent Agrostis capillaris, buck’s-horn plantain Plantago coronopus (abundant), bulbous buttercup Ranunculus bulbosus, common stork’s bill Erodium cicutarium, sheep’s-fescue Festuca ovina, crested dog’s tail Cynosurus cristatus and the lichen Cladonia rangiformis (abundant). Common milkwort Polygala vulgaris is also recorded here. Sand martin Riparia riparia breed within the southern cliffs and eider Somateria mollissima have nested on the shore. This is believed to be only the second record of nesting eider in Wales. Fulmar Fulmarus glacialis, herring gull Larus argentatus, raven Corvus corax , peregrine Falco peregrinus and chough Pyrrhocorax pyrrhocorax breed along the cliffs. Cormorant Phalacrocorax carbo use the cliffs for roosting and most of these are likely to come from the nearby breeding colony at Craig yr Aderyn (Bird Rock) SSSI. At high tide grey seal Halichoerus grypus, harbour porpoise Phocoena phocoena, bottlenose dolphin Tursiops truncatus and common dolphin Delphinus delphis have all been recorded within the site. Grey seal on haul-out have been noted at Sarn y Bwch and otter Lutra lutra is thought to use the intertidal area for feeding. Remarks: The site is adjacent to the Pen Llín a’r Sarnau Special Area of Conservation and Broadwater SSSI. The site supports otter, grey seal, harbour porpoise, common dolphin and bottlenose dolphin, species which are listed under Schedule 5 of the Wildlife and Countryside Act 1981, as amended and under Annex II (with the exception of the common dolphin) and IV of the European Directive 92/43/EEC on the Conservation of Natural Habitats and of Wild Fauna and Flora. Parts of the site above mean high water lie within the Snowdonia National Park.

Species mentioned