At a glance
Cors Erddreiniog (LL77 7UR) — large calcareous fen NNR on central Anglesey. Marsh helleborine and orchids (June–August), otter, water rail, snipe. Free. Path network with some boardwalk — wellies needed. No on-site facilities. Car recommended (3 miles from Llangefni).
About Cors Erddreiniog
Central Anglesey holds a hidden landscape of wetlands and fens — and Cors Erddreiniog is the finest of them. Fed by calcium-rich groundwater flowing through glacial deposits, this calcareous fen is one of the rarest habitats in Wales: alkaline, wet, and supporting plants that cannot survive in the acid bogs that dominate the Welsh uplands. In late June and July, the fen meadows are spectacular with orchids — marsh helleborine, southern marsh orchid, fragrant orchid — and the drone of specialist insects that depend on them.
The fen's channels hold otters; the reed beds conceal water rail, sedge warbler, and snipe. On an island more often celebrated for its red squirrels and coastline, Cors Erddreiniog is a quieter and more specialist reward — a National Nature Reserve that most visitors to Anglesey never find, and that rewards the effort of finding it with a botanically rich landscape unlike anything else on the island.
Find it on the map
Frequently asked questions
Cors Erddreiniog (the name translates roughly as "the fen of the blackthorn") is a large calcareous fen in central Anglesey — one of the largest and most botanically important fens in Wales. It is a National Nature Reserve (NNR) and a Site of Special Scientific Interest (SSSI) managed by Natural Resources Wales. Calcareous fens are rare habitats worldwide: they form where groundwater rich in calcium carbonate (from limestone bedrock or glacial deposits) flows through low-lying ground, creating alkaline wet conditions quite different from the acid bogs more typical of upland Wales. Cors Erddreiniog covers several hundred hectares and includes open fen, fen meadow, reed beds, carr woodland, and small pools, making it one of the most diverse wetland habitats in north Wales.
Cors Erddreiniog supports a remarkable range of wetland wildlife. For plants, the fen is most famous for its orchids: marsh helleborine (Epipactis palustris) flowers in late July and August in spectacular numbers in the fen meadows; southern marsh orchid, early marsh orchid, and fragrant orchid are also present. Other notable plants include grass-of-Parnassus, marsh pennywort, and several nationally scarce sedges. For birds, water rail are heard (their pig-like squealing call) year-round in the reed beds; sedge warbler, reed bunting, grasshopper warbler, and snipe breed in the fen; in winter, bittern have been recorded. Otter are present on the fen channels and pools and are best looked for at dawn and dusk. Nationally important invertebrate populations include fen-specialist beetles, hoverflies, and the rare fen raft spider (Dolomedes plantarius), which is one of Britain's rarest spiders.
June and July are the best months for orchids — marsh helleborine in particular is at its peak from late June into early August, and the fen meadows can be spectacular with flowering plants at this time. Late May and June are also good for breeding birds: reed bunting, sedge warbler, grasshopper warbler, and snipe are all active and visible. Autumn (September–October) brings migrants through the reed beds and the fen takes on a golden quality with the turning reed and sedge. Winter visits are atmospheric but the ground is at its wettest and most difficult — wear wellies and expect mud. The fen's botanical interest extends throughout the growing season (April–September), making it an excellent destination for plant enthusiasts in any summer month.
Cors Erddreiniog has a path network that allows access to the principal habitats without crossing the wettest areas of the fen. Some paths are surfaced or boardwalk; others are rough tracks across fen meadow. Wellingtons (or waterproof walking boots) are strongly recommended even in dry summer weather — the fen is groundwater-fed and remains wet throughout the year. The NNR has no visitor facilities (no car park, toilets, or interpretation centre on site), so come self-sufficient with a map (OS 1:25,000 Explorer 263 covers Anglesey). The Small car park or roadside pull-in near the LL77 7UR postcode gives the most direct access. The interior of the fen is not accessible without expert guidance.
Cors Erddreiniog is one of several important nature reserves on Anglesey but is distinct in character from the others. Newborough Forest (NNR) and Newborough Warren are famous for red squirrels, Corsican pine, and the dune system leading to Llanddwyn Island — a landscape of dry pine woodland and coastal dune. Cors Erddreiniog is a contrasting wetland habitat — flat, open, and botanical rather than dramatic. South Stack on Holy Island offers spectacular sea-cliffs and seabird colonies (puffin, razorbill, chough). Together, these reserves demonstrate the remarkable diversity of habitats packed into Anglesey: dry heath, blanket bog, coastal dune, sea-cliff, and calcareous fen all within a small island. Anglesey's geological diversity (including limestone outcrops that create the alkaline conditions at Cors Erddreiniog) underpins this habitat richness.