Alkaline fens are soligenous in nature, being fed by the rise of springs or an inflow of surface water run-off from surrounding slopes. The vegetation is characterised by peat or tuft-producing small sedge and brown moss communities developed on permanently waterlogged soils with a calcareous water supply. These species-rich habitats, both for bryophytes and flowering plants, are unevenly and locally distributed in Wales, with particularly important examples found on the Anglesey fens, the Lleyn fens and at Cwm Cadlan (in SE Wales).
