The River Teifi flowing through raised bog habitat at Cors Caron Bog, near Tregaron, West Wales. Raised bogs develop from a lake or flat marshy area. Over centuries there is a progression from open lake, to marsh, until peat deposit fills the lake. Eventually peat builds up from decaying sphagnum moss to a level where the land surface is too flat for ground or surface water to reach the centre of the wetland and a raised bog is produced.