The source of the River Severn
An area of marshland characterised by peat bog forms the source of the River Severn. The source of the River Wye can also be found on the same mountain (Plynlimon). Plynlimon is 610 metres (a.s.l.) and receives on average over 2000 mm of rain per year. It has an impermeable geology comprised of Ordovician - Silurian Period (c. 450 million years ago) shales, grits and munstones. This impermeable geology contributes to the 'flashy' storm hydrographs recorded in the upper catchment of the River Severn in Wales.