Pen Pumlumon Arwystli is the second highest point on the Plynlimon massif, a part of the Cambrian Mountains in the county of Ceredigion, Wales. The summit is grassy, and is marked by 3 large ancient cairns. The views include Rhos Fawr, Drygarn Fawr, Pen y Garn to the south and Aran Fawddwy, Glasgwm, Tarrenhendre and Tarren y Gesail to the north. The longest river in Britain, the River Severn, has its source on a lower boggy plateau to the east of the summit.