The Cambrian Mountains cover much of the east of the county; this large area forms part of the desert of Wales. In the south and west, the surface is less elevated. The highest point is Pumlumon at , other Marilyns include Pen y Garn and Llan Ddu Fawr. On the slopes of Pumlumon five rivers have their sources: the Severn, the Wye, the Dulas, the Llyfnant and the Rheidol, the last of which meets the Afon Mynach in a plunge at the Devil's Bridge chasm. The largest river is the River Teifi which forms the border with Carmarthenshire and Pembrokeshire for part of its length. The towns of Lampeter, Llandysul, Newcastle Emlyn and Cardigan are all in the Teifi Valley, and each has communities on each side of the river, in different counties. Other significant rivers include the River Aeron which has its estuary at Aberaeron, and the River Ystwyth and the River Rheidol both of which reach the sea in Aberystwyth harbour.