The shire is almost wholly mountainous, although there are some fertile valleys in the east. The highest point (county top) is Moel Sych at , whose summit lies at the triple county boundary point of Montgomeryshire, Denbighshire and Merionethshire in the Berwyn Mountains. The county top of Denbighshire, Cadair Berwyn at , is less than a kilometre away. Its main rivers are the River Severn (which flows east into Shropshire) and the River Dyfi (which flows west into the Irish Sea). Lake Vyrnwy is a reservoir supplying Liverpool.