Snowdonia National Park, Welsh Parc Cenedlaethol Eryri, national park in Gwynedd county and Conwy county borough, northern Wales, with an area of 838 square miles (2,171 square km). It is best known for its mountains, composed largely of volcanic rock and cut by valleys that show the influence of Ice Age glaciers. Snowdon mountain’s summit, Yr Wyddfa, in the northwestern part of the park, is the highest peak in England and Wales, with an elevation of 3,560 feet (1,085 metres). A rack-and-pinion railway (opened 1896) runs from Llanberis to the summit. Farther south Cader Idris (“Chair of Idris”), a long mountain ridge, reaches a height of 2,927 feet (892 metres) at Pen-y-Gader.