mountain in northern Wales that is the highest point in England and Wales and the principal massif in the Snowdonia mountains. It is located in the county of Gwynedd and the historic county of Caernarvonshire. Snowdon consists of about five main peaks that are connected by sharp ridges and between which lie cirques (scooped-out basins). The highest of these peaks is Yr Wyddfa, which reaches an elevation of 3,560 feet (1,085 metres). Snowdon is composed mainly of slates and porphyries that date from the Ordovician Period (490 million to 443 million years ago). Intense glaciation has affected the entire locality, producing the ridges, cirques, and numerous small lakes that nestle in the lower valleys radiating out from Snowdon. The mountain is ascended by a rack-and-pinion railway that runs from Llanberis to the summit of Yr Wyddfa. Snowdon is the chief attraction of Snowdonia National Park.