Geology and landscape

The rocks that form Yr Wyddfa are approximately 450 million years old, dating to the Ordovician period. During the Ordovician, the area that is now North Wales lay near the edge of the microcontinent Avalonia, submerged beneath the Iapetus Ocean. Intense submarine volcanic activity — associated with a fault system known as the Beddgelert Fracture Zone — deposited enormous thicknesses of volcanic material.

The dominant rock type is rhyolitic tuff: the compacted ash from violent pyroclastic eruptions. These ash-flow deposits, known as the Snowdon Volcanic Group, are in some places up to 500 metres thick. They are accompanied by flows of rhyolite lava and, at lower levels, basalt — a characteristic bimodal volcanic suite typical of continental margin settings. The subsequent Caledonian Orogeny — the mountain-building event that closed the Iapetus Ocean — subjected these rocks to intense folding, faulting, and low-grade metamorphism, and created the copper-lead-zinc mineralisation that would later be exploited in the Britannia Copper Mine.

The dramatic landscape visible today was carved over approximately two million years of repeated glaciation, culminating in the Devensian glaciation that peaked around 20,000 years ago. At that point the entire mountain — and most of Britain north of a line through the Thames Valley — was buried beneath an ice sheet.

The glaciers carved the landscape with extraordinary precision. Valley glaciers excavated the U-shaped passes — Llanberis Pass, Nant Gwynant, Nant Colwyn — deepening and widening pre-existing river valleys into the broad, flat-floored corridors visible today. Cirque (or cwm) glaciers occupied the mountain’s eastern hollows, scouring out the classic armchair-shaped basins — Cwm Glaslyn, Cwm Llan, Cwm Clogwyn — that now hold Snowdon’s lakes. The ridges between adjacent cirques were sharpened into the knife-edge arêtes of Crib Goch and Crib y Ddysgl; the summit itself was given the classic pyramidal form of a nunatak — a peak that projected above the surrounding ice.

The result is a mountain that reads almost as a geology and glaciology textbook made visible: cwms, arêtes, a pyramidal peak, ribbon lakes in hanging valleys, and the characteristic asymmetry of glacially modified terrain.

Yr Wyddfa is surrounded by a series of lakes occupying glacially excavated basins at different elevations. Each has a distinct character. Llyn Llydaw — the largest, at 45 hectares and 58 metres deep — occupies the main cwm east of the summit and is famous for its causeway and the Horseshoe reflection. Its water carries a slight copper colouration from centuries of mining runoff and a naturally elevated copper content in the bedrock. Glaslyn — directly below the summit crags at 39 metres deep — has a distinctive blue-green tint from dissolved copper compounds, intensified during the mining era. Its colour is striking and gives the lake its Welsh name, meaning Green Lake or Lake of the Blue Spring.

At lower elevations, Llyn Padarn in the Llanberis valley is a Site of Special Scientific Interest partly because of its population of Arctic char — a relict fish species stranded in the lake when it was isolated from the sea at the end of the last Ice Age. The fish has changed little from its glacial-period ancestors and is considered a living indicator of the lake’s ancient origins. Llyn Cwellynon the western flank is managed as a reservoir.

Yr Wyddfa does not stand alone. It is the highest point of a massif of interconnected peaks. Garnedd Ugain (also known as Crib y Ddysgl, 1,065 m) is Wales’s second-highest summit — a mere 20 metres lower — and forms part of the famous Snowdon Horseshoe ridge walk. Crib Goch (923 m) is the narrow, exposed eastern arête — one of Wales’s most iconic ridges and a serious Grade 1 scramble. Y Lliwedd (898 m) forms the southern arm of the Horseshoe and is associated in legend with the sleeping warriors of King Arthur.

The Snowdon Horseshoe — traversing Crib Goch, Garnedd Ugain, Yr Wyddfa, and Y Lliwedd in a single circuit — is consistently rated among the finest mountain walks in Britain and, by some accounts, in Europe. It requires technical scrambling on Crib Goch and is appropriate only for experienced walkers with good navigation and a comfortable head for heights.