Crib Goch ridge and the valley of Cwm Uchaf, north-eastern Snowdon Massif. This landscape is composed of some of the highest, and therefore youngest, sequences of Ordovician volcanic rocks which are exposed in Snowdonia today (The Snowdon Volcanic Group). In the foreground and in the green tilted cliffs behind, coarse grained, thick beds of basaltic, volcaniclastic sediments of the middle part of this group (the Bedded Pyroclastic Formation) were erupted from the chain of volcanic islands onto Avalonia but were largely laid down in a marginal marine environment where they were re-worked by shoreline and marine processes. A very thick unit of massive, acid ash flow tuffs, the Lower Rhyolitic Tuff Formation, sits just beneath these rocks (out of sight here) and forms most of the steep cliffs south of Llanberis Pass, but also most of the peaks of Snowdon and Y Lliwedd to the south, as well as the cliffs of Cwm Idwal, in the Glyder Range to the north. The steep back wall of Crib Goch ridge in the background is formed by a probably contemporaneous rhyolite intrusion, which in some places clearly shows hexagonal