The First Signs of Ordovician Volcanism in North Wales
Looking due west from the summit of Aran Fawddwy (2971ft/907m) in the Aran Mountains near Bala. Rhobell Fawr, the rough, broad, dome shaped mountain at centre right in sunlight, is a 2408ft/734m high mountain which sits just to the east of the main Harlech Dome structure and the Rhinog Mountains, which are seen along the skyline here, and were described on the previous page. The main summits in the distance are, from left to right, the broad domes of Diffwys and Y Llethr, the flat topped Rhinog Fach and more pointed Rhinog Fawr (both seen on the previous page) and the knobbled ridge of Moel Ysgyfarnogod on the far right-hand skyline). Whilst the distant Rhinog Mountains are composed mainly of coarse sedimentary grits and sandstones of Cambrian age, the nearer summit of Rhobell Fawr is entirely different, and represents the eroded remains of a sub-aerial basaltic volcano which erupted onto the land surface of the Avalonian micro-continent as part of the first series of eruptions in the Early Ordovician Period (the late Tremadoc age Rhobell Volcanic Group). Not only does Rhobell Fawr still preserve many of these sub-aerially erupted basaltic lava flows, but it also exposes many of the underlying dolerite and diorite dykes and other igneous intrusions from which most of these lavas were erupted. These intrusions rose up through a series of mainly north-south oriented fissures which probably formed zones of crustal weakness along which lava was able to force its way to the surface. However, some of these fractures and associated faults also radiate out across the uplifted dome like structure of the Harlech Dome and may therefore also represent the leaking of magma through cracks in the uplifting dome itself. Although Rhobell Fawr represents the earliest known volcanic site in the Ordovician volcanic sequence in North Wales it is far from the last. It is the changes in location and timing of the following episodes of volcanism in North Wales, and there effects on the landscape of Snowdonia, that we will continue to investigate in the following pages.