The Arenig Mountains
Isolated Peaks Formed by Resistant Volcanic Rocks on the North-eastern Margin of the Harlech Dome.
The Arenig Mountains lie to the north of the Aran Range (as we saw in the photo on the previous page) but are relatively isolated peaks which stand proud above the mostly bleak, rolling moorland to the west of Bala Lake. Here they are viewed from the north-west, from the main A4212 Trawsfynydd to Bala road, so we are looking at the opposite, north-western side of the range, compared to the previous photo. Arenig Fawr (2801ft/854m is the twin peaked mountain, whilst Moel Llyfnant (2461ft/ 751m) lies out of view to the far right. As can be seen in this photo the main Arenig peaks look to be composed of sub-horizontal rock bands. These are in fact layers of volcanic ash, ash flow and air-fall tuffs, and associated igneous intrusions that form most of the Llyn Conwy Formation, which is again the northward continuation of the thick (and quite extensive!) Aran Volcanic Group we saw in the upper face of Cadair Idris, and along the crest of the Aran Mountains in the previous two pages. Once again, it is the relatively resistant nature of these thick sequences of volcanic and intrusive rocks which have resulted in the formation of nearly all the higher mountains of southern and eastern Snowdonia, whereas most of the surrounding countryside of lower moorland and valleys are underlain by the softer, more easily eroded marine mudstones and siltstones which continued to be deposited in the back-arc basin environment (the Serw and Nant Hir Formations here, which are the equivalent of the similar Nant Francon Formation sediments further north in the Moelwyn, Snowdon, Glyder, Carneddau and other mountains, which we will see later). Whilst the Early Ordovician volcanoes mostly erupted onto the sea floor, through fissures in the underlying, stretched and thinned continental crust of Avalonia, in some locations the accumulations of volcanic rocks, and the injection of thick intrusive igneous rocks were voluminous enough to break the ocean surface, as is clearly indicated by their localised surface erosion and re-working by ocean waves and currents at some localities.