Moelypenmaen

Also within

Gwynedd County

Geology & landscape

Geological Period: Ordovician

Landscape processes: Karst, Volcanic / Igneous

Description

Description Moelypenmaen is of special scientific interest because it provides fine exposures of intermediate extrusive volcanic rocks belonging to the Llanbedrog Volcanic Group. This group records the third of four phases of magmatic activity which occurred on Llín during mid-Caradoc (Upper Ordovician) times. The site is locatedi approximately 3.5 km south-east of Nefyn and incorporates a craggy hill, rising to an altitude of 153 m, on which are situated several small, long-disused quarries and numerous natural rock outcrops. The Llŷn Volcanic Complex, of Caradoc age, consists of predominantly silicic volcanic rocks and genetically related sub-volcanic granitoid intrusions. The rocks can be subdivided, on the basis of their geochemistry, into subalkaline and peralkaline types, the latter being especially significant as they indicate magma generation in a tensional tectonic setting. The craggy outcrops on Moelypenmaen expose a series of Ordovician silicic tuffs and agglomerates together with intercalated lavas of basic to intermediate composition. Of special significance are trachyandesitic lava flows which show a geochemical affinity with the peralkaline granitic rocks of the area. This affinity is the basis for ascribing an Ordovician age to the intrusions and the interpretation that they represent a sub-volcanic intrusive complex. It is also the basis for arguing that the intrusive rocks have, like the lavas, evolved through fractional crystallization of basaltic magma.

Species mentioned