The geology of Anglesey

© Craven & Pendle Geological Society


Cae'r Sais

Serpentinites / Metagabbros

At Cae'r Sais (SH 268 772) there is a small abandoned road-side quarry which is easily located on route from South Stack to Rhoscolyn. There is a school nearby with good parking in a layby. This locality is of interest because it preserves one of the best exposures of metagabbro and serpentinite in Wales. Serpentinite-metagabbro complexes are unusual in the UK. Here their origin, tectonic setting and geochemical affinity remain uncertain. This is not surprising since these these ultrammafic rocks are difficult to provenance as they are altered. Another issue surrounding the ultramafic rocks of the New Harbour Group is whether they are intrusive or have actually been slid into the sediments.

At outcrop the serpentinite is seen to be typically dark green with a scaly foliated surface. Prior to metamorphism, the original rock may have been related to dunites and harzburgites - both of which contained olivine in abundance. Rumour has it that Napoleon Boneparte had a table-top made out of this serpentinite!


Across the track lies good exposures of the pale green metagabbro. These rocks referred to as ‘altered gabbros’ by Greenly (1919) seem to have a penetrated the serpentinites. Even though the original gabbro has been metamorphosed, the original igneous texture is in part intact. Petrological studies of these rocks show a distinct basaltic character suggesting that they were part of an ancient ophiolite complex. Phillips, (1989) likened their chemistry to the serpentinites and gabbros found at the mid-Atlantic ridge.


Oceanic igneous rocks are not often preserved in the geological record. Perhaps we are fortunate to have examples at Cae’r Sais and similar rocks at the Lizard, Cornwall. The fact that the serpentinites and metagabbros are easily accessibe and lithologically unusual will undoubtedly continue to make this site of interest to petrologists. It is considered one of the best sites for studying the plutonic, metamorphosed igneous rocks of the New Harbour Group.

CPGS members at the Serpentinite outcrop