The geology of Anglesey

© Craven & Pendle Geological Society

Rhoscolyn Headland

Rhoscolyn Headland

As at South Stack lighthouse, the Rhoscolyn headland is composed of rocks belonging to the South Stack Group and as such is likely to be Cambrian in age (Collins and Buchan 2004). This is also the type locality for the Rhoscolyn Formation which is the youngest strata in the South Stack Group.

The main interest on the Rhoscolyn headland is the presence of a prominent NE plunging fold known as the Rhoscolyn Anticline (Anticlin Rhoscolyn). Whilst the core of the anticline runs beneath the Coastguard Lookout, it can be viewed in spectacular cliff sections to the south-west. The photograph above shows the precarious nature of the exposures so care must be taken at all times. It is a remarkable field laboratory for the study of three dimensional fold geometries and cleavage-bedding relationships therefore making it a major attraction to A-Level geologists and undergraduates.

Quartz-veined schistose mudstones of the Rhoscolyn Formation

Similarly to South Stack, these bed at Rhoscolyn are made up of turbidite psammites (quartzites) and pelites (schistose mudstones). Although metamorphosed and highly deformed, there are sections where sedimentary structures can be observed e.g. flute casts at Bwa Gwyn (White Arch). These are sedimentary scour-and-fill structures formed on an ancient sea-bed by high-energy turbidity currents. If you look closely you may observe other sedimentary structures such as cross-bedding and graded bedding especially in the quartzites of the Rhoscolyn Formation. The photograph above shows spectacular folding on the south-east limb of the Rhoscolyn Anticline. The view is down-plunge to the north east.

On leaving the area surrounding the Coast Guard lookout where the core of the plunging Rhoscolyn Anticline is best exposed, follow the footpath north west towards St. Gwenfaen's Well.

Porth Saint

Follow the headland around now heading north-east towards Porth Saint. This locality is very colourful in part due to the rusty colouration of the some of the beds. The southern side of the bay exposes mainly turbidites whilst the northern side is in quartzitic sandstones which can be seen to be folded into a gentle syncline. Note that the haematite bed lies along a rust-stained fault plane.

A little further north still lies Bwa Gwyn (the White Arch). This locality lies in the Rhoscolyn Formation and is predominantly composed of white quartizitic rocks (massively bedded sandstones that have been subjected to low grade metamorphism). Curiously there is a grindstone wheel on top of the arch indicating that this was once the scene of industrial activity.

A the base of the cliff next to the white arch is a small sea cave. In the roof of this cave is an enormous flute cast (a sedimentary scour-and-fill structure). Unfortunately I don't have a photograph to show you. May be next time!

Bwa Gwyn (White Arch)

To summarise: the coastline at Rhoscolyn is one of the best places in the UK to study the effect of folding and cleavage formation on such a large asymmetric fold. This sequence of bedded sedimentary rocks was deposited in a turbidite fan environment. Research work on these rocks proved conclusively that the South Stack Group is older than the overlying New Harbour Group.