The geology of Anglesey

© Craven & Pendle Geological Society

South Stack Lighthouse, Anglesey

South Stack Lighthouse
Type section for the South Stack Group. Video Clip: South Stack

A visit to the cliffs at South Stack lighthouse is a popular destination for sightseers on Anglesey. It is also a major attraction to ornithologists for viewing guillemots, razorbills, kittiwakes and puffins. As the photograph (right) shows, South Stack Lighthouse is also of interest to photographers. For the geologist, it is a good place to see Greenley's "Bedded Sucession", a part of the Monian Supergroup of Holy Island. The rocks here are represented by sandy turbidites (psammites) and schistose mudstones (pelites) of the South Stack Group. Overlying the SSG is the New Harbour Group followed by the Gwna Group melánge (which is more intriguing if a little less well understood).

Anticline in the South Stack Formation

Having left the cars the best way to view these rocks is to descend the 400 steps to the bridge connecting South Stack Lighthouse to Holy Island. The cliffs show some of the best exposures of folded metasedimentary rocks in the UK. The photograph (right) shows an anticline in the South Stack Formation below the lighthouse, whilst the photograph below is taken near the bottom of the steps and shows intricate folding in the Pelites (Schistose Mudstones) of the Holyhead Formation.

Edward Greenly (1919: Memoir of the Geological Survey) described these rocks as an ‘amazing revelation’ because of the spectacular fold structures. The folded turbidites are clearly seen in the cliffs, alongside the steps and on South Stack itself. There are dramatic large scale folds with small scale folds superimposed upon them. Cleavage is also present in the fine-grained rocks.

Schistose mudstones (Pelites) by the steps

The age of the South Stack Group (the basal unit of the Monian Supergroup) has been controversial. Previously the age of the rocks have been constrained between Neoproterozoic and Lower Ordovician times. This was determined by the presence of fossils in the melánge blocks and an unconformably overlying Upper Arenig sequence. Gibbons, Horák (1990) and Strachan (2000) had tied the age of these rocks to late Neoproterozoic and Early Cambrian times. However, work on detrital zircon in the South Stack Formation (Collins & Buchan 2004) suggest that deposition of these rocks took place between Mid Cambrian and late Arenig times, i.e. between c. 500 and 475 Ma i.e. not Precambrian after all! By inference the New Harbour Group and Skerries Formation are likely to be Cambrian age as well. Recent research in 2011 suggest that the Gwna Group may be the oldest Precambrian in southern Britain (somewhere between 600 - 800 MA).

Tectonic slice the accretionary prism