The geology of Anglesey

© Craven & Pendle Geological Society

View of Snowdonia from Llanddwyn Island

Newborough (Niwbwrch)

Pillow Basalt Lavas of Llanddwyn Island

(SH 390 630)

Back in the 16th Century Newborough (in Welsh: Niwbwrch) was the County town of Anglesey. By the car park and beach there is plenty of marram grass which in years gone by formed the basis of a thriving industry producing matting, nets and rope. Much of the area is now a Nature Reserve and if the beach doesn't appeal to you there are splendid walks in the 810 hectares of woodland.

Pillow lavas

When you arrive at Newborough take the Forestry Road leading to a Newborough Warren car park (£2). There are toilets at the car park. Walk west along the beach to a narrow tidal isthmus projecting south-west from the Anglesey coastline. It is perhaps a good place for couples to visit because the name Llanddwyn means "The church of St. Dwynwen". She happens to be the Welsh patron saint of lovers!

Llanddwyn remains attached to the mainland at all but the highest tides and is a European Blue Flag Winning beach. It provides excellent views of the Snowdonia National Park and the Lleyn Peninsula and is part of the Newborough Warren National Nature Reserve. No hammering please.

Having walked to Llanddwyn Island you will see some of the best examples of oceanic basaltic pillow lavas in the UK. It is the type locality of Greenly’s ‘Spilitic Lavas’, which form part of the Gwna Group mélange - the youngest unit in the Monian Supergroup.

Limestone and brecciated basalt
Tyfry Formation (Green sandstones)
Bedded Jaspery Cherts
Dolerite Dyke

On the island the rocks are steeply dipping to vertical lavas, limestones and other sedimentary rocks trending the length of the isthmus in a NNE - SSW direction. This site is excellent for demonstrating the nature of basaltic pillow lavas as erupted on the sea bed. There is evidence to suggest that they have a deep water origin as they are associated with cherts and micritic limestones. The pillow structures allow the way-up of the sequence to be worked out and in this case they seem to young towards the south-east. There are sandstones, shales, and superb red jaspery cherts whose age has proved to be quite controversial since possible Cambrian micro-fossils were discovered in them.

The Gwna Group on Llanddwyn Island is much more than just a series of spectacular pillow lavas. There are dolerite dykes, and rocks belonging to the Tyfry Formation (dark green fine-grained sandstones, Limestone commonly mixed with basalt (pale pink and cream limestones frequently mixed with brecciated basalt), and Bedded Jaspery Cherts (perhaps the best example of deep-water cherts in North Wales).

The tectonic setting of these beds is still an area of intense discussion. The basaltic lavas and breccias are perhaps ancient remnants of the ocean floor that may have got caught up in the accretionary prism in the subduction zone. The Tyfry Formation may have been laid down on the sea bed in a basin setting above the accretionary prism.

Summary

The pillow lavas at Llanddwyn are as good as it gets in Great Britain and as such they have been designated the type locality for these kind of rocks.

In combination with the limestones and lavas, Llanddwyn Island is one of the most spectacular field localities in the UK.