Geology of Anglesey

Enjoy Your Visit

Brittania Bridge

Introduction (Cyflwyniad)
Anglesey or Anglesea (Welsh: Ynys Môn pronounced “Uh-niss Morn”), is an island and county off the northwest coast of Wales. The island is separated from the mainland by the Menai Strait. The link to the mainland is made by two bridges, the original Menai Suspension Bridge, built by Thomas Telford in 1826 as a road link, and the Britannia Bridge, carrying the A55 and the North Wales Coast Railway line. The original Britannia Bridge was commissioned because of the need for a rail route to link London with Dublin. It was engineered by Robert Stephenson in 1850. Following a fire in 1970, the Britannia Bridge was reconstructed. Scaled down versions of both the Menai & Britannia bridges are in use in Conwy.

Historically, Anglesey has long been associated with the Druids. In c. AD 60 the Roman General Suetonius Paullinus, determined to break the power of the druids, attacked the island, destroying the shrine and the sacred groves. The Romans called the island Mona. Following the Romans, the island was invaded by Vikings, Saxons, and Normans before falling to King Edward I of England, in the 13th century.

Llanddwyn Island Pillow Lavas

From the 18th century onwards Anglesey became important for two reasons:

a) Copper deposits as mined at Parys Mountain.

b) Sailing to Ireland.

One of the highlights of a visit to Anglesey is the varied natural history. Botanists and ornithologists are attracted to Anglesey's magnificent coastline, gardens and country parks. Anglesey also offers a wealth of opportunities for walkers, especially as the landscape is gently undulating and is therefore suitable for all ages and abilities.

For the more energetic there are a good selection of golf courses, climbing and horse riding. Fishing is varied and the beaches are excellent. Many of the beaches are unspoilt and award winning e.g. European Blue Flag Resort Seaside Awards go to Llanddwyn (Newborough), Trearddur Bay, Traeth Mawr, Cemaes Bay, Benllech and Llanddona. With all of these opportunities in mind, one thing is for sure - the geology of Anglesey is as good as anything else Anglesey has to offer (or better than. . . . !).

The reason for setting up a week-end of geology in Anglesey is two fold:

1. To visit a geological locality that I was introduced to as a student in 1974 (see photo below) at Crewe & Alsager College by Dave Scott and Dave Beesley.

2. To have a look at the nearest exposures of Precambrian / Cambrian rocks to where CPGS is based in NE Lancashire.

© CPGS 1974 - PK under the hand!

Geological background
The geology of Anglesey was originally described and mapped by Edward Greenly - The Geology of Anglesey (1919). Memoir of the Geological Survey of Great Britain, No.78, H.M.S.O., London, 980pp (2 vols). Since then, Anglesey has attracted much attention over the years, and still continues to interest geologists today.

It was Greenly who first coined the term Mona Complex to describe the Precambrian rocks of Anglesey. Subsequent workers, in particular, Shackleton (1950's to 1970's) and more recently Gibbons and Horák, have contributed greatly to our understanding of the geology of Anglesey. The map below shows where the Precambrian rocks are located in terms of the three main terrane boundaries and also which localities we visited.

Precambrian rocks can be seen to occupy over half the island's interior in four separate districts. They have considerable geological importance, are very well preserved, and are a good example of processes operating at the boundaries of crustal plates. These fault-bounded blocks or terranes are collectively recognised today as the Monian Composite Terrane. This terrane is made up of smaller crustal fragments known as the Monian Supergroup, the Coedana Complex and the Blueschist Belt. These are overlain by Palaeozoic volcanic and sedimentary rocks.

The geology of Anglesey fits into a plate tectonic model making the complex volcanic, metamorphic and palaeogeographic story much easier to understand.

Field Localities

O.S. Maps:
1:25 000 Explorer 262 Anglesey West

1: 25 000 Explorer 263 Anglesey East.

Barber, A.J. & Max, M.D. (1979): A new look at the Mona Complex (Anglesey, North Wales). Journal of the Geological Society, London, 136, pp. 407–432

Carney, J.N., Horák, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D. (2000): Precambrian Rocks of England and Wales. Geological Conservation Review Series, Joint Nature Conservation Committee, Peterborough, 20.

Collins, S.A. and Buchan, C. (2004): Provenance and age constraints of the South Stack Group, Anglesey, UK: U-Pb SIMS detrital zircon data. Journal of the Geological Society, London, 161, pp. 743–746

Conway, J. (2010): Rocks and landscapes of the Anglesey Coastal Footpath. Produced by GeoMon Anglesey Geopark. ISBN 0-9546966-3-8

Dallmeyer, R.D. & Gibbons, W. (1987): The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40Ar/39Ar mineral ages of the Penmynydd Schists. Journal of the Geological Society, London, 144, pp. 843–850

Gibbons, W., Horák, J.M. (1990): Contrasting metamorphic terranes in northwest Wales. In: D'Lemos, R.S., Strachan, R.A. & Topley, C.G. (eds) The Cadomian Orogeny. The Geological Society, London, Special Publications, 51, pp. 315–328.

Gibbons, W., Tietzsch-Tyler, D., Horák, J.M. & Murphy, F.C. (1994): Precambrian rocks in Anglesey, southwest Llyn and southeast Ireland. In: Gibbons, W. & Harris, A.L. (eds) A revised correlation of Precambrian rocks in the British Isles. Geological Society, London, Special Reports, 22, 75–83.

Gibbons, W. & Horák, J.M. (1990): Contrasting metamorphic terranes in northwest Wales. In: D'Lemos, R.S., Strachan, R.A. & Topley, C.G. (eds) The Cadomian Orogeny. Geological Society, London, Special Publications, 51, pp. 401–423.

Gibbons, W. & Horák, J. M. (1996): The evolution of the Neoproterozoic Avalonian subduction system: Evidence from the British Isles. In: Nance, R.D. & Thompson, M.D. (eds) Avalonian and related peri-Gondwanan terranes of the circum-Atlantic. Geological Society of America Special Papers, 304, pp. 269–280.

Greenly, E., (1919): The geology of Anglesey. Memoir of the Geological Survey of the U.K. [2 vols, 980 pp.].

Horák, J.M., (1993): The Late Precambrian Coedana & Sarn Complexes. PhD thesis, University of Wales, Cardiff.

Horák, J.M., & Evans, J. A. (2011): Early Neoproterozoic limestones from the Gwna group, Anglesey. Geol. Mag. 148 (1), pp. 78-88

Kawai, T., Windiey, B.F., Terabayashi, M., Yamamoto, H., Maruyama, S., Isozaki, Y., (2006): Mineral isograds and zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a subduction-accretion complex. Journal of Metamorphic Geology 24, pp. 591-602.

Kawai, T., Windley, B.F., Terabayashi, M., Yamamoto, H., Maruyama, S., Omori, S., Shibuya, T., Sawaki, Y., Isozaki, Y., (2007): Geotectonic framework of the blueschist unit on Anglesey-Lleyn, UK, and its role in the development of a Neoproterozoic accretionary orogen. Precambrian Research 153, pp. 11-28.

Kawai, T., Windley, B.F., Terabayashi, M., Yamamoto, H., Isozaki, Y., Maruyama, S. (2008): Neoproterozoic glaciation in the mid-oceanic realm: An example from hemi-pelagic mudstones on Llanddwyn Island, Anglesey, UK. Gondwana Research 14, pp. 105-114

Maruyama, S., Kawai, T., Windley, B.F. (2010): Ocean plate stratigraphy and its imbrication in an accretionary orogen: the Mona Complex, Anglesey - Lleyn, Wales, UK. Geological Society, London, Special Publications, Vol. 338, pp. 55-75

Shackleton. R.M. (1975): Precambrian rocks of North Wales. In: Harris. A.L., Shackleton. R.M., Watson. J., Downie. C., Harland, W.B. & Moorbath. S. (eds) A correlation of Precambrian rocks in the British Isles. Geological Society. London Special Reports. 6, pp. 76—82.

Tatham, D. 2009. Field Excursion to Anglesey. Deformation, Rheology & Tectonics Conference, a joint venture between the universities of Liverpool and Manchester.

Tietzsch-Tyler, D. & Phillips, E.R. (1989): Correlation of the Monian Supergroup in NW Anglesey with the Cahore Group in SE Ireland. Journal of the Geologicl Society, London, 146, pp. 417–418

Treagus, J. E., Treagus, S.H. Droop, G.T.R. (2003): Superposed deformations and their hybrid effects on the Rhoscolyn Anticline unravelled. Journal of the Geological Society, London, 160, pp. 117–136

Treagus, J. E. & Treagus, S.H. (2013): The Rocks of Anglesey's Coast. Published by Gwasg Carreg Gwalch. ISBN: 978-1-8452-209-1

Treagus, J. (2008): Anglesey Geology - a field guide. ISBN 0-9546966-2-X

Wood, M. (2012): The Historical development of the term 'Melange' and its relevance to Precambrian geology of Anglesey and the Lleyn Peninsula in Wales, UK. Journal of Geography 121 (1) 168 - 180 2012.