GEOLOGY OF ANGLESEY
Field Meeting : Friday to Sunday, 10 - 12 June 2011

Itinerary

1. Friday
Lligwy Bay [SH 492 873]; Parys Mountain [SH 437 904]

2
. Saturday
Trwyn y Parc [SH 374 937]; Llanbadrig [SH 375 945]; Ogof Gynfor [SH 378 947]

3
. Sunday
Llanddwyn Island [SH 405 634]; Marquess of Anglesey Monument [SH 536 716]

Site 1. LLIGWY BAY
Lligwy Bay contains the only Old Red Sandstone (ORS) locality in North Wales and is well exposed in the northern part of the bay. At the opposite end there is Carboniferous (Visean) limestone (Loggerheads Limestone Formation) and an intriguing feature known as the Lligwy Bay Disturbance.

Lligwy Bay Disturbance

Lligwy Bay Disturbance, Traeth Lligwy

Lower Old Red Sandstone

Lower Old Red Sandstone, Traeth Lligwy

Site 2. Parys Mountain Video Clip: Parys Mountain
Parys Mountain has been confirmed as a site of prehistoric mining, and there are also some indications of Roman activity. Extensive economic production of ore eventually resulted in the mine becoming (allegedly) the world’s largest copper mine in the 1780s. Until 1800 most mining was by open cast, but from 1810, Cornishmen opened up significant underground workings. By 1900 all significant mining activity had ceased.

Parys Mountain Copper Mine

CPGS members examining the centre boss at Parys Mountain

Site 3. Cemaes Bay: Video Clip: Cemaes Bay
(Trwyn y Parc, Llanbadrig, and Ogof Gynfor)

The walk from Cemaes to Ogof Gynfor highlights national and internationally important geological sites. At Trwyn Y Parc there are a number of 'Miocene pipes' in the Gwna Limestone, from which some of them have yielded Miocene fossil spores. In addition there are Precambrian stromatolites preserved in the very pure limestones of Gadlys Quarry. On the beach of Porth Padrig Ordovician strata are exposed along with the White Lady - a small sea stack made of white quartzite. The Llanbadrig peninsular is the informal type locality for the Gwna melange, a spectacualr and chaotic assemble of clasts from very small to very large! The cliff section at Ogof Gynfor is an important site for demonstrating the unconformity between Gwna rocks and Arenig Ordovician sandstones.

Miocene Pipe

Miocene Pipe in Gadlys Quarry, Trwyn y Parc, near Cemaes Bay

Ordovician Slates

Cleaved Ordovicain slates in Porth Padrig (no graptolites found)

Gwna Melange

Gwna Melange on Llanbadrig peninsula (Hammer for scale)

Dolerite Dyke

Dolerite dyke (hammer for scale) cutting through the Gwna Limestone

Unconformity at Ogof Gynfor

Church of St. Patrick's graveyard with the Ordovican / Precambrian unconformity
in the distance at Ogof Gynfor

Site 3. Llanddwyn Island (Video Clip: Llanddwyn Island)
Llanddwyn Island is a narrow tidal isthmus projecting south-west from the Anglesey coastline. The contrast of rock types with their distinctive colours and textures produces one of the most dramatic geological assemblages in the British Isles.

Llanddwyn means The church of St. Dwynwen. She is the Welsh patron saint of lovers, making her the Welsh equivalent of St. Valentine. Her feast day, the 25th January, is often celebrated by the Welsh with cards and flowers, just as 14th February is for St. Valentine. The island boasts two lighthouses. The older lighthouse, Tŵr Mawr has returned to service following the decommissioning of Tŵr Bach in 1975. Tŵr Bach featured prominently in the Demi Moore film ‘Half Light’ released in 2006.

The island records an entire plate tectonic event, from the creation of the ocean floor as a mid-ocean ridge, seen here as pillow lavas, through its journey across the ocean basin where it picked up sediments, to its burial and metamorphism as plates collided and the sediment sank down into a deep ocean trench i.e., we are looking at slices of ocean floor in an accretionary prism associated with a subduction zone (Kawai et al., 2008).

Pillow Lava

Pillow lava outcrop in Newborough Forest

Basaltic Pillow Lava

Basaltic pillow lavas on Newborough beach

Oceanic Cherts

Oceanic sedimentary rocks inc. red jasper cherts

Altered Pillow Lava

The grey and red (jasper) are altered basaltic pillow lavas
which are capped by Gwna limestone above

Gwna Melange

Dramatic and colourful assortment of rocks belonging to the Gwna Melange

Chaotic assortment of clasts

Chaotic and colourful assortment of clasts in the Gwna Melange

Site 4. Blueschist (Marquess of Anglesey's Column)
The 27 m tall column is composed of local fossiliferous limestone from Moelfre. It was built in 1816 in memory of Lord Paget, the Marquess of Anglesey of ‘Battle of Waterloo’ fame. From the top of the column there are good views of Plas Newydd, the stately home of the current Marquess of Anglesey and the Menai Strait. If you choose to climb the column there is a charge.

The blueschist exposed in the crags around the base of the column are Precambrian in age and as such are one of the oldest and quite probably the oldest assemblage of blueschists in the world. It is formed under high pressure and low temperature. Usually these rocks continue their journey down the subduction zone where their blueschist mineralogy is obliterated.

Blueschist

Blueschist outcrop below the Marquess of Anglesey's Column

Miscellaneous

Pebbles

Colourful assortment of pebbles on Porth Padrig beach, near Cemaes Bay

Onward in excellent weather

CPGS members on Saturday, 11 June 2011 en-route
from Cemaes Bay to Trwyn y Parc