Anglesey. As soon as I saw that name I knew I had to go. This is an exotic terrane (Monian) that occurs in every OU geo-unit as an example of something or other. I read on, the talented teacher Paul Olver as leader and the dependable Chris Sadler doing all the organising. The modest cost included evening talks and a hotel offering good-sized meals, en-suite bathrooms for washing rocks and tolerance to boots. I went and it was really good fun.

So, on a sunny Saturday, we set off over Telford’s bridge, giving us a fine view of the new double-decker Britannia bridge (trains below, cars above). On the NW side of the straits we stopped at Church Island (SH552 717).

The gravestones were mostly of purple Bethesda slate. They were all blemished by ovoid green marks. These are organic in origin and an imaginary line drawn from one to the next along the long axis gives the bedding plane.

The Menai Strait is a NE/SW trending sinistral strike slip fault. Beside the water there was an exposure of mylonites. These enclosed more massive pods, which are interpreted as relict pillow lavas. Paul postulated that these pillow lavas of the Gwna Group were obducted. Prolonged and intense deformation associated with the late Cambrian/early Ordovician had produced these structures. As with other Precambrian rocks on Anglesey they are not related to the rocks found on the other side of the straits but correlate with the Precambrian in Rosslare and in Newfoundland. From the Ordovician onwards, the stratigraphy corresponds with that of mainland Wales.

Our route now took us through Llanfairpwllgwyngyllgogogoch, the name proudly displayed on every business, seemingly increasing the length of the village.

At Porth Isallt Bach, Treaddur Bay, (SH 250 792) we encountered another Precambrian exposure, this time from the New Harbour Group. This is an attractive shiny green chlorite muscovite phyllite, with parasitic structures superimposed on larger folds which again plunged NE. The chevron folding was so tight that quartz had formed in a crack along the axial plane resulting transpositional (pseudo) bedding.

We took lunch sitting on the cliffs of Holy Island, looking out to sea and the South Stack lighthouse. There were choughs and sea birds hanging in the wind and some of us saw puffins. Memories of the 2001 Symposium came back as we watched the car ferry ‘Ulysses’ coming in from Dublin and disappearing around the headland in the direction of the unseen ferry port.

After lunch, the geological purpose of our visit was to look back at the cliffs from the South Stack lighthouse island. We left the lighthouse itself to the ordinary tourists (no time, actually!). What we could see in the cliffs was a complex anticlinorium, i.e. an anticline with a series of broad folds containing lots of complex little parasitic folds. These were rocks of the South Stack Series.

There were two lithologies here: white metaquartzites that were relatively competent and green chloritic phyllites that were softer and more ductile. There were quartz tension gashes in the phyllites and the metaquartzites were pinched out into boudins and rotated.

The phyllites were of the same pelitic composition as those of the New Harbour Series in Treaddur Bay, but were older. Both series are believed to have originated as continental shelf sediments lain down on the northern margin of the sea associated with the Gwna Group. This sea was being subducted along its southern margin at the same time that the Iapetus Ocean was opening even further north.

The axes of the folds here were NE-SW like the Caledonian orogeny and so at one time were thought to be Caledonian, but the orientation was just coincidence. The trend for the Cadomian and Caledonian is the same everywhere. The grade of metamorphism here is higher than would have been expected for the Caledonian and the folds are therefore believed to have been formed by the time of the Cadomian orogeny, about 600 – 500 Ma.

Another aspect of the rocks to be noticed was that the minor folds had the same plunge as the major fold they were sitting on. This demonstrates ‘Pumpelly’s Law’. In order to find the axis of the bigger fold one should look down the plunge and look at the smaller folds.

Paul reminded us of the verse “Big fleas have little fleas upon their backs to bite ‘em and little fleas have smaller fleas and so ad infinitum”. For fleas read folds.

The next stop was an unscheduled one to a quarry in the Rhoscolyn area of south Holy Island where there had been a gabbroic intrusion into the pelites of the New Harbour Series. The original intrusion of massive rock had not been sheared. The local name for it is ‘verdantique’ and it has been used as a decorative stone for statues and pillars in churches.

However, hot fluids had altered the first outcrop that we saw. They had serpentinized the augite pyroxene and plagioclase to bright green epidote and sheet-like amphiboles. The rock was an unusual one, similar to the serpentinite that many of us are familiar with from the Lizard in Cornwall, but with shiny talc shear planes from the two metamorphic events already mentioned. These events had taken it to a higher metamorphic grade. Flaky minerals like talc and chrysotile are generally favoured in metamorphism because they can stand the pressure and occupy the space better than stubby ones, which don’t fit.

There was a large metamorphic aureole of epidotised hornfels around the original gabbroic intrusion. Chris also found a purple and grey hornfels that Paul could not identify without sectioning and further investigation. We wondered whether it was new to science. Somebody suggested the name ‘Sadlerite’, after it’s discoverer! I wonder who?

The final location of the afternoon was a column built in memory of the Marquis of Anglesey, of Battle of Waterloo fame. From the top of the column we could see Plas Newydd, the stately home of the current Marquis of Anglesey. The column itself was made of fossiliferous local limestone, but we had in fact come to see the ridge of glaucophane schist on which it was built.

The schists had been subjected to high pressure, low temperature metamorphism by being taken down a subduction zone. They bore a suite of distinctive minerals – glaucophane, lawsonite and pumpellyite. Glaucophane is a blue/grey amphibole that is characteristic of the blueschist facies. Just as the highly deformed rocks that we had seen at Church Island that morning had originally been Mid Ocean Ridge Basalt (MORB), so had these. However, the Church Island MORB had been subjected to shallower, cataclastic shearing, whereas these had been subjected to regional burial metamorphism.

Both schists are believed to have once been pillow lavas and we were invited to look for relict pillows; lensoid shapes infilled with minerals with the foliation flowing around them.

After dinner that evening, Paul gave a talk to help put all of what we had seen into the context of the plate tectonics of Anglesey from 1000 Ma to the present, no mean feat!

Sunday dawned with the smell of fresh rain, electrifying skies and swirling clouds which seemed to be getting and striking closer as we neared our first destination as described by George Gibbons.

A lovely wet morning, entirely suitable for a long beach walk from Newborough Warren Car Park (£2) to Llanddwyn Island. This is in Gwna Group territory of Terrane 3 (in Paul’s classification) and we were soon gazing in wonderment at some wet and glistening Pillow Lavas that were well weathered but quite undeformed. They were greenish in colour with Jasper-containing red flint in the interstices. Very striking. Absence of chalk-derived material between pillows indicated that they had formed at depth, beyond the carbonate dissolution point. Right now we could see (under Paul's prodding ) that they look upside down, and have been turned through at least 100^o since deposition which was in the early Cambrian, according to recent examination of micro fossils in the jasper chert. A further walk across the wet sand brought us to exposures of the Tyfry Beds, vertically cleaved grey green grits or greywackes. Their orientation differed from the pillows the area having been deformed in the Cadomium orogeny.

Now the long walk back across the sands to the cars (and the car park loos).

Our second stop was at Porth Trecastell a charming little bay which we could enjoy the more as the weather was by now warm and sunny. This is on Terrane 2 of the Anglesey jigsaw and the exposures here are Penymynydd schists of pre - Cambrian age, in fact, the oldest rocks on Anglesey. These are older than 640ma, as around that time they were intruded by a granite that could be dated. They were definitely wrinkled and also faulted, as befits ancient rocks. On one side of the fault there were black Graphitic schists that were definitely novel in my limited experience. They must have originated in a life-supporting environment, probably marine. But on both sides of the fault there were a variety of schists with both calcite and quartz intrusions and we could have stayed for another age with our little hammers, but Lunch called…..

Lunch was taken on the cliff top by the church and graveyard at Llanbadrig, a place of some antiquity, on the North Coast of the island.

After lunch, the major structure of the Gwna Melange was examined. This somewhat evocative French word implies a right old muddle! Which it was!

On the western side from the church, there were very large to small blocks of quartzite, sandstone, limestone, phyllite, serpentinite and jasper all set in a fine grained matrix. To the east of the church, the rocks were much the same with one very large quartzite block and a rather indistinct unconformity.

This assemblage has caused much head scratching as to origin. Greenly (1919) thought that the cause was tectonic; Shackleton (1969) interpreted it as an olistostrome, a sedimentary deposit caused by large scale submarine slumping. The latest theory, Barber (1998?), suggests that the “mixture” is a result of a “burp” of sediments etc rising during subduction. Room for more theories, perhaps!

The next location was not far away, at Penrhynmawr, near Cemaes Bay. This was a coastal disused quarry trending ESE to WNW where there are massive Pre-Cambrian limestones, light coloured and relatively unfossiliferous as far as could be seen, except for some stromatolites seen on weathered limestone surfaces by the shore. The shore side where the stone had not been worked showed heavy weathering. The remains of a limekiln were evidence that the stone was calcined for agricultural use. Of interest was the change at the limit of the workings where limestone became confined with black graphitic schists. The interpretation of this junction suggests that the limestone is one large block within the melange, set in metamorphosed schists. This was supported by the amount of debris of a non-limestone nature found on the quarry floor including pieces that could have been from volcanic ash layers, bits of jasper and sandstone etc.

After this yet another interesting site, tired and happy, the group made tracks back to Bangor.

Monday dawned as wet as Sunday without thunder and lightning but with incessant drizzle and mist. In view of the conditions Paul and I decided it would be better to curtail the trip and miss out the ‘Old Red Sandstones’ so we headed for the Carboniferous Limestones at Trwyn Penmon just to the north east of Beaumaris. When we arrived the rain was still falling and the lighthouse bell was tolling every couple of minutes. The limestone here is a lime mud with corals, brachiopods and some bivalves. We spent ½ hour scrabbling around the rocks looking for specimens in the outcrop whilst trying to stay dry and upright on the slippery surface.

We then moved onto the geological museum and shop, Stone Science, near Pentraeth. The displays include a brief walk through time with rocks and fossils and the environments with collections of fossils, minerals and stones from all over the world. Children were kept amused by a cartoon video about dinosaurs. Having looked at the displays and browsed around the shop, we were made a cup of tea and as the skies had now cleared, we ate our sandwiches in the back garden amongst the large variety of chicken and Indian Tee Pees. By one o’clock we said our good byes and started to make our separate ways back home after a very interesting weekend.

I would like to thank the volunteers, Janet, Lynn, George and John who have contributed to this write up and thanks again to Paul for leading the trip.

As for the ‘Sadlerite’ I am still waiting to hear the results of the intensive research being carried out by — Sue Vernon.