(i) Short discussion on the importance of Anglesey to Plate Techtonics, led by Margaret Wood.
(ii) Marquess of Anglesey's Column in Llanfair PG to see the result of the pillow lavas of the oceanic crust being
altered deep within an oceanic trench at a destructive plate margin. Outcropping beneath the Column, they
have become Glaucophane Schist demonstrating that they had suffered pressures at about 30 km depth in the
crust but not high temperatures. In Europe, similar materials would have gone down much deeper and, under
higher temperatures, would have formed eclogites.
(iii) Anglesey's Precambrian rocks are structurally controlled and occur in many places as accretionary prisms.
Anglesey demonstrates Ocean Plate stratigraphy of West Pacific type - the same as Japan is experiencing
today. The great thing about Anglesey is that the rocks are well exposed across the strike on the coast.
Llanddwyn Island is a complete but very small plate which is now less than one kilometre in length but if
extended along the faults, etc., would be 13 km long. The eastern end is the product of a constructive
plate margin and exhibits fresh-looking pillow lavas and evidence of the original sea bed. The enclosed jasper
has yielded poor specimens of fossil filaments . (Better Precambrian fossils are to be found in the limestones
at Cemaes Bay on the north coast of Anglesey and these have yielded stromatolites and vesicularities,
The spillite (pillow material) extends across the island north-eastwards where it has been metamorphosed to
green and blue (glaucophane) schist. You will also see the explosive material that initiated the pillow
extrusions. Crossing the island is material which would have been at the centre of our plate. There are ice-
rafted boulders in this material showing evidence of a late stage Precambrian glaciation - however these are
inaccessible and so will not be seen. At the far end of the island is material which was deposited in the trench
at a destructive plate margin and here you can see spectacular rocks of all types which slipped down the
trench - not too far, so as to form the melange - this is the world type section for this rock and where the
melange was first identified and named by Edward Greenly in his 1919 British Geological Suvey memoir.
(iv) South Stack: Led by Dr Stewart Campbell, (Head of Earth Science for Wales at CCW )
Walk up to the top of the cliff steps and MW will give a short introduction to what you will see. These rocks
were considered to be the oldest Precambrian in Anglesey, when it was believed that Anglesey's rocks
were part of a sedimentary succession. They are now considered to be younger than the other Precambrian
rocks on Anglesey and and have been given Cambrian age from zircon dating. The problem was solved when it
was realised that the Precambrian rocks, apart from igneous intrusions, were accreted onto the hanging
wall of a destructive plate margin. So the oldest rocks, (being the first to be accreted), are on top of
successively younger rocks that were forced beneath older rocks in the accretionary sequence, as the ocean
plate was forced down the trench. Hence, of the 3 groups on Anglesey, (Gwna, New Harbour and South
Stack), originally thought to be Precambrian, only the Gwna is Precambrian while the New Harbour, (not yet
dated), and South Stack groups are now considered to be of Cambrian age. The South Stack rocks consist
of shallow water sediments, (showing worm casts and bioturbation in places); they are metamorphosed to
meta-sediments and are spectacular. They demonstrate rocks that formed during the latter stages of a
'dying' trench and show the effects of folding in the meta sandstone, with large scale gentle folds and faults
contrasting sharply with the schistose rocks produced by the original mudstones. Superb folding and some
bondage can be seen on Ynys Lawd and the adjacent mainland together with a Tertiary age dolerite dyke.
There is a cafe and carpark nearby where you could get a snack.