GeoMôn

Anglesey Geopark

 

During July 2009 GeoMon hosted a group of students from Jena University, Germany.  Professor Georg Buchel and his staff from the university were partcularly interested in the island's Precambrian rocks.  The group had booked themselves a stay at Anglesey's Youth Hostel in Llanberis after agreeing an itinary with Margaret Wood.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Itinary - The excursion began at Llanberis at 10am.      

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                     

 

 

 

               (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,

                      (Cynaobacterial origin).

                     

                      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.

 "We very much appreciated your kind and professional guidance.  Those days on Anglesey were highlights on our tour".

 

Reinhard Gaupp (Jena University)

 

 "Many thanks for arranging the trip to Anglesey at such short notice.  I have heard from Georg Buchel that he  was overwhelmed by you friendship and professional guidance".

 

Frau Dr. Frey

 

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Field Trip: Jena University, Germany