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For many people, Anglesey is just along the highway to Ireland, or the last bastion of the Welsh language and culture. That is incorrect on both counts. Anglesey is a geologically fascinating place, and while there is a strong Welsh identity here, it merely reflects what is found elsewhere in North Wales.
The main roads through North Wales to Anglesey pass through the rugged mountains of Snowdonia and out onto a low plain at Menai Strait. Compared to the spectacular views in the mountains, Anglesey seems especially featureless and flat. But this fecund, rolling landscape, largely supporting sheep and mixed farming and increasing tourism, hides a long and complex geologic history.
Large faults divide the island into three blocks of very different Precambrian to early Cambrian rocks. The southern block contains volcanic rocks and sedimentary rocks deposited on the sea floor. The central block contains schist, gneiss, and granite that date from the continental Cadomian collision of about 600 million years ago. The northern block contains sedimentary rocks, volcanic ash and sandstone, crumpled during the Caledonian collision. Ordovician and younger rocks, including the Old Red Sandstone and Carboniferous Limestone, drape over these blocks in places.
Of special interest to this EarthCache is the great expanse of Red Wharf Bay, where the cliffs have excellent exposures of limestone laid down in the equatorial seas of Carboniferous times. The cliffs are not obvious from the village, but lie north along the beach. Here the limestone is typical of the grey Carboniferous Limestone; well layered in parts and rich in fossils.
About one kilometre to the north, just below a campsite, are impressive sandstone pillars in the limestone (see Waypoint). Geologists believe the pillars fill sinkholes dissolved in the limestone when it was raised above sea level; then, later, sand filled the sinkholes.
The top of the limestone is a raised beach, about three metres above present sea level, cut into the limestone during an interglacial period about 120,000 years ago when sea level was higher than at present. Cold conditions returned about 100,000 years ago and frost shattering produced light-coloured debris that plasters the cliffs above the raised beach. This debris, called head, contains rock fragments shattered by frost, which then flowed over the raised beach during the spring thaws. Glaciers re-advanced from the Irish Sea and laid down a red till above the head. South of Red Wharf Bay, the road climbs across the Berw Fault back onto the Precambrian rocks of the southern block.
The Carboniferous Limestone Series at Red Wharf Bay is of Visean age. It is part of the so-called Clwyd Limestone Group, around 330 million years old, and is a diverse range of limestone facies with subordinate sandstone and mudstone, and exhibiting what is known as 'dolomitisation'.
Dolomitisation is the process by which an original calcium carbonate rock is converted into the calcium magnesium carbonate, either wholly or in part. This process often destroys embedded fossils, original sedimentary structure, etc., but occasionally there will be alteration of only part of the rock, e.g. in some limestones only the smallest grains are converted to dolomite, the larger crystals and shell fragments remaining unaltered or merely developing a thin dolomitic skin. On the other hand, in some rocks all fossil remains are dolomitised and only the matrix is unaltered.
Dolomitisation may take place at any time during or after the deposition of the calcareous sediment. It appears to be in many cases the result of a reaction between hypersaline marine brines and calcite, e.g. in the deeper parts of coral reefs dolomitisation has proceeded to a remarkable extent.
To claim this EarthCache:
(a) Give a more precise estimate of the period known as Visean
(b) Identify the longer period to which the Visean period belonged
(c) What forms of life might you expect to find as fossils?
(d) Visit the sandstone pillars and estimate the size of the most prominent ones you find
(e) What evidence can you see and describe that betrays the location and axis of the Berw Fault line?
Finally, there is neither requirement nor obligation to post pictures, but it would make the cache page more attractive and interesting if visitors could in due course add a picture or two of the locality.
Additional Hints
(Decrypt)
(q) Erdhverf n ivfvg gb gur cvyynef, rvgure nybat gur ornpu, vs gur gvqr vf bhg, be ol n ebhgr bs lbhe pubbfvat; (r) pna or nafjrerq sebz gur pne cnex ng TM.