On Saturday morning, 27th April, 27 members of the Cork Geological Association gathered in Portmagee to begin a two-day trip along the north coast of Valentia Island. Our leader and guide was Prof. Ken Higgs of UCC. As we drove across the bridge there was every prospect of a fine sunny day with just a fresh westerly breeze.
The main rocks on the island belong to the Valentia Slate Formation, the oldest rock formation of the Old Red Sandstones in the whole Munster Basin.These Devonian rocks were deformed during the Variscan Orogeny towards the end of the Carboniferous Period about 300 million years ago. This tectonic movement caused folding, faulting and even uplift of the rocks. In the finer-grained rocks the great pressure produced the slate which gives the formation its name. The excellent handout provided by our leader explains the geology of the area in great detail.
Our first stop was at the well-known Valentia Slate Quarry which was worked from 1816 to 1911 and re-opened again in 1998. The slabs of slate were cut by large saws powered by steam engines. The old spoil heaps are also searched for suitable material. In the cliff face at the entrance the purple siltstones dip gently to the northwest while the cleavage dips steeply to the southeast.
Next we moved down to the fossil trackway on the shore at Dohilla. Here we see the footprints of a tetropod (translates as 'four feet') the earliest known animal to walk on land. It's the second oldest evidence in the world of such a four-legged vertibrate moving from water on to land and was discovered in 1993 by a Swiss geology student, Ian Stossel. Prof. Higgs explained all about the size and shape of the tetrapod, how he moved across the muddy coastal plain and how the fossil prints can be dated to around 385 million years ago. The trackway appears on a bedding plane of purple sandstone and siltstones and the beds in the area are gently folded.
We then moved a short distance away to a new tetrapod trackway which was discovered in 2011 at Coosdallisk. Again the fossil prints are in fine-grained sandstone but they seem to belong to a smaller or maybe younger animal who also moved more slowly along the sand flats beside a river channel.
After lunch in Knightstown we made a short visit to the museum housed in the old schoolhouse. Then followed the long walk to Reenadrolaun Point. Here the younger beds of the Valentia Slate Formation consist mainly of fine-grained purple siltstones. Their steep cliffs show the stark erosive effect of the Atlantic storm waves. Here also is a mafic volcano tuff and an unusual pebbly conglomerate with rare metamorphic clasts.
The rocks of the Valentia Slate Formation on Trawaginnaun Bay were the subject of a disussion on whether they showed bedding or cleavage. Near Fort Point Lighthouse we saw some small Myriapod trackways called Diplichnites in the siltstone. A myriapod is a type of small arthropod that resembles a millipede and probably lived in small ponds on the river floodplains.
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