OUGS FIELD EXCURSION TO CLOGHER HEAD 5 September, 1999 30 miles north of Dublin near Drogheda.

Picture, between 500 and 400 million years ago, a "European" continent and an "American" continent, having much in common with our currently existing ones, and with an Atlantic-sized ocean, the Iapetus* Ocean, separating them. Imagine this ocean closing. Place Clogher Head in "Europe", the Cooley Peninsula, a few kilometres to the north, in "America", and somewhere in between put the Iapetus Suture with oceanic crust descending southward beneath the overriding rocks of Clogher, not only this but picture another slab descending to the north. It is thought that this continent-continent collision was oblique and closed, rather like a zipper, from the north-east first. Today, Ireland, along with Scotland, Newfoundland and parts of Scandinavia are unusual in that they now possess parts of both these ancient continents.

Various geophysical methods have provided much evidence for a deep structural feature. However, there is much controversy as to where the actual suture lies though it probably runs north of Clogher Head and continues south-west across Ireland to the Shannon estuary and beyond.

Two distinct marine faunas are found in the Ordovician of Ireland. One in the south and east has characteristically "European" fossils, the other in the west has an "American" affinity. This is convincing evidence for the existence of a large ocean. As time passed, into the Silurian, these faunas tended to merge (ocean closing). Unfortunately at Clogher head fossil evidence is not abundant locally and we saw none on the day.

Just south of the harbour on the shore are greywackes, sandstones and mudstones of the Lower Silurian Clogher Head Formation, its age determined from graptolites found in the formation immediately above. As mentioned earlier the rocks are on the overthrust slab so they were neither deeply buried nor greatly deformed but they are folded and cleaved. We were able to identify and follow the original sedimentary layers of sand and silt around the folds and see how the cleavage was refracted reflecting the different compositions of the layers. Adrian showed us a good example of tectonic rippling, formed by interference between bedding and cleavage.

There was much evidence for forces operating obliquely. For example, when we looked closely at folds we saw that the cleavage, normally parallel to the axial plane of a fold, was not actually axial planar but a little askew. There were slightly rotated lenses of quartz which gave a sinistral (left-handed) sense of shear. Almost all the small-scale structures that we saw had this left-handed sense which corresponds well with the main Caledonian movements.

The rocks were crossed by lamprophyre dykes, intrusive rocks, very calcic, very basic, with little or no quartz and abundant ferromagnesian minerals. It is said that they will fizz like limestones if acid (dilute HCl) is poured on them. As no-one happened to have any acid with them the theory went untested! In places within the dykes there were very curious, cone-shaped forms about 30cm across, called witches hat structures, which were caused by shearing.

As we walked toward Clogher Head we took on a wider view and discussed later events. As Adrian pointed out there are no great deposits of Tertiary sediments in the Irish Sea, though for example, the Mournes to the north had had over 4km of rock stripped off them. Where have the sediments gone? Adrian postulated Shannon-sized rivers flowing west towards the Porcupine Basin.

At Clogher Head we climbed out onto a large fold, the Clogher Head Anticline, and were able to see that the fold type changed along the coast from symmetrical to asymmetrical.

In Red Man's Cove, the red siltstones of the Formation named after it overlie the green Clogher Head Formation. Here the folds illustrated pressure from the side. Slickensides, fine, parallel grooves which develop on surfaces that have slipped against each other, indicated contrasting directions of movement on opposite limbs, but there was no evidence for slip along the top (axis). Other folds had beds that were thin in the upright limbs and widened across the top.

Finally we were fast-forwarded in time to the recent past and man's early influence. In a small cove were areas of reddening where fires had been lit against the rock. As Adrian admitted this could have happened last week but we preferred to think of it happening thousands of years ago. Lying close by on the ground were rounded cobbles with percussion marks. Such cobbles were not typical of this part of the coast. They probably came from several kilometres to the south and were surely imported by man. It is accepted that the rocks of Newgrange came from Clogher Head and had been transported by sea and up the Boyne, to their destination. Were we seeing evidence of pre-historic workings? The cobbles, incidentally, had a nice relevance for those who were on the recent OUGS trip to Schull and visited the Bronze Age mines on Mount Gabriel.

In such a small area there is a great deal to see and it is well worth a visit.

Our sincere thanks to Adrian for giving us his time and expertise so unstintingly, and to all those who helped to make this a memorable trip.

Susan Pyne

* In Greek mythology Iapetus , a titan, was the father of Atlas who gave his name to the Atlantic.