Monday 9th July 2001

The field trip was run as part of the OUGS Symposium programme held in Dublin City University. It was led by Dr. Ian Somerville of University College Dublin. There was a good attendance of about forty people from England, Ireland, Scotland, Wales and the United States. A truly international field.

Loughshinny is a picturesque fishing hamlet tucked away on the coast between Rush and Skerries. The morning was bright and sunny, an unusual combination, in this mediocre summer to date and with the prospect of a cracking field trip we made our way across the beach in Loughshinny Bay to our first stop.

The cliff facing north was a stunning scene of folded beds showing extreme deformation. The beds form part of the Loughshinny Formation (Lower Carboniferous c. 330-350 Ma) and consist of well-bedded grey limestones interbedded with black shales. The folding occurred later, at the end of the Carboniferous during the Hercynian Orogeny (c.290 Ma ago). The axial planes of the folds (anticlines and synclines) tilt to the south, with overturned northern limbs. As we stuck our noses into to the cliff face we spotted some fossils, crinoids, bivalves and corals, in some of the beds.

Conscious of the need to keep ahead of the tide, we traversed back across the beach and took the cliff-top path and headed north. On the way we could look down onto the foreshore where the folding had resulted in clearly discernible whale-back ridges.

Our next major stop was The Coppermine Fault. A small bay developed here as a result of the weakness induced by the fault. The beds dipping south on the northern limb contain large vugs (cavities) and we saw well-formed quartz crystals within. As the Coppermine suggests, copper minerals are found in this area and an adit mine in the adjoining field was worked in the last century. Green malachite was evident in some of the quartz but I did not see any evidence of the other copper mineral, azurite found here.

As we made our way north of the fault zone, the raingear and sweaters were coming off as the Sun got higher in he sky. I am not making this up, it was the best day of the summer.

We were now traversing the massively bedded limestones of the Drumanagh Member of the Loughshinny Formation. The beds, over 5m thick, are coarse-grained, fossiliferous breccias which have been dolomitized. There was an abundance of large brachiopods and crinoids. Further north, the dolomitized limestones continue and you can see banded quartz-filled vugs, known as zebra quartz.

The next area of interest was an inlier of argillaceous limestone with an abundance of Gigantoproductus brachiopod shells. Further north, the limestone showed disturbed bedding and slumping and there are large blocks and boulders of massive pale-grey mud-mound facies with laminar spar-filled cavities.

Only some of the party made a very quick trip to the final stop on the northward traverse, to the Smugglers Cave. The roof of the cave is made of a conglomerate of greywacke pebbles and vein quartz. In the lower part of the conglomerate there are some limestone pebbles but above that the rocks are from a continental deposit probably formed in a fluvial channel.

We quickly made our way back to the car park at the harbour for a quick lunch, followed by a dash to Portrane for the next leg of the field trip. I would like to offer a word of thanks to Dr. Somerville for a really enjoyable and informative morning. The geology was spectacular and this was further enhanced by the comprehensive and well-produced field notes distributed by Dr. Somerville.

Michael Andrews