Field Trip to Galway and Connemara - July 1998

Leaders - Sadhbh Baxter and Brian Callaghan, Geology Dept, University College, Galway

A small but select group met on a rainy Saturday morning at the start of what was to be a wonderful weekend of sun, scenery and spectacular geology. The weather cleared as we travelled in convoy to Leckavrea, passing distinctive white outcrops of Bennabeola quartzite on the way. Here we had our first glimpse of the Cleggan Boulder Bed (Middle Dalradian), a glacio-marine tillite which can be traced across Northern Ireland and into Scotland. It suggests that this area was in high latitudes in the Southern hemisphere about 600 - 700 million years ago. The ‘drop-stone’ boulders of granite and dolomite have been elongated by deformation so that they are hardly recognisable at this site. What was most impressive was the sight of thin sheets of fine-grained granites which had intruded the tillite.

In the distance a prominent dark ridge stood out from gently sloping fertile fields. At Gur, our second stop, we were able to investigate the banded amphibolite which formed the ridge, and the overlying Lakes Marble Formation which produced the fertile fields. We could not fail to be impressed with the superb folding in the marbles and there were spontaneous ‘oohs’ all round. Locally the amphibolite had been thrust over the younger marbles, the thrust plane being clearly visible.

At Stop 3, near Finny, Ordovcian pillow basalts generated a debate on ‘way-up’, based on the shape of the pillows and elongated gas vesicles. These basalts suggest that a chain of volcanic islands lay to the South of the Laurentia coast in early Ordovician times. Sediment accumulating in the adjacent subsiding marginal basin provided us with the rocks of what is now known as the South Mayo Trough.

End Ordovician folding was followed by a northward marine transgression during Silurian times. From Finny Church, travelling in a SE direction, a sequence of rocks beginning with andesites (unconformably overlying the basalts) and fluviatile sandstones, followed by quartzite with worm burrows, flagstones with shelly marine fauna and culminating in turbidites, superbly display a slow marine transgression in this area in Silurian times. A discussion of conjugate fault sets, seen in the turbidites, rounded off the day.

Day 2, which concentrated on the 400Ma old Galway Granite Batholith, saw us tracing the contact with the adjacent metagabbro gneiss suite. It was interesting and surprising to note that there was no sign of differential erosion between the 2 rock types. The granite itself varied in grain size and abundance of K-feldspar and plagioclase feldspar from place to place, and also in the degree of alignment of the minerals.

The surface expression of the Shanawona fault was examined at Leitir Mor, and the relative ages of events worked out using cross-cutting dolerite dykes. This fault separates the western and central granite blocks. The central block has been upthrown by several km relative to the outer blocks, revealing a deeper level of the batholith. Some spectacular examples of rapikivi texture were seen in the central megacrystic granites, where white plagioclase rims had formed around pink K-feldspars.

The infamous Costelloe-Murvey granite, with its high uranium content was examined and provided us with samples of fluorite. Perhaps the greatest debate of the day was generated by the ‘mixing and mingling’ zone at Casla and Cnoc na Greine. Here melts of differing compositions have cooled without total mixing, forming sheets and enclaves. Discussion of magma temperatures, viscosities and diffusion rates helped us to understand how such a zone formed, and ended the weekend on a high. The hotel car park at Cnoc na Greine was declared a national monument before we all retired inside for refreshment.

Many thanks to our leaders for a very well-organised field trip.

Bettie Higgs