Report on our excursion to examine the Rocks of Northern Irelandl

14th - 21st September 2002

Leaders: Various

We thought we would go with GO so we went. Lulsgate airport confiscated my scissors but left Jim with his, so any blisters were going to be plastered up after all.

Arriving at Aldergrove International Airport west of Belfast we picked up our very grubby downmarket Avis van and headed north to Ballycastle on the Antrim coast, through a grey day but with the sun trying to break through. Mrs McCarry gave us a warm welcome and plied us with cups of coffee and tea while we sorted out the accommodation. As Jan was not going to be with us when we visited the Giant’s Causeway we elected to spend the afternoon there doing our own thing and hopefully think of some intelligent questions for our leader when the time came! A lovely afternoon and the ice-cream was gorgeous too!

The cliffs near the Giants Causeway

Sunday morning, with Mark Cooper of the Geological Survey, we looked at a Dalradian inlier – Dalradian rocks of 600 million years surrounded by basalt rocks of 60 million years. As good an example of an inlier as it gets. Here we discovered that the old rocks had been overturned and we had been looking at the bottom limb of a nappe formed during the collision of Gondwana and Laurentia when the Iapetus disappeared.

And that was only the first morning! We still had to cope with the van breaking down before our next stop. After valiant efforts at pushing, pulling, rocking, shouting and cheering we were mobile again and off to Pan’s Rock – a small beach site. This outcrop was Carboniferous in age. The bottom bed was both overlain by fine grained marine sediments and burrowed. These in turn were covered by shaley beds and then more massive sandstone beds – a clear case of coarsening upwards succession. The sandstones contained detrital coal and the finer beds had imprints of leaves and bark. Internal structures showed tabular cross beds and nothing that would associate it with a meandering river or the sea. Interbedded with the sandstones were some rubbly horizons which turned out to be calcrete – a fossil soil formed on exposure to the air – and the top calcrete seemed to have a karstic appearance. The interpretation of these beds was that of channel fill within a braided river with episodes of exposure when the river took a different course and as we were in the Carboniferous it had a marginal setting probably on a pro-grading delta.

And so to bed after our evening meal. Sandra and Ian had a shorter time for rest. They were the ones who got up early to drive back to Belfast with our suspect van and exchanged it for another slightly better one. But they were back in time to take us to meet Dr Paul Lyle of the University of Ulster at Larrybane Bay and to walk the rope bridge over to Carrick-a-Rede, which means Rock in the Road. It seems that this is the way the salmon come and the fishermen can string their nets between the rock and the shore to catch them. This little island was the vent of a small explosive volcano which erupted at the start of the Tertiary volcanics. We could also see dykes which had been intruded during the later quiet phase of the igneous activity and this seemed to follow the curve of the vent. Monday afternoon we went with Paul to the Giant’s Causeway but more of that elsewhere.

The Carrick-a-Rede bridge, volcanic plug intruded into chalk.

Chalk and downfaulted basalt on White Park Bay

Looking down on the Giants Causeway

Tuesday morning, with the new bus behaving itself, saw us meeting Dr Mike Simms late of the University here in Bristol at Magheramorne Quarry on the shore of Lough Larne. Here he demonstrated that the chalk had been well and truly karstified in the 10 million year interval between the end of the Cretaceous and the start of the Tertiary Volcanics and the top of the chalk would have looked like Guilin province in China today – pinnacles and valleys filled with flints and clay. During the afternoon we went to Waterloo Bay where another intriguing possibility was revealed to us. Graeme will tell you about that. The last stop was Portree Beach where the fossil hunters amongst us had a great time searching in the toe of a land slip.

Lava filling karst surface

Wednesday was the day for visiting Mike’s domain in the museum in Belfast and transferring to our other centre in Newcastle. Like all museums, behind the scenes there was a mass of specimens and wonderful things housed in very cramped conditions. We took the opportunity to view the exhibits out in the public galleries, but we had to be quick because everyone had to clear out at 12.30pm. The reason? Princess Anne was arriving later that afternoon and the building had to be searched and declared safe. Even Mike had to vacate the building and so he took us to Cultra shore, on the south side of Belfast Lough where we looked at Carboniferous and the only Permian in Ireland.

Thursday we reported for duty at Helen’s Bay on the south bank of Belfast Lough. From here Dr Bernard Anderson who worked at Queen’s University, was going to initiate us into the mysteries of accretionary prisms and suspect terranes down through the Ards Peninsula. Well, that was the plan, but he had to miss out several of his localities – either we talked too much or the rocks were very slippery or we took some time to understand. All three I think! Enough to say that we were looking at repeated slices of ocean floor scraped up when Iapetus closed, including pillow lavas, black shales with graptolites and lots of greywacke deposited by turbidity currents coming off the rising hinterland. From here we travelled to the southern point of the peninsula where we caught the ferry to make our way back to Newcastle. As the treasurer I was a bit taken aback that the ferryman was very keen to believe that we were all oldies and therefore entitled to free passage. The thought drifted into my mind of Charon and the river Styx but we did survive.

Pillow lavas at Helens Bay

Bernard Anderson on overturned base of greywacke, channelled into top of preceding bed.

The last day was spent in the company of Dr Jack Preston also late of Queen’s. At 83 years old he lead the way and was more than able to walk uphill and talk at the same time! He told us about cauldron subsidence, cone sheets and ring dykes. The Tertiary volcanic activity youngs to the west – I thought that this suggested a hot spot. After all, the Laurentian plate was advancing to the east so that the volcanic activity would get younger westwards. Wrong again! Our first stop was at Bloody Bridge where we followed the stream up to a quarry. It was a lovely day and a very pretty spot. I confess! I looked at all the exposures as we climbed and saw the different rock but could not put together a story except that the chemistry of the magma can change and therefore the rock type. And also the reverse. In the past geologists had mapped two different granites in the east of this province but now there is only one – the chemistry is the same but different minerals developed. Furthermore they have found that a cone sheet goes right round granites one and two so now granite number one has disappeared from the maps.

Mourne granite intruding country rocks.

Our last port of call was Green Harbour Beach near Glasdrumman. Jack had said that the dykes were older than the granites and here were dykes aplenty. We started at dyke 55 but the diagram he handed out goes up to 93! There were dykes of all kinds – dolerite, porphyritic, some with well formed crystals, amygdaloidal, some with xenoliths, some were intruded by later dykes and some pointed to differentiation within the magma chamber. From the palaeomagnetic directions it appeared that there were flurries of activity with long intervals between – the dykes came in ‘bundles’ as it were.
 

And that was it. We got up early the next day to fill up with diesel, get the van back by 9.00am, and prepare to do battle over the grotty bus we were first given. In the event they deducted one day’s hire. Then we discovered they were throwing the fleet away the following week! That’s life. A jolly good week. Thanks to everyone for their efforts especially Graeme and Babs and Sandra our two drivers. I shall make those two pay us next time – they enjoy it so much.