A POTTED GEOLOGY OF NORTHERN IRELAND

Northern Ireland geology is, for such a relatively small area, both ancient and modern, diverse and is, at the same time picturesque and dramatic. This is a necessarily short piece that could, in common with many other accounts, be very much longer. So don’t look for too much detail.

On the broad scale Northern Ireland is made up of five main areas. The northeast, which contains Britain’s largest freshwater body, Lough Neagh. This area stretches from the north coast south to the Lagan valley, and from the East Coast to the scarp slopes of the Bann valley. It comprises of tertiary basalt’s split into the upper and lower basalts with a distinct interbasaltic bed. The upper and lower basalt’s are a fairly primitive olivine basalt. The interbasaltic bed contains Laterite, Bauxite and Lithomarge. There is also some rhyolitic lava, some volcanic plugs and some pyroclastic rocks. On the north coast is Northern Irelands most popular tourist attraction, the Giants Causeway. This is a spectacular outcrop of columnar tholeiitic basalts. There are a number of layers of the outcrop rising from sea level to about 700ft and forming the dramatic cliffs of north Antrim for which the area is noted. There are several eroded volcanic plugs scattered throughout this area, the most notable being Slemish. This now isolated hill is oval in shape, and is 1000 metres by 300 metres, rising some 200m above the surrounding peneplain. The north coast cliffs are matched by the cliffs that tower above the capital city of Belfast. The view from the cliffs above Belfast is alone worth the trip to Northern Ireland.

Lough Neagh (Pron. Nay) is the largest fresh Water Lake in the British Isles, apart from it’s size it has little else to commend it, not being particularly scenic. It is a tectonic feature with the river Bann running into it from the south and again out of it to the north, emptying into the sea near Portrush.

The Southern Uplands Fault crosses the North Channel from Scotland and traces a line southwest along the line of the Lagan Valley. The fault continues in the south-westerly direction across Ireland and into the Atlantic Ocean in the area of Galway. South of the S.U. Fault down to the Tertiary Igneous Mountains is a southwest trending belt of Lower Palaeozoic rocks. These consist mainly of shale’s that have little relief. This belt of shales is reminiscent of the sort of deposition that would occur in the shallow waters along continental margins. In fact this belt of shales does follow a suture which runs along the line from Carlingford to Shannon. The suture is not visible at the surface and is only revealed by deep earth seismic profiling. This is the collision zone between the north and south coasts of the now disappeared Iapetus Ocean. So Northern Ireland also has a continental collision zone, a subduction zone to go with it, and the volcanics that you would expect to find in such an area. Another main feature of interest in the Co. Down and Co. Armagh is the swarms of Drumlins. This is world class site for the study of these supposedly glacial features. It appears to me that an area under a glacier or at the ablation zone of a glacier is an unlikely spot for the deposition of clay sized particles given the high energy of the glacial melt waters. (That’s a personal opinion) Anyway, if you want Drumlins, we have hundreds of them.

The southernmost part of Northern Ireland is the area of highest relief. The Mourne Mountains rise out of the relatively flat Palaeozoic peneplain to the north. Unlike the basalts just 30 or so miles to the north, this area is tertiary granite. It is an area of considerable scenic beauty as well as the water catchment area that supplies the water for the most densely populated areas of Northern Ireland. One of the most notable geological features in this region is Slieve Gullion. This is a now extinct volcano and has around it’s peak an easily seen and complete ring dyke. I warn you however, that it is strenuous walk up there. South of this area is Carlingford Lough, a typical Fjord and then into the Republic Of Ireland.

To the west of Lough Neagh are the Tyrone Uplands. This is an area of highly complex geology. The area has moderate relief and poor soils on Old Red Sandstones and falls away to the gentle lowlands of the Erne Basin on Carboniferous Limestones. In spite of the poor soils, this area entirely agricultural and sparsely populated. The low, rolling topography gives this area of Northern Ireland a beauty and serenity that few other places in Britain can match. Don’t bother so much about the rocks here, just take in the peace and quiet.

The last major area in Northern Ireland is that region to the west of the BannValley, the Sperrin Highlands. Here again the geology is highly complex and cut by numerous NE to SW trending faults. The rocks in this area are markedly older than those east and south of it. They are Moinian and Dalradian schists. These rocks are associated with the very similar rocks found in the north of Scotland and contain varying grades of metamorphic minerals. These rocks are part of the Caledonian Orogeny that affected a belt of the Earth’s surface stretching from Scotland through Northern Ireland across the Atlantic Ocean and into the Appalachian Mountain Belt in the eastern United States.

That concludes this very short history of the geology on Northern Ireland. Geologically we have seen a very violent and turbulent past. Thankfully, nature here is, for at least the being quiescent. And so it should be for the foreseeable future, but, no geologist would bet on the Status Quo for too long.

Bibliography. Much of the text and the map above was extracted from the Geological Survey of Northern Ireland. By H.E.Wilson M.Sc. Published by Her Majesty’s Stationery Office 1972.

M E Moore
Dromara
Co.Down
Northern Ireland
m1611@netscapeonline.co.uk