'The Present is the Key to the Past'
Sir Charles Lyell 1797-1875
The story of Northern Ireland’s rocks is one of a journey through time and place, as our seemingly unchanging Earth has altered its appearance as land masses came and went and physically moved position. What is now the land forming Northern Ireland tells a complex geological history as this part of the Earth has moved through many different climatic zones, sometimes occurring as dry land, at other times submerged below past oceans. Our geological history is summarised in the Table at the end of this section.
ANCIENT OCEANS AND MIGHTY MOUNTAINS
The oldest, and some of the most widespread rocks of Northern Ireland form the uplands of the Sperrins and parts of north-east Antrim. These hills are the remains of an ancient mountain chain, once larger and higher than the Himalayas are today. Formed from sediments laid down in basins on the North American side of a long-disappeared ocean known as Iapetus, they show that not only did part of Northern Ireland once form part of that continent, but also that tremendous collisions have occurred to produce mountains.
The sediments which accumulated on this ancient ocean floor have been preserved as the extensive series of sandstones and shales, forming the lowlands of counties Down and Armagh. Once again, evidence contained in these rocks shows that they represent sections of the seabed that originated from the ‘North American’ side of this lost ocean. During a later phase of mountain building, melting of the crust created large magma chambers that then cooled to form granitic rocks deep below the Earth’s surface. Erosion has now exposed these, and the rocks can be seen today forming the uplands which include Slieve Croob and Slieve Gallion.
DESERTS AND TROPICAL OCEANS
Desert conditions prevailed during the Devonian period, with this area once again situated on dry land. The erosion of the now vanished mountain chain and volcanoes yielded sediments, which in turn produced new rocks – the ‘pudding stone’ conglomerates of Cushendun and red sandstone forming low ground on the northern part of the Clogher Valley.
Returning marine conditions during the Carboniferous resulted in the development of extensive limestone, some of which bear evidence of ancient reefs and tropical seas. Some of the limestones in County Fermanagh have subsequently been partially dissolved by the action of rainwater to form a distinctive karst landscape. Cave systems have developed, with associated surface features including limestone pavement, sinking streams and dry valleys. Some of the finest examples are to be found in the area of the Marble Arch show-caves.
Major sea-level change and tectonic movements led to progressive shallowing of the ocean later in the Carboniferous, until eventually great river deltas dominated the area. Vast thicknesses of sand and mud were deposited, with the resulting rocks now forming much of the upland blocks of Slieve Beagh and southwest County Fermanagh. Accumulated organic remains from these times were also preserved in places and form the small coalfields of Ballycastle and Coalisland.
While Northern Ireland largely remained as dry land during the next geological periods, the Permian and the Triassic, a notable climatic change occurred. Now positioned approximately where Sudan is today, arid desert conditions prevailed. River and dune systems deposited sand in low lying areas, producing the sandstone now seen at Scrabo and across the Lagan Valley.
Large lakes that may have been fed by seawater were susceptible to drying out under the intense evaporation, resulted in great thicknesses of red mudstone in places sandwiching massive beds of rock salt, the accumulated residue left as the salty water disappeared in the heat. While these soft rocks generally do not make a dramatic impact on landscape, their use by the salt mining industry continues to this day in the Carrickfergus area.
THE AGE OF THE DINOSAURS
Sea level rose during the Jurassic period, the legacy being grey mudstone and limestone now found mainly around the County Antrim coast. They can yield interesting fossils, but their most visible contribution has been in promoting instability of the overlying rocks. Landslips of various types, both active and inactive are well seen on the Antrim Coast Road at
Minnis, north of Ballygally, and Garron Point respectively.
Whilst there is evidence of uplift and erosion, marine conditions returned through much of the succeeding Cretaceous. Initially sandstones formed but these were overlain by the visually striking white limestones (originally called chalk). Formed from microscopic remains of marine organisms, they record warm, clear sea conditions. The hardness of this limestone has made it resilient to erosion such that our white cliffs in County Antrim, unlike those at Dover which are of the same geological age, are standing fast against the action of the sea. Unfortunately, however, although this was the age of the dinosaurs, no significant fossils of them have been found to date in Northern Ireland (only a handful of isolated bones from the underlying Jurassic rocks have been found). These mudstones and limestones are however full of fossils of marine organisms, including rare bones of Ichthyosaurs and Plesiosaurs (late Triassic and Jurassic).
By the end of the Cretaceous a land mass broadly recognisable as Ireland had continued its northward movement to a position similar to present day southern France. Weathering of the limestone surface created a karst landscape with sinkholes and caves present. These features are visible today at the top of much of the chalk.
ERUPTING VOLCANOES
The next major geological event was to be literally explosive. The opening up of the North Atlantic, a process continuing today, was accompanied by widespread volcanic activity. Scenes like those seen presently in Iceland produced a great basalt plateau, the eroded remains of which dominate County Antrim and parts of County Londonderry. Successive lava flows covered the land, sometimes producing dramatic landscapes as at the Giant’s Causeway, while some molten bodies failed to reach the surface but have been exposed by subsequent erosion. These have often resulted in striking features and include Fair Head and Ramore Head on the north coast. Elsewhere combinations of surface volcanic action and intruded molten magma resulted in the formation of the Slieve Gullion complex with its remarkable ‘Ring of Gullion’, while Tardree in County Antrim is the result of explosive surface volcanic activity. Mention must also be made of the Mountains of Mourne, which were formed at this time as molten granitic magma emplaced just below the land surface. The granite has been exposed at the surface by erosion over millions of years.
This widespread volcanic activity was accompanied and followed by frequent shifts in the relative levels of the land. This faulting, often reactivating much older crustal weaknesses, has produced a range of features. Large-scale faulting over the central area of Northern Ireland resulted in a depression (or basin) now occupied by Lough Neagh. Elsewhere, faulting has often brought rocks of different ages to similar levels so that cliff scenery on the north coast shows dramatic lateral transitions from basalt to chalk.
Lough Neagh and other basins acted as a natural sink for accumulating sediments during the latter part of the Palaeogene. In the past, there was much interest in these rocks as along with the mud and sand, much organic material, especially wood, was also deposited to form lignite around southern Lough Neagh and also at Ballymoney.
IN THE DEEP FREEZE
The Pleistocene brought a major change in climate and saw the last major geological events to occur in Northern Ireland. Although thought of as the Ice Age, ice was in fact only present for relatively short periods of time albeit still amounting to many thousands of years. The impact on our landscape cannot be overestimated with ice sheets thousands of metres thick moving over the land eroding and redepositing vast amounts of material several times. The evidence we have of these events on land is all around us and dates mainly from the last major glacial phase of the Pleistocene. The action of flowing ice in many lowland areas formed drumlin and ribbed moraine fields, particularly in Counties Down, Armagh, Tyrone and Fermanagh. Upland areas were generally smoothed, giving the characteristic outline of much of the Mountains of Mourne for example.
As the ice melted a range of landforms developed including terminal moraines that mark positions where glaciers halted on their retreat (these are different to the ribbed moraines mentioned above), eskers which form from rivers within the ice, and deltas where glacial rivers poured into enormous ice-dammed lakes. Such features are particularly well seen north and south of the Sperrin’s between Cookstown and Omagh and in the Dungiven to Strabane area.
AFTER THE ICE HAS GONE
While the major control of landscape character since the Ice Age has been the impact of human activities, natural processes are ongoing. Changes in relative sea-level have formed the raised beach on which the Antrim coast road has been built, with stranded caves and intriguing stranded coastal landforms near Ballintoy Harbour. The most notable developments have been the growth of coastal dune complexes as at Magilligan in County Londonderry to the northwest and Murlough Bay in County Down to the southeast, and initiation and expansion of lowland and upland peat bogs such as at Garry Bog and Garron Plateau in County Antrim.
More information on the geology of Northern Ireland beyond is provided through links at ‘finding out more’.
