Geology of Northern Ireland

The story of Northern Ireland's rocks is one of a journey through space and time, as our seemingly unchanging earth has altered its appearance as land masses come and go and physically move their position. What is now the solid land forming Northern Ireland tells complex 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 on Table 1 below.


The oldest widespread rocks of Northern Ireland form the uplands of the Sperrins and 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 American side of a long disappeared ocean known as the Iapetus, they show that not only did part of Ireland once form part of that continent, but also that tremendous collisions have occurred, producing mountains out of materials formed on ocean floors.

Other sediments which accumulated on this ancient ocean floor have been preserved as the extensive series of sandstone and shale, forming the lowlands of Down and Armagh. Once again, evidence contained in these rocks shows that they actually represent sections of the seabed which originated from the American side of this lost ocean. During a second phase of mountain building, granitic rocks were emplaced below the earths surface. Erosion has exposed these, and can be seen today as the uplands which include Slieve Croob. Slieve Gallion mainly dates from this period.


Desert conditions prevailed during the Devonian period, with Northern Ireland once again present as dry land. The erosion of a 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 north 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 coral-fringed oceans. These can be seen in the Marlbank area of Fermanagh where they are testament to the great thickness which these reefs achieved. Some of the limestones in Fermanagh have subsequently been partially disolved by the action of rainwater. Cave systems have developed, with associated surface features including sinking streams and dry valleys. Some o the finest examples are to be found in the area of the Marble Arch show-caves.

Progressive shallowing of the ocean occurred later in the Carboniferous, until eventually great river delta's dominated the area. Great thicknesses of these sand and mud are preserved, forming much of the upland blocks of Slieve Beagh and west Fermanagh. Accumulated organic remains from these times, have also been preserved in places and form the small coalfields of Ballycastle and Coalisland.

While Northern Ireland remained dry land during the next geological period, the Triassic, a notable climatic change occurred. Now positioned approximately where Sudan is today, arid desert conditions prevailed. Occasional flood events deposited sand in shallow lakes, surrounded by dunes, producing the sandstone now seen at Scrabo and within the Lagan Valley. A gradual return to shallow marine basins, susceptible to drying out under the intense evaporation, resulted in great thicknesses of red mudstone in places sandwiching massive beds of salt, the residue left as sea-water disappeared in the heat. While these soft rocks generally do not make a dramatic impact on landscape, exploitation, continuing today, has left evidence of a salt mining industry in the Carrickfergus area.


Seas deepened during the Jurassic period, their legacy being grey mudstone and limestone now found mainly around Antrim coast. They can yield interesting fossils but their most visible contribution has been in promoting instability of overlying rocks. Landslips of various types, both active and stable, are well seen on the Antrim coast road at Minnis, north of Ballygally, and Garron respectively.

Marine conditions were maintained through much of the succeeding Cretaceous. Initially sandstone formed but these were overlain by the visually striking white limestones. Formed from microscopic remains of marine organisms, they record warm, clear sea conditions. The hardness of this chalk has made it resilient to erosion such that our white cliffs in 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 have been found to date.

By the end of the Cretaceous, a land mass broadly recognisable as Ireland, had continued its northward movement to a position similar to southern France today. 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.


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 similar to those in presently in Iceland, produced a great basalt plateau, the eroded remains of which dominate Antrim and parts of 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', while Tardree hill in Antrim is the result of surface volcanic activity alone. Mention must also be made of the Mountains of Mourne which were formed at this time as molten granite was emplaced just below the land surface and now exposed by erosion.

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 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 Tertiary. There has been much interest in these rocks as, along with the mud and sand, much organic material, especially wood, was also deposited, forming lignite. This brown coal may be exploited in the future, around southern Lough Neagh and also at Ballymoney.


The Pleistocene brought a major change in climate. 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 landscape cannot be overestimated. Ice masses thousands of meters thick, moved over the land eroding and re-depositing vast amounts of material. Ice action in many lowland areas formed drumlin belts, particularly in Down and Armagh, while many upland areas were generally smoothed giving the characteristic outline of much of the Mourne Mountains.

As ice melted, a range of landforms developed including moraines, marking positions where glaciers halted on their retreat, eskers, formed by rivers within the ice and deltas, where rivers poured into enormous ice dammed lakes. Such features are well seen between Cookstown and Omagh and also in the Dungiven area.

While the major determinant of landscape character since the Ice Age has been the impact of human activities, natural processes are ongoing. Changes in relative sea-level has 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 and Murlough and initiation and expansion of lowland and upland peat bogs which can be seen at Garry Bog and Garron Plateau.

QUATERNARY 1.6 - 0.01 Blown sand, peat
Sand, gravel and boulder clay
Beach, lake, river and peat bogs
Glacial deposits from glaciers and ice-caps
Modern humans.
Woolly mammoth.
TERTIARY 65 - 1.6 Clay, lignite
Basalt lava
Marsh and lake
Fissure eruptions
Earliest Hominids.
Widespread mammals.
First primitive apes.
Main bird groups present.
CRETACEOUS 135 - 65 White Limestone Marine conditions with accumulating planktonic material Extinction of dinosaurs, plesiosaurs and ammonites. Early flowering plants.
JURASSIC 195 - 135 Mudstone with minor limestone Marine conditions Dinosaurs dominant on land, plesiosaurs in oceans and pterosaurs in air. Early mammals.
TRIASSIC 250 - 195 Red mudstone with salt
Coastal lagoons
Shallow water in continental desert
First dinosaurs and large marine reptiles. First flies. Ammonites common.
PERMIAN 290 - 250 Limestone and sandstone Desert conditions with occasional marine influence Reptiles spread on land. Insects spread. Conifers common.
CARBONIFEROUS 355 - 290 Sandstone and minor coal
Coastal lagoons
Marine with coral reefs
Amphibians spread. Shark-like fishes.
Early trees and reptiles appear.
DEVONIAN 410 - 355 Sandstone, mudstone
Continental desert with periodic floods First amphibians. Fern-like plants on land.
SILURIAN 438 - 410 Shale and sandstone Mainly deep marine conditions

Occasional volcanic activity
First land plants. Armoured, jawless, fishes common.
ORDOVICIAN 510 - 438 Early fishes appear. Graptolites, trilobites and brachiopods common in oceans.
CAMBRIAN 570 - 510 Not present Dominance of trilobites in seas and development of early shelled forms.
PRECAMBRIAN 4,600 - 570 Metamorphic rocks: schist, marble Sedimentary and some igneous rocks changed by later metamorphism Early multi-celled animals.
Early bacteria and algae
© National Museums Northern Ireland 2003-19

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