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Home > NIEA > Protected Areas > Areas of Outstanding Natural Beauty > Ring of Gullion AONB > Geology within Ring of Gullion AONB

Geology within Ring of Gullion AONB

Last updated: 19 January 2010


The Ring of Gullion is the most spectacular example of a ring-dyke intrusion in the British Isles. The rocks of the area are complex and have featured in international geological debate over the past 60 or so years. The site has attracted geologists from all over the world and featured in a number of theories that have been put forward to explain the unusual rock relationships. Some of these theories have now become an accepted part of geological science.

The oldest rocks in the area formed in an ancient ocean more than 400 million years ago during the Silurian period. Masses of molten granitic rock or magma, were later intruded into these rocks, which underlie Newry town and much of the Slieve Gullion area. These granites are some 390 million years old and date from a major period of mountain building in Ireland.

In the Tertiary period, commencing some 65 million years ago, the area once again became the centre of volcanic activity. The sequence of events is complex but probably began with the development of a very large volcano of which little now remains. Volcanoes also erupted in the south of the area and the remains of volcanic necks can be found in the hills around Forkhill. Along the contact zone between the Silurian rocks and the Newry granite a roughly circular fracture developed into which was intruded a series of acidic lavas. These cooled to form very hard granophyre and felsite rocks - in fact two 'rink dykes'.

Slieve Gullion itself is more recent than the ring dykes and is made up of layers of granitic and basaltic rocks. There has been some debate as to their origins. One suggestion is that a huge explosive eruption of the volcano created a vast crater, or caldera, into which lavas were excluded in layers. Another more plausible explanation is that the lavas were extruded in layers. There is evidence of many highly unusual features developed by the interaction of basic and acidic magmas.

In more recent times the landscape has been shaped by the action of glaciers during successive Ice Ages. Glaciers exploited existing weaknesses in the rocks (faults and softer rocks) to erode deep valleys through the Ring of Gullion. The upstanding hills were glacially scoured leaving craggy outcrops (roches mountonnees), boulder strewn slopes and rocky ridges and hollows. The valley bottoms were in-filled with glacial deposits forming rounded drumlins, streamlined by the flowing ice. The 'tail' of Slieve Gullion, which itself forms the 'crag', is seen at Dromintee, and is a ridge of boulder clay deposited in the wake of Slieve Gullion as it was overrun by ice moving from the north.