Glendesha
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Geology & landscape
Geological Feature: Geological Fault, Rock Outcrop · Geological Period: Paleogene · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Glendesha Tertiary igneous The area is of special scientific interest because of its geology, which is seen in outcrop along the western side of Slievebrack. The Slieve Gullion complex, of which Glendesha is a part, is the finest example of a Tertiary igneous centre in Ireland and is among the best topographic expression of a ring-dyke system in the British Isles. The rocks found here are of international geological importance, having played an important role in a number of theories related to the development of and interaction between igneous rocks. The Slieve Gullion complex developed in an area of crustal weakness, previously exploited by the much older Newry granite, around 400 million years old. In Palaeogene times, some 56-58 million years ago, it was the site of a major volcano of which there is now little evidence. Subsidence of this central unit was related to a ring fault, some 20 km in diameter, the latter providing a natural weakness exploited by intrusive igneous rocks. This produced the ring-dyke complex which has been exposed, through erosion of the softer surrounding rocks, to reveal the near-circular system known as the Ring of Gullion. The final phase saw activity return to the central area with apparently layered igneous rocks, both acid and basic in nature. Past debate on the formation of the central complex has been largely resolved. The layered structure apparent today appears to have developed as a series of igneous units intruded or injected into pre-existing rocks, rather than building up as a succession of extruded rock bodies. The Slieve Gullion complex is historically important as it has featured in a number of major geological debates on the nature of igneous rocks and the processes by which they can be formed. The main rock exposures found in Glendesha consist of the porphyritic felsite, part of the ring-dyke which here forms the steep hillside of Slievebrack. A vent agglomerate series is also present, pre-dating the felsite and representing an explosive phase caused by degassing. The violent explosion ripped the older Newry granodiorite from the wall of the ring during this cataclysmic degassing. The debris-filled vent, together with the main ring fault itself, was then invaded by the molten felsite. The felsite shows southerly inclined flow structures that track the contact with the vent agglomerate at its base. Occasional units of basic igneous rocks are present, representing the earliest eruptive materials from the Slieve Gullion volcano. This location provides a fuller understanding of the geological history of the Slieve Gullion Ring complex. https://www.daera-ni.gov.uk/publications/glendesha-assi