Ring of Gullion Geodiversity Profile
Outline Geomorphology and Landscape Setting
The use of a cultural overlay in defining Landscape Character Areas (LCAs) means that they frequently subdivide natural physiographic units. It is common therefore for significant geomorphological features to run across more than one LCA. It is also possible in turn, to group physiographic units into a smaller number of natural regions. These regions invariably reflect underlying geological, topographic and, often, visual continuities between their component physiographic units, and have generally formed the basis for defining landscape areas such as AONBs. It is essential therefore, that in considering the 'Geodiversity' of an individual LCA, regard should be given to adjacent LCAs and to the larger regions within which they sit. In the original Land Utilisation Survey of Northern Ireland, Symons (1962) identified twelve such natural regions.
This LCA lies within the region described as the Igneous Massifs of Down and Armagh. This region consists of two igneous complexes that rise above the broken foothills of a largely Silurian basement and are separated by the fault-guided and glacially modified inlet of Carlingford Lough. To the northeast are the rounded, Tertiary granite domes of the Mourne Mountains, whilst to the southwest are the mixed Caledonian and Tertiary igneous intrusions that comprise the ring dyke complex surrounding the central mass of Slieve Gullion. Separating the two are the drumlin covered Newry lowlands and the fault-guided Newry River.
The Ring of Gullion is situated on the southern border of Northern Ireland, and is a distinctive circle of hills around the striking landmark of Slieve Gullion. The landscape of the Ring of Gullion shows a remarkable coincidence with, and control by,, the underlying geology and is a truly 'geological landscape'. As such it is an internationally recognised example of a ring dyke complex. The underlying country rock is the Caledonian Newry Granodiorite, into which have been intruded a series of Tertiary igneous rocks to form a ring complex approximately 10km across. Subsidence of the central block produced alternating beds of granophyre and dolerite that now form the steep slopes of Slieve Gullion. The central mountain is surrounded in turn by a poorly drained lowland underlain by the Newry Granodiorite. Beyond this, the intrusive rocks, mainly granophyres and felsites, form a ring of rugged hills. Gaps in the ring are invariably associated with faults that offset the hills, for example at Camlough. To the southeast of Slieve Gullion is a 'tail' of glacial deposits laid down in the wake of the complex and in response to regional ice flow from the north at the end of The Midlandian. The scenic value and landscape quality of the area has been recognised by its designation as an AONB. For further information on this area and the Tertiary history of Ireland, see Davies and Stephens (1978)
Pre-Quaternary (Solid) Geology
The stratigraphy of this area is made up of the mapped formations in the table, the youngest of which usually overlie the oldest.
Stratigraphic Table (youngest rocks at the top of the table)
|Tertiary - various intrusives (Slieve Gullion Complex), Lower Basalt Formation - 50 to 60 million years old|
|Late Caledonian - Newry Complex - 400 million years old|
|Lower Palaeozoic - Gala Sandstone, Hawick Group - 450 - 400 million years old|
The area is centred on Slieve Gullion (ASSI 198), the LCA boundary roughly following the Newry Granodiorite - Lower Palaeozoic country rock contact (exposed in Cam Lough Quarry, ESCR Site 422). This is an exceptional area in terms of geology.
The area of Slieve Gullion is dominated by a 11km diameter ring complex or series of arcuate fissures along which various intrusions have occurred. The ring complex is of Tertiary age and is centred on the south-western end of the Caledonian Newry Granodiorite (ESCR Site 421). Thus part of the complexity of the geology is the fact that young (Tertiary) volcanic and subterraneanen intrusives occur in the same place as older Caledonian (probably Devonian).
Key Sites include the porphyritic granophyre with Newry xenoliths that is exposed at Crooked Road (ESCR Site 37). A dolerite plug cuts the Newry Granodiorite and is itself cut by granitic veins (Mullaghbane South, ESCR Site 40).
Along the inner arc of these northwestern breccias and extending intermittently on around the ring, the main ring dyke of porphyritic granophyre occurs. Exposures include Crooked Road (ESCR 37); Mullaghbane North (ESCR 38) and Mullaghbane Central (ESCR 39). Porphyritic felsites with vent agglomerate are seen at Glendesha (ESCR Site 41).
At Cloghinny (ESCR Site43) evidence of magma mixing can be observed and the Slieve Gullion ASSI (198) as a whole covers the Tertiary igneous features at Slieve Gullion itself, Sarah Daly's Bridge and Forest Quarry.
The Tertiary central complex of Slieve Gullion is emplaced within an almost detached lobe of the older Caledonian 'Newry' Granodiorite at its SW end Little remains of the basaltic shield volcano which once covered the site, but in the volcanic hearth now exposed by erosion one can see evidence for subsidence of the volcanic pile along a ring-fault some 20 km in diameter. This probably resulted in a summit caldera, though movement on the ring fault probably resulted from resurgence as well as subsidence. Movement of an acid magma up the SW quadrant of the ring-fault accompanied by explosive degassing produced spectacular vents filled with fragments of country rock. These vent agglomerates can be traced from Mullaghbawn Mtn. round to Slieveboiea; in places they are composed almost entirely of Newry Granodiorite or Lower Palaeozoic greywackes, elsewhere they carry large slumped masses and smaller fragments of lava. The acid magma congealed in the ring-fracture and in the vents as a Porhyritic Felsite; flow structures within the rock indicate a steep outward dip to the ring-dyke - the outer felsite, but the vent intrusions emplaced within the bounding ring-fault are irregular in outcrop and apparently sheet-like with a dip of flow bands to the SW. Confocal banding in the rock points to feeding channels within the ring-dyke. In places the Porphyritic Felsite contains broken phenocrysts and shows eutaxitic flow banding; some disruptive vesiculation of the rising magma may have produced ash flow material now welded and compacted to a tuffisite. A slightly later Porphyritic Granophyre ring-dyke occupies the ring-fault in the remaining quadrants. It may be a multiple intrusion with a xenolithic core to the NW near Lislea.
Quaternary (Drift) Geology
Northern Ireland has experienced repeated glaciations during the Pleistocene period that produced vast amounts of debris to form the glacigenic deposits that cover >90% of the landscape. Their present morphology was shaped principally during the last glacial cycle (the Midlandian), with subsequent modification throughout the post-glacial Holocene period. The Late Midlandian, the last main phases of ice sheet flow, occurred between 23 and 13ka B.P. from dispersion centres in the Lough Neagh Basin, the Omagh Basin and Lower Lough Erne/Donegal. The clearest imprint of these ice flows are flow transverse rogen moraines and flow parallel drumlin swarms which developed across thick covers of till, mostly below 150m O.D. during a period that referred to as the Drumlin Readvance. At the very end of the Midlandian, Scottish ice moved southwards and overrode parts of the north coast. Evidence for deglaciation of the landscape is found in features formed between the glacial maximum to the onset of the present warm stage from 17 and 13ka B.P. - a period of gradual climatic improvement. Most commonly these are of glaciofluvial and glaciolacustrine origin and include: eskers, outwash mounds and spreads, proglacial lacustrine deposits, kame terraces, kettle holes and meltwater channels (McCarron et al. 2002). During the Holocene, marine, fluvial, aeolian and mass movement processes, combined with human activities and climate and sea-level fluctuations, have modified the appearance of the landscape. The landforms and associated deposits derived from all of these processes are essentially fossil. Once damaged or destroyed they cannot be replaced since the processes or process combinations that created them no longer exist. They therefore represent a finite scientific and economic resource and are a notable determinant of landscape character.
The Drift Geology map for this LCA confirms the strong control exerted by the underlying geology on the landscape. This is revealed as a series of concentric, drift free ridges running around the centrals core of Slieve Gullion. Lowland areas within the ring are covered by Late Midlandian till, which at a regional scale flowed southeastwards across the region from a centre in the Lough Neagh Basin. It is unclear, however, whether any of this till was derived from locally centred ice masses towards the end of the Midlandian. In terms of its Quaternary history, this area is relatively unexplored, especially in comparison with the nearby Mournes Massif, and it would benefit from targeted research into its glacial and post-glacial history. The Drift map does, however, highlight numerous local pockets of alluvium within the ring complex, indicative of the broken terrain and intricate drainage pattern of the area.
198 SLIEVE GULLION
The Ring of Gullion is one of the best ring-dyke systems in the British Isles. Slieve Gullion itself is the finest example of a Tertiary igneous centre in Ireland. The rock exposures on the mountain and surrounding areas are of international geological importance. Slieve Gullion is one of the largest heathlands in Northern Ireland. The variety of heathland types reflects the different environmental conditions on the site, with the most extensive community dominated by Heather Calluna vulgaris. On the lower slopes there is a natural transition from upland communities, to lowland heaths and acid grasslands down into a series of small wetlands. These 'basin fens' are very diverse and of high conservation value. Some of the lowland heath communities are especially important for Northern Ireland.
Other sites/units identified in the Earth Science Conservation Review
422 Cam Lough Quarry
Caledonide-Igneous. Newry Igneous Complex. Evidence of relationships between contact metamorphosed Silurian country rock and late Caledonian microgranitic and granodiorites of SW pluton. Exposures of Tertiary Slieve Gullion Ring Dyke.
421 Dublin Road Bridge
Caledonide- Igneous. Access to exposed contact between 2 plutons of Newry Igneous Complex. Evidence of variety and relative timing of intrusion of igneous rocks.
37 Crooked Road
Tertiary igneous Slieve Gullion series - demonstrates central and outer margin of the porphyritic granophyre with abundant xenoliths of Newry granodiorite. Structures suggest multiple phase emplacement of granophyre.
38 Mullaghbane North
Tertiary igneous Slieve Gullion series - excellent sections through the 'ring' showing relationship between porphyritic felsite (early ring-dyke), the porphyritic granophyre (late ring-dyke), vents and basic intrusions.
39 Mullaghbane Central
Tertiary igneous Slieve Gullion series - section shows altered Newry granodiorite within the ring-dyke.
40 Mullaghbane South
Tertiary igneous Slieve Gullion series - section shows a dolerite plug cutting the Newry granodiorite and cut in turn by granitic veins. The age relationships are disputed.
Tertiary igneous Slieve Gullion series - excellent exposure of porphyritic felsite, showing contact with vent agglomerate, the latter comprised mainly of Newry granodiorite.
Tertiary igneous Slieve Gullion series - key locality in dispute over nature of the areas geology - 'pillow lavas' were subsequently reinterpreted as the mixing of 2 contrasting magmas, dolerite and granophyre.
This LCA encompasses almost all of the Slieve Gullion AONB (1991). This designation is indicative of the scenic quality of the landscape.