Slieve Gallion 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 Central Uplands of Tyrone and Fermanagh. This area is defined in the north by the fault-guided scarp that forms the southern edge of the Sperrin Mountains. Below this are plateau lands that decrease in height and complexity to the south, before rising again to the lower slopes of Slieve Beagh. Below ca 350m the landscape is dominated by thick drift deposits, including prominent drumlin fields, dead ice features and glaciofluvial deposits - often capped by blanket peats. Some hills rise above the general level of the plateau, most notably the basalt-capped outlier of Slieve Gallion. The southwestwards trending Clogher Valley effectively divides the southern section of the upland into two blocks, one lying between Tempo and Pomeroy and the other centred on Slieve Beagh.
Slieve Gallion is a prominent peak on the far eastern fringe of the Sperrins. It is a volcanic plug and has a distinctive profile, with a steep-sided, flatter summit than the surrounding mountains and an uneven slope profile. The solid geology of the Slieve Gallion area is particularly complex, with igneous rocks of volcanic origin surrounded by areas of granite. Granite extends up onto the southern slopes of Slieve Gallion, producing a particularly scenic and unusually verdant landscape on the shores of Lough Fea. The upland plateau to the west of the summit is dominated by extensive blanket bog, small rounded loughs and the conifer plantations of Davagh Forest. A resistant band of limestone crops out at the foot of Slieve Gallion and is deeply dissected by numerous streams within deep, narrow valleys. The lower slopes of Slieve Gallion have a diverse landscape pattern, with an irregular patchwork of fields, punctuated by small blocks of woodland. Many of the narrow glens are densely wooded. There is extensive sand and gravel quarrying on the fringes of Lough Fea. Areas of glacial deposits on the upland plateau are affected and the scenic qualities of this unusual and attractive combination of landscape elements is threatened by the quarries, machinery and the associated roads. A key element in the landscape is the Lough Fea area in the west of the LCA that is of importance in understanding the recent glacial history of Northern Ireland and contains an assemblage of deglacial features including eskers, end moraines, outwash plain and kettle depressions. It is extremely unusual to find all features associated with this ice disintegration so well preserved in such close proximity. The area also has important peatland habitat including Teal Lough ASSI/candidate SAC.
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. The older formations can be upside down (tectonically inverted).
Stratigraphic Table (youngest rocks at the top of the table)
| Tertiary - dolerite dykes and sills, about 60 million years old |
|---|
| Cretaceous - greensands and limestones, about 100 million years old |
| Triassic - Sherwood Sandstone Group - about 240 million years old |
| Carboniferous - Iniscairn, Altagoan, Desertmartin, Meenymore, about 350 million years old |
| Late Caledonian - Slieve Gallion Granite - about 400 million years old |
| Early Caledonian - Tyrone Plutonic Complex - about 450 million years old |
| Ordovician - Tyrone Volcanic Group - about 450 million years old |
| Moinian - Corvanaghan - about 1000 million years old |
Comprises a large number of rock types of many ages in complex faulted, unconformable and intrusive relationship.
Key Sites
Moinian - Corvanaghan Formation
Silliminate - mica schists with migmatite lenses and bands. Exposed in the cores of anticlines (the classic example being the Ballinderry Anticline) and in eroded fault-blocks. Crops out on the crags and open moorland of Oughtmore Mountain (ESCR Site 312) and also in the Central Tyrone Inlier around Eagles Rock (ESCR Site 313).
Early Caledonian - Tyrone Plutonic Complex
Olivene gabbros that intrude early dolerites. Gabbros comprise coarse pyroxene - plagioclase - olivene, banded and poikiloblastic types with rafts of dolerite. Underlies the central - southwestern tract, about 15% of the total LCA area. Deformed volcanic tuffs and chert crop out at Bonnety Bush (ESCR Site 431).
Ordovician (Lower Palaeozoic) - Tyrone Volcanic Group
The Tyrone Volcanic Group lavas occur in a fault-bound block in the north of LCA41, and on the north-eastern edge of the Slieve Gallion Granite, in faulted contact with Carboniferous. The lavas are part of an ophiolitic succession - fragments of oceanic crust that have been pushed up from the ocean into a continental sequence of rocks. They comprise lava flows and pillow lavas metamorphosed during their emplacement in the Caledonian Orogeny. These lavas are generally degraded and not used in construction. Good-quality soils can develop above theses rocks where they are not enriched in copper. Crops out at ESCR Site 433: Sruhanleanantawey Burn, also Slieve Gallion itself (ESCR Site 438).
Carboniferous (Upper Palaeozoic, from oldest to youngest)
A lower succession comprises Iniscairn Formation (Tournaisian red breccias, conglomerates and sandstones). Crops out at the type section (ESCR Site 253). The Drumard Member is seen in the streams of Iniscairn (ESCR 253). The Desertmartin Formation crops out at Keenaght Water (ESCR Site 259).
Caledonian, Variscan and post-Variscan aged faults of many orientations cross the area. Tight folding in common in the Corvanaghan schists.
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 more than 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. 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 shows that the largely drift-free upland of Slieve Gallion is encompassed to the north, east and south by extensive till deposits. Thes are from the late Midlandian expansion of the Lough Neagh ice sheet. However, the area of greatest interest in terms of its Quaternary geomorphology and geology is the major deglacial complex that lies to the west and southwest of the uplands. The Lough Fea Deglacial Complex occupies a substantial area (18.6km2) in the LCA, with smaller areas in LCAs 25 and 40. An intermittently stagnant ice front has left a classic series of deglacial landforms around the Lough Fea area. Initial ice halts are marked by moraine ridges south of Lough Fea, at Lissan and Grouse Lodge, where there is also evidence of sediment deposition within a waterbody. Retreat continued to positions at Mill Lough and north of Davagh Eskers, again marked by moraine ridges. During this latter phase, subglacial channels fed sediment and meltwater under the ice mass and over the Teal Lough and Brackagh outwash plains, which were ice-free by this time. The line of this channel is marked by sections of the Davagh esker. It is extremely unusual to find all features associated with this ice disintegration so well preserved in such close proximity.
The east of the LCA also touches on the Moneymore Deglacial Complex that consists of discontinuous sand and gravel mounds and spreads in a north/south oriented belt in the Lough Neagh lowlands between Moneymore, Coalisland and Dungannon, Co. Tyrone. The deposits consist of a very limited subaquaous fan deposit (0.5km2) at the border with LCA 49. This Complex is much more extensive in LCAs 42 and 49, where it is described more fully.
Key Elements
ASSIs
088 TEAL LOUGH PART II
Physiographical interest in the active peat processes. The blanket bog developed on a free draining fluvio-glacial sand and gravels due to iron pan formation impeding drainage. The bog contains a well developed pool and hummock complex and supports a Sphagnum rich bryophyte carpet with several rare species. The Oligotrophic lough is characteristic of base-poor lakes on peat. The overall habitat supports a notable upland peatland insect fauna.
011 TEAL LOUGH & SLAGHTFREEDEN BOGS
Three sites of undisturbed upland raised bog within blanket bog. Fine hummock and pool complex at Teal Lough. Bog pools contain uncommon plant species. High sphagnum cover.
Deglacial Complexes
LOUGH FEA DEGLACIAL COMPLEX
The Lough Fea area is a classic grouping of associated deglacial landforms that can subdivided into discreet assemblges. This LCA includes a number of important sites including the Lough Teal outwash plain with moraine ridges and the Brackagh outwash plain. The third major element, the Davagh esker lies mainly in LCA 25.
Other sites/units identified in the Earth Science Conservation Review
312 Oughtmore Mountain
Precambrian. Outcrops of Corvanaghan Formation, showing lithology and structural features.
313 Eagles Rock
Precambrian. Central Tyrone Inlier. Outcrops of Corvanaghan Formation, showing lithology and structure.
431 Bonnety Bush
Caledonian - Igneous. Tyrone Plutonic Group. Exposures of deformed Ordovician volcanic tuff and ferruginous Chert.
259 Keenaght Water
Conglomerates and pebbly sandstones overlain by Desertmartin Limestone Formation. Two limestone beds at the top of the section contain quartz-rich veins with coral and brachiopod fossils.
433 Sruhanleanantawey Burn
Caledonide-Igneous. Outcrops of black shale interbedded with mixed volcanic and volcaniclastic (tuff, phyllite and chert) of the Tyrone Volcanic Group. The shale contains graptolite fossils.
438 Slieve Gallion
Caledonide-Igneous. Good access site to the granodiorite and rhyolitic porphyry intrusions of Slieve Gallion.
352 Slieve Gallion
peat stratigraphy
253 Iniscarn
Carboniferous. Exposures of stratotype for Iniscarn Sandstone Formation and succeeding Drumard Member of Altagoan Formation.
AONB
Except in the far east, almost all of the LCA lies within the Sperrin AONB (1968) and is indicative of its scenic quality.
