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 North Derry Uplands and Sperrin Mountains. This region has a composite geological structure. In the north, the North Derry Plateau is wholly developed on basalt and defined by a steep, unstable escarpment to the west and a set of structural benches dipping gently to the east. Southwest of this plateau land, and beyond the Glenshane Pass, schists and quartzites form the rounded, whaleback ridges of the High and Low Sperrins. The incised, steep-sided valleys of rivers such as the Glennelly and Owenkillew accentuate the southwestwards, Caledonian structural trend of the Mountains. Late Glacial ice recession from around the mountains and the creation of temporary ice-dammed lakes has left valley floors and slope foot zones mantled in thick, complex glaciofluvial deposits. Northwest of the Sperrins is a dissected block of country underlain by schists that forms the Loughermore-Altahullion hills and the Middle Faughan basin.
South Sperrin includes the upland river valley of the Owenkillew and Owenreagh Rivers and the broad ridges to the south of the Glenelly valley, including the summits of Spaltindoagh (410m) and Mullaghmore (554) to the north of the Owenkillew River and Crocknamoghil (335m) to the south. The broad upland ridges of the Sperrins in this area form a backdrop to the valley landscapes. The mountain skyline is open, with upland grasses and rocky screes on the slopes leading to the summits. The valley slopes are deeply undulating and dissected by tributary burns flowing in rocky, open channels. The character and pattern of the landscape changes gradually from the valley floor to the upper moorland slopes. The slopes of the upper Owenkillew and Owenreagh River valleys are characterised by a patchy mosaic of derelict pastures and scrub and poorly-drained land is often infested with rushes. The lower river valleys, to the west of the confluence of the Owenkillew and Owenreagh Rivers, have a more secluded, pastoral character. In the east of the LCA the landscape can therefore be summarised as one of broad rounded ridges with deep, branching gullies and fast-flowing upland streams. Within these valleys, meandering rivers are a focus for views and irregular mounds of glacial till and glaciofluvial sands and gravels often subdivide narrow floodplains. However, in the west of the area, the landscape opens out into a wide, gently sloping basin at the junction of the Glenelly and Owenkillew rivers that forms a scenic contrast to the steeply rising ground of the heather covered, rounded main Sperrin range. This lowland contains a range of glaciofluvial and claciolacustrine deposits that culminate in the elevated deltaic complex at Gortin. Although this lies largely in LCA 26 to the southwest, it exerts a considerable visual influence on this LCA and does much to establish the landscape character of its southwestern margin. The very south of the LCA also includes part of the Murrins deglacial complex.
Pre-Quaternary (Solid) GeologyThe 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)Carboniferous - youngest strata, about 350million years old |
Owenkillew |
Barony Glen - Omagh Sandstone Group |
Ordovician - about 450 million years old |
Tyrone Volcanic Group |
Un-named Caledonian metabasites |
Laght Hill Tonalite |
Dalradian (Neoproterozoic) - oldest strata, about 600 million years old |
Mullaghcairn |
Glengawna |
Glenelly |
Dart |
This LCA is dominated by Dalradian metamorphic rocks disposed as a NE-SW trending anticline, parallel to the Omagh Thrust to the southeast.
Key outcrops are out around Barnes Gap (ESCR Site 320), in Golan Burn (Golan Burn Member exposed: ESCR Site 322, Glendarragh Burn - ESCR Site 323). The Ordovician Tyrone Volcanic Group comprises a series of tuffs, lava flows, pillow lavas (mapped separately in the middle of the Tyrone Volcanic Group as the Copney Pillow Lavas in some areas), volcaniclastic breccias and rhyolites. Occurs along the southeastern edge of LCA24, north of the Omagh Thrust. Caledonian (early Ordovician) magmatic intrusives are represented by the highly deformed Laght Hill biotite - hornblende tonalite is found within Ordovician volcanic rocks. Near Newtownstewart, Carboniferous sandstones are recorded at Middletown Burn (ESCR Site 248). Carboniferous pebbly sandstones, sandstones and siltstones found in the far western protuberance of LCA24. Rests unconformably on Dalradian to the north and in faulted contact with the same to the south.
Two tectonic phases have affected the area: consequently NE-SW faults, folds, intrusions and minor fabrics posses this orientation. The Omagh Thrust clips the southern edge of LCA24.
Quaternary (Drift) GeologyNorthern 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 epoch. 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 mountainous character of much of the LCA is reflected in the high proportion of drift-free upland, often with a blanket of peat, that is shown on the Drift Geology map of the area. Although there is a partial cover of Late Midlandian till, derived from ice that moved up the valley from the west/southwest, the most interesting features are the deglacial sediments that were deposited towards the end of the Midlandian. The LCA contains most of The Owenkillew River Valley Complex running southwestwards through the centre of the LCA. Small, isolated outliers occur in LCA 26 to the southwest. The complex consists of sand and gravel spreads and hummocky moraine related to westward ice retreat along the valley. The complex was formed late in the deglaciation of Northern Ireland and low relief landforms on both sides of the river at Gorticashel are interpreted as outwash and hummocky moraine relating to an ice front which retreated westwards along the valley. Deeply incised meltwater channels cut in rock and drift are numerous. Overall, the landforms record ice margin retreat from the Sperrin Mountains to the north generally west and south towards the Foyle/Strule valleys and the Omagh basin. The Owenkillew valley is visually intact and presents an extremely scenic rural landscape within the Sperrins uplands. Development of the sand and gravel resource could affect the hydrology of the river as well as increase silt deposition on the river bed.
There is a small element of The Glenelly River Complex in the northeast of the LCA, but this glaciolacustrine and morainic complex is largely restricted to LCA28, where more detail can be found.
A limited area of The Murrins Complex (3.3km2) occurs in the south of the LCA, although most of this is located in LCAs 25, 26 and 43 to the south, The complex is characterised by proglacial outwash, frontal moraines, small retreat moraines, local high level deltas and discontinuous feeder eskers and local kettling and meltwater erosion.
Key Elements Deglacial Complexes/Sand and Gravel ResourcesGLENSAWISK BURN PROGLACIAL DELTA (Murrins Complex)
This area of the central Fintona Hills occupies a limited area in the extreme south of the LCA and is of unique importance in understanding the complexity of deglacial processes in Northern Ireland. Flat spreads of deltaic and glaciofluvial sediments within the Glensawisk Burn valley record the lowering of a lake bounded by bedrock slopes to the east, west and north. The evidence for ice withdrawal through topographic corridors in this area highlights the complexity of the pattern of ice breakup towards the end of the deglacial cycle, as ice cover disintegrated and became restricted to discrete areas in the Lough Neagh, the Omagh and the Lough Erne Basins. The Delta overlaps with LCA 26.
OWENBRACK/CLOGHFIN RIVER VALLEY PROGLACIAL ASSEMBLAGE (MURRINS COMPLEX)
This limited set of deposits straddle LCAs 24 and 25 and consist of a well-defined assemblage of sand and gravel delta and outwash surfaces deposited in a pro-glacial lake. They show that sediment was carried by meltwater derived from an ice front situated on high ground to the south. This meltwater must have been dammed by an ice front on lower ground to the north. The landforms are generally pristine and this intactness is an important landscape attribute. It is of high scientific importance because it provides evidence for the pattern of ice mass fragmentation as the ice thinned.
The Owenkillew River Valley Complex
See above.
Other sites/units identified in the Earth Science Conservation Review320 Barnes Gap
Precambrian. Deeply incised glacial overflow channel, containing good outcrop of Glenelly Member.
322 Golan Burn
Precambrian. Only exposure and stratotype for calcareous Golan Burn Member within Glenelly Formation. Exposure of Carboniferous unconformity at base of Owenkillew Sandstone Group.
323 Glendarragh Burn
Precambrian. Southern Highland Group. Exposures of top of Glengawna Formation with base of succeeding Mullaghcarn Formation.
325 McNally's Burn
Precambrian. Omagh Fault. Accessible exposure of contact between Tyrone Volcanic Group and metamorphic Dalradian rocks.
248 Middletown Burn
Carboniferous. Newtownstewart Outlier. Fossiliferous exposures of Omagh Sandstone Group.
AONBAll of the LCA lies completely within the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.