Bessy Bell and Gortin 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 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.
The Bessy Bell and Gortin landscape is a distinctive, scenic and much visited part of the North West; the twin peaks of Bessy Bell and Mary Gray form a gateway to the south of the Sperrins. The high summits of Mullaghcarn (542m), Slieveard (419m) and Bessy Bell (420m) are outliers to the south west of the principal Sperrins range. They are divided by the scenic valley of the River Strule, which flows northwards from Omagh towards the Foyle. In common with the rest of the Sperrins, the high summits of Mullaghcarn and Slieveard have a dramatic, mountainous appearance, with distinct, sharp ridges and rocky summits. The slopes are littered with grey scree and carved by steep, fast-flowing burns, which flow in deep gullies. A long ridge extends from the main mountain block to the northwest, enclosing the undulating valley of Cappagh Burn and its branching tributaries. Its sequence of lower summits, Ballnatubbrit Mountain, Beauty Mountain and finally, Mary Gray, form a scenic backdrop to views along the Strule Valley. To the west of the Strule, Bessy Bell, and the neighbouring smaller summits of Deer's Leap and Forster's Mountain, have a more rounded character, in common with the foothills to the north of the Sperrins. The open summits are capped with open moorland, with a transition to marginal pastures and richer farmland on the lower slopes. From the confluence with Cappagh Burn to Newtownstewart, the Strule River meanders within a deeply incised, wooded channel. The river channel has been dammed to create a sequence of loughs on the valley floor.
A key geomorphological and scenic element in the landscape is the flat-topped, steep-sided glaciolacustrine mound at Gortin. This forms a striking topographic feature visible from part of the Ulster Way, which also passes directly through the landform complex. The large, deeply dissected landform is perched high on the wooded (plantation) and peat-covered southern slopes of the flat-bottomed Owenkillew valley, with the steep delta foreslope forming a spectacular backdrop to Gortin town. The stark contrast of the flat, notched, delta skyline with the surrounding hills is unique in Northern Ireland for scenic value, visibility and intactness. The only comparable feature is the Carey delta, Co. Antrim.
In the east of the LCA, the Murrins deglacial complex is aesthetically unique. This is because it is generally visually intact, the impact of the moraine-outwash assemblage on the landscape is very marked and landscape contrasts are provided by the juxtapostion of sand and gravel and bedrock and differences in ridge orientation and topography type. Moraines and feeder eskers contrast with intervening alluvial flats, meltwater channels and kettleholes.
This is a highly accessible and scenic landscape. Mullaghcarn and Gortin fall within the Sperrins AONB, but Bessy Bell and the adjacent Baronscourt Estate are classified as part of the Sperrins Foothills Area of Scenic Quality. The landscape as a whole is in good condition and is extremely sensitive to change and the mountain slopes form a backdrop to the long river views. Key elements in the landscape include:
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)
Carboniferous - about 350 million years old |
|---|
Greenan |
Claragh |
Owenkilllew |
Omagh |
Ordovician - about 450 million years old |
Tyrone Volcanic Group |
Dalradian (Neoproterozoic) - about 600 million years old |
Un-named metabasites |
Mullaghcairn |
Glengawna |
Glenelly |
Dart - Claudy |
Dungiven |
Newtownstewart |
This LCA is dominated by Dalradian (Neoproterozoic) strata of the South Tyrone succession as it passes north into the Co. Londonderry succession.
Two tectonic phases have affected the area: the Caledonian (Ordovician - Silurian) and Variscan (end Carboniferous). Consequently NE-SW faults, folds, intrusions and minor fabrics posses this orientation. The Omagh Thrust clips the southeastern edge of LCA26, where it is exposed at Mountfield Old Quarry (ESCR Site 324).
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 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 upland character of this LCA is reflected in the large areas of drift-free bedrock shown on the Drift Geology map. This also shows extensive lowland deposits of Late Midlandian till associated with an ice mass that was centred on the Omagh Basin. However, the Quaternary deposits that are of greatest geomorphological and geological significance are those laid down during the deglaciation of the area. This LCA contains significant elements of three such deglacial complexes that are important scientifically and for their sand and gravel resources.
The Murrins Complex occupies 3.4km2 in the east of the LCA. The Murrins complex as a whole is located in the lowland area between Carrickmore to the southeast and the upland massif of Mullaghcarn and adjacent hills to the northwest. The northern part of the complex terminates at the foothills of the Sperrin Mountains near Greencastle. Associated esker ridges up to 8 km long occur to the south at Beragh, Coolesker, Seefin and Cloghfin. Sediments are mainly superimposed upon bedrock to the west and north, and glacial till to the east and 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. The complex also lies to the south and east in LCAs 43 and 25, with outliers to the southwest in LCAs 22 and 23.
The Gortin Proglacial Delta (part of the Glenelly Valley Complex) occurs south of Gortin town, where steep-sided, flat-topped deltaic deposits record the ponding of proglacial lakes at progressively lower levels as ice withdrew along local river valleys. Esker ridges demonstrate subglacial transport of sediments into the delta complex. The landform assemblage probably contains a major reserve of sand and gravel and may be vulnerable to future commercial exploitation. However, the delta surfaces are generally pristine.
The Foyle Valley Glaciofluvial Complex, occurs as a limited number of deposits (5.5km2) primarily as glaciofluvial terraces along the valley of the Strule and ending at the Deer Park moraine and outwash,, approximately 75% of which lies in LCA 27 to the North. Most of the Foyle Complex is in LCA 27 and is described in detail there
Key Elements
ASSI
DEER PARK MORAINE AND OUTWASH (Foyle Valley Complex)
A massive sharp-crested moraine ridge extends westwards from Newtonstewart and merges on its northern flank with a flat-topped terrace that lies along the western bank of the Strule River. Two esker ridge fragments are located to the south of the main ridge. Landforms mark a major ice stillstand during final retreat towards the Omagh Basin. Steep ice contact slopes and deeply entrenched meltwater channels produce dramatic landscapes of high aesthetic quality.
Deglacial Complexes
The Murrins complex
The Murrins complex records a sequence of sedimentary events associated with southwestern ice retreat from the southern Sperrin Mountains into the central part of the Omagh Basin and can be classified as being of unique importance on a Northern Ireland scale.
GLENSAWISK BURN PROGLACIAL DELTA (Murrins Complex)
This area of the central Fintona Hills occupies the extreme east 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 24.
STRULE RIVER VALLEY OUTWASH COMPLEX (Foyle Valley Complex)
Landforms along the flanks of the Strule River valley are part of a suite of features that document the final withdrawal of ice from the Foyle valley and its tributaries southwards into the Omagh Basin. Deposits along the Strule consist of meltwater-dissected outwash and are the most visible landscape feature to be seen from the main Omagh - Strabane road. Exploitation has been limited to occasional ad hoc extraction, and the landscape of this pleasant, rolling, agricultural area is generally intact. This contrasts with features immediately to the north in the Newtownstewart area, where extensive destruction of glaciofluvial landforms is evident.
GORTIN PROGLACIAL DELTAS (GLENELLY VALLEY COMPLEX)
The Gortin complex is one of the largest scale examples of a proglacial delta landform assemblage in Northern Ireland. The lakes were ponded by ice fronts located in the lower Owenkillew, upper Owenkillew and Owenreagh river valleys. Eskers meltwater channels indicate that ice pressure was from the south (Omagh Basin). The Gortin Loughs now occupy kettle holes.
Other sites/units identified in the Earth Science Conservation Review
324 Mountfield Old Quarry
Precambrian. Exposure of Omagh Fault. Evidence of nature of Grampian Terrane and Midland Valley Terrane boundaries.
AONB
The eastern half of LCA lies in the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.
