Lough Bradan 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 Western River Basins, but grades westwards in to the Plateau and Valley Lands of Fermanagh. This region consists essentially of the connected river systems that drain the Carboniferous and Old Red Sandstone plateau of County Tyrone, as well as the foothills of the Sperrin Mountains to the east and Donegal to the west. The region extends from the Omagh Basin in the south, northwards along the lower Foyle valley. The Omagh Basin has particular significance as an ice centre during the Late Midlandian and is now largely covered by a complex mixture of glaciofluvial sands and gravels and drumlins overlying Rogen moraines. When the headwaters of these river systems rise together they have in the past been responsible for serious flooding at the bottleneck of Strabane. Although this has been mitigated by extensive drainage control works in and around the town.

Lough Bradan extends from NE Fermanagh to the Lough Bradan Forest in West Tyrone. The area is characterised by valleys and steep drumlins on the lowlands, with a transition to an undulating sandstone plateau. The plateau is slightly elevated and fingers of this more resistant rock extend out into the drumlin lowlands. To the northeast, the land rises to the massive rounded summits of Pollnalaght (268m), Dooish (341m) and Tappaghan Mountain (333m). In lowland areas, there is an alternating pattern of forest and marginal pastures, dissected by small rivers that link the areas of bog, damp woodland and meadow that lie between the drumlins. The patchwork of small, angular fields breaks down to scrub and open grazing on the fringes of the moorland. The upland has a simpler landscape pattern, with rough grazing, peat cutting and large forestry plantations such as that of Lough Bradan Forest. Land quality in the lowlands varies considerably. The well-drained land around the Glendarragh Valley is intensively farmed, however, the sandstone soils are of poorer quality and forestry has become increasingly important. In the north of the LCA Bollagh Mountain is an area of karstification (see Key Elements, below). Quarries are found in this area (on the Dunaree Hills) and may in some cases be a catalyst for the downgrading of upland moorland. Quarry scars, poorly designed screening mounds and equipment are all visual eyesores. New quarries require landscape and visual assessment prior to site selection and existing abandoned quarry sites would benefit from restoration.

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)

This LCA is dominated by Carboniferous rocks of the Kesh - Omagh succession in faulted contact with Devonian and Neoproterozoic.Tertiary dolerite dykes extend through the area.

Basal Carboniferous conglomerates and sandstones of the Kesh - Omagh succession rest unconformably on Devonian and Silurian in the Glendurragh River (ESCR Site 208). The Claragh Sandstone Formation is exposed (ESCR Site 210, together with Termon River Limestone Member and Bin Mountain Sandstone Member) of Giant's Cave. The overlying Bin Mountain Sandstone is seen in Binnawooda Spring (ESCR 211). The topmost part of the Ballyshannon Limestone comprises the Ederny Limestone Member: its contact with the lowest member of the Bundoran Shale (the Skea Sandstone) can be seen at Ederny Quarry (ESCR Site 217). These two members can also be observed at ESCR Site 219 (Shanvin): conglomeratic Skea Sandstone can be seen in the scarp outcrops beside Drummahon Lake (ESCR Site 220). Mullaghmore Sandstone is exposed in Dromore Big Quarry (ESCR Site 221) and at Montiaghroe (ESCR Site 223). Overlain by the Drumchorick Siltstone Member of the Benbulben Shale in the Bannagh River (ESCR Site 221). Benbulben Shale is exposed (ESCR Site 224) of the Killygarry River. The Greenan Sandstone Formation is exposed in the type section at Greenan Mountain (ESCR Site 228) and passes up into Ballinamallard mudstones. The Slievebane Group contains the Drumlish Conglomerate, exposed in Straduff Quarry (ASSI117), an exceptional rock formation being usually found in Canada not Europe.

The Omagh Thrust and associated Castle Archdale Fault intersect the southeastern side of LCA14. These ENE-WSW oriented faults were active in the Variscan (end Carboniferous) when all the rocks of the area were deformed.

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 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 iceflows 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 two very distinct units. To the east are largely drift-free uplands blanketed in extensive areas of peat. To the west are drumlin covered lowlands underlain by Late Midlandian till. Drumlin orientation and till fabric indicate ice flow from the west (Davies and Stephens 1978). Streamlined rock ridges in the uplands of the LCA also indicate this flow direction.

Within Northern Ireland drumlins take a variety of forms; some are rounded in plan, although the majority are elongated in the direction of ice flow. Some have sharp crests, whereas others are more whaleback in profile. Although most drumlins are composed of glacial till or tills, a small number are 'drumlinoid features' are rock-cored and some are composed of sand and gravel. Where drumlins are rock cored there may have been significant frost shattering prior to their shaping by ice flow. It is possible therefore to see tails of shattered debris within till leading away from the feature in the direction of flow (Davies and Stephens 1978). It is generally accepted that the drumlins of Northern Ireland were formed by deposition beneath fast flowing ice. In the majority of cases this has resulted in a thick layer of Upper (younger) Till overlying a core of Lower (older) Till. This pattern has been observed across Northern Ireland, apart from a limited area in the north of County Down, where Hill(1971) observed drumlins composed only of Lower Till. The precise temporal relationship between the two tills has not been definitively resolved, but Davies and Stephens (1978) refer to an organic layer between the tills in County Fermanagh that has been dated at 30 500 ± 1170/1030 years B.P. and shelly material between the tills on the Ards Peninsula dated at 24 050 ± 650 years B.P.. However, these deposits only indicate that the Lower Till is older than the dates obtained.

It can be argued that an equally important component of any 'drumlin landscape' are the similarly numerous inter-drumlin hollows. The majority of these hollows would have held open water from local runoff at the end of the Pleistocene. Whilst some continue to exist as isolated small loughs, many have now been infilled by sediment washing off the surrounding drumlins. This has created typically flat-bottomed, marshy areas between the drumlins that are subject to seasonal inundation. Much of the infilling probably occurred early in the Holocene, as the landscape adjusted to increasingly temperate conditions. However, erosion may also have been accelerated in historical times, when rural population densities were considerably higher and much of the lowland landscape of Northern Ireland was more intensively cultivated. Whatever the stimulus for erosion and deposition, the sediments within these hollows typically contain an important record of local environmental change.

Key Elements

ASSI

117 STRADUFF

The Upper Carboniferous rocks of the Drumlish Conglomerate Formation that are exposed in Straduff Quarry were formed when eroded sediments were deposited by braided streams in the proximal part of an alluvial fan. This formation is exceptional in that its closest equivalents occur in northeast Canada and not in the British Isles where it is thus unique. The Scarce Blue-tailed Damselfly Ischnura pumilio has been recorded from the site. This is the rarest species of damselfly in Northern

Karst Features

LOUGH LEE - BINNAWOODA AREA

Karstification of limestones and calcareous sandstones has occurred in the area of Lough Lee and Bin Mountain, County Tyrone. Of the two areas Lee karst is only of local significance and has been extensively modified by hydrological works. The Binnawooda doline - sinkhole field and Fairy Water Rising are considered to be of significant importance as they are the only known karstified sandstones in Northern Ireland.

Deglacial Complexes/Sand and Gravel Resources

FOYLE VALLEY COMPLEX

There is a small area of outwash (0.1km2) belonging to the Foyle Valley deglacial complex in the northeast of the LCA.

Other sites/units identified in the Earth Science Conservation Review

208 Glendurragh River

Carboniferous. Exposure of basal unconformity of Omagh Sandstone Group and most lithologies typical of this Group.

210 Giant's Grave

Carboniferous. Quality exposure of stratotype of Termon River Limestone Formation. Exposures of Claragh Sandstone and Bin Mountain Sandstone Formations. Some fossils.

211 Binnawooda Spring

Carboniferous. Exposure of contact between Bin Mountain Sandstone and underlying Claragh Sandstone Formations. Some fossils.

212 Tullyard

Carboniferous. Exposure of stratotype of Tullyard Conglomerate Member.

213 Rushindoo

Carboniferous. Exposure of stratotype of Rushindoo Oolite Member.

217 Ederny Quarry

Carboniferous. Exposure of top strata of Balyyshannon Limestone Formation, including stratotype for Ederny Limestone Member. Overlain by Skea Sandstone Member of Bundoran Shale Formation.

219 Shanvin

Carboniferous. Exposure of stratotype for Skea Sandstone Member. Illustrative outcrop of Ederny Limestone Member.

220 Drummahon Lane

Carboniferous. Unique occurrence of coarse conglomerates in outcrop of Skea Sandstone Member. Fossiliferous limestone blocks.

221 Bannagh River

Carboniferous. Exposure of varied lithologies of Mullaghmore Sandstone Formation and of contact with succeeding fossiliferous Drumchorick Siltstone Formation.

222 Dromore Big Quarry

Carboniferous. Exposure of Dromore Sandstone Member of Mullaghnore Sandstone Formation.

223 Montiaghroe

Carboniferous. Stratotype of Drumskinny Sandstone Member of Mullaghmore Sandstone Formation.

224 Killygarry River

Carboniferous. Exposure of stratotype of Drumchorick Siltstone Formation and contact between this and succeeding Benbulben Shale Formation plus long sections of Benbulben Shale Formation.

228 Greenan Mountain

Carboniferous. Exposure of stratotype of Greenan Sandstone Formation. Some fossils that date the rocks.