Clogher Valley Lowlands 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 is a distinctive lowland area 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.
The Clogher Valley Lowlands are confined between the sandstone ridges of Brougher Mountain to the north, and Slieve Beagh to the south. It is a broad distinctive corridor of undulating lowland, including a low watershed between the Colebrooke and Tempo river valleys and the Clogher Valley, which contains the Upper Blackwater River. The lowland is covered with boulder clay drumlins of varying sizes and long winding eskers of sand and gravel. There are also occasional solid rock outcrops. Between the drumlins and ridges are areas of flatter land, linked by streams and rivers. The Clogher Valley is thus a distinct physical region, defined by the Fintona Hills to the north and the Slieve Beagh highlands to the south. Variability of landform size, alignment and proximity produces a scenically diverse landscape. Commercial exploitation of aggregate deposits has been minimal and the landscape is thus virtually intact. The valley is a designated Scenic Route and is a major tourist gateway to the Fermanagh Lake District.
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 dyke, about 60 million years old |
|---|
| Carboniferous - southwestern (Fermanagh) succession, about 350 million years |
| Meenymore (with basal Carnmore Sandstone) |
| Dartry |
| Glencar |
| Benbulben Shale |
| Mullaghmore Sandstone (east only) |
| Bundoran Shale |
| Ballyshannon Limestone (including Waulsortian Limestone) |
| Clogher Valley |
| Ballyness |
| Carboniferous - northeastern (Tyrone) succession |
| Topped Mountain |
| Maydown |
| Devonian - Gortinfinbar, about 400 million years old |
| Silurian - Lisbellaw, about 420 million years old |
This LCA is dominated by fossiliferous Carboniferous rocks of the Fermanagh succession to the south and of the Kesh - Omagh succession to the north. These Carboniferous strata overlie Silurian (greywackes and shales), exposed in small outcrops.
Carboniferous Clogher Valley Formation marine limestones and shales, together with underlying Ballyness Formation conglomerates, are exposed (ESCR Site 199) at Cole Bridge Stream section.
Ballyshannon Limestone (dark, fossiliferous limestones with occasional interbedded argillaceous beds) are exposed at ESCR Site 200, Mullaghsillogagh Quarries.
All the above successions were deformed in the Variscan (end Carboniferous) phase of tectonics. The NE-SW Tempo-Sixmilecross Fault clips the northwestern edge of LCA17. The NE-SW Clogher Valley Fault runs through the centre of LCA17.
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 it to be mainly underlain by Late Midlandian till deposited by ice that moved across the region from a centre to the northeast in the Omagh basin. In doing so, it left behind a legacy of flow orientated drumlins in the lowland areas. In recent years however, researchers have also identified in the central and southern Omagh Basin, a great many subglacial diamict (till) ridges that lie transverse to the southwestwards Late Midlandian ice flow (e.g. Knight and McCabe 1997). These ridges can be anything from 0.5 - 2.5km in length and 100 - 450m wide and have been interpreted as rogen moraines. Some of ridges were streamlined and overprinted by subsequent drumlin development, while others remained unaffected. In some cases eskers were draped across the ridges during final deglaciation. This combination of subglacial bedforms is used to suggest that during the last deglacial phase ice masses were highly mobile and that flow was episodic due to variations in the subglacial thermal regime. One consequence of this was a high sediment flux to the ice margins that in turn generated significant glaciofluvial complexes. Within this LCA there are significant elements of two such deglacial complexes.
The Tempo Valley Delta Complex (6.1 km2 of a total of 11.9km2) runs along the northwestern margin that overlaps with LCA16. The delta complex of the Tempo Valley is situated on the south side of the Fintona Hills and extends northeast-southwest from Glengesh to Lisbellaw. The valley margins are defined by the bedrock uplands of the Fintona Hills to the west. It records a range of ice marginal and ice contact sediments including glaciolacustrine outwash spreads, deltas and moraines. Deep meltwater channels through the axis of the Tempo Valley probably reflect meltwater and/or glacial lake drainage to the southwest. These deposits are superimposed upon bedrock in the uplands, and subglacial diamict of rogen moraine morphology in the lowlands. These rogen bedforms relate to earlier northeast-southwest ice flow from ice centres north of the Fintona Hills. Ice retreated east and southeast from the main axis of the Fintona Hills. The Tempo Valley delta complex remains largely intact and aesthetically it is excellent.
The Clogher Valley Subglacial and Ice-Marginal Complex is the most important deglacial component in this LCA. The Clogher Valley trends northeast-southwest and is defined as the area of low ground (70-110 m elevation) between the Fintona Hills to the north and northwest, and Slieve Beagh to the south. It opens in a southwest direction into the Lough Erne Basin and terminates around Maguiresbridge where the valley reaches 10 km wide. The Clogher Valley complex dominates the central part of the valley and forms an assemblage of oriented ridges of ice marginal and subglacial sand, gravel and diamict. The sediment system is discontinuous and extends from Ballygawley to outside the valley at Lisnaskea. It is 36 km long and up to 4 km wide, covering an area of 80 km2. A small area in the northeast overlaps with LCA 16 and the sand and gravel resources extend marginally into LCAs 18, 44 and 45.
In the north east of the LCA is a small areas of Ballygawley Delta Complex (1.5km2) that lies mainly in LCA 44.
Key Elements
Deglacial Complexes
TEMPO VALLEY DELTA COMPLEX (6.1 km2)
The Tempo Valley delta complex is classified as being of high importance on a Northern Ireland scale. This is due to its importance in showing that ice retreated to the east and southeast. This is very different to the direction of ice advance. The localised occurrence of concretions and mud curls appears to be unique to this area, and have not been extensively reported in similar settings elsewhere in Northern Ireland. The pattern of deglaciation is rather different to that originally envisaged by Charlesworth. Ice retreat was far more dynamic, and involved two interacting ice margins after ice broke up over the axis of the Fintona Hills.
LOUGH EYES MORAINES
This is an important component of the Tempo Valley delta Complex that lies almost completely in this LCA. Prominent ridges aligned northwest - southeast mark standstill positions of an active ice front during ice retreat into the western Clogher Valley. Large-scale meltwater channels accentuate ridge morphology. The complex is unique in Northern Ireland because of its extent and ridge size. Landforms in the Lough Eyes area are intact and contribute to a generally high landscape quality. Lough Eyes itself is an attractive landscape focus and adds diversity within the ridge dominated complex. Containment of the complex within the Tempo valley heightens the contrast with bedrock uplands to the west.
CLOGHER VALLEY SUBGLACIAL AND ICE-MARGINAL COMPLEX
Landforms within the Clogher Valley comprise an extensive suite of subglacial and ice-marginal features and include ice-marginal sand and gravel ridges, eskers and outwash spreads formed towards the end of the last ice age. Together, they provide a rare association within Northern Ireland with good evidence for the break up of regional ice into very localized ice masses and the persistence of an isolated ice mass within the valley following the breakup of ice over the Fintona Hills. Deltaic landforms around Ballymackilroy record an early stage in the westward retreat of ice into the valley. The landform assemblage within the valley indicates that this localized ice mass underwent further fragmentation and that final ice stagnation took place around Clogher as well as in the western part of the valley
Landforms reflect a complex pattern of active ice processes, ice stagnation and meltwater erosion produced during the last stages of the deglacial cycle. They can be subdivided into four main areas: the Ballymackilroy deltas, the proglacial Kilgreen and Drumhirk outwashes, and the Fivemiletown eskers The only known commercial sand and gravel workings are at Drumhirk and are largely disused.
BALLYMACKILROY delta and outwash (Ballygawley DELTA complex)
The area around Ballymackilroy in the northeast of the LCA overlaps with LCAs 16, 44 and 45 and is of importance in understanding the recent glacial history of Northern Ireland. It consists of a well-defined assemblage of sand and gravel delta and outwash surfaces deposited in a pro-glacial lake. Landforms around Ballymackilroy record sand and gravel deposition following the break up of ice over the Fintona Hills and subsequent ice withdrawal towards the south, into the Clogher valley.Sand and gravel landforms around Ballymackilroy are generally pristine and this intactness is an important landscape attribute. The outwash spreads are a major component of the landscape as seen from the main -Omagh road. For a fuller description of the Ballygawley Complex see LCA 44.
Other sites/units identified in the Earth Science Conservation Review
199 Cole Bridge Stream
Carboniferous. Exposure of stratotype for both Ballyness and Clogher Vally Formations. Miospores and sparse fossils.
200 Mullaghsillogagh Quarries
Carboniferous. Exposure of fossiliferous Ballyshannon Limestone formation.
11 Derryvree
Dated organic detritus from below Late Midlandian till. Exposed stratigraphy of drumlin with in situ freshwater organic horizons representing a phase of open Tundra conditions from around 30,000 Years ago.
