Slieve Beagh 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.

Between Fivemiletown and Rosslea there is an extensive area of rolling sandstone uplands, rising to the rounded summit of Slieve Beagh (380m). To the south of the summit, there is a prominent escarpment of Upper Limestone capped by gritstone, which outcrops below the sandstone. The northern edge of the uplands has a broken surface, with flat-topped hills and rounded ridges separated by deep valleys and punctuated by attractive small rounded lakes. The southern escarpment comprises a long line of hills and summits which command tremendous views of the lowlands, for example at Carnrock viewpoint. Steep, south facing slopes are interrupted by small streams in steep wooded glens. The southern escarpment slopes were historically in agricultural use and ladder field patterns on the escarpment are a local feature. The extensive blanket bog on the summit hills and flat ridges has been subject to agriculture, peat cutting and afforestation. The landscape can therefore be summarised as one of extensive rolling hills deeply dissected by long river valleys and bounded by a prominent escarpment on the south; the massive rounded summit of Slieve Beagh is dominant, with panoramic views over adjacent lowlands.

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.

Stratigraphic Table (youngest rocks at the top of the table)

This LCA is dominated by fossiliferous, Carboniferous sedimentary rocks of the Fermanagh succession. It contains the only known outcrop of the Alderwood Mudstone. Contains the ASSI of Fardross Stream (ASSI 141, ESCR Site 233) where the Bundoran - Meenymore succession is exposed. The Ballyshannon Limestone (massive dark, fossiliferous limestones) is exposed at Slatmore Quarry (ESCR Site 231). The Dartry Limestone (well-bedded fossiliferous limestones with chert nodules and chertified beds) is exposed at Ballagh Quarry (ESCR Site 204).

The Meenymore Formation (100m) comprises shales and limestones with subordinate gypsum beds and nodules. A mapped sandstone, the Carnmore Sandstone Member occurs at the base. The Meenymore underlies two thirds of LCA18: it is exposed at Tircar Stream (ESCR Site 207), Ashfield Mountain Bar (ESCR Site 237), Rockfield Quarry (ESCR Site 205) and Carnmore Quarry (with Carnmore Sandstone Member: ESCR Site 206).

Alderwood Mudstone (Asbian - Brigantian). Occurs adjacent to the Clogher Valley Fault, southeast of Fivemiletown. The only mapped occurrence of this formation is exposed in ESCR Site 207 (Tircar Stream).

All the above successions were deformed in the Variscan (end Carboniferous) phase of tectonics. The NE-SW Clogher Valley Fault runs through the north of LCA18.

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 confirms the presence of large, drift free upland areas that are now largely blanketed in peat. These areas would have been overridden and scoured in Late Midlandian times by ice that moved southeastwards across the region. This same ice left extensive spreads of till on the flanking slopes of the upland areas and on the surrounding lowlands. During the deglaciation of the area, there is evidence to the north in the Clogher Valley of significant fluvioglacial activity as the ice wasted The northern slopes of Slieve Beagh overlap with the sands and gravels of the Clogher Valley Subglacial and Ice-Marginal Complex, which comprises an assemblage of oriented ridges of ice marginal and subglacial sand, gravel and diamict.

Key Elements

ASSI

1141 FARDROSS STREAM (ESCR 233)

Fardross Stream cuts through rocks that range in age from approximately 339 million years old (M.y.) to some 335 M.y. and span five distinct geological formations in the Tyrone Group. It is extremely rare to find the boundary between five formations, so this is an exceptional exposure. The rocks, and the fossils they contain, indicate that they were laid down in a marine environment: as water depth and distance from land slowly increased through time, sand and mud deposition gave way to limestones. Fossils include colonial and solitary corals and brachiopods.

Other sites/units identified in the Earth Science Conservation Review

237 Ashfield Mountain Bar

Carboniferous. Outcrop of lithology of Meenymore Formation. Wide variety of marine and fluvio-deltaic sediments. Occurrence of diagnostic fossils at several horizons.

214 Ballagh Quarry

Carboniferous. Exposure of stratotype for Ballagh Limestone Member of Dartry Limestone Formation.

205 Rockfield Quarry

Carboniferous. Exposure of contact between Dartry Limestone and Meenymore Formations.

206 Carnmore Quarry

Carboniferous. Stratotype for Carnmore Sandstone Member of Meenymore Formation.

231 Slatmore Quarry

Carboniferous. Isolated most easterly outcrop of Ballyshannon Limestone Formation. Sparse fossils of corals.