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The Knockmore Scarpland Geodiversity Profile

Last updated: 22 January 2010
  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.

picture of Knockmore limestone escarpmentThis LCA lies within the region described as the Plateau and Valley Lands of Fermanagh. This is a series of sharply defined plateau blocks separated by steep-sided, glacially deepened, lake strewn valleys. The morphology of the Carboniferous uplands is largely controlled by the presence of a series of gritstone caprocks. Beneath these, well-jointed limestones have allowed the development of extensive subterranean drainage systems as well as a variety of surface karst phenomena for which the region is internationally renown.

The Knockmore limestone escarpment dominates the skylines in west Fermanagh. The rugged karst relief has been emphasised by glacial action and includes limestone pavements, cliffs, potholes, sink holes and gorges. At Knockmore, 100m cliffs descend into a fringe of ash and hazel woodland. To the south, Belmore Mountain has a broader landscape pattern and is capped by conifer plantations. The more enclosed, intimate glen landscapes have a mixture of small loughs, patchy fields, scrub woodland and scattered houses. Small streams disappear into the limestone at potholes, emerging at springs on the lower slopes. Larger rivers cut through the rock to form waterfalls, spectacular gorges and caves. A number of loughs lie along the scarp edge, fringed with reeds and alder. On the lower clay soils, wet meadows and flushes are common. On the upper slopes, soil cover is thin. Limestone areas generally support grassland, whereas acid grassland and heath is associated with sandstone outcrops. The key elements in the landscape are a dramatic series of karst features, including scarps, extensive cliff faces, gorges, caves, limestone pavements and loughs.

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)
Tertiary - dolerite dyke - about 60 million years old
Carboniferous Formations - about 350 million years old
Dergvone & Carraun
Glenade Sandstone & Bellavalley
Meenymore (includes Quarry Sandstone Member at base)
Dartry Limestone (inc Knockmore Limestone Member near base)
Glencar Limestone
Benbulben Shale
Mullaghmore Sandstone
Budoran Shale
Ballyshannon Limestone

This LCA is comprised of fossiliferous Carboniferous sedimentary rocks with the exception of the Tertiary dolerite dyke: of Silees River (diverse silicified fossil fauna) occurs in this LCA. Cliffs of Magho, expose Glencar and Dartry (limestone) formations in ESCR Site 189. All the Carboniferous strata were folded and faulted during the end Carboniferous phase of Variscan tectonics.

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 (short ridges or mounds of sand and gravel deposited during the melting of glacial ice), 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 clearly distinguishes between the drift-free, peat blanketed limestone scarps to the west and south, and the till covered scarp foot zone that fringe the Sillees valley to the east. The till is late Midlandian in age and carry a dispersed cover of drumlins. These indicate a complex ice flow pattern in the south below Belmore Mountain. The general pattern of flow was southwestwards from the Lough Erne lowlands, but on encountering Belmore, some was deflected to the southwest, some crossed westwards onto the Ballintempo Uplands and some was deflected northwestwards to run along below the escarpment.

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.

Key Elements
  ASSIs

144 BOHO

Boho is the only example of a joint controlled maze cave in Northern Ireland. With 1.5km of explored passage, it is the seventh longest cave system in NI. The site also contains karst features such as episodic surface rivers and an episodic waterfall, a limestone gorge and a river bed with a series of bank risings. Chambers within the cave system are decorated by calcite straws, stalactites, stalagmites, flowstone, curtains, gour pools and canopies.

111 LARGALINNY (ca 25%, shared with LCA 4)

The geological interest of the site is centred on the silicified fossil fauna from the Glencar Limestone Formations at Carrick Lough,which are of international importance.The scarp and dip slope topography displayed by the Glenade Sandstones are of physiological interest and provide the underlying growing conditions for the diversity of plants and habitats throughout the site.

201 WEST FERMANAGH SCARPLANDS (ca 67%, shared with LCA 4)

West Fermanagh Scarplands has a diverse range of geological and physiographical features. These support a range of habitats and associated vegetation communities of unparalleled significance in Northern Ireland. The rocks of the area are some 335 million years old and date from the Carboniferous, a time when Ireland lay near the equator. The Limestone formations at Knockmore Hill are particularly significant. The Knockmore area supports a range of surface karst topography including some of the finest limestone pavement in Northern Ireland. Three major cave systems also occur within the site, with over 14km of surveyed passage in total.

Other Karst Features

The dominant geomorphological features of this LCA are associated with the development of extensive surface and subsurface karst systems. In total, Western Fermanagh contains approximately 52 square kms of karst landscape developed predominantly in limestones of Viséan (Lower Carboniferous) age. picture of Boho cavesThere are hundreds of cave entrances and 30+ km of explored cave passage. The bulk of these features are contained in two main belts of exposed upland limestone, the Marlbank - Cuilcagh Mountain Region and the Belmore, Ballintempo and Tullybrack Uplands. Almost the entire latter region occurs in this LCA. Its Karst Geomorphology is strongly influenced by the Meenymore Formation and the Glenade Sandstone that form an impermeable cap to the Tullybrack and Belmore Mountain uplands. These formations overlie the Dartry Limestone exposed on the eastern flank of Tullybrack, in the Boho valley and on the southern and eastern flanks of Belmore. The Dartry Limestone is underlain by the Glencar Limestone, and the Dartry - Glencar contact is exposed close to the floors of the major valleys. The major underground karst features are developed on the eastern flank of Tullybrack Mountain and in the Boho valley. They comprise four major cave systems: Pollaraftra Cave, Noon's Hole - Arch Cave, the Reyfad System, and Boho Caves. Associated with this underground drainage are many classic surface karst features. Detailed descriptions can be found in the report on the Karst Geomorphology of Northern Ireland prepared for the DoE by T. Fogg and J.G. Kelly (1995).

REYFAD-CARRICKBEG (ca 67%, the remainder in LCA 4)

The quality of geomorphological and geological features in the Reyfad-Pollnacrom-Polltullybrack cave system make it arguably the most important underground karst site in Northern Ireland. In a regional context, it contains the most extensive system of passages, has the greatest volume of passage and attains the greatest depth of passage from sink to the lowest explored point in the system. The clastic sediments are the most extensive underground glacial/post-glacial deposits in Ireland. Speleothem deposits are extensive and varied and a wide range of passage cross-sections and smaller features of erosional morphology are present. There is great potential for further research and exploration including investigation of the hydrology of the catchment. Well developed surface karst features include limestone pavements, dry valleys, solution and collapse dolines, relict caves and potholes.

NOON'S HOLE - ARCH CAVE

This small area (3.5 km2) contains some of the best underground karst in Northern Ireland. The cave system has the deepest series of unbroken vertical shafts in Ireland, both active and fossil phreatic horizontal passages, a major active stream passage with dramatic speleothem deposits and a classic, free draining, contact controlled resurgence. The belt of limestone pavement trending north from Noon's Hole to Pollaraftra is the most extensive and best developed in Northern Ireland. These underground and surface karst features which are of National significance.

KNOCKMORE - POLLARAFTRA

The impressive cliffs of Knockmore Hill form an dramatic backdrop to the northern edge of the Tullybrack uplands. The Pollaraftra system is the one major cave system in the area and the best example of a fault controlled cave in Northern Ireland. Within the system important features include a wide range of calcite deposit forms and clastic sediments with features (mammalian bones and the glacial flour clays) of interest. Solution and collapse dolines, dry valleys and limestone pavement are abundant and form part of a unique suite of surface karst features.

Other sites/units identified in the Earth Science Conservation Review

189 Magho Cliffs

Carboniferous. Tyrone Group. Thick layers of Benbulben Shale, Glencar Limestone and Dartry Limestone Formations. Some fossils. Spectacular cliffs.

292 Silees River

Palaeontological. Very rich and diverse fossils, with silicified marine faunas.