 | | Section on Sruh Croppa River showing contact between Corry Member at top of Bellavally Formation (hammer rests on upper surface) and limestones and mudstones of Derreens Limestone Member at base of Carraun Shale Formation. |
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| Site Type: | Crag, Crags | | Site Status: | PART ASSI | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H123280 | | Bing maps: | 54.20128,-7.81151 | | Google maps: | 54.20128,-7.81151 | | Rocks |
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| Rock Age: | Carboniferous (Namurian, Visean) | | Rock Name: | Bellavally Formation, Briscloonagh Sandstone Formation, Carraun Shale Formation, Dartry Limestone Formation, Dergvone Shale Formation, Glenade Sandstone Formation, Gowlaun Shale, Lackagh Sandstone Formation, Leitrim Group, Meenymore Formation, Tyrone Grou | | Rock Type: | Limestone, Mudstone, Shale | | Interest |
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| Fossil Groups: | Coral, Goniatite | | Other interest: | Marine sediments |
| Summary of site: |
| Basins are large depressions in the sea bed, usually adjacent to landmasses, in which sediments accumulate through time. They can persist for considerable periods and throughout their existence they reflect conditions in the ocean and, to some extent, on the neighbouring land. The evolution of a basin will show conditions ranging from shallow water, when the basin is filled almost to sea level (or even temporarily drying completely), very deep water and all depths between these extremes. Room for new sediment is made by subsidence of the basin floor. The nature of the sediment generally reflects the conditions in the region and long-lived basins also record the evolving nature of life in the seas, both on the sea bed and swimming and drifting in the waters above. | | Cuilcagh Mountain tells the story of one such basin during the early Carboniferous period. It extended over an area of 1,400 km² for a period of around 8 million years, from around 337 to 329 million years ago. | | Its rocks give a detailed record of events and contain plentiful fossils, particularly goniatites, which assume great importance. Goniatites were early relatives of squid and octopuses that occupied coiled shells; their rapid evolution produced many species that makes time zones easy to define. | | The rocks of Cuilcagh Mountain are essentially a record of the sediments forming the Leitrim Group and are by far its best sequence. From the unconformity at the top of the Dartry Limestone (the last formation of the Tyrone Group), to the summit of the mountain, 580 m of Leitrim Group sediments can be seen. Blanket bog and plant cover limit exposure on the middle slopes but outcrops dotted across the northern face of the mountain combine to form a near complete record. | | The base of the group is the base of the Meenymore Formation which comprises a thick series of shales with occasional beds of mudstone, sandstone and limestones rich in algal remains. It is also subject to dramatic lateral thinning in places (e.g. at Trien, where it is reduced to only 2m) and even vanishes completely to the west near the border with the Republic of Ireland. | | The Glenade Sandstone Formation lies above the Meenymore rocks and, in marked contrast, is a medium to coarse-grained grey to buff-coloured sandstone showing cross bedding and quartz pebble beds, both evidence of shallow water conditions. Thin shale partings divide the sandstones. | | In the west bank of the Sruh Croppa River at Tromogagh, there is clear exposure of the top of the Glenade Sandstone and the lowest beds of the succeeding Bellavally Formation. These rocks show a regularly repeated pattern of shales, mudstones, sandstones, limestones and evaporites. The repeats (known as cycles) are responses to a series of subsidences along a fault, each marking the transition from deeper water (shale) to emergence and evaporation (evaporite). Unlike equivalent rocks over the border in the Connaught Coalfield the upper cycles are fossiliferous and contain goniatites, clearly indicating that the junction between the Asbian and Brigantian fossil stages lies somewhere in the middle of the Bellavally, perhaps marked by a pebbly sandstone called the Doobally Sandstone Member. The Carraun Shale Formation, a fossil-rich series of black, shaley mudstones and siltstones with diagnostic thin, fine-grained limestones, follows. Its goniatites confirm the presence of the Brigantian almost to the top of the Carraun where new species appear clearly indicating a Pendleian age, the first of the Namurian (the European Upper Carboniferous). | | These outcrops are in the upper reaches of the northerly flowing tributaries of the rivers sinking at the base of the Meenymore Formation into the Dartry Limestone, at Cat’s Hole and Monastir Sink far down the mountain. At these higher levels, approaching the impressive, near vertical summit cliff line, exposure improves and the last four formations present long outcrops. | | Immediately above the Carraun Shale is the much thicker Dergvone Shale Formation, a dark grey to blue-black iron rich shale, weeping rusty water, largely unfossiliferous but with some richly fossiliferous, thin limestones and nodules. The goniatites in these levels are Pendleian in age. About 30m below the top of the Dergvone Shale is an easily differentiated unit of sandstones, siltstones and shales forming the Lacoon Sandstone Member, important because its sedimentary structures show clear evidence of rapidly flowing shallow water. | | Above the Dergvone Shale is the Briscloonagh Sandstone Formation, fine to medium grained, with siltstones and mudstones near the base becoming coarser and thicker bedded towards the top. The lower beds weather into a series of pronounced ‘steps’ that form a feature across the base of the summit cliffs. | | The Gowlaun Shale Formation above is largely a dark grey, poorly fossiliferous mudstone, grading upwards into the Lackagh Sandstone Formation which forms the topmost cliffs and the long, narrow and flat summit of the mountain. The base of the sandstone is laminated and crumbly with few fossils but it quickly becomes a medium- to coarse-grained pale brown to grey-white sandstone. The upper beds of the cliff and flat summit are full of white quartz pebbles, sometimes scattered through the rock, sometimes concentrated into beds. The presence of substantial fragments of fossilized wood indicates an inshore delta surface; the widely fluctuating sea level and the intensity of current flow in the braided distributaries is reflected in deeply incised, filled channels, at least one of which cuts completely through the sandstone into the Gowlaun Shale. | | Nowhere else in Northern Ireland can the evolution of a basin over a period of 8 million years from the late Visean into the Namurian be seen so clearly. It records submarine and terrestrial faulting, widely fluctuating water depths and salinities and a diverse set of animal communities. The goniatites, because they evolved so rapidly and are plentiful in the rocks, enable refined dating of all the major events. This combination of factors is exceptional, making Cuilcagh a vitally important (indeed, a key) record of the events of these times. | | The extensive peat drainage that formerly threatened the surface morphology and underlying cave systems of Cuilcagh’s northern face has now ceased and conservation programmes have apparently stabilized the situation. There appear to be no further threats. |
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