| Site Type: | Cliff | | Site Status: | local interest | | Council area: | Fermanagh & Omagh District Council | | Grid Reference: | H0056,H087579 | | Bing maps: | 54.45297,-7.99998 | | Google maps: | 54.45297,-7.99998 | | Rocks |
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| Rock Age: | Carboniferous (Asbian, Visean) | | Rock Name: | Benbulben Shale Formation, Dartry Limestone Formation, Glencar Limestone Formation | | Rock Type: | Limestone, Mudstone, Shale | | Interest |
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| Fossil Groups: | Brachiopod, Coral, Crinoid, Polyzoan | | Other interest: | Marine sediments |
| Summary of site: |
| In an area like Northern Ireland, draped with glacial deposits and further masked by vegetation, it is unusual to find large and continuous rock outcrops - so a cliff 9km long, cresting at over 300m, offers rare geological promise. The cliffs at Magho dominate the southern shore and skyline of Lower Lough Erne and command immense views across the lough far into counties Donegal, northern Fermanagh and Tyrone. The Magho viewpoint, approached through Lough Navar Forest, is one of Northern Ireland’s greatest topographic surprises as the forest opens and the land falls away to reveal a panoramic landscape, rich in variety and detail, and ideal for interpretation. | | The cliff itself climbs out of a steep mass of unstable ground, largely landslip and scree banked against its base, with its toe in the lough at the east end. This heap obscures all but isolated outcrops to a height of around 150m. Above this, the rock faces are spectacular and close to vertical. | | Only three formations are present on the escarpment and because they are close to horizontal (inclined to the south between 5 and 10 degrees) they appear as stripes running along the face of the cliff. From base to top, these are the upper part of the Benbulben Shale Formation, the Glencar Limestone Formation and the Dartry Limestone Formation. | | The Benbulben Shale is largely buried under the debris stacked against the base but at Polaphuca there are two streams with continuous sections right up to the base of the Glencar Limestone. Here the rocks are dark grey to black shales, some lime-rich, with a profusion of fossils ranging from corals, crinoids and bryozoa to a rich diversity of brachiopods. There are thin, dark-coloured lenticular limestones throughout the shale but they form less than 10% of the total thickness. | | There is no sharp boundary between the Benbulben Shale and the overlying Glencar Limestone but the proportion of limestone to shale increases progressively until they are roughly equal in the Glencar. The limestones are dark-coloured and muddy, reaching thicknesses of up to 20cm, and they are interbedded with shales that become increasingly muddy. Pockets of richly fossiliferous rock occur throughout, with faunas tending to be dominated by brachiopods. The base of the Glencar Limestone is not seen in the cliff because it is concealed by the upper slopes of scree but it climbs out of the dead ground as the lower part of the exposed cliff up to a line at about 230m which marks the base of the Dartry Limestone. | | Only the lower part of the Dartry Limestone is present at Magho and there is a marked change from the Glencar Limestone as the proportion of limestone increases rapidly. In contrast with the Dartry limestones further south on Cuilcagh, they are black to dark grey and characterised by chert nodules taking the form of irregular layers, lenses, rod-like forms and large masses, all broadly following the bedding planes. Thin, brittle shales that stink when hammered separate these beds. Fossils are less common here and appear to be restricted to species able to tolerate more hostile conditions, largely brachiopods and bryozoans. The Dartry Limestone forms the top tier of the cliff and extends back behind the summit into the forest. The top of the formation does not occur here. | | All three formations are of Asbian age, confirmed by their fossils, dating between 336 and 340 million years ago. They appear to have been deposited in relatively deep water but still within depths penetrated by light. The supply of muddy sediment that dominated the Benbulben Shale appears to have become intermittent in Glencar times, allowing shelly faunas to become important sediment providers. The onset of the Dartry sees much mud incorporated into limestones and although chert (finely crystalline silica) nodules may have an organic origin, the fauna as a whole is restricted and the fetid smell suggests a more anoxic (oxygen depleted) environment. During Asbian times the entire area was equatorial. | | The cliff is currently an impressive, continuous and uninterrupted sweep of rock and any proposed development that might blemish this unspoiled state should be resisted. |
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