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Knockmore CliffFermanagh
Knockmore Cliff on SE corner of Knockmore hill, Co. Fermanagh.
Summary Full report
Site Type: Cliff
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H083407, H093500
Rocks
Rock Age: Carboniferous (Asbian, Visean)
Rock Name: Dartry Limestone Formation, Knockmore Limestone Member, Tyrone Group
Rock Type: Breccia, Limestone
Interest
Minerals: Micrite
Fossil Groups: Brachiopod, Polyzoan
Other interest: No data, Marine sediments

Summary of site:

Knockmore Cliff is the rugged 2km rim that forms the skyline around this prominent hill. It is unusual because the undercut base creates extensive overhangs into limestones that show an absence of bedding.

About 337 million years ago this area was sea bed, part of the margin of a giant continent then creeping northwards over the equator at the rate of a few millimetres each year. The general water depth was around 130m, very close to the depth limit of light penetration. Faulting and subsidence affected parts of this sea floor and on the sinking sides of these faults a series of steep sided mounds, constructed from what appears to have been inorganic lime mud, grew upwards keeping pace with the rate of subsidence. Their summits reached into the zone of surface water affected by storm waves, which curbed further upward growth. Unconsolidated mud cannot normally support a slope of more than a few degrees but these mounds had flanks at up to 40 degrees from the horizontal and reached maximum heights of around 120m above the sea bed. How they grew is still a mystery because there is no evident biological or other framework. The only structures within the mud are irregular cavities, from a few millimetres to a few centimetres maximum size, filled with fine sediment and calcite. Between the mounds, and draped on their flanks, are accumulated fragments of consolidated mud, presumably detached by wave action from the mounds and fine grained lime muds. In contrast to the almost barren muds of the mound cores, these deposits were crowded with life, particularly lamp shells (brachiopods) and moss animals (bryozoa). This is the interpretation of the fine muds and breccias of the Knockmore Limestone Member of the Dartry Limestone Formation in this area. The mounds are formed of grey to pale-brown, very fine grained limestones and the breccias (accumulated angular fragments) and the bedded micrites between them are identical in colour. Knockmore Cliff is the stratotype for the Knockmore Limestone Formation and as such is the standard reference for all outcrops of similar age and rock type in the area. It is therefore of primary importance, as well as being a very fine example of carbonate mounds in a striking landform.


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