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Glendurragh River, LackFermanagh
PHOTO TO BE ADDED Glendurragh River section, north of Lack, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H275672
Rocks
Rock Age: Carboniferous (Tournasian)
Rock Name: Omagh Sandstone Group
Rock Type: Mudstone, Sand, Sandstone
Interest
Fossil Groups: Bivalve, Fish, Gastropod, Miospore
Other interest: No data, Fluvial sediments

Summary of site:

This section, in the Glendurragh River north of Lack, occupies a rare window through the generally pervasive cover of superficial deposits that obscure the bedrock of the area. In the northern bank of the river, 20m downstream of the bridge, the basal beds of the Omagh Sandstone Group can be seen lying on the ancient foundation of Dalradian metamorphic rocks of the Mullaghcarn Formation. The first 3m are conglomerates, pebbly sandstone, finer sandstones and silts, all purple to reddish-brown in colour. They are followed by more sandstones, some cross-bedded (beds building outwards in shallow water conditions) with pebbly beds, the pebbles being of white quartz. At the base, the beds tilt west at up to 35 from the horizontal but generally the dip is a gentler 15. Above this level are more sandstones, mostly gritty, interbedded unpredictably with ancient sandy soils.

About 100m downstream, the sediments become finer - sandstones, siltstones and dark organic mudstones, with the odd ancient soil - and their colour changes from purple-reds to greys and grey greens. Many sandstone beds are coarse at their base, becoming finer towards the top - a sign of rapid deposition. There are no obvious fossils but in the last 200m of the section four grey mudstones were sampled for their microscopic content; processing yielded over 30 species of plant spores. These are often valuable for dating purposes and here they indicate a biozone (a relative time zone defined by fossils) of Tournaisian age (the first epoch of the Carboniferous period), around 360 million years ago. At the western extremity of the outcrop, fossils appear sporadically: bivalve molluscs, sea snails, fish scales and teeth accompanied by plentiful plant remains. All these rocks were formed in a low, coastal landscape crossed by rivers and subject to periodic flooding, forming temporary lakes. Soils formed and were colonized by a rich flora, only to be washed away by migrating rivers or inundated by floods. Only the western extreme of the outcrop suggests the encroachment of the sea towards the end of Omagh Sandstone times. At the end of the Tournaisian, earth movements lifted and warped these rocks and erosion stripped the topmost beds before they subsided into the ocean basins of the next Carboniferous epoch, the Visean. This locality is important because it exposes the base of the Omagh Sandstone and the unconformity with the ancient Dalradian rocks beneath. The spore floras add to this significance providing clear evidence of their Tournaisian age. There are no immediate threats to the section.


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