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Largy QuarryFermanagh
Largy Quarry, Co. Fermanagh, showing large scale, low angle cross-laminations in coarse-grained sandstones with pebble beds, of the Topped Mountain Sandstone Formation.
Summary Full report
Site Type: Quarry (disused)
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H299476
Rock Age: Carboniferous (Visean)
Rock Name: Ballyreagh Conglomerate Formation, Kilskeery Group, Topped Mountain Sandstone Formation
Rock Type: Conglomerate, Mudstone, Sand, Sandstone
Other interest: cross-bedding, dessication cracks, ripple marks, No Data

Summary of site:

The importance of this quarry lies in its exposure of around 20 m of sandstones and conglomerates representing the upper part of the Topped Mountain Sandstone Formation, an enormously thick formation (estimated at around 1000 m) of late Visean age, formed towards the close of the early Carboniferous period roughly 335 million years ago.

The rocks in the quarry are tilted 20 degrees from horizontal to the north west and consist mainly of coarse-grained sandstones with a high proportion of feldspar grains and they range in colour from pale grey and green to medium brown. There are also a few beds of pebbly sandstones and conglomerates. Cross-lamination can be seen in some beds and crude ripple marks associated with desiccation cracks are also present. The pebbles and cobbles in the conglomerates are mostly greywackes (mixed grainsize sandstones) of Ordovician or Silurian age which distinguishes them from other conglomerates with which they could be confused. The rocks were deposited in a rapidly subsiding, land-locked basin in a dry, hot, harsh climate on the margin of a supercontinent, Laurentia, slowly drifting northwards towards the equator. This is the best section through the rocks at the top of the Topped Mountain Sandstone and is therefore an important section in the geological history of Northern Ireland.

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