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Lissan WaterLondonderry
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Cookstown District Council
Grid Reference: H815809
Rock Age: Triassic (Scythian)
Rock Name: Lissan Conglomerate Formation, Sherwood Sandstone Group
Rock Type: Breccia, Conglomerate
Other interest: No data, No Data

Summary of site:

This limited outcrop in the north bank of the Lissan Water river exposes a thickness of around 6 m of breccia, whose angular fragments are mostly pinkish granite but with a small proportion of schist, vein quartz and sandstone. The largest pieces are around 15 cm across and all are set in a matrix of what appears to be weathered granite

This is the best exposure of the Lissan Conglomerate Formation, part of the Sherwood Sandstone Group of the Triassic period. The formation was already known from borehole cores drilled at Ballyloughan, where the breccias occur 300 m above the base, on top of a considerable thickness of conglomerate with well-rounded pebbles and cobbles, again up to 15 cm across. The pebbles, in contrast to the breccia fragments, are of quartzite, sandstone, mudstone, porphyry, tuff and jasper with only a small proportion of granite. The increasing ratio of granite through time suggests that a local granite (perhaps the Slieve Gallion granodiorite) was being stripped of its rock cover throughout early Triassic times to form local hills. At the time of their formation, around 250 million years ago, these rocks were part of a hot desert on the east coast of a giant supercontinent called Pangea. They were just about in the tropics of the time. As the best surface expression of this formation, these rocks deserve to be preserved.

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