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Iniscarn Townland, near DesertmartinLondonderry
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Magherafelt District Council
Grid Reference: H8290
Rock Age: Carboniferous (Tournasian)
Rock Name: Iniscarn Sandstone Formation, Tyrone Group
Rock Type: Arkose, Conglomerate, Sand, Sandstone
Other interest: cross-bedding, No Data, alluvial fan

Summary of site:

The stratotype (see glossary) of the Iniscarn Sandstone Formation lies in the Iniscarn Water and the river valley, 800m to the north west. It is in these sections that the best and most typical rocks of the formation can be seen. To properly define a rock division, ideally its base should be exposed, but the block of rock in the Iniscarn area is isolated on all sides by faults and there is no sign of its basal beds.

The rocks are red-brown, coarse-grained, feldspar sandstones with pebble beds and sandy conglomerates, tilted to the north between 10 and 40 from the horizontal. The conglomerate beds average around 50cm thick and are separated by soft sandstones, mottled red and grey and muddier sandstones. Further west the full thickness of the formation is around 400m, but at Iniscarn it is probably greater. These rocks were formed in a landlocked, equatorial basin in something near to desert conditions, soon after the start of the Carboniferous period (during the early Chadian stage almost 350 million years ago). The landscape was carved into the ancient Dalradian metamorphic rocks that formed the surrounding high ground. It was these weathered and shattered rocks that provided the sediment that was washed into the basin by infrequent torrential downpours. The breccias tended to accumulate on the fans of debris spilling into the lowlands from mountain wadis while the fine sands and muds were transported further out into the open centre. These sediments grade into the next formation, the Altagoan, which shows that the conditions persisted, largely unchanged, into its lowest part. Rock could be obscured or exposed by the shifting of river deposits; otherwise there are few threats to the area.

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