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Benvan, looking north towards Murlough Bay, Co. Antrim.
Summary Full report
Site Type: Crag, Crags
Site Status: PASSI
Grid Reference: D205415
Rock Age: Precambrian (Dalradian)
Rock Name: Argyll Group, Owencam Formation
Rock Type: Green beds, Pelitic rock, Psammite, Semi-pelitic rock
Other interest: No data, No Data

Summary of site:

This series of crags and rocky knolls, roughly between the 150m and 250m contours, overlook Murlough Bay and Fair Head to the north west and expose a series of metamorphic rocks. The rocks are bedded, betraying their origins as sediments, but they are now coarse, feldspar-rich grits, and pearly-quartz schists and quartz-mica schists, with a few thin beds of green chloritic schists. These are among the oldest Dalradian rocks in north-east Antrim belonging to the Owencam Formation of a larger subdivision of the Middle Dalradian, the Argyll Group.

Although these rocks were not heated to very high temperatures or subjected to the most severe pressures (they belong to the low grade Greenschist Facies) they have been much altered in appearance and there is clear evidence of at least two deformation phases, as reflected in the cleavage (splitting) planes which give the rock a ‘herringbone’ appearance in places. The original sediments were a series of coarse sandstones, silts and muddy shales deposited over 600 million years ago on the shallow continental shelf of a nearby supercontinent. There is evidence that the soft sediments were initially unstable and in a series of mass slides (turbidity currents) swept into deeper water. The metamorphism occurred much later, around 400 million years ago, in a widespread phase of mountain building called the Caledonian orogeny. This violent period also created huge regional folds and Benvan lies close to the axis of one of them - the Altmore Anticline - which can be traced across the North Channel to Scotland. The site is of national importance because it shows the range of rock types and metamorphic fabrics of the Owencam Formation. The site is owned by the National Trust and lies within the confines of the Murlough Bay Nature Reserve. It is currently rough grazing on heather moorland with sufficient exposure to permit hammering. There are no immediate threats to the site.

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