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Banagher Glen QuarryLondonderry
Disused limestone quarry (Dungiven Formation), Banagher Glen, Co. Londonderry.
Summary Full report
Site Type: Quarry (disused)
Site Status: PASSI
District:
Grid Reference: C669048
Rocks
Rock Age: Precambrian (Dalradian)
Rock Name: Dungiven Formation
Rock Type: Limestone
Interest
Minerals: Calcite
Other interest: No data, No Data

Summary of site:

On the west bank of the Owenrigh River, about 5.5km south-south-west of Dungiven, is the much overgrown Banagher Glen Quarry. It was formerly worked for limestone but the later accumulations of scree, now thickly vegetated, partly obscure the faces that were originally 25m high. Typically the metamorphic rock is a grey to bluish-grey recrystallized limestone with clear banding thought to be an indication of original sedimentary bedding. At the base of the north face of the quarry the limestones contain thin beds of quartz schist. The northern limit of the limestone appears to terminate against an ore-like mass incorporating metamorphosed lavas, perhaps equivalent to pillow lavas seen nearby in the east bank of the river. All these rocks are part of the Dungiven Formation, a sequence of rocks formed during an important transition in the Dalradian with equivalents in Co. Donegal (Culdaff Limestone), Co. Antrim (Torr Head Limestone) and in Scotland (Loch Tay Limestone).

The Dalradian supergroup is the last phase of the Precambrian in Scotland and Ireland and the Dungiven Formation is the last deposit of the Middle Dalradian, dating to approximately 600 million years ago. At that time the area was on a rapidly thinning crustal margin of the Laurentian supercontinent. After a long period of settled conditions, the thinning crust was breached by major volcanic episodes, some heaping pillow lavas on to the sea bed by submarine eruption. The result was a mixture of limestone, sandy sediments, lavas, volcanic ashes and other volcanic rocks. Such mixtures characterise the Dungiven Formation. Recent research has interpreted some of the limestones as breccias formed perhaps by volcanic disruption after the sediments had been cemented and hardened but before the entire Dalradian was metamorphosed in late Caledonian times (around 400 million years ago, during the early Devonian). The degree of regional metamorphism was low, as evidenced by minerals of the Greenschist facies. The continental plate collisions in Caledonian times were also responsible for the massive structural upheaval of the region, and the Banagher Glen Quarry is near the core of a major structure, the Sperrin overfold. This is the finest site showing the black brecciated limestones of the Dungiven Formation and is of national importance. Currently the only threat is from further erosion and overgrowth but there could be future pressure for use as a landfill site (which should be resisted) or for further working (which should be managed to retain the geological interest).


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