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Eagles RocksTyrone
Eagles Rocks, Davagh Forest, Co. Tyrone.
Summary Full report
Site Type: Crag, Crags
Site Status: PASSI
District:
Grid Reference: H717838
Rocks
Rock Age: Precambrian (Moinian)
Rock Name: Central Tyrone Inlier, Corvanaghan Formation
Rock Type: Arkose, Pelitic rock, Psammite, Quartzite, Semi-pelitic rock
Interest
Other interest: boudinage, No Data

Summary of site:

The rocks at this locality are amongst the oldest seen in Northern Ireland and their history and relationship to other ancient rocks are still matters of debate. This area of around 40km² of the Corvanaghan Formation is therefore of national importance.

These rocks form part of an area called the Central Tyrone Inlier (an inlier is an area of rocks completely surrounded by younger rocks) that has been the subject of continuous discussion since its original recognition during mapping in 1843. They were initially described simply as metamorphic rocks, then as schists and gneisses (types of metamorphic rocks) “Probably of Pre-Cambrian…..age” (before the appearance of fossils visible to the naked eye). Later work refined these descriptions and Hartley, in 1933, ventured an opinion that they belonged to the Dalradian (late Precambrian, around 600 million years old). The name Corvanaghan Formation was first applied by the Geological Survey of Northern Ireland in the 1960s. In 1996 a radiometric date of around 650 million years was obtained from veins of a coarse grained igneous rock cutting through the metamorphic rocks. Since the metamorphics have to be older than the veins cutting through them, they have to be older than 650 million years. Recent opinion suggests dates closer to 1,000 million, which pushes them back beyond the Dalradian into the Moinian stage of the Proterozoic. The rocks of the area are quartz-rich schists and gneisses with areas of mafic minerals (pyroxenes and olivines) and lenses of white and clear quartz. They are strongly banded, parallel with the dominant ‘grain’ of the schists and are inclined between 25º and 45º to the west. The banding is the result of metamorphism, not bedding, but in the quartz and feldspar-rich rocks there are linear structures that may represent original bedding planes. The rocks in general have been much deformed but because they are quartz-rich they have insufficient clay minerals to preserve the generations of cleavage needed to give a detailed history. The rocks were originally sandstones, some with feldspar grains, laid down presumably in a shallow ocean in remote times but the presence of metamorphic greenstones suggests that they were injected, before metamorphism, by molten rocks of basaltic composition. One authority believes them to resemble ancient basement Grenvillian rocks similar to those in North America. If this is so, they are more closely related to the basement rocks of southern than northern Britain. Sometime before 470 million years ago the rocks were altered by intense heat (close to melting) and pressure and during this period there were at least three periods when the hot rocks were folded. Since the Central Tyrone Inlier is defined by faults on all sides, its relationship to other rocks in the area is impossible to determine. The main outcrops are in maturing forest and every effort should be made to ensure that their important features remain accessible and visible.


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