Summary of site:
The Glengawna Formation is poorly exposed everywhere in Northern Ireland but this area of scattered outcrops in Glenlark, 10km slightly north of east of Gortin, shows more of the succession than anywhere else and has been designated as its ‘stratotype’ (see glossary).
The Glengawna Formation is Precambrian in age and forms part of the Dalradian Supergroup. The supergroup is subdivided, in date order from the base, into the Argyll and Southern Highland Groups; in the south Sperrins, the Glengawna is the third of four divisions of the Southern Highland Group, with the Glenelly Formation beneath and the Mullaghcarn above. These rocks are ancient, almost 600 million years old and predate the evolution of fossils visible to the naked eye.
The rocks are seen intermittently in Glendarragh Burn (where they are in contact with the top of the Glenelly Formation) and extend across the Glenlark River to the southern slopes of the valley. The formation is around 400m thick and because it is in the overturned, southern limb, of an immense overfold, the succession is upside down, so that the older rocks appear to be on top. Here they are described in age sequence.
The basal beds of the Glengawna are flaggy, black, graphitic mica-schists, in marked contrast to the silvery, pelitic and mica schists of the older Glenelly Formation. They pass into silvery, green, pelitic (of shale composition) rocks and pale grey psammites (quartz-rich metamorphic rocks). The top of the formation is seen in a stream, deeply incised into glacial sands and gravels on the south side of the valley. Here the topmost beds of pale grey psammites, mica schists and graphitic schists give way to flaggy psammites and altered silty shales of the Mullaghcarn Formation.
The intensity of metamorphism in regionally metamorphosed rocks can be gauged by the new minerals formed in the process, and these rocks have a mineralogy that places them in a category called the greenschist facies, indicating a low grade of change.
The rocks were originally sediments, a mixture of shales and sandstones of varying degrees of purity, pouring into a subsiding ocean basin alongside a giant supercontinent that straddled the south pole of the time (then ice-free). The graphite schists are an unusual feature. Graphite is a soft, low pressure form of pure carbon (the ‘lead’ in ordinary pencils) and carbon is normally associated with organic life. In this case it is thought that organic carbon sank with muds on to an oxygen-starved ocean floor where it could not decay. The source of the carbon is unknown.
Although the rocks were formed 600 million years ago, they were not metamorphosed until around 470 million years ago, during the Ordovician period, when they were deeply buried in a phase of continental collision. It was at this time that the rocks were deformed, both on a regional and local scale. These immense forces shaped the Sperrin Overfold (40km across), overturning the rocks of this area as the southern limb of the fold was overridden. The schistocity (the ‘grain’ of mineral alignment) is inclined to the north west and there are two (or possibly three) other phases of small scale deformation.
This area offers the best outcrops (though admittedly sporadic) of the Glengawna Formation and shows its relationship with the formations immediately below and above. As a stratotype, it is an essential component of the geological history of Northern Ireland and should be preserved.