Summary of site:
This site is important because it contains one of the best exposures of the Ballykelly Formation to be seen in Northern Ireland. The formation is part of the Dalradian Supergroup, ancient rocks formed around 600 million years ago, before the time of clearly evident fossil life. There are two divisions of the Dalradian, the Argyll Group below and the Southern Highland Group above. This latter group has four subdivisions and the Ballykelly Formation is the third in time sequence.
In the quarry, extensive clean and relatively fresh exposures reveal a considerable thickness of altered (metamorphosed) greywackes (a special type of sandstone) and shales or mudstones (changed to phyllites). Despite the metamorphism, original sedimentary structures are still clearly evident and the greywackes form thick beds (up to 1 m) separated by the thinner greenish-blue phyllites. The greywackes show a range of grain sizes and some are very coarsely granular.
The beds are steeply inclined, around 60 degrees from the horizontal, and a distinct cleavage, the results of the stresses of metamorphism, dips more gently at around 50 degrees.
Greywackes form when bedded sediments lying on a coastal shelf become unstable (due to gravity, earth tremors or similar phenomena) and slip down the continental slope into in a discrete and internally mixing body that eventually settles in deep water. In this case, around 600 million years ago, the event took place on the fringe of a gigantic continent situated at the South Pole (then ice-free). The rocks were metamorphosed over a considerable area about 465 million years ago in a time when the continent collided with an active volcanic arc. They were crumpled into immense folds 30 to 40 km wide that persist across Northern Ireland into Scotland. On the scale of metamorphic alteration these rocks are considered low grade i.e. little changed.
The quarry is threatened by landfill, probably unofficial at the time of the survey, and flooding but if it becomes an official landfill site it should be possible, with careful planning, to preserve some faces for future educational and research use. Parts of the quarry could even have a future as a resource of good quality building stone.