Summary of site:
This old quarry is significant because it contains one of the few good exposures of ancient Dalradian rocks of the Londonderry Formation. It also shows something of the history of their deformation as they were regionally metamorphosed.
The Londonderry Formation is the youngest formation of the Southern Highland Group, the final group of the Dalradian in this area. It was formed around 600 million years ago.
On the single face of this quarry (3-4m high and 40m long) a succession of alternations between orange brown, coarse psammite (a metamorphic rock rich in quartz) and a greenish-grey, silvery phyllite (fine grained, often silky, metamorphic rock rich in clay minerals) is clearly exposed. Pits in the rock result from the weathering of lime-rich particles.
The rocks form part of a giant syncline (a downwarp of the Earth’s crust), the Lough Foyle Syncline, and examination of the structures caused by deformation during their regional metamorphism shows it to be complimentary to and contemporary with, the equally gigantic Sperrin Fold to the south east. Original bedding planes can be recognised inclined to the north and north west and the progressive fining of sediments towards the top in the psammite beds show that they are the right way up (i.e. not overturned in the fold). Cleavage cutting across the bedding is typical for the area but it is thought to succeed an earlier deformation now completely obliterated.
The rocks were originally sediments washing into a marine coastal basin caused by rifting as the gigantic supercontinent of the time began to pull apart. Sands and muds, washed from the barren land surfaces, accumulated rapidly in this offshore structure, thickening as it subsided and deepened. Due to earth tremors or similar destabilising events, these sands and muds detached and slid down the gently inclined continental slope, mixing as they plunged into deeper water where they settled as sediments of mixed grainsize, called greywackes.
Between 465 and 470 million years ago, during the Ordovician period, a phase of mountain building (the Grampian orogeny) buried, heated and pressurized these rocks, altering them to the form in which we now see them. It was during the crustal stresses of this episode that the regional folding creating the Lough Foyle Syncline (30km across) and the Sperrin Fold (30+ km across) occurred together with the phases of minor folding and cleavage seen in the quarry.
The fine, clean face in this quarry is the best outcrop of the Londonderry Formation in an area rapidly losing more minor sites to filling and development. This exceptional outcrop should be preserved for future reference and research.