 | | Pillow lavas (Dungiven Formation), Strabane Quarry. |
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| Site Type: | Quarry (disused) | | Site Status: | ESCR | | Council area: | Derry City & Strabane District Council | | Grid Reference: | H349982 | | Bing maps: | 54.83084,-7.45691 | | Google maps: | 54.83084,-7.45691 | | Rocks |
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| Rock Age: | Precambrian (Dalradian) | | Rock Name: | Argyll Group, Dungiven Formation, Tayvallich Subgroup | | Rock Type: | Green beds, Lava, Pillow lava | | Interest |
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| Summary of site: |
| Metamorphic rocks are some of the most difficult to interpret and they present geologists with major challenges. The high pressures and temperatures involved in metamorphism often obliterate the original features of a rock. All that then remains as evidence of its original appearance and characteristics is the suite of minerals created and stabilised at the pressures and temperatures achieved during metamorphism. This is particularly true of the rocks around Strabane. | | The rocks of the area belong to the Dungiven Formation, a division of the Argyll Group, the earliest rocks of the ancient Dalradian supergroup, more than 600 million years old. In this quarry the dominant rock type is a green amphibolite schist coloured by hornblende, chlorite, zoisite and epidote (all greyish-green minerals) with albite feldspar and biotite mica. This suite of minerals betrays a volcanic origin but the metamorphism has so changed the rock that its original appearance has been obliterated except for “ghosts” of pillow lavas which survive in the southern side of the lane to the quarry and in the quarry itself at the northern end and on the overhanging western face. There are limestone streaks in the rocks enclosing the pillows. Pillow lavas are pillow shaped and sized masses formed by the extrusion of lavas from fissures and vents on the sea bed. The lava chills immediately on contact with the sea water and inflates with the injection hotter lava while the outer skin remains briefly plastic. It then breaks off to join the heaps of other pillows as it solidifies. | | There is evidence to suggest that the limestones associated with the lavas are equivalent to the Culdaff Limestone of Donegal and the Loch Tay Limestone and Tayvallich Limestone of Scotland, suggesting a broad structural belt across the entire region. From this evidence it is possible to loosely reconstruct the geography and events of this time. The rocks were formed near the South Pole on the subsiding margins of a fracture separating a new continent, Laurentia, from the giant parent continent of Gondwana. The stretching of the crust created first thinning, then breaching of the sediments on this sea floor, finally releasing volcanic rocks into the marine basin. These volcanic rocks, perhaps originally lavas with associated volcanic dust and debris now survive as green amphibolite schists and the still recognisable pillow lavas. In later Dalradian times the volcanism subsided and sandy sediments reasserted their dominance as Laurentia (incorporating what are now parts of North America, Greenland and north west Europe) commenced its long, imperceptibly slow progress northwards. It was a collision of this continent with another 140 million years later that caused the metamorphism of the Dalradian rocks. | | This site is important because it contains the earliest pillow lavas in the Dalradian rocks of Northern Ireland which act as a marker horizon for comparisons. A giant overfold affects all the Dalradian rocks of the Sperrin Mountains. This quarry is on the northern side of the fold and may correlate with pillow lavas at Craig on the southern, overturned, limb of the same fold. It could therefore be a key site to our understanding and interpretation of geological events in the Dalradian and the deformation and metamorphism of the rocks during the Grampian orogeny. | | This quarry has not been worked for a considerable time and is now hemmed in by housing. It has suffered the fate of many others in similar circumstances, dumping of all kinds of rubbish, old cars etc and there are now mature trees with established tangles of undergrowth. Access is also a problem since the lane is blocked by a locked gate. Substantial clearing will be needed to fully restore geological interest but the importance of the site will justify the effort, particularly if the site could find an additional use without compromising the key outcrops. |
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