 | | Pillow lavas (Dungiven Formation) at Craig, Co. Tyrone. |
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| Site Type: | Crag, Crags, Pit | | Site Status: | ESCR | | Council area: | Derry City & Strabane District Council | | Grid Reference: | H523981 | | Bing maps: | 54.82843,-7.1862 | | Google maps: | 54.82843,-7.1862 | | Rocks |
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| Rock Age: | Precambrian (Dalradian) | | Rock Name: | Dungiven Formation | | Rock Type: | Basalt, Lava, Limestone, Pillow lava | | Interest |
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| Summary of site: |
| Regional metamorphism alters the original fabric of rocks to such a degree that its features are often difficult or impossible to recognise, leaving interpretation to speculative opinion based on composition, structure and spatial relationships. The importance of this locality lies in the survival of easily recognised pillow lavas that have survived metamorphism with most of their original features intact. | | On both sides of the Carrickayne Road at Craig there are small crags which expose rocks of the Dungiven Limestone Formation (previously the Dungiven Formation) - ancient, regionally metamorphosed rocks of late Precambrian age, belonging to the Dalradian stage. The Dungiven Limestone Formation is the youngest formation in this area of the Argyll Group (Middle Dalradian). | | Pillow lavas are piles of elongated pods or pillows formed when lavas erupt into water, most commonly through fissures in the sea bed. Rapid chilling of the surface of the lava creates an envelope, inflated from the lava source until the pillow form is complete, at which point it detaches and settles on to either the sea bed or previously formed pillows. At this stage the interior is still molten and plastic, so its shape adjusts to the pillows below. Typical features of pillow lavas are: chilled, fine grained margins; convex upper surfaces; concentric, radial fractures; and, often, a hollow core. Rarely do individual pillows exceed 1m in diameter. | | The crags around Craig exhibit ancient pillow lavas and, despite metamorphism which has altered the mineralogy of the original basalt and stretched the deposit during metamorphism, the original structures have survived with remarkable detail. Individual pillows, up to 1m in diameter, show recognisable chilled margins with enclosed vesicles (originally gas bubbles) and radial fractures. Sagging (i.e. accommodating their shape to the pillows below) is also evident. Material between the pillows appears to have been sediment composed of grains similar in composition to the lavas. | | The massive crystalline limestones typical of the Dungiven Limestone Formation are seen in a disused stone pit, 150m east of the buildings, topped by a coarse psammite (originally sandstone) and black pelitic schist (originally shale or mudstone). It is this deposit that the pillows inundated. | | At the close of the Middle Dalradian, over 600 million years ago, lime deposits were forming on a shallow continental shelf. Periodically sand and mud from nearby land was washed in. Volcanic activity in the area breached this sea bed and created the pillow lava pile now seen around Craig. | | The exceptional preservation of the ancient pillow lavas here is unrivalled in Northern Ireland, offering text book examples for study. Their context and relevance to wider stratigraphy are a vital element in the research base essential for an understanding of Sperrins geology. | | With the possible exception of the stone pit, which could be at risk from dumping, there are no obvious threats to the geological interest of the site. |
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