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Garvagh BurnTyrone
Garvagh Burn, Glenelly, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
Grid Reference: H621937
Rock Age: Precambrian (Dalradian)
Rock Name: Garvagh Bridge Member, Glenelly Formation, Southern Highlands Group
Rock Type: Green beds, Limestone
Other interest: No data, No Data

Summary of site:

The Dalradian metamorphic rocks are among the oldest found in Northern Ireland. They date either side of 600 million years ago, a time before fossils recognisable to the naked eye had evolved. Because they are so highly altered, they are also difficult to interpret, which is why it is useful to be able to recognise and trace their divisions. This site is important because one of these divisions is defined here, giving it the status of a stratotype (the standard of comparison for all other rocks of this division).

The Dalradian is divided into two major groups, the older Argyll Group and the younger Southern Highland Group. This later group is subdivided into four formations, from the base: the Dart, Glenelly, Glengawna and Mullaghcarn. The stratotype at Garvagh Burn is of the Garvagh Bridge Member, the last of four subdivisions of the Glenelly Formation. The member is best developed in an abandoned quarry where the full thickness of 18m can be seen. It consists of beds of lime-rich, altered mudstone and schist containing silvery mica, well-formed albite (a mineral of the feldspar group) crystals, the green mineral chlorite and needles of almost black tourmaline. Between the beds are bands and lenses of grey, medium-grained, crystalline limestones, sometimes showing calcite grains. The rocks were formed almost at the South Pole (then ice-free) about 600 million years ago as the giant continent of Gondwana rifted and shed the supercontinent of Laurentia. As the two separated and the crust between them stretched and thinned, a marine basin was formed which was rapidly silted by unstable sediments sliding from the crustal shallows into deeper water. The Garvagh Bridge Member represents a period of stable, shallow water conditions at the margin of the basin in which limestones could form. These deposits survived as sedimentary rocks until about 465 million years ago when they were caught in a continental collision that thrust them deeper into the earth’s crust. There they were heated and compressed on a regional scale, causing the metamorphic transformation we now see. As part of that process the rocks across Northern Ireland were incorporated into a giant fold that ‘fell over’ to the south east, inverting the south-easterly limb. The rocks at Garvagh Burn are part of this structure, so the succession is upside down. Smaller scale, metamorphic structures are also superimposed on these rocks. Stratotypes are the essential components of the geological history of an area and this member is consequently of national importance and should be protected. The management of this site should ensure that no further dumping occurs and that rubbish already accumulated is removed.

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