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Carrickaness Bridge, The Black WaterTyrone
Main falls, 50m NW of Carrickaness Bridge, The Black Water, Co. Tyrone; fall-maker and vertical face comprises 2.3m of calcareous sandstones of Drumowen Sandstone Member (stratotype) of Ballyshannon Limestone Formation.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Omagh District Council
Grid Reference: H2974
Rocks
Rock Age: Carboniferous (Visean)
Rock Name: Ballyshannon Limestone Formation, Drumowen Sandstone Member, Ederny Limestone Member, Tyrone Group
Rock Type: Sand, Sandstone
Interest
Other interest: No data, Marine sediments

Summary of site:

For rocks to be correlated across country, they must be described in sufficient detail for comparisons to be made and they must be given an identity, usually a name of a place where they occur, linked to a descriptive term for the rock. The best, most complete outcrop of a sedimentary rock, ideally where its base and top can be defined, is called a ‘stratotype’ and at all other exposures of the rock it is the standard of comparison.

The sections at Carrickaness Bridge on the Blackwater River form the stratotype of the Drumowen Sandstone Member of the Ballyshannon Limestone Formation. The rock immediately above it is the Ederny Limestone Member of the same formation. North of Carrickaness Bridge on the Blackwater is a series of falls and the rock forming the resistant step in the river is the Drumowen Sandstone. Here it consists of 2.6m of calcareous, medium grained limestone in well defined beds. On freshly hammered surfaces it is pale brown to pale grey in colour but weathered faces have a pink stained, pocked appearance. Vertical burrows of a trace fossil up to 35cm long penetrate through the base of the bed into the crinoidal limestone beneath. At Carrickaness Bridge, on the south bank, almost 3m of the Ederny Limestone Member can be seen resting on the Drumowen Sandstone, clearly defining its top. Here the lime component of the sandstone has been etched by ground water but fine beds (laminations) with some cross laminations (which suggest shallow water deposition) are clearly apparent, much disturbed by sediment feeding animals and, as at the first location, vertical burrows are present. Sandstones in the Ballyshannon Limestone Formation are rare and this limited occurrence may represent sedimentation in an area close to the shoreline, where a supply of sand was available. There is a suggestion that, while the sandstone was accumulating in-shore, the Crockanaver Limestone Member was forming in shallow water well out into open water at the same time. The outcrops at Carrickaness Bridge are the stratotype for the Drumowen Sandstone Member of the Ballyshannon Limestone Formation and are therefore of scientific importance requiring designation. River works are the only likely threat.


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