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Little River and Slate Quarry BridgeTyrone
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Summary Full report
Site Type: Stream section
Site Status: ASSI
District: Cookstown District Council
Grid Reference: H72977268, H727727
Rock Age: Silurian, Ordovician (Ashgill, Cautleyan, Hirnantian, Llandovery)
Rock Name: Killey Bridge Formation, Tirnaskea Formation
Rock Type: Mudstone, Sand, Sandstone, Shale
Fossil Groups: Bivalve, Brachiopod, Graptolite, Trilobite
Other interest: Caledonian, fault, No Data

Summary of site:

These two adjacent sites, exposing rocks of Ordovician and earliest Silurian age, formed around 440 to 439 million years old, are of national and international importance. They incorporate the type locality of the Tirnaskea Formation and a large number of fossils new to science were found here. The rocks have been considerably disturbed by thrusting and folding and exposure is so limited that relationships can only be determined by reference to the age of fossils and rock types.

The two formations found here are the Killey Bridge below and the Tirnaskea above. The Killey Bridge Formation is a grey, lime-rich mudstone with an exceptionally large, diverse and informative fossil fauna that proves its age to be Cautleyan, a stage of the Ashgill series, close to the end of the Ordovician period. The overgrown quarry was the source of most of the fossils and together with the river exposure provided 60 species entirely new to science. Most are small brachiopods, bivalves and members of a group of trilobites called trinucleids with very distinct head shields but there are many other groups in the fauna. Species are divided up into closely related groups called genera (singular, genus). For example Tyrannosaurus rex, the famous dinosaur, has a typical name reflecting this; the species name is rex and the genus to which it is allocated is Tyrannosaurus. There could be more than one species of Tyrannosaurus in which case a different species name would be allocated. The scientist who first described the genus created both names so this was not simply the type specimen (the first described specimen) of the species rex, it was also the genotype for the genus Tyrannosaurus. The old quarry is the type locality for two internationally important genotypes. The Tirnaskea Formation is intermittently exposed in Tirnaskea stream where it consists of dark grey mudstones containing relatively few fossils belonging to a restricted set of groups. Towards the top the rocks change colour to pale green mudstones which grade into blue-grey shales with graptolites and it is these graptolites that give the first indication that Silurian rocks are present. The Killey Bridge rocks were formed on the outer edge of the continental platform in fairly deep water. From here they appear to have slid into much deeper water carrying their fossils with them. All the fossils belong to the Cautleyan stage and there is no fossil evidence to support the presence of the later Rawtheyan stage. The Tirnaskea Formation rocks have fewer fossils from a smaller number of groups and include graptolites. This would suggest deeper and possibly cooler water (although the rocks formed in the tropics) reflecting polar glaciations at this time. Their Northern Ireland significance rests on the exceptional exposure of late Ordovician rocks and their fossil content, enhanced by the stratotype (the defining exposure) of the Tirnaskea Formation. Internationally this is a key site which gave vital information on the rich faunas of the Cautleyan stage and expanded them with over 60 new species. Authorities studying the pal'ontology of this time are frequent visitors to the locality. It is a designated Area of Special Scientific Interest (ASSI) and therefore protected. The south bank of the Little River has been further quarried from 1992 exposing more of the Killey Bridge Formation and, while this has improved access to the rocks, such activity needs to be carefully managed and monitored. Vegetational cover of small but key outcrops is a problem and periodic careful clearing to keep sections fresh is recommended.

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