Tircreven ASSI

The stream section at Tircreven is important because of its geology, in particular an exposed series of Jurassic rocks, including the type section of the Tircreven Sandstone Member, within a sequence of the Waterloo Mudstone Formation. Fossil rich sections are found at a number of locations through the site (both of Jurassic and Cretaceous ages) but of particular importance is the material found at the base of the Cretaceous age rock making this the only site in Northern Ireland at which a Cretaceous basal conglomerate contains abundant macro-vertebrate remains.

The Jurassic rocks here belong to the Lower Lias, specifically the Sinemurian stage, dating from 195 million years ago with a total thickness of around 52m.

The lowest Cretaceous material belongs to the Hibernian Greensands Formation and at Tircreven Burn this is quite different from rocks thought to be of equivalent age developed in East Antrim.

The fossils include the bivalve Gryphaea arcuata together with phosphatic faecal pellets, fish teeth and other fish bones. The latter material forms a notable ‘bone bed’ at the base of these Cretaceous rocks.

The age of this ‘bone bed’ is uncertain. The basal 'bone-bed' contains small rolled fish teeth and bones together with reworked Gryphaea arcuata and columnals of the crinoid or sea-lily Isocrinus sp. both of which are undoubtedly derived from the underlying Jurassic rocks.

In addition there are small gastropods and bivalves, some 1-3 mm in size, which may either be derived from the Lias or may perhaps be contemporary and thus of Cretaceous age. However a number of other fossils including the echinoid Micraster ex gr. gibbus, the bivalves Gryphaeostrea canaliculata and Limatula semisulcata together with teeth of the shark Squalicorax falcatus can all definitely be assigned to the Cretaceous period. The only other fish remains formally identified to date are teeth of Oxyrhina, a Cretaceous chimaeroid, a cartilaginous fish closely related to the sharks.

Tircreven appears now to be the only site on the western side of the Antrim Plateau to expose the basal beds of the Cretaceous succession. While the age of the basal conglomerates here, and indeed of much of the rest of the Cretaceous series at this site, are not entirely clear and their correlation to other sites in Northern Ireland presents difficulties, this highlights the geological significance of the locality.