Summary of site:
A small and isolated area of Carboniferous limestones and shales, perhaps 1 sq km in extent, at Castle Espie, gives an enigmatic glimpse of the final stages of the early Carboniferous. These rocks, the Castle Espie Limestone Formation, were formerly exposed in working quarries, now flooded; no bedrock can now be seen.
A litter of blocks adjacent to the quarries and on the foreshore shows pink and red stained crystalline limestones, many crowded with plates of crinoids. Nineteenth-century accounts describe up to 40ft of limestones in beds of variable thickness, interspersed with shales and resting on a bed of purplish-red and green shales exceeding 6ft in thickness. The Formation was described as resting unconformably on the ancient upturned Silurian rocks of the surrounding area, but nowhere was the contact seen. Fossils are frequent but now fragmented. Brachiopods, corals, crinoids and bryozoa can be recognised and, exceptionally, pieces of large conical cephalopods of the genus Rayonnoneras, its only location in Northern Ireland. With the microscopic foraminiferan Saccaminopsis, these fossils indicate a Brigantian age, the last stage of the Lower Carboniferous, and the only major open water outcrop east of Carganamuck Quarry in Co. Armagh.
This locality tells of the closing stages of the Lower Carboniferous when the tropical sea finally encroached on to, and overwhelmed, low hills of intensely folded Silurian rocks. A profusion of life inhabited these sea beds, including giant cephalopod molluscs (related to octopuses) with straight conical shells that sometimes exceeded a metre in length. Specimens can be seen in the Ulster Museum collection.
Despite the present paucity of exposure at the site, this is a key locality in the history of the inundation of the area by tropical seas as it crossed the equator around 335 million years ago. Its fossil faunas are not fully documented but future pumping or excavation should reveal vital new material and information.