Lisdoo ASSI

The green mudstones at Lisdoo contain diagnostic miospores indicative of the Upper Devonian period. This site was one of the first to yield fossils of undisputed Devonian age from Northern Ireland.

The pit at Lisdoo exposes green mudstones which, when examined microscopically, are seen to contain preserved plant miospores representing some 29 different plant types. This site was one of first to yield fossils of undisputed Devonian age from Northern Ireland.

The rocks at Lisdoo belong to the Shanmullagh Formation. An absence of diagnostic macro- or micro-fossil materials in these rocks has resulted, in the past, in uncertainty over their age. The green mudstones at sites such as Lisdoo have been used to date these rocks which here indicate an Upper Devonian age some 370 million years old.

The green mudstones were deposited in a shallow lake which periodically received small amounts of sand either blown in by wind action from the surrounding desert or, in the case of thicker beds, due to transport by temporary rivers.