Slieve Anierin is an important natural heritage site due to unbroken sequence of Carboniferous marine fossils present in the rock layers spanning the Namurian (326-315 million years ago) and lower Westphalian (313-304 million years ago) stages of the Silesian (series).
In her landmark study "The Palaeontology of the Namurian rocks of Slieve Anierin, County Leitrim, Eire", Patricia Yates (1962) demonstrated a "remarkable extent" of Namurian marine fauna bands, abundant with goniatite-Bivalvia, at Slieve Anierin.