Sliabh an Iarainn, at the southern end of this mass, and east of Lough Allen, is a flat topped mountain with a prominent and steep grit slope, easily mistaken for the summit from a distance, when in fact another of shales form a small residual outlier overlying this grit. There is a thick obscuring mantle of peat bog and glacial drift below the steep grit slope, with heather and peat bog forming a thick mantle over most of the upland plateau at the summit. Rocks are typically horizontal or gently dipping, except in land-slipped areas. Impressive landslides have occurred along the western face, and at the south-western and south-eastern corners of the mountain, indicating an appreciative magnitude of land-slipping.