The summit of Blackstairs Mountain looks down on rolling hills and the neighbouring peaks of the Blackstairs Mountains. Some geological features found here include scree cones, blockfields, ice-moulded rock, tors, and peat cover. Many of these were formed during the last ice age when glaciers were at their largest and thickest. About 24,000 years ago, ice flowed down the valleys and mountains like frozen rivers, covering the landscape. Blackstairs Mountain peak is one of the few in County Wexford that stood out above the ice.
Scree cones are collections of loose rocks on the mountain slopes, formed by periods of freezing and thawing weather that weakened the rock, causing it to break away and move downslope due to gravity. Blockfields can also be seen on the mountain. These piles of loose rocks were formed by similar processes to scree cones, but these rocks have remained in place as there is less of a slope. Rock outcrops that have been flattened and polished by the action of the glaciers as they flowed over their tops are known as ice-moulded bedrock and are also present.
Perhaps the most striking geological feature found here are the large granite tors, known locally as Brans, formed in place as ice, and freezing and thawing broke down the surrounding rock. As well as granites, you may also see other rocks such as schists and slates on the surface to the east, on the Wexford side of the slopes of Blackstairs range.