The shape of the hill was modified during the ice ages as the Usk Valley glacier flowed past it to its north. A small glacier nourished by windblown snow from the plateau excavated the hollow on the eastern side of Blorenge which is known as The Punchbowl. Cwm Craf on the hill's north-eastern slopes has a cirque-like form though probably never harboured a full-grown glacier. Small landslips affect Blorenge's slopes, the most well-known of which is the Tumble through which the B4246 climbs towards Keeper's Pond.