Much of Llanfoist is built on the Llanfoist-Nevil Hall moraine, deposited by a receding Usk valley glacier during the last ice age. The wooded pool in the foreground does not appear on Ordnance Survey maps yet is an enduring feature, filling a large hollow in the moraine. Cwm Craf on the front face of Blorenge beyond is likely to be a nivation hollow rather than a glacial cirque.