Twmpa or Lord Hereford's Knob is a mountain in south-east Wales, forming a part of the great northwest scarp of the Black Mountains. It lies 3 km west of the border with England, and around 7 km south of Hay-on-Wye. To the northeast lies the Gospel Pass through which runs a minor road between Hay and the Llanthony Valley. A ridge known as Darren Lwyd tapers away for about 3km to the southeast of the summit.
In common with neighbouring hills, Twmpa is composed of alternate layers of sandstone and mudstone dating from the Devonian period and ascribed to the Lower Old Red Sandstone. Its lower slopes are formed from the mudstone-dominated St Maughans Formation whilst the upper part of the hill is composed of sandstone-dominated Senni Formation rocks. At the junction between the two is a prominent calcrete known as the Ffynnon Limestone.