The River Ogney, runs through the town and joins the streams and Boverton Brook which flow in from Eglwys Brewis in the northeast; these then merge and become the Afon Colhuw, which meanders down the Colhuw meadows before discharging through an outfall into the sea. The Llantwit Major area is built on a range of different levels and the town itself is sloping. At the lower coastal level is the flat, glacial Colhugh Valley, marked by steep cliffs on both sides, leading to a pebble beach. The beach, located to the south of the town (), has the remains of an ancient old stone wall from the large Iron Age hill fort, Castle Ditches. The steep cliffs at Llantwit which allow walks along the coast to St Donat's Castle and Atlantic College have undergone dramatic erosion in recent years: in many places, the rock structure has collapsed in piles, particularly on the eastern face.