The preglacial course of the River Teifi in southwestern Wales was one of incised meanders, and when the ice retreated, the river cut its new channel across several of the meanders, eroding gorges in the rock floor. A seismic refraction line parallel to the gorge at Cenarth, and across the postulated preglacial meander, shows the Ordovician rock floor to be overlain by 200 feet of glacial deposits, with the deepest point at 30 feet O.D. or 228 feet below the surface. Comparison with results at other points in the valley indicates a gradient of about 8 1/2 feet per mile between Cenarth and Castle Malgwyn, a logical increase upstream from the five feet per mile between Malgwyn and the estuary.