AbstractGravity and seismic surveys over the estuaries of the rivers Dovey, Dysynni, Glaslyn and Mawddach have indicated that the rock floor lies at depths of from 100 to 1,250 ft (30 m to 380 m) below the present surface. The methods of survey and difficulties of interpretation are discussed, and the significance of the results is assessed in conjunction with those already published for the Teifi estuary and with preliminary results obtained by seismic reflection profiling in. Cardigan Bay. It is concluded that all the valleys with the exception of the Glaslyn could have been carved during Pleistocene times, but that the much deeper buried valley of the Glaslyn, extending south of Portmadoc across Morfa Harlech, is probably of earlier origin.