AbstractAn Ashgill and Llandovery sedimentary succession north‐west of the Tywi Axis has been remapped at a scale of 1:10000. A revised lithostratigraphy is suggested, based on more rigorous sedimentology, and is integrated with earlier biostratigraphic work. The lithostratigraphy is compared with that from other parts of the Welsh basin and the sediments are related to palaeogeography and tectonics.Ashgill sediments indicate a shelf–slope environment which became shallower during the Hirnantian due to glacio‐eustatic regression. During the early Llandovery, sedimentation took place in deeper water mainly on a slope, while middle and upper Llandovery sediments record deposition in a slope to basin enviro