The deep structure of the Vale of Glamorgan is investigated using recently acquired seismic data. Reflection sections from a commercial seismic survey in the Vale enable the Carboniferous Limestone, Old Red Sandstone and Silurian sequences to be mapped in the subsurface. The Old Red Sandstone sequence thins westwards under the southern part of the Vale, and in the southwest corner of the Vale the base of the Old Red Sandstone is interpreted to overstep across the underlying Silurian sequence in an analogous manner to its regional overstep in west Wales. The Precambrian basement surface is not clearly represented on the sections but may coincide with the base of a reflective sequence observed in places along one of the reflection lines. Basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines. Basement depths remain uncertain to 1–2 km but appear to be greater under the central part of the Vale than under the western part and along the south coastal zone. A linked system of Variscan forethrusts is traced to outcrop in the core of the Cowbridge anticline but none involves major displacement. The Ty'n-y-Nant-Moel Gilau fault system of the South Wales coalfield is shown to be a component of the linked fault system and its current net extension is attributed to Mesozoic reactivation of a Variscan thrust involving only limited displacement. The individual thrusts appear to connect to a basal thrust that may extend northwards into or under the South Wales coalfield. The amount of displacement along the basal thrust cannot be determined reliably but it may exceed 10 km and involve large scale repetition of Palaeozoic sequences under the Vale.

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