In the northwestern part of the Wessex Basin of southern England, interpretation of seismic reflection and recently released borehole data provides detailed information of the major unconformity at the base of the sedimentary-fill. Seismic mapping of the top of Variscan basement reveals a concealed, faulted, dome-like Palaeozoic basement culmination or structural ‘high’ lying within the larger Cranborne–Fordingbridge High, southwest of Shaftesbury. This structure profoundly affected the deposition of the overlying Permo-Triassic sequence. The culmination, composed of Carboniferous Limestone, lies to the south of northwards-throwing, basement-controlled, syn-sedimentary faults which form the southern margin of the Mere Basin. In this area, the main phase of faulting occurred during Permo-Triassic times with little or no faulting during the Jurassic. Triassic strata onlap the basement culmination which was overstepped and finally submerged in late Triassic times. The structure has much in common with the basement ‘highs’ at Bruton and in the Mendips and provides further evidence for the existence of such features close to the main lines of Mesozoic faulting within the Wessex Basin. It is considered that differential subsidence related to syn-depositional normal faulting was the dominant factor controlling development of the highs. The simple infilling of a pre-existing post-orogenic topography played a minor role.