An 18 km long N–S seismic reflection profile has been acquired across the faulted boundary between the Alston Block and the Northumberland Trough. A NW–SE geological cross-section has been constructed across the trough by integrating this line with other data, and a Tricentrol seismic profile confirms the suitability of this model. The trough has a markedly asymmetric form, with a thickness of more than 4.2 km of Dinantian strata adjacent to its faulted southern margin. The present day surface faults (Stublick, Ninety Fathom) are related to Variscan and later inversion and transpression, and do not everywhere correspond precisely to the earlier syn-depositional normal faults. The basin is believed to have formed in response to reactivation of the Iapetus Suture in a dominantly N–S extensional stress field. Basin evolution has been analysed by fault restorations and subsidence modelling. The fault restorations indicate an upper crustal extension factor of 1.15 to 1.19. Subsidence modelling indicates a whole crustal extension factor of 1.30, with similar sub-crustal lithospheric extension. It is possible that the difference between the inferred extension factors is due to non-uniform (increasing downwards) lithospheric extension, but the uncertainties inherent in both methods are such that this cannot yet be confirmed.