Coastal sections observed between Watchet and Kilve contain some of the best exposed normal fault systems in the UK. The normal faults which formed as a result of extension, during the formation of the Bristol Channel Basin show evidence of reactivation. The reactivation of the faults in the Bristol Channel Basin is thought to have occurred in the late Cretaceous and Tertiary due to contraction attributed to the Alpine collision which formed the Alps.
The rocks exposed in the cliffs of the coastal section belong to the Mercian Mudstone Group and the Penarth Group of the Late Triassic, and the Lower Lias of the Lower Jurassic. The faults offset the beds and along the fault plains gypsum has been spread, which record slickensides indicating the movement of the faults.
The photo shows one of the faults which show evidence of mild inversion in the Bristol Channel Basin. The normal fault has clearly offset of the red Triassic rocks of the Mercian Mudstone Group in the footwall and the pale grey Penarth Group in the hanging wall. Along the fault plain is a thin layer of gypsum. This fault has been reactivated and has been interpreted as an inversion structure due to the evidence of contraction, which is highlighted by the tight folding in the hanging wall against the fault. The contraction cannot be accommodated on the original fault due to the steep angle of the fault, therefore folding occurs in the hanging wall. Occasionally later reverse faults called shortcut faults form to accommodate the stress caused by the contraction but this was not observed at this location.
