The extent to which persistent, rather than transient, fissures (wide planar voids) can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon-bearing fluids. Variscan (late Carboniferous) faults cutting Dinantian (Lower Carboniferous) limestones on the Gower peninsula, South Wales, host clear evidence for fissures up to several metres wide. Evidence includes dendritic hematite growth and elongate calcite growth into open voids, spar ball and cockade breccia formation, laminated sediment infill and void-collapse breccias. Detailed mapping reveals cross-cutting geometries and brecciation of earlier fissure fills, showing that fissures were formed during, rather than after, active faulting. Fissures therefore probably formed by geometric mismatch between displaced fault walls, rather than by solution widening along inactive faults.