Moho undulations from the analyses of the gravity anomalies over Brittany computed by incorporating various improvements in the classical filtering technique have been compared with deep seismic refraction data. By progressively low-pass filtering the gravity anomalies with increasing cut-off wavelengths, some distinct new anomaly trends associated with deep structures have become prominent. Comparisons with heat flow and wide-angle seismic reflection results confirm that the sources of these anomalies are located at the lowest part of the crust. In Brittany, the image of the Moho undulations reveals the presence of a prominent N130° Moho downwarp cutting across the superficial geological trends. On the surface, this trend corresponds to an active topographic uplift and indicates that the deep crustal bulge is still building up. This structure is bounded by N130° dextral faults that are known to accommodate the actual closure of the Bay of Biscay. West of Brittany, this shortening is also at the origin of an east–west oriented bulge previously mapped with the same technique. This bulge limits and postdates the subsidence of Mesozoic and Cenozoic basins of the Celtic Sea.