A modern road bridge across the Afon Alwen is proposed to replace the existing A5 Pont Melin Rûg, a masonry arch structure built in 1792. However, progress on engineering design has been hampered by problematic ground conditions caused by a previously unrecorded, 40–50 m thick sequence of soft to very soft glaciolacustrine silts (Rûg Silts). The remarkable depth of the Rûg Silts and the need to locate a foundation stratum for the proposed new bridge prompted the application of geophysical techniques. A high-resolution seismic reflection survey was performed to image the subsurface conditions at the proposed bridge site to a depth of 100 m below ground level and provide a comprehensive understanding of the geological profile. The seismic survey successfully delineated the bottom of the Rûg Silts, identified a basal unit of very dense, heterogeneous glaciofluvial gravels and delimited bedrock of Silurian Nantglyn Flags. Subsequently, a single deep borehole confirmed the findings of the geophysical survey and importantly, the investigations proved a possible foundation horizon for the proposed new bridge at a depth of 45.80 m in the basal gravel unit. The seismology provided the largest and most continuous source of geological information across the site and consequently had the greatest influence on the deliberations on foundation selection and optimization. This case history illustrates the benefits of using reflection seismology in areas of complex glacial geology. It also demonstrates the benefits of geophysical investigations to geotechnical assessments of local ground conditions.