Eight reversed seismic refraction profiles have been recorded across the Phanerozoic Williston sedimentary basin that overlies the margins of the Superior and Wyoming Archean cratons and the Proterozoic Trans-Hudson orogen. These data have been modeled using two-dimensional ray-tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals. Within the southern extension of the Superior craton margin (Thompson belt) a high-velocity (~6.4 km/s) lid overlies a narrow region of high electrical conductivity, and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge (Thompson fault) of the craton margin. Complex zones of low-velocity material and the North American Central Plains electrical conductivity anomaly are concluded to lie within the southern extension of the Reindeer--South Indian Lakes terrain, a component of the Trans-Hudson orogen. A high-velocity (>7.0 km/s) lower crustal layer, which represents a zone of complexity separating ''normal'' crust from the upper mantle, extends throughout the region. The crust is relatively thick, with depths to Moho varying smoothly between 41 and 48 km. There is no evidence in the relatively low-resolution seismic refraction data for abrupt changes in the level of the Moho, but there may be a minor (3--4 km) thinning of the crust near the center of the Williston Basin. ¿American Geophysical Union 1987