The results of three crustal refraction lines on the western margin of Baffin Bay and one in Lancaster Sound are described. The refraction measurements in Baffin Bay along with earlier refraction, gravity, magnetic, and seismic reflection data are used to define the boundary between continental and oceanic crust. The results suggest that the transition from continental to oceanic material takes place in about 30 km. The seismic refraction data also suggest a sedimentary basin on the continental shelf with at least 6 km thickness of sediment which, however, thins rapidly near Baffin Island. This basin is truncated under the slope by either a basement high or carbonate rocks. Lancaster Sound is filled by about 10 km of sediments that could be either of Mesozoic or Paleozoic age based on comparisons with velocities in nearby wells. The sedimentary and structural characteristics of Lancaster Sound are discussed and related to the concepts of sea-floor spreading and continental drift.