High-resolution seismic-reflection data were used to determine the subsurface structure and origin of a topographic scarp in the Snake River Plain, which has been ascribed to surface faulting by some investigators. The survey consisted of three lines crossing the scarp along the southern and eastern edges of Clay Butte near Mud Lake, Idaho. Considering the poor seismic-reflection reputation of thick basalt flows, the resulting common depth point seismic sections were of excellent quality. We conclude from these seismic sections that the Clay Butte scarp is not primarily the result of tectonic faulting. It is very likely the edge of a basalt flow and is covered with eolian sand which obscures the edge of the flow. The seismic data also reveal stratigraphic thickness detail on the scale of about 3 m from several reflection horizons.