Thick sand-argillaceous deposits filling the paleorelief roughness are widespread in the south of the Irkutsk amphitheater. The origin of these deposits is still debatable: Some researchers relate them to eolian processes, and others consider them sediments of a dammed paleolake. A series of sand-argillaceous sections was studied in order to refine the genesis of deposits in the area of possible flooding. Results of these studies and laboratory analyses of the deposits argue for their lacustrine genesis and a drastic change in sedimentation conditions during the early formation of the paleolake. The tectonically induced instantaneous Badarma landsliding to the Angara River valley is assumed to have caused the damming of the main rivers in the study area.