The three Tharsis Montes are about apart. They show a distinctive northeast-southwest alignment that has been the source of some interest. Ceraunius Tholus and Uranius Mons follow the same trend to the northeast, and aprons of young lava flows on the flanks of all three Tharsis Montes are aligned in the same northeast-southwest orientation. This line clearly marks a major structural feature in the Martian crust, but its origin is uncertain.