This big canyon was Kasei Vallis, one of the clearest manifestations of ancient flooding anywhere on Mars. A single glance at a satellite photo and it was obvious that a very large flood had run down Echus Chasma once upon a time, until it reached a break in its great eastern wall, perhaps a graben. The water had turned right down this valley and smashed through it with fantastic force, eroding the entrance until it was a smooth curve, slopping over the outside bank of the turn and ripping at joints in the rock until they were a complex gridwork of narrow canyons.
